membranes: transport, potentials, and...

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© Copyright 2012, John P. Fisher, All Rights Reserved

Membranes: Transport, Potentials, and Muscle

Adapted From:

Textbook Of Medical Physiology, 11th Ed. Arthur C. Guyton, John E. Hall

Chapters 4, 5, 6, 7, and 8

John P. Fisher


Lipid Barrier of the Cell Membrane •  All molecules may

eventually pass through the cell membrane, but the rate of transport varies greatly with the properties of the molecule

•  Small (<100 Da), hydrophobic, nonpolar, and neutral molecules all pass quickly

•  Larger, hydrophilic, and

charged molecules are highly impermeable •  These molecules may

pass through the membrane using protein channels

Chemical Composition of Extracellular and Intracellular Fluids

EXTRACELLULAR INTRACELLULAR

Hydrophobic Molecules O2, CO2, N2, Benzene

Large, Neutral, Polar Molecules H2O, Urea, Glycerol,

Glucose, Sucrose

Charges Molecules (Ions) H+, Na+, HCO3

-, K+, Ca++, Cl-, Mg++

2


EXTRACELLULAR

Na+ 142 mEq/l 10 mEq/l K+ 4 mEq/l 140 mEq/l Ca++ 2.4 mEq/l 0.0001 mEq/l Mg++ 1.2 mEq/l 58 mEq/l Cl- 103 mEq/l 4 mEq/l HCO3

- 28 mEq/l 10 mEq/l Phosphates 4 mEq/l 75 mEq/l SO4

-- 1 mEq/l 2 mEq/l Glucose 90 mg/dl 0 to 20 mg/dl Amino Acids 30 mg/dl 200 mg/dl Proteins 2000 mg/dl 16000 mg/dl Phospholipids 500 mg/dl 2 to 95 mg/dl pO2 35 mmHg 20 mmHg pCO2 46 mmHg 50 mmHg pH 7.4 7.0

INTRACELLULAR

Lipid Barrier of the Cell Membrane Chemical Composition of Extracellular and Intracellular Fluids •  All molecules may

eventually pass through the cell membrane, but the rate of transport varies greatly with the properties of the molecule

•  Small (<100 Da), hydrophobic, nonpolar, and neutral molecules all pass quickly

•  Larger, hydrophilic, and

charged molecules are highly impermeable •  These molecules may

pass through the membrane using protein channels


Lipid Barrier of the Cell Membrane Basic Structure of the Cell Membrane

•  Thin film of noncovalently bound lipids and proteins

•  Most of these molecules are free to move within the membrane

•  Encloses the cell and defines its boundaries

•  Maintains essential differences, such as chemical and ionic, between the cell’s cytosol and the extracellular environment

lipid bylayer containing proteins

membrane components are mobile

3


Lipid Barrier of the Cell Membrane Basic Structure of the Cell Membrane •  There are approximately 5 million lipid molecules

in 1 µm2 area of a lipid bilayer

•  Lipid molecules are amphiphilic, meaning they contain a hydrophilic and a hydrophobic domain

•  Typically formed from phospholipids •  Hydrophilic head contains polar, water

soluble groups •  Hydrophobic tail contains long fatty acids

•  14 – 24 carbons + carbonyl group •  One tail has one or more cis-double

bonds

•  Forms micelles or bilayers, depending upon structure and concentration

fatt

y ac

id

phosphate

choline

glycerol

fatt

y ac

id

lipid

Hydrophilic (water soluble)

Hydrophobic (lipid soluble)


Lipid Barrier of the Cell Membrane

Guyton & Hall. Textbook of Medical Physiology, 11th Edition

Transport Through the Cell Membrane

•  Many molecules, including ions and large molecules do not permeate the cell’s plasma membrane

•  Transport of these molecules occurs through a membrane bound protein

•  Transport can be accomplished by •  Diffusion

•  Simple diffusion •  Facilitated diffusion

•  Active transport

4


Diffusion Diffusion Through Protein Channels •  Substances can move by simple diffusion through protein channels that span the lipid

bilayer that makes up the cell membrane •  Channels are often selectively permeable •  Channels are sometimes gated

•  Sodium Channels •  Protein channel, 0.3 by 0.5 nm in diameter •  Strongly negatively charged, allowing dehydrated sodium ions to pass through

the membrane

•  Potassium Channels •  Protein channel, 0.3 by 0.3 nm in diameter •  Uncharged, allowing hydrated potassium to pass, but preventing larger hydrated

sodium to pass through


Diffusion Diffusion Through Protein Channels •  Critical Parameters

•  Molecule diameter : Pore diameter •  Bulk flow : Solute diffusion

•  Governing equation •  Steady state molar flux, J

W is a hydrodynamic coefficient* V0 is the bulk fluid velocity c0 is the solute concentration at the pore entrance cL is the solute concentration at the pore exit Pe is the Peclet Number *W depends upon pore geometry as well as particle – pore interactions

Pe

PeL

eecccWVJ

−

−

−

−=

1)/(1 0

00

r0

rs

r

5


Diffusion Diffusion Through Protein Channels

•  Peclet Number, Pe = WV0L/HD∞

•  Compares bulk flow to solute diffusion •  L is the membrane thickness •  H is a hydrodynamic coefficient

•  For Pe << 1 •  Diffusion dominates •  J = HD∞/L(c0 – cL)

•  For Pe >> 1 •  Bulk flow dominates •  J = WV0

r0

rs

r


Diffusion Gated Protein Channels •  The “gating” of gated channels may be controlled

•  Voltage Gated Channels •  The molecular conformation of the protein channel changes in response to the

electrical potential across the cell membrane •  Sodium channels in action potential propagation

•  A strong negative, intracellular charge causes gate to remain closed •  Loss of this charge opens the gate and allows influx of sodium ions •  This, and similar, channels typically exhibit “all or none” response

•  Chemical Gated Channels

•  The molecular confirmation of the protein channel changes in response to ligand binding

•  Acetylcholine channel in neuron synapes: Acetylcholine binds the membrane channel, providing a negatively charged pore about 0.65 nm in diameter

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Diffusion Gated Protein Channels •  Ion current flow through protein

channels has typically been described using a patch-clamp technique

•  Here, a micropipette with a tip diameter of 1 to 2 µm is brought into contact with a single cell

•  Next a small suction force is applied to the other end of the micropipette, sealing the cell membrane to the micropipette tip



Diffusion Facilitated Diffusion

•  Facilitated transport is a non-energetic consuming process (no ATP is consumed) where conformational changes in the membrane bound protein shuttle extracellular molecules into the cell •  Conformational changes occur with or without a bound solute •  Conformational changes occur due to natural thermal fluctuations (Brownian

motion) •  Transport is regulated by the number of facilitated transport proteins and their rate

of conformational change •  Transport is specific between receptor and ligand •  Transport occurs in both directions across the membrane, but net transport in down

the concentration gradient

extracellular

intracellular bound change unbound delivery

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Diffusion Facilitated Diffusion •  Model

•  Release is much faster than conformational change •  Intracellular solute concentration is low •  Equilibrium state is reached between S + P and C •  All membrane bound protein is either free or in a complex

C S*P S + P S* + P

PSESCPS kk

k

k

+⎯→⎯⎯→⎯⎯⎯←

⎯→⎯+

−

** 32

1

1

23 kk >>

0* ≈SCPS ←

→+

CPPT +=


Diffusion Facilitated Diffusion •  Complex formation

•  Complex dissociation

•  Thus where

C S*P S + P S* + P

m

m

KSSVJ

+= TPkV 2max =

1

21

kkkKm

+= −

)(11 CPSkSPkV Tf −==

CkCkVd 21 += −

8


Diffusion Facilitated Diffusion Glucose Transport •  Glucose must enter the cell cytoplasm for metabolism, but it cannot diffuse through

the cell membrane nor the pores of the membrane •  A glucose transporter utilizes facilitated transport to carry glucose across the cell

membrane •  The transporter shuttles glucose in either direction (into or out of the cell) but the

high concentration of extracellular glucose ensures that the overall flux of the nutrient is into the cell

Amino Acid Transport •  Most amino acids also enter cells by facilitated transport

bound change unbound delivery


Diffusion Water Transport •  Water transport into eukaryotic cells is very rapid – up to 100x the volume of red

blood cells per second – but is also highly regulated so that the net water flux approaches zero

•  Cellular water content is largely regulated by osmosis •  The concentration of water is controlled by the relative amount of molecules on

either side of the selectively permeable cell membrane •  If molecular content is too high within the cell, water will move into the cell

to reduce overall molecular concentration •  If molecular content is too low within the cell, water will move out of the cell

to increase overall molecular concentration •  Thus, the key parameter is molecular concentration

•  Osmolality (osmoles/kg) describes molecular concentration, and is simply calculated by multiplying the molality by the number of dissociable groups in that molecule

•  Similarly, osmolarity (osmoles/l) is calculated by multiplying molarity by the number of dissociable groups in that molecule

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Active Transport Types of Active Transport

•  Similar to facilitated transport, however energy is required •  Primary Active Transport: Energy is used directly, as in the consumption of ATP

to ADP •  Secondary Active Transport: Energy is derived from another source, such as a

concentration gradient established by primary active transport •  Transport can occur against the concentration gradient across the plasma membrane •  Transport typically utilizes membrane bound carrier proteins •  Transport can be regulated by the relative number of ligands and / or receptors

•  Therefore, transport can be saturated

ATP ADP


Active Transport Primary Active Transport

•  Ions cannot permeate the plasma membrane

•  Ions are moved across the plasma membrane by ion channels

•  Ions are also moved across the plasma membrane, even against a concentration gradient, by active transport proteins

•  Unregulated flux of ions would equilibrate a diffusion gradient with an ionic gradient

EXTRACELLULAR

Na+ 142 mEq/l 10 mEq/l K+ 4 mEq/l 140 mEq/l Ca++ 2.4 mEq/l 0.0001 mEq/l Mg++ 1.2 mEq/l 58 mEq/l Cl- 103 mEq/l 4 mEq/l HCO3

- 28 mEq/l 10 mEq/l Phosphates 4 mEq/l 75 mEq/l SO4

-- 1 mEq/l 2 mEq/l Glucose 90 mg/dl 0 to 20 mg/dl Amino Acids 30 mg/dl 200 mg/dl Phospholipids 500 mg/dl 2 to 95 mg/dl pO2 35 mmHg 20 mmHg pCO2 46 mmHg 50 mmHg pH 7.4 7.0

INTRACELLULAR

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•  A Na+-K+ pump is found in nearly all eukaryotic cells •  3 Na+ are pumped out of the cell, while 2 K+ are pumped into the cell, with both

transports occurring against the established concentration gradient

Na+

K+

ATP

ADP

P

Na+

P

Na+

P

K+

K+

P

Na+

Na+

Na+ Na+

K+

K+

extracellular

intracellular

K+



•  Other forms of primary active transport include

•  Calcium Pump: Pushes Ca++ either out of the cell, or into organelles particularly the sarcoplasmic reticulum and mithochondria

•  Hydrogen Pump: Gastric glands release H+, in the form of HCl, as a part of digestive secretions; kidney renal tubules release H+ into the urine to control overall body hydrogen ion concentration

ATP ADP

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Active Transport Secondary Active Transport

•  Secondary active transport can take two forms

•  Cotransport: Established concentration gradient of substance X also carries substance Y in the same direction (down the concentration gradient of X)

•  Examples include sodium cotransport of glucose, sodium cotransport of amino acids, sodium-potassium cotransport of 2 chlorides, and potassium cotransport of chloride

•  Countertransport: Established concentration gradient of substance X carries substance Y in the opposite direction (up the concentration gradient of X)

•  Examples include sodium countertransport of calcium as well as sodium countertransport of hydrogen


Membrane Potentials and Action Potentials Membrane Potentials

•  Electrical potentials exist across the membranes of virtually all cells of the body

•  Nerve and muscle cells are capable of generating rapidly changing electrochemical impulses at their membranes, and these impulses are used to transmit signals along the nerve or muscle membranes

•  In glandular cells, macrophages, and ciliated cells, local changes in membrane potentials also activate many of the cells' functions

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Membrane Potentials Membrane Potentials Are Caused by Diffusion •  K+ is high inside a nerve fiber membrane,

but K+ is low outside the membrane

•  Ignoring other ions, the large K+ gradient causes K+ to diffuse outward through the membrane •  K+ carry positive electrical charges to

the outside, thus creating electropositivity outside the membrane and electronegativity inside because of negative anions that remain behind

•  The potential difference ultimately is balanced with the concentration difference and further K+ transport is stopped

•  In the normal mammalian nerve fiber, the potential difference created by the K+ concentration difference is about 94 mV, with negativity inside the fiber membrane



Membrane Potentials Membrane Potentials Are Caused by Diffusion •  Na+ is high outside the membrane, while

Na+ is low inside the membrane

•  Ignoring all other ions, Na+ diffuses to the inside and creates electropositivity inside the membrane and electronegativity outside •  Membrane potential is in opposition to

that created by K +

•  Again, the potential difference is then balanced by the concentration difference

•  In the normal mammalian nerve fiber, the potential difference created by the Na+ concentration difference is about 61 mV positive inside the fiber


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Membrane Potential Nernst Equation

•  The diffusion potential level across a membrane that exactly opposes the net diffusion of a particular ion through the membrane is called the Nernst potential for that ion

•  The magnitude of this Nernst potential is determined by the ratio of the concentrations of that specific ion on the two sides of the membrane

•  The greater this ratio, the greater the tendency for the ion to diffuse in one direction, and therefore the greater the Nernst potential required to prevent additional net diffusion


Membrane Potential Nernst Equation

•  The membrane potential, or Nernst potential, for a single ion is given as

•  Vs potential difference •  R gas constant •  T absolute temperature •  F Faraday’s constant •  z charge on the ion •  Sin intracellular ion concentration •  Sout extracellular ion concentration

RT zF

Vs = ln Sout

Sin ( )

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Membrane Potential Goldman-Hodgkin-Katz Equation

•  Under the influence of sodium, potassium, and chloride ions, the Nernst Equation expands to the Goldman-Hodgkin-Katz Equation

•  Notes about the GHK Equation •  K+, Na+, and Cl- are the most significant ions in nerve and muscle fibers •  Permeability controls significance •  Note sign convention •  Cl- does not change greatly during transmission of a nerve impulse, so it plays a

rather minor role in this phenomena

Vm = - ln RT F

PK+ CK+in + PNa+CNa+in + PCl- CCl-out ( ) PK+ CK+out + PNa+CNa+out + PCl- CCl-in


Membrane Potential Resting Membrane Potential

•  In vivo, and in the presence of normal physiological conditions, the resting membrane potential of mammalian cells varies from -60 to -90 mV

-

- -

-

-

- +

+

+

+ +

+

+

+ +

+

+ +

Negative Potential

- - - -

- -

15


Membrane Potential Measuring Membrane Potential

•  A small pipette filled with an electrolyte solution is impaled through the cell membrane to the interior of the fiber

•  A second electrode, called the "indifferent electrode," is placed in the extracellular fluid

•  The potential difference between the inside and outside of the fiber is measured using an appropriate voltmeter

•  For recording rapid changes in the membrane potential during transmission of nerve impulses, the microelectrode is connected to an oscilloscope



Membrane Potential Measuring Membrane Potential

•  To create a negative potential inside the membrane, only enough positive ions to develop the electrical dipole layer at the membrane itself must be transported outward

•  Thus an incredibly small number of ions needs to be transferred through the membrane to establish the normal "resting potential" of -90 mV inside the nerve fiber

•  Only about 1/3,000,000 to 1/100,000,000 of the total positive charges inside the fiber needs to be transferred

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Membrane Potential Resting Membrane Potential of Nerves •  The resting membrane potential of large nerve fibers when not transmitting nerve

signals is about -90 mV •  The potential inside the fiber is 90 mV more negative than the potential in the

extracellular fluid on the outside of the fiber

•  Maintenance of the resting potential •  Na+-K+ pump influences both ion concentration and electropotential

•  Leakage of potassium and sodium •  A channel protein exists in the nerve membrane through which K+ and Na+

can leak (K+ Na+ Leak Channel) •  The emphasis is on K+ leakage because, on average, the channels are 100

times more permeable to K+ than to Na+


Membrane Potential Origin of Resting Membrane Potential •  Potassium Diffusion Potential

•  If K+ is the only ion to move through the membrane (channel), and since there is a high ratio of K+ inside to outside (35:1), the resting potential inside the fiber would be equal to -94 mV

•  Sodium Diffusion Potential •  There is a slight permeability of the nerve

membrane to Na+ due to the K+ Na+ leak channel

•  In addition, the ratio of Na+ from inside to outside the membrane is 0.1, and this gives a calculated Nernst potential for the inside of the membrane of +61 mV

•  Combining with the GHK equation, the resting potential is -86 mV


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Membrane Potential Origin of Resting Membrane Potential •  Na+-K+ Pump

•  The 3 Na+ pumped outside and the 2 K+ pumped inside causes continual loss of positive charges from inside the membrane; this creates an additional -4 mV on the inside beyond that which can be accounted for by diffusion

•  The net membrane potential with all these factors operative at the same time is about -90 mV



Nerve Action Potential Nerve Signals are Transmitted by Action Potentials •  Nerve signals are rapid changes in the

membrane potential that spread rapidly along the nerve fiber membrane

•  Each action potential begins with a sudden change from the normal resting negative membrane potential to a positive potential

•  Each action potential ends with an almost equally rapid change back to the negative potential

•  The action potential moves along the nerve fiber until it comes to the fiber's end


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Nerve Action Potential Nerve Signals are Transmitted by Action Potentials •  Resting Stage

•  Initial state where the membrane is polarized due to the -90 mV potential

•  Depolarization Stage •  Membrane suddenly becomes very

permeable to Na+ ions, allowing Na+ into the cell

•  The -90 mV state is immediately neutralized by the inflowing Na+

•  Potential rapidly rises in the positive direction: depolarization

•  Repolarization Stage •  Na+ channels begin to close and the K+

channels open more than normal •  Rapid diffusion of K+ to the exterior re-

establishes the normal negative resting membrane potential: repolarization



Nerve Action Potential Voltage-Gated Na+ and K+ Channels

•  Voltage-gated Na+ channel drives the action potential

•  Voltage-gated K+ channel speeds the repolarization of the membrane

•  Voltage-Gated Na+ Channel •  Channel has two gates, one near the

outside of the channel called the activation gate and one near the inside called the inactivation gate

•  At rest (-90 mV) the activation gate is closed

•  When the membrane potential rises from -90 mV, at approximately -70 to -50 mV, the activation gate opens (activated state) increasing the Na+permeability as much as 500 to 5000 times


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•  Voltage-Gated Na+ Channel •  However, this same change in

potential closes the inactivation gate 0.0001+ seconds later (inactivated state)

•  Inactivation gate will not reopen until potential returns to resting level

•  Thus, Na+ channel will not reopen until repolarization

•  The membrane potential begins to recover back toward the resting membrane state, which is the repolarization process




•  Voltage-Gated K+ Channel •  During resting state, K+ gate is closed •  As the resting membrane potential

rises from -90mV, K+ gate slow begins to open

•  Opening not fully effective until Na+ inactivation gate has already begun to close

•  Thus decreasing Na+ flux in and increasing K+ flux out, speeds repoloarization


20


Nerve Action Potential Measuring the Effect of Voltage on Voltage-Gated Channels: The "Voltage Clamp." •  The Voltage Clamp research led to a quantitative understanding of Na+ and K+

channels: Hodgkin and Huxley received the Nobel Prize for this work

•  Setup: Two electrodes are inserted into the nerve fiber •  One measures the voltage of the membrane potential and one conducts electrical

current into or out of the nerve fiber

•  An experimental voltage is determined and the electronic portion of the apparatus injects either positive or negative electricity through the current electrode into the cell



Nerve Action Potential Measuring the Effect of Voltage on Voltage-Gated Channels: The "Voltage Clamp." •  When the membrane potential is suddenly increased by this voltage clamp from -90

mV to 0 mV, the voltage-gated Na+ and K+ channels open

•  To counterbalance the effect of Na+ and K+ transport, electrical current is injected automatically through the current electrode of the voltage clamp to maintain the intracellular voltage at 0 mV •  The current injected must be equal to but of opposite polarity to the net current

flow through the membrane channels - an oscilloscope records current flow


21


Nerve Action Potential Experimental Setups for the Voltage Clamp Experimental Cell •  Large nerve fibers isolated from crustaceans, especially in the giant squid where

axons approach 1 mm in diameter, allow easy experimentation Experimental Ion Conditions •  When Na+ is the only ion in the solution, the voltage clamp measures current flow

only through the sodium channels •  When K+ is the only permeant ion, current flow only through the potassium channels

is measured Experimental Ion Blockers •  Na+ channels can be blocked by the tetrodotoxin toxin applied to the outside of the

cell membrane •  K+ channels can be blocked by tetraethylammonium ions when it is applied to the

interior of the nerve fiber.


Nerve Action Potential Initiation of the Action Potential •  A positive feedback loop opens Na+ channels

•  As long as the membrane of the nerve fiber remains undisturbed, no action potential occurs in the normal nerve

•  If any event causes enough initial rise in the membrane potential from -90 mV towards 0 mV, the rising voltage itself causes many voltage-gated Na+ channels to begin opening

•  An action potential will not occur until the initial rise in membrane potential is great enough to create the positive feedback loop •  This occurs when Na+ entering the fiber becomes greater than the number of K+

ions leaving the fiber •  A sudden rise in membrane potential of 15 to 30 mV usually is required: the

threshold for stimulation

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Nerve Action Potential Propagation of the Action Potential •  A local circuit of current flow from the

depolarized areas of the membrane to the adjacent resting membrane areas •  Positive charges are carried by the

inward-diffusing Na+ through the depolarized membrane and then for several millimeters in both directions along the core of the axon

•  Positive charges increase the voltage for a distance of 1 to 3 mm inside the large myelinated fiber to above the threshold voltage value for initiating an action potential

•  The action potential travels in all directions

away from the stimulus until the entire membrane has become depolarized



Nerve Action Potential Propagation of the Action Potential •  All-or-Nothing Principle

•  Once an action potential has been elicited at any point on the membrane of a normal fiber, the depolarization process either

•  Propagates •  Or

•  Does not propagate

•  This is the all-or-nothing principle, and it applies to all normal excitable tissues

•  Occasionally, the action potential reaches a point on the membrane at which it does not generate sufficient voltage to stimulate the next area of the membrane

•  When this occurs, the spread of depolarization stops

23


Nerve Action Potential Re-establishing Na+ and K+ Gradients •  Action potential transmission only slightly reduces Na+ and K+ concentrations

•  For a single action potential, this effect is so minute that it cannot be measured •  100,000 to 50,000,000 impulses can be transmitted by large nerve fibers

before the concentration differences reach the point that action potential conduction ceases

•  Nevertheless, Na+ and K+ concentrations need to be re-established •  Na+ and K+ concentrations are re-established by the Na+-K+ pump •  Pump requires ATP •  Na+-K+ pump is strongly stimulated when excess Na+ accumulate inside the

cell membrane


Nerve Action Potential Action Plateau in Some Action Potentials •  Sometimes, the potential remains on a plateau near the peak of the spike potential

for many milliseconds •  In heart muscle fiber the plateau lasts for as long as 0.2 to 0.3 s and sustains

heart muscle contractions for 0.2 to 0.3 s

•  The plateau is caused by •  Voltage-activated Na+ channels (fast channels)

•  Opening of fast channels causes the spike portion of the action potential •  Voltage-activated Ca++-Na+ channels (slow channels)

•  Slow, prolonged opening of Ca++-Na+ channels allows Ca++ to enter, which is largely responsible for the plateau portion

•  Voltage-gated K+ channels •  Slower than usual to open, often not opening very much until the end of the

plateau

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Rhythmicity of Some Excitable Tissues Rhythmicity •  Repetitive self-induced discharges occur normally

•  Rhythmical beat of the heart •  Rhythmical peristalsis of the intestines •  Neuronal events such as the rhythmical control of breathing

•  For spontaneous rhythmicity to occur, the membrane even in its resting state must be permeable enough to Na+

•  In the heart’s rhythmical control center the resting potential is -60 to -70 mV, too high to keep Na+ and Ca++ channels totally closed •  Procedure: Some Na+ and Ca++ flow inward, which increases the membrane

voltage in the positive direction, which further increases membrane permeability •  Still more Na+ and Ca++ flow inward, and the permeability increases more, and

so on, until an action potential is generated •  Then, at the end of the action potential, the membrane repolarizes

•  After another delay of milliseconds or seconds, spontaneous excitability causes depolarization again, and a new action potential occurs spontaneously


Rhythmicity of Some Excitable Tissues Rhythmicity •  The membrane of the heart control

center does not depolarize immediately after it has become repolarized

•  Toward the end of each action potential, and continuing for a short period thereafter, the membrane becomes excessively permeable to K+

•  The excessive outflow of K+ carries tremendous numbers of positive charges to the outside of the membrane, leaving inside the fiber considerably more negativity than would otherwise occur


25


Rhythmicity of Some Excitable Tissues Rhythmicity •  This continues for nearly a second after the

preceding action potential is over, thus drawing the membrane potential nearer to the K+ Nernst potential •  This is a state called hyperpolarization •  As long as this state exists, self-re-

excitation will not occur

•  But the excess K+ conductance gradually disappears, thereby allowing the membrane potential again to increase up to the threshold for excitation

•  Then, suddenly, a new action potential results, and the process occurs again and again



Nerve Fibers Anatomy of Nerve Fibers

•  A myelin sheath is deposited around the axon by Schwann cells in the following manner •  Schwann cell membrane, containing

the lipid sphingomyelin, envelops the axon and wraps around many times

•  Lipid is an electrical insulator that decreases ion flow through the membrane about 5000-fold

•  No ions can flow through the thick myelin sheaths of myelinated nerves

•  Between two Schwann cells, a 2 to 3 µm uninsulated area remains where ions flow through the axon membrane (Node of Ranvier)

•  Action potentials occur only at the nodes and are conducted from node to node (saltatory conduction) Guyton & Hall. Textbook of Medical Physiology,

11th Edition

26


Nerve Fibers Consequences of Saltatory Conduction

•  Velocity of nerve transmission in myelinated fibers increases 5- to 50-fold by causing the depolarization process to jump long intervals along the axis of the nerve fiber •  Velocity of conduction from 0.25 m/s in

very small unmyelinated fibers to 100 m/s in very large myelinated fibers

•  Conservation of energy (only the nodes depolarize) allows perhaps 100 times less loss of ions •  Requires little metabolism for re-

establishing the Na+ and K+ concentration differences

•  Quick repolarization due myelin membrane insulation and the 50-fold decrease in membrane capacitance



Eliciting the Action Potential Action Potential Initiation

•  Any factor that causes Na+ to begin to diffuse inward through the membrane in sufficient numbers can set off automatic regenerative opening of Na+ channel •  Mechanical disturbance of the membrane •  Chemical effects on the membrane •  Passage of electricity through the membrane

•  All these are used at different points in the body to elicit action potentials •  Mechanical pressure to excite sensory nerve endings in the skin •  Chemical neurotransmitters to transmit signals from one neuron to the next in

the brain •  Electrical current to transmit signals between successive muscle cells in the

heart and intestine

27


Eliciting the Action Potential Excitation of a Nerve Fiber by a Negatively Charged Metal Electrode

•  Experimentally, electricity is applied to the nerve or muscle surface through two

small electrodes, one of which is negatively charged and the other positively charged •  The excitable membrane becomes stimulated at the negative electrode

•  Effect of the Experimental Approach

•  Negative current from the electrode decreases the voltage on the outside of the membrane to a negative value nearer to the voltage of the negative potential inside the fiber

•  This decreases the electrical voltage across the membrane and allows Na+ channels to open, resulting in an action potential

•  At the positive electrode, the injection of positive charges on the outside of the nerve membrane heightens the voltage difference across the membrane rather than lessening it

•  This causes a state of hyperpolarization, which actually decreases the excitability of the fiber rather than causing an action potential


Eliciting the Action Potential Threshold for Excitation and Acute Local Potentials

•  The level required to elicit an action potential is called the threshold level

•  A weak negative electrical stimulus may not be able to excite a fiber •  These local potential changes are called

acute local potentials, and when they fail to elicit an action potential, they are called acute subthreshold potentials

•  A stronger stimulus induces a stronger acute local potential

•  When the voltage of the stimulus is increased, there comes a point at which excitation does take place


28


Refractory Period After an Action Potential Refractory Period •  A new action potential cannot occur in an excitable fiber as long as the membrane is

still depolarized from the preceding action potential •  Na+ channels, Ca++ channels, or both have become inactivated

•  No amount of excitatory signal applied to these channels at this point will open the inactivation gates

•  The only condition that will allow gates to reopen is for the membrane potential to return to or near the original resting membrane potential level

•  Then the inactivation gates of the channels open, and a new action potential can be initiated

•  The period during which a second action potential cannot be elicited, even with a strong stimulus, is called the absolute refractory period •  This period for large myelinated nerve fibers is about 1/2500 s •  Therefore, one can readily calculate that such a fiber can transmit a maximum of

about 2500 impulses / s


Inhibition of Excitability Inhibition •  Membrane-stabilizing factors can decrease excitability

•  A high extracellular fluid Ca++ concentration decreases membrane permeability to Na+ and simultaneously reduces excitability

•  Local Anesthetics •  Among the most important stabilizers are the many substances used clinically as

local anesthetics, including procaine and tetracaine •  Most act directly on the activation gates of Na+ channels, making it much more

difficult for these gates to open, thereby reducing membrane excitability •  When excitability has been reduced so low that the ratio of action potential

strength to excitability threshold (safety factor) is reduced below 1.0, nerve impulses fail to pass along the anesthetized nerves

29


Physiologic Anatomy of Skeletal Muscle Contraction of Skeletal Muscle

•  About 40% of the body is skeletal muscle

•  About 10% is smooth and cardiac muscle

•  Basic principles of contraction apply to all these different types of muscle


Physiologic Anatomy of Skeletal Muscle Skeletal Muscle Cell Biology

•  Satellite cells: Resident progenitor cells (unipotent stem cell) which matures either into another satellite cell or a myoblast

•  Myoblast: Mononucleated cells that fuse to form myotubes

•  Myotubes: Multinucleated, function cell of skeletal muscle

•  Myofibers: Organized structures of myotubes

30


Skeletal Muscle Cell Biology

•  During skeletal muscle regeneration, satellite cells become activated and proliferate •  Once activated, they can repopulate the

quiescent reserve or differentiate into myoblasts

•  The absolute number of satellite cells decreases with age

•  The differentiated cells, fuse with other myoblasts to form myotubes

•  These myotubes then mature to form myofibers

•  In the muscle, myofibers are stacked and bound by connective tissue that contain nerves and blood vessels

Physiologic Anatomy of Skeletal Muscle

Proliferation

Activation

Fusion

Maturation

Preservation


Physiologic Anatomy of Skeletal Muscle Skeletal Muscle Cell Biology •  Skeletal muscle is composed of muscle cells, connective tissue, blood vessels, and

nerve fibers •  Skeletal muscle is both highly vascularized and innervated

•  Primary function is contraction, force generation, and induction of movement

•  The parenchymal cell is a multinucleated muscle fiber •  Thread like structures aligned in the axial direction of the cell •  Responsible for muscle contraction •  Contains contractile filaments, regulatory proteins, and supporting structures

•  Contractile filaments include actin filaments and myosin filaments

31


Physiologic Anatomy of Skeletal Muscle Skeletal Muscle Fiber

•  All skeletal muscles are composed of numerous fibers ranging from 10 to 80 µm in diameter

•  In most skeletal muscles, each fiber extends the entire length of the muscle

•  Most fibers are usually innervated by only one nerve ending, located near the middle of the fiber


muscle fiber

actin

myosin


Physiologic Anatomy of Skeletal Muscle Sarcolemma

•  The sarcolemma is the cell membrane of the muscle fiber

•  The sarcolemma consists of a true cell membrane and an outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fibrils

•  At each end of the muscle fiber, the sarcolemma fuses with a tendon fiber, and the tendon fibers in turn collect into bundles to form the muscle tendons that then insert into the bones


actin

myosin

32


Physiologic Anatomy of Skeletal Muscle Myofibrils

•  Each muscle fiber contains several hundred to several thousand myofibrils

•  Each myofibril is composed of about 1500 adjacent myosin filaments and 3000 actin filaments


myofibril

actin

myosin



•  Myosin and actin filaments are partially interdigitated, and thus cause the myofibrils to have alternating light and dark bands

•  I bands (light and isotropic to polarized light) contain only actin filaments

•  A bands (dark and anisotropic to polarized light) bands contain myosin filaments and overlapping actin filaments’ ends

•  The entire muscle fiber has light and dark bands, as do the individual myofibrils, giving skeletal and cardiac muscle a striated appearance


33



•  Ends of the actin filaments are attached to a Z disc

•  Composed of filamentous proteins, different from actin and myosin

•  From Z disc, these filaments extend in both directions to interdigitate with the myosin filaments

•  Z disc passes crosswise across the myofibril, attaching myofibrils to one another across the entire muscle fiber


myofibril

actin

myosin



•  The portion of the myofibril that lies between two successive Z discs is called a sarcomere

•  When the muscle fiber is at its normal, fully stretched resting length, the length of the sarcomere is about 2 µm •  At this length, the actin

filaments overlap the myosin filaments, and the tips of the actin filaments are just beginning to overlap one another


sarcomere

actin

myosin

34



•  The side-by-side relationship between the myosin and actin filaments is maintained by a large number of filamentous molecules of a protein called titin

•  Each titin molecule is about 3x106 Da, which makes it one of the largest protein molecules in the body •  Filamentous structure allows it to be springy •  Acts as a framework to hold myosin and actin filaments in place so that the

contractile machinery of the sarcomere will function



•  Many myofibrils are suspended side by side in the muscle fiber

•  The spaces between the myofibrils are filled with intracellular fluid called sarcoplasm •  Contains large quantities of potassium, magnesium, and phosphate, plus

multiple protein enzymes •  Contains tremendous numbers of mitochondria that lie parallel to the myofibrils

•  Supply the contracting myofibrils with large amounts of ATP •  Contains an extensive reticulum, called the sarcoplasmic reticulum (SR)

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General Mechanism of Muscle Contraction Muscle Contraction Mechanism

•  An action potential (AP) travels along a motor nerve to its endings on muscle fibers

•  At each ending, the nerve secretes a small amount of the neurotransmitter acetylcholine •  Acetylcholine acts on a local area of the muscle fiber membrane to open multiple

acetylcholine-gated channels through protein molecules floating in the membrane

•  Opening of the acetylcholine-gated channels allows large quantities of Na+ to diffuse to the interior of the muscle fiber membrane

•  Na+ flux initiates an AP at the membrane


General Mechanism of Muscle Contraction Muscle Contraction Mechanism

•  Once initiated, the AP travels along the muscle fiber membrane in the same way that APs travel along nerve fiber membranes

•  AP depolarizes the muscle membrane

•  AP causes SR to release large quantities of stored Ca++

•  Ca++ initiate attractive forces between the actin and myosin filaments, causing them to slide alongside each other, which is the contractile process

•  After a fraction of a second, Ca++ are pumped back into the SR by a Ca++ membrane pump, and they remain stored in the SR until a new muscle action potential comes along

•  Removal of Ca++ from the myofibrils causes the muscle contraction to cease


36


Molecular Mechanism of Muscle Contraction Sliding Filament Mechanism of Muscle Contraction •  Sacromere exists in relaxed and contracted states

•  Relaxed: Ends of the actin filaments begin to overlap one another

•  Contracted: Ends of actin filaments overlap one another to their maximum extent

•  Z discs have been pulled by the actin filaments up to the ends of the myosin filaments

•  Actin filaments slide due to forces generated by dynamic bonds between myosin and actin filaments

•  When an action potential travels along the muscle fiber, this causes the sarcoplasmic reticulum to release Ca++ that rapidly surround the myofibrils

•  Ca++ activate the forces between the myosin and actin filaments, using ATP, and contraction begins



Molecular Mechanism of Muscle Contraction Myosin Filament •  Myosin filament is composed of multiple

myosin molecules, about 480,000 Da

•  Myosin molecule is composed of six polypeptide chains •  Two heavy chains, each 200,000 Da •  Four light chains, each 20,000 Da

•  Heavy chains structures •  Two heavy chains wrap spirally around

each other to form a double helix •  One end of each of these chains is folded

bilaterally into a globular polypeptide structure called a myosin head

•  There are two free heads at one end of the double-helix myosin molecule


37


Molecular Mechanism of Muscle Contraction Myosin Filament •  Light chain structures

•  The four light chains are also part of the myosin head, two to each head

•  Chains control the function of the head during muscle contraction

•  Myosin filament is made up of 200+ myosin molecules •  Tails of myosin molecules are bundled

together to form the body of the filament •  Many heads of the molecules hang outward

to the sides of the body •  Part of the body of each myosin molecule

hangs to the side along with the head, thus providing an arm that extends the head outward from the body

•  The protruding arms and heads together are called cross-bridges



Molecular Mechanism of Muscle Contraction Cross-bridges •  Each cross-bridge is flexible at two hinges

•  One where the arm leaves the body of the myosin filament

•  One where the head attaches to the arm

•  The hinged arms allow •  Heads either to be extended outward or to

be brought close to the body •  Hinged heads participate in the actual

contraction process

•  The myosin filament itself is quite twisted so that each successive pair of cross-bridges is axially displaced from the previous pair by 120°

•  Myosin head acts as an ATPase enzyme, cleaving ATP and using the energy to drive contraction


no myosin heads exist in the center

of the myosin filament

38


Molecular Mechanism of Muscle Contraction Actin Filament

•  Complex composed of actin, tropomyosin, and troponin •  Tropomyosin molecules that fit in the grooves between the actin strands •  Attached to end of each tropomyosin molecule is a troponin complex that

initiates contraction •  Double-stranded, helical F-actin protein molecule •  Composed of polymerized G-actin molecules, about 42,000 Da

•  Each G-actin contains one ADP •  ADP molecules are thought to be active sites on which the cross-bridges of

myosin filaments interact to cause muscle contraction •  The active sites are staggered, giving one active site every 2.7 nm

•  Each actin filament is about 1 µm long •  Actin filaments bases are inserted strongly into the Z discs •  Ends of actin protrude in both directions to lie in the spaces between the myosin

molecules


Molecular Mechanism of Muscle Contraction Troponin-Tropomyosin Complex

•  Attached intermittently along tropomyosin is troponin

•  Troponin is actually a complex of three loosely bound protein subunits, believed to attach tropomyosin to actin •  Troponin I has a strong affinity for actin •  Troponin T has a strong affinity for tropomyosin •  Troponin C has a strong affinity for calcium ions

•  It is hypothesized that the active sites on actin filament in a relaxed muscle are inhibited or physically covered by the troponin-tropomyosin complex

•  In excess Ca++, the inhibitory effect of the troponin-tropomyosin is lost •  It is hypothesized that Ca++ binds troponin C, the

troponin complex undergoes a conformational change that tugs on the tropomyosin molecule, uncovers the active sites of the actin, and allowing myosin cross-bridge contraction


39


Molecular Mechanism of Muscle Contraction Walk-Along Theory of Contraction

•  After Ca++ activation, the heads of the cross-bridges from the myosin filaments become attracted to the active sites of the actin filament, and causes contraction to occur

•  Contraction is hypothesized to occur due to a walk-along mechanism •  When a cross-bridge head attaches to an active site,

the attachment causes the head to tilt toward the arm and drags actin filament - the power stroke

•  After tilting, the head then automatically breaks away from the active site

•  Next, the head returns to its extended direction •  The head then combines with a new active site farther

down along the actin filament •  The head tilts again to cause a new power stroke

•  Each cross-bridges acts independently •  Number of cross-bridges in contact with the actin

filament determines the force of contraction



Molecular Mechanism of Muscle Contraction ATP is the Energy Source for Contraction •  Before contraction, myosin heads of the cross-bridges bind with ATP

•  Myosin ATPase cleaves ATP and leaves ADP plus phosphate ion, bound to the head

•  Myosin head extends perpendicularly toward the actin filament

•  When troponin-tropomyosin complex binds Ca++, actin active sites are uncovered, and the myosin heads then bind with these active sites on actin •  Due to this binding, myosin head undergoes a conformational change, tilting

toward the arm of the cross-bridge - providing the power stroke for pulling actin - and then energy for the power stroke is provided by the conformational change due to ATP cleavage

•  Once myosin head has undergone the conformational change (power stroke), ADP and phosphate ions are released

•  A new molecule of ATP binds myosin, causing the detachment of the myosin head from actin

40


Myosin - Actin Overlap Determines Tension •  At no overlap, developed tension is zero •  As overlap increases, tension increases •  At a sarcomere length of 2.2 µm, the actin filament

has overlapped all the cross-bridges of the myosin filament but has not yet reached the center of the myosin filament

•  With further shortening, the sarcomere maintains full tension until 2.0 µm

•  Below 2.0 µm, the ends of the two actin filaments begin to overlap each other in addition to overlapping the myosin filaments •  Two Z discs of the sarcomere abut the ends of

the myosin filaments •  Ends of the myosin filaments are crumpled •  Strength of contraction decreases rapidly and

approaches zero

Molecular Mechanism of Muscle Contraction



Effect of Muscle Length on Force of Contraction •  Force generation in whole muscle is similar to a

single muscle fiber, although differences exist •  Whole muscle has a large amount of connective

tissue •  Sarcomeres in different parts of the muscle do

not always contract the same amount

•  When muscle is approximately at normal resting length (a sacromere length of 2.0 µm), it contracts with maximum force

•  At increased lengths, the increase in tension that occurs during contraction (active tension) decreases

Molecular Mechanism of Muscle Contraction


41


Molecular Mechanism of Muscle Contraction Velocity of Contraction

•  Muscle contracts the fastest with no load

•  As load increases, velocity decreases

•  Once load equals the maximum load of the muscle, velocity of contraction equals zero



Molecular Mechanism of Muscle Contraction Sources of Energy for Muscle Contraction

•  ATP is consumed during muscle contraction, and typical muscle tissue contains enough ATP to maintain contraction for only 1 to 2 seconds - after that point, the ADP product needs to be rephosphorylated into ATP

•  Sources of rephosphorylation •  Phosphocreatine - provides 5 to 8 seconds of additional contraction •  Glycogen - glycolysis provides another 60 seconds of contraction •  Oxidative metabolism - oxygen consuming reactions provide long term energy

for muscle contraction

42


Molecular Mechanism of Muscle Contraction Whole Muscle Contraction •  Isometric contraction - muscle length does not

change during contraction

•  Isotonic contraction - muscle tension does not change during contraction

•  Different muscles contract at different rates depending upon their typical function •  Ocular - eye movement •  Gastrocnemius - limb movement •  Soleus - whole body support


§  Rates of muscle contraction are largely determined by the relative amounts of fast and slow muscle fibers §  Fast fibers - large fibers, large SR, high amounts of glycolytic enzymes,

less vascularized (less blood for oxidative metabolism), few mitchondria §  Slow fibers - small fibers, small nerve fibers, highly vascularized,

increased mitochondria, high amounts of myoglobin


Excitation of Skeletal Muscle Neuromuscular Junction

•  The nerve fiber forms a complex of branching nerve terminals that invaginate into the surface of the muscle fiber but lie outside the muscle fiber plasma membrane •  Structure is called the motor end plate •  Schwann cells insulate it from the

surrounding fluids •  Invaginated membrane is the synaptic

gutter or synaptic trough •  Space between the terminal and fiber is

the synaptic space or synaptic cleft •  Space is 20 to 30 nm •  Subneural clefts increase the

surface area for the synaptic transmitter


43


Excitation of Skeletal Muscle Neuromuscular Junction

•  The axon terminal contain many mitochondria ATP for the synthesis of acetylcholine •  Acetylcholine excites the muscle fiber

membrane •  Acetylcholine is synthesized in the

cytoplasm of the terminal, but it is absorbed rapidly into many small synaptic vesicles

•  Acetylcholinesterase exist in the synaptic space and destroy acetylcholine a few milliseconds after it has been released from the synaptic vesicles



Excitation of Skeletal Muscle Secretion of Acetylcholine by the Nerve Terminals

•  When a nerve impulse reaches the neuromuscular junction, about 125 vesicles of acetylcholine are released from the terminals into the synaptic space

•  When an action potential spreads over the terminal, voltage-gated Ca++ channels open and allow Ca++ to diffuse from the synaptic space to the interior of the nerve terminal

•  Ca++ are believed to exert an attractive influence on the acetylcholine vesicles, drawing them to the neural membrane adjacent to the dense bars

•  The vesicles then fuse with the neural membrane and empty their acetylcholine into the synaptic space by the process of exocytosis Guyton & Hall. Textbook of Medical Physiology, 11th Edition

44



•  Acetylcholine-gated ion channels exist in the muscle fiber membrane, near the mouths of the subneural clefts •  Each receptor is a protein complex of 275,000 Da •  Five subunit proteins, two alpha proteins and one each of

beta, delta, and gamma proteins •  The channel remains constricted until two acetylcholine

molecules attach respectively to the two alpha subunit proteins, when a conformational change occurs to open the channel

•  The opened acetylcholine channel has a diameter of about 0.65 nm, large enough to for Na+, K+, and Ca++, anions are repelled due to negative charge •  Na+ dominate the flux as it is in excess in the extracellular

space and it is attracted to the negative potential inside the cell

•  Na+ flux creates a local positive potential change inside the muscle fiber membrane, called the end plate potential

•  End plate potential initiates an action potential, that spreads along the muscle membrane and causes muscle contraction




•  Acetylcholine, once released into the synaptic space, continues to activate the acetylcholine receptors as long as the acetylcholine persists

•  Acetylcholine is removed rapidly by two means •  Acetylcholinesterase, which is attached mainly to the spongy layer of fine

connective tissue that fills the synaptic space between the presynaptic nerve terminal and the postsynaptic muscle membrane

•  Diffusion out of the synaptic space

45


Excitation of Skeletal Muscle End Plate Potential and Excitation of the Skeletal Muscle Fiber •  Na+ flux causes the electrical potential inside the fiber to

increase as much as 50 to 75 mV, creating a local potential called the end plate potential

•  End plate potentials may be too weak to elicit an action potential due to external regulation •  Curare - a drug that blocks the gating action of

acetylcholine on the acetylcholine channels by competing for the acetylcholine receptor sites

•  Botulinum toxin - a bacterial poison that decreases the quantity of acetylcholine release by the nerve terminals

•  End plate potentials that pass the threshold level cause enough Na+ channels to open so that the self-regenerative effect of more and more Na+ flowing to the interior of the fiber initiates an action potential



Excitation of Skeletal Muscle Acetylcholine Formation and Release •  Small, 40 nm vesicles are formed by the Golgi apparatus of a motoneuron in the

spinal cord •  Vesicles are then transported to the neuromuscular junction

•  Acetylcholine is synthesized in the nerve fiber terminal and transported into the vesicles

•  An action potential arrives at the nerve terminal, and opens Ca++ channels in the membrane of the nerve terminal •  [Ca++] increases, which increases the rate of fusion of acetylcholine vesicles with

the terminal membrane •  Fusion causes many vesicles to rupture, allowing exocytosis of acetylcholine into

the synaptic space - about 125 vesicles usually rupture with each action potential

•  After opening Na++ channels, acetylcholine is split by acetylcholinesterase into acetate ion and choline

•  For continued, long term function of the neuromuscular junction, vesicles are reformed in the terminal nerve membrane and ready for more acetylchoine transport

46


Excitation of Skeletal Muscle Acetylcholine Stimulants •  Drugs that stimulate the muscle fiber by acetylcholine-like action

•  Methacholine, carbachol, and nicotine act like acetylcholine •  However, these drugs are not destroyed by cholinesterase or are destroyed so

slowly that their action often persists for many minutes to several hours

•  Drugs that stimulate the neuromuscular junction by inactivating acetylcholinesterase •  Neostigmine, physostigmine, and diisopropyl fluorophosphate inactivate the

acetylcholinesterase in the synapses so that it no longer hydrolyzes acetylcholine

•  Drugs that block transmission at the neuromuscular junction •  Curariforms compete with acetylcholine for acetylcholine receptors and thus can

prevent passage of impulses from the nerve ending into the muscle


Excitation of Skeletal Muscle Muscle Action Potential •  The initiation and conduction of action potentials in nerve fibers (Chapter 5) and is

mostly the same mechanism in skeletal muscle fibers

•  Resting membrane potential •  About -80 to -90 mV in skeletal muscle fibers •  About -80 to -90 mV in large myelinated nerve fibers

•  Duration of action potential •  1.0 to 5.0 msec in skeletal muscle fibers •  0.2 to 1.0 msec in large myelinated nerve fibers

•  Velocity of conduction •  3 to 5 m/s in skeletal muscle fibers •  40 to 65 m/s in large myelinated nerve fibers

47


Excitation of Skeletal Muscle Propagation of Muscle Action Potential •  Skeletal muscle fibers are so large that

action potentials spreading along the surface membrane cause almost no current flow deep within the fiber

•  Excitation-contraction coupling •  Interior transmission of action

potentials occurs along transverse tubules (T tubules) that penetrate all the way through the muscle fiber from one side of the fiber to the other

•  T tubule action potentials cause release of Ca++ inside the muscle fiber in the immediate vicinity of the myofibrils, and these Ca++ then cause contraction



Excitation of Skeletal Muscle Transverse Tubule-Sarcoplasmic Reticulum System •  The sarcoplasmic reticulum (SR) contains an excess of

Ca++ which is released when an action potential (AP) occurs in the adjacent T tubule •  The AP of the T tubule causes current flow into the

SR cisternae

•  The AP causes rapid opening of Ca++ channels through the membranes of the cisternae and their attached longitudinal tubules •  Ca++ channels remain open for a few msec,

allowing Ca++ to be released into the sarcoplasm surrounding the myofibrils to cause contraction

•  Once Ca++ release occurs, muscle contraction continues as long as Ca++ remain in high concentration •  An SR Ca++ pump pushes Ca++ from the myofibrils

back into sarcoplasmic tubules •  Also, calsequestrin binds and concentrates Ca++ in

the SR


48


Excitation of Skeletal Muscle Transverse Tubule-Sarcoplasmic Reticulum System •  Ca++ resting cytosol concentration (< 10-7 M) is too low

to elicit contraction •  Troponin-tropomyosin complex keeps the actin

filaments inhibited and maintains a relaxed state of the muscle

•  Full excitation of the T tubule and SR system causes Ca++ release resulting in concentrations up to 2×10-4 M •  Immediately thereafter, the Ca++ pump depletes

[Ca++] •  The total duration of this Ca++ pulse in the usual

skeletal muscle fiber is about 0.05 sec •  In heart muscle, the Ca++ pulse lasts 0.33 sec

because of the long duration of the cardiac action potential

•  Muscle contraction occurs during the Ca++ pulse •  For contraction to continue, a series of Ca++ pulses

must be initiated by a continuous series of repetitive action potentials



Contraction and Excitation of Smooth Muscle Smooth Muscle

•  Smooth muscle is composed of small fibers, usually 1 to 5 µm in diameter and only 20 to 500 µm in length •  Skeletal muscle fibers are up 30x greater in diameter and several 100x as longer

•  Many of the same principles of contraction apply to smooth muscle as to skeletal muscle •  The same attractive forces between myosin and actin filaments cause contraction

in smooth muscle as in skeletal muscle •  The internal physical arrangement of smooth muscle fibers, however, is very

different

49


Contraction and Excitation of Smooth Muscle Types Smooth Muscle

•  Multi-Unit Smooth Muscle •  Composed of discrete, separate smooth muscle

fibers •  Each fiber operates independently of the others and

often is innervated by a single nerve ending •  The outer surfaces are covered by a thin layer of

collagen and glycoprotein that helps insulate the separate fibers from one another

•  Each fiber can contract independently of the others, and their control is exerted mainly by nerve signals

•  Examples include ciliary muscle and iris muscle of the eye



Contraction and Excitation of Smooth Muscle Types Smooth Muscle

•  Unitary Smooth Muscle (Syncytial and Visceral) •  A mass of hundreds to thousands of smooth muscle

fibers that contract together as a single unit •  The fibers usually are arranged in sheets or bundles •  Cell membranes are adherent to one another

(including gap junctions) allowing for force transmission, ions flux, and action potentials conduction

•  Examples include the walls of most viscera of the body, including the gut, bile ducts, ureters, uterus, and many blood vessels


50


Contraction and Excitation of Smooth Muscle Contraction Mechanisms in Smooth Muscle

•  Smooth muscle is similar to skeletal muscle •  Contains both actin and myosin filaments •  Contraction process is activated by Ca++ •  ATP provides the energy for contraction

•  There are major differences between smooth muscle and skeletal muscle •  Physical organization

•  Smooth muscle does not contain the normal troponin complex found in skeletal muscle contraction

•  Excitation-contraction coupling •  Control of the contractile process by calcium ions •  Duration of contraction •  Amount of energy required for contraction



•  Smooth muscle is irregular compared to skeletal muscle

•  Large numbers of actin filaments are attached to dense bodies •  Dense bodies are attached to the cell membrane and dispersed

inside the cell - they serve the same role as the Z discs in skeletal muscle

•  Some membrane bound dense bodies of adjacent cells are bonded together by intercellular protein bridges - these bonds transmit force from cell to cell

•  Myosin filaments are interspersed among actin filaments •  Approximately 15x less myosin than actin •  Myosin filaments have sidepolar cross-bridges, where the

bridges on one side hinge in one direction and those on the other side hinge in the opposite direction

•  Sidepolar cross-bridges allow myosin to pull an actin filament in one direction on one side while simultaneously pulling another actin filament in the opposite direction on the other side

•  This allows smooth muscle cells to contract as much as 80% of their length, compared to < 30% in skeletal muscle

Guyton & Hall. Textbook of Medical

Physiology, 11th Edition

51



•  Most smooth muscle contraction is prolonged tonic contraction, sometimes lasting hours or even days

•  The rapidity of cycling of the myosin cross-bridges is slow in smooth muscle

•  As little as 1/10 to 1/300 that in skeletal muscle •  In addition, the time that the cross-bridges remain attached to the actin

filaments is long compared to skeletal muscle

•  Only 1/10 to 1/300 as much energy is required to sustain the same tension of contraction in smooth muscle as in skeletal muscle •  This sparsity of energy utilization by smooth muscle is exceedingly important to

the overall energy economy of the body, because organs such as the intestines, urinary bladder, gallbladder, and other viscera often maintain tonic muscle contraction almost indefinitely



•  Time course of contraction •  Contraction begins 50 to 100 msec after excitation •  Reaches full contraction about 0.5 sec later •  Declines in contractile force 1 to 2 sec later •  Contraction time for smooth muscle varies from 0.2 to 30 sec

•  The initiation of contraction in response to Ca++ is much slower than in skeletal muscle

•  The slow onset of contraction and prolonged contraction is caused by the slowness of attachment and detachment of the cross-bridges with the actin filaments

52



•  Force of muscle contraction •  The maximum force of contraction of smooth muscle is often greater than that of

skeletal muscle due to the prolonged period of attachment of the myosin cross-bridges to the actin filaments

•  4 to 6 kg/cm2 cross-sectional area for smooth muscle in comparison with 3 to 4 kg/cm2 for skeletal muscle

•  Latch mechanism •  Once smooth muscle contraction is initiated, the amount of continuing excitation

and energy (ATP consumption) is typically greatly reduced •  Yet, muscle maintains its full force of contraction •  Allows maintenance of prolonged contraction with little nerve excitation and

energy consumption •  Stress-relaxation

•  The original force of contraction may be returned within seconds after it has been elongated or shortened

•  A change (ñò) in urinary bladder fluid volume augments bladder pressure (ñò), but within 15 to 60 sec the original pressure returns


Contraction and Excitation of Smooth Muscle Regulation of Contraction by Calcium Ions •  The initiating stimulus for most smooth muscle contraction is an increase in Ca++

•  Yet smooth muscle does not contain troponin

•  Combination of Ca++ with calmodulin •  Smooth muscle cells contain a large amount of calmodulin •  Calmodulin initiates contraction by activating the myosin cross-bridges

•  Ca++ bind with calmodulin •  Calmodulin-Ca++ complexes activate myosin kinase, a phosphorylating

enzyme •  One of the light chains of each myosin head (the regulatory chain) becomes

phosphorylated in response to this myosin kinase •  Due to phosphorylation, the myosin head can bind repetitively with the

actin filament and proceed through the entire cycling process of contraction

53


Contraction and Excitation of Smooth Muscle Regulation of Contraction by Calcium Ions •  Cessation of contraction

•  When Ca++ concentration falls below a critical level contraction stops in the reverse manner

•  Reversal requires myosin phosphatase, located in the fluids of the smooth muscle cell, to split the phosphate from the regulatory light chain

•  Once dephosphorylated, cycling stops and contraction ceases

•  Hypothesis for regulation of latch mechanism •  When myosin kinase and phosphatase are strongly activated, the cycling

frequency is great •  As the activation of the enzymes decreases, the cycling frequency decreases

•  Also, enzyme deactivation allows the myosin heads to remain attached to the actin filament

•  As the number of heads attached to the actin determines the static force of contraction, tension is maintained

•  Little energy is used because ATP is not degraded to ADP except on the rare occasion when a head detaches


Contraction and Excitation of Smooth Muscle Nervous and Hormonal Control •  Skeletal muscle is activated almost exclusively by

the nervous system, whereas smooth muscle can be activated by the nervous system, hormones, mechanical stimulation, as well as other means

•  Autonomic nerve fibers that innervate smooth muscle generally branch diffusely on top of a sheet of muscle fibers •  Fibers form so-called diffuse junctions that

secrete their transmitter substance into the matrix coating of the smooth muscle nm to µm away

•  Nerve fibers often innervate only the outer layer, and action potential conduction or diffusion of the transmitter substance propagates contraction


54


Contraction and Excitation of Smooth Muscle Nervous and Hormonal Control •  Most terminal axons have multiple varicosities

distributed along their axes •  At these points the Schwann cells are interrupted

and allow transmitter secretion

•  The vesicles of the autonomic nerve fiber endings contain acetylcholine in some fibers and norepinephrine in others, and other substances

•  Acetylcholine and norepinephrine are utilized, but they are never secreted by the same nerve fibers •  If acetylcholine is excitatory, norepinephrine is

inhibitory •  If acetylcholine is inhibitory, norepinephrine is

usually excitatory

•  The type of receptor determines whether the smooth muscle is inhibited or excited, and also determines which (acetylcholine or norepinephrine) is effective in causing the excitation or inhibition



Contraction and Excitation of Smooth Muscle Membrane Potentials and Action Potentials

•  Smooth muscle membrane potential differs, but is typically -50 to -60 mV (30 mV higher than skeletal muscle)

•  Unitary smooth muscle has the same action potentials as skeletal muscle (and myelinated nerve fibers) •  Spike action potentials - typical •  Slow wave potentials - self-excitatory •  Plateau action potentials - allow for prolonged

contraction

•  Smooth muscle has few voltage gated Na+ channels, but many voltage gated Ca++ channels •  Ca++ channels open slowly and remain open

longer, accounting for the slow AP in smooth muscle


55


Contraction and Excitation of Smooth Muscle Smooth Muscle Contraction Without Action Potentials

•  Other factors cause approximately ½ of all smooth muscle to contract without AP initiation

•  For vascular smooth muscle, relaxation occurs with low levels of O2, high levels of CO2, and high levels of H+

•  Circulating hormones (norepinephrine, epinephrine, acetylcholine, angiotensin, vasopressin, oxytocin, serotonin, histamine) can cause smooth muscle excitation or inhibition - depending upon the receptor •  Hormones act to regulate ion channel gating, thus initiating depolarization or

hyperpolarization •  Hormones act to initiate SR Ca++ release •  Horomones initiate intracellular signaling cascades


Contraction and Excitation of Smooth Muscle Source of Calcium Ions in Smooth Muscle Contraction

•  In smooth muscle, most Ca++ is transported into the cell rather than released from the SR •  Contraction is therefore delayed somewhat,

compared to skeletal muscle

•  Some smooth muscle do have significant SR located just beneath caveolae •  Caveolae allow propagation of AP (similar to T

tubules) and the release of Ca++ from SR •  As SR content increases, the rate of contraction

increases

•  In contrast to skeletal muscle, smooth muscle force of contraction can be regulated by Ca++ concentration •  Concentration is not in great excess, and

therefore can be consumed


membranes: transport, potentials, and...

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