bio 130 chapter 12 notes

Nervous System: Nervous Tissue

(Chapter 12)

Lecture Materials

for

Amy Warenda Czura, Ph.D.

Suffolk County Community College

Eastern Campus

Primary Sources for figures and content:

Marieb, E. N. Human Anatomy & Physiology 6th ed. San Francisco: Pearson Benjamin

Cummings, 2004.

Martini, F. H. Fundamentals of Anatomy & Physiology 6th ed. San Francisco: Pearson

Benjamin Cummings, 2004.

Neural Tissue

-3% of body mass

-cellular, ~20% extracellular space

-two categories of cells:

1. Neurons: conduct nervous impulses

2. Neuroglia / glial cells: nerve glue,

supporting cells

Organization of Nervous System

1. Central Nervous System (CNS)

-spinal cord, brain

-function: integrate, process, coordinate

sensory input and motor output

2. Peripheral Nervous System (PNS)

-all neural tissue outside CNS

-function: carry info to/from CNS via nerves

Nerve = bundle of axons (nerve fibers) with

blood vessels and CT

-cranial nerves " brain

-spinal nerves " spinal cord

Divisions of PNS:

1. Sensory/Afferent Division

-sensory receptors ! CNS

A. Somatic afferent division

-from skin, skeletal muscles, joints

B. Visceral afferent division

-from internal organs

2. Motor/Efferent Division

-CNS ! effectors

A. Somatic Nervous System

-voluntary nervous system

-to skeletal muscles

B. Autonomic Nervous System (ANS)

-involuntary nervous system

-to smooth & cardiac muscle, glands

1. Sympathetic Division

- fight or flight

2. Parasympathetic Division

- rest and digest

(tend to be antagonistic to each other)

Histology of Nervous System

Neuron / Nerve cell

-function:conduct nervous impulses (message)

-characteristics:

1. Extreme longevity

2. Amitotic (exceptions: hippocampus,

olfactory receptors)

3. High metabolic rate: need O2 and glucose

Amy Warenda Czura, Ph.D. 1 SCCC BIO130 Chapter 12 Lecture Notes

Structure:

-large soma / perikaryon

-large nucleus, large nucleolus (rRNA)

-many mitochondria, ribosomes, RER, Golgi:

(#ATP, #protein synthesis to produce

neurotransmitters)

-Nissl bodies: visible RER & ribosomes, gray

-neurofilaments = neurofibrils, neurotubules

(internal structure)

-no centrioles

-2 types of

processes:

(cell extensions)

1. Dendrites:

-receive info

-carry a graded potential toward soma

-contain same organelles as soma

-short, branched

-end in dendritic spines

2. Axon:

-single, long

-carry an action potential away from soma

-release neurotransmitters at end to signal

next cell

-long ones = nerve fibers

-contains:

-neurofibrils & neurotubules (abundant)

-vesicles of neurotransmitter

-lysosomes, mitochondria, enzymes

-no Nissl bodies, no Golgi (no protein

synthesis in axon)

-connects to soma at axon hillock

-covered in axolemma (membrane)

-may branch: axon collaterals

-end in synaptic terminals or knobs

-may have myelin sheath: protein+lipid

-protection

-insulation

-increase speed of impulse

CNS: myelin from oligodendrocytes

PNS: myelin from Schwann cells

Axoplasmic transport

-move materials between soma and terminal

-along neurotubules on kinesins

-Anterograde transport = soma ! terminal

(neurotransmitters from soma)

-Retrograde transport = terminal ! soma

(recycle breakdown products from used

neurotransmitters)

Some viruses use retrograde transport to

gain access to CNS (Polio, Herpes,

Rabies)


-presynaptic cell sends message along axon to

axon terminal

-postsynaptic cell receives message as

neurotransmitter

Neurotransmitter = chemical, transmits signal

from pre- to postsynaptic cell across

synaptic cleft

Synaptic knob = small, round, when

postsynaptic cell is neuron, synapse on

dendrite or soma

Synaptic terminal = complex structure, at

neuromuscular or neuroglandular junction

Synapse

-site where neuron

communicates with

another cell:

neuron or effector

Structural classification of neurons:

1. Anaxonic neurons:

-dendrites and axon look same

-brain and special sense organs

2. Bipolar neurons:

-1 dendrite, 1 axon

-soma in middle

-rare: special sense organs,

relay from receptor to neuron

3. Unipolar neurons:

-1 long axon, dendrites at one

end, soma off side (T shape)

-most sensory neurons

4. Multipolar neurons:

-2 or more dendrites

-1 long axon

-99% all neurons

-most CNS

Functional Classification of Neurons:

1. Sensory/Afferent neurons

-transmit info from sensory receptors to CNS

-most unipolar

-soma in peripheral sensory ganglia

Ganglia = collection of cell bodies in PNS

A. Somatic sensory neurons

-receptors monitor outside conditions

B. Visceral sensory neurons

-receptors monitor internal conditions

2. Motor/Efferent neurons

-transmit commands from CNS to effectors

-most multipolar

A. Somatic motor neurons

-innervate skeletal muscle

-conscious control or reflexes

B. Visceral/Autonomic motor neurons

-innervate effectors on smooth muscle,

cardiac muscle, glands, adipose

3. Interneurons / Association neurons

-distribute sensory info and coordinate motor

activity

-between sensory and motor neurons

-in brain, spinal cord, autonomic ganglia

-most are multipolar

Neuroglia =supporting cells

Neuroglia in CNS

-outnumber neurons 10:1

-half mass of brain


1. Ependymal cells

-line central canal of spinal

cord and ventricles of brain

-secrete cerebrospinal fluid

(CSF)

-have cilia to circulate CSF

-CSF: cushion brain, nutrient & gas exchange

2. Astrocytes

-most abundant CNS

neuroglia

-varying functions:

a. blood brain barrier:

processes wrap capillaries, control

chemical exchange between blood and

interstitial fluid of brain

b. framework of CNS

c. repair damaged neural tissue

d. guide neuron development in embryo

e. control interstitial environment: regulate

conc. ions, gasses, nutrients, neurotransmitters

3. Oligodendrocytes

-wide flat processes wrap

local axons = myelin

sheath

-1 cell contributes myelin to many

neighboring axons

-lipid in membrane insulates axon for faster

action potential conductance

-gaps on axon between processes/myelin =

Nodes of Ranvier, necessary to conduct

impulse

-white, myelinated axons = white matter

4. Microglia

-phagocytic

-wander CNS

-engulf debris, pathogens

-important CNS defense

(no immune cells or antibodies)

Cells in the CNS Neuroglia in PNS

1. Satellite cells

-surround somas in ganglia

-isolate PNS cells

-regulate interstitial environment of ganglia

2. Schwann cells

-myelinate axons in PNS

-whole cells wraps axon,

many layers

-Neurilemma: bulge of schwann cell,

contains organelles

-Nodes of Ranvier between cells


-vital to repair of peripheral never fibers after

injury: guide growth to original synapse

-some hold bundles of unmyelinated axons

+ + + +

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-- - - - ----

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Neurophysiology

Neurons: conduct electrical impulse

-requires transmembrane potential = electrical

difference across cell membrane

-cells: positive charge outside (pump cations

out) and negative charge inside (proteins)

Voltage = measure of potential energy

generated by separation of opposite charges

Current = flow of electrical charges (ions)

Cell can produce current (nervous impulse)

when ions move to eliminate the potential

difference (volts) across the membrane

Resistance = restricts ion movement (current)

(high resistance = low current); membrane

has resistance, restricts ion flow/current

Ohms Law: current = voltage resistance

Current highest when voltage high and

resistance low

Cell voltage set at -70mV but membrane

resistance can be altered to create current

Membrane resistance depends on permeability

to ions: open or close ion channels

Cell must always have some resistance or ions

would equalize, voltage = zero,

no current generated = no nervous impulse

Membrane ion channels:

-allow ion movement (alter resistance)

-each channel specific to one ion type

1. Passive channels (leak channels)

-always open, free flow

-sets resting membrane potential at -70mV

2. Active channels

-open/close in response to signal

A. Chemically regulated/ Ligand-gated

-open in response to chemical binding

-located on any cell membrane

(dendrites, soma)

B. Voltage regulated channels

-open/close in response to shift in

transmembrane potential

-excitable membrane only: conduct

action potentials (axolemma,

sarcolemma)


C. Mechanically regulated channels

-open in response to membrane

distortion

-on dendrites of sensory neurons for

touch, pressure, vibration

When channel opens, ions flow along

electrochemical gradient:

-diffusion (high conc. to low)

-electrical attraction/repulsion

Sodium-Potassium Pump:

-uses ATP to move 3 Na+ out 2 K+ in

(70% of neuron ATP for this)

-runs anytime cell not conducting impulse

-creates high [K+] inside and high [Na+]outside

When Na+ channel opens:

- Na+ flows into cell:

1. Favored by diffusion gradient

2. Favored by electrical gradient

open channel = $resistance = #ion flow/current

When K+ channel opens:

- K+ flows out of cell:

1. Favored by diffusion gradient only

2. Electrical gradient repels K+ exit

- Thus less current than Na+

Channels open = resistance low = ions move

until equilibrium potential: depends on

-diffusion gradient

-electrical gradient

Equilibrium Potential

For K+ = -90mV

For Na+ = +66mV

Open channel ! current ! graded potential

Graded potential = localized shift in

transmembrane potential due to

movement of charges in to /out of cell

Na+ channel opens = Na+ flows in,

depolarization (cell less negative)

K+ channel opens = K+ flows out,

hyperpolarization (cell more negative)

Graded potentials:

-occur on any membrane: dendrites and somas

-can be depolarizing or hyperpolarizing

-amount of depolarization or hyperpolarization

depends on intensity of stimulus:

# channels open = # voltage change

-passive spread from site of stimulation over

short distance

-effect on membrane potential decreases with

distance from stimulation site

-repolarization occurs as soon as stimulus is

removed: leak channels & Na+/K+ pump

reset resting potential

Graded potential = localized change in

transmembrane potential, not nervous

impulse (message)


If big enough depolarization = action potential

= nervous impulse = transmission to

next cell

Action potentials:

-occur on excitable membranes only

(axolemma, sarcolemma)

-always depolarizing

-must depolarize to threshold (-55mV) before

action potential begins

(voltage gated channels on excitable

membrane open at threshold to

propagate action potential)

- all-or-none : all stimuli that exceed

threshold will produce identical action

potentials

-action potential at one site depolarizes

adjacent sites to threshold

-propagated across entire membrane surface

without decrease in strength

The Generation of

an Action Potential

-55 mV

1. Depolarization to threshold:

- a graded potential depolarizes local

membrane and flows toward the axon

- if threshold is met (-55mV) at the hillock, an

action potential will be triggered

2. Activation of sodium channels and rapid

depolarization:

- at threshold (-55mV), voltage-regulated

sodium channels on the excitable

membrane open

- Na+ flows into the cell depolarizing it

- the transmembrane potential rapidly changes

from -55mV to +30mV

3. Inactivation of sodium channels and

activation of potassium channels:

- at +30mV Na+ channels close and K+

channels open

- K+ flows out of the cell repolarizing it

4. Return to normal permeability:

- at -70mV K+ channels begin to close

- the cell hyperpolarizes to -90mV until all

channels finish closing

- leak channels restore the resting membrane

potential to -70mV

(Handout)

Restimulation only when Na+ channels closed:

influx of Na+ necessary for action potential

Absolute Refractory Period = -55mV

(threshold) to +30mV, Na+ channels open,

membrane cannot respond to additional

stimulus

Relative Refractory Period = +30mV to

-70mV (return to resting potential), Na+

channels closed, membrane capable of

second action potential but requires

larger/longer stimulus (threshold elevated)

Cell has ions for thousands of action potentials

Eventually must run Sodium-Potassium pump

(burn ATP) to reset high [K+] inside and

high [Na+] outside

(Death = no ATP, but stored ions can

generate action potentials for awhile)

Propagation of Action Potentials

-once generated must be transmitted length of

axon: hillock to terminal

-speed depends on:

1. Degree of myelination

2. Axon diameter

1. Myelination

A. Continuous Propagation:

-unmyelinated axons

-whole membrane depolarizes and

repolarizes sequentially hillock to

terminal

-only forward movement; membrane

behind always in absolute refractory

period


B. Saltatory propagation

-myelinated axons

-depolarization only on exposed

membrane at nodes

-myelin insulates covered membrane

from ion flow

-action potential jumps from node to

node: faster and requires less

energy to reset

Continuous Propagation Saltatory Propagation

2. Axon diameter

-larger axon ! less resistance ! easier ion

flow ! faster action potential

A. Type A Fibers

- 4-20m diameter

- myelinated (saltatory propagation)

- action potentials 140m/sec

- carry somatic motor and somatic

sensory info

B. Type B Fibers

- 2-4m diameter

- myelinated (saltatory propagation)


- carry autonomic motor and visceral

sensory info

C. Type C Fibers

- < 2m diameter

- unmyelinated (continuous propagation)


- carry autonomic motor and visceral

sensory info

Myelination:

-requires space, metabolically expensive

-only important fibers large and myelinated

-occurs in early childhood

-results in improved coordination

Multiple Sclerosis = genetic disorder, myelin

on neurons in PNS destroyed !

numbness, paralysis

Synapse = junction between transmitting

neuron (presynaptic cell) and receiving

cell (postsynaptic cell), where nerve

impulse moves from one cell to next

Two types:

1. Electrical Synapse

-direct contact via gap junctions

-ions flow directly from pre to post cell

-less common synapse

-in brain (conscious perception)

2. Chemical synapse

-most common


-pre and post cells separated by synaptic cleft

-presynaptic neuron releases neurotransmitter

to trigger effect on post synaptic cell

-dynamic: facilitate or inhibit transmission,

depends on neurotransmitter:

1. Excititory Neurotransmitters =

-depolarization

-propagate action potential

2. Inhibitory Neurotransmitters =

-hyperpolarization

-suppress action potential

Propagation across chemical synapse always

slow but allows variability

Events at a Synapse:

e.g.Cholinergic Synapse(Acetylcholine as neurotransmitter)

(Handout)

Neurotransmitter Mechanism of Action

1. Direct effect on membrane potential

2. Indirect effect on membrane potential

(Handout)

(Handout)

Post synaptic potential = graded potential

caused by a neurotransmitter due to

opening or closing of ion channels on

post synaptic cell membrane

Two types:

1. Excititory Post Synaptic Potential (EPSP)

-causes depolarization

2. Inhibitory Post Synaptic Potential (IPSP)

-causes hyperpolarization

-inhibits postsynaptic cell (need larger

stimulus to reach threshold)

Multiple EPSPs needed to trigger action

potential in post cell axon

EPSP summation:

1. Temporal summation

-single synapse fires repeatedly: string of

EPSPs in one spot

-each EPSP depolarizes more until

threshold reached at hillock


2. Spatial summation

-multiple synapses fire simultaneously

-collective depolarization reaches threshold

Facilitated = depolarized; brought closer to

threshold by some sort of stimulus, less

stimulus now required to reach threshold

(e.g. caffeine)

Post Synaptic Potentiation:

-repeat stimulation of same synapse

conditions synapse, pre cell more easily

stimulates post cell to threshold (repetition)

Most nervous system activity results from

interplay of EPSPs and IPSPs to

promote differing degrees of facilitation

or inhibition to allow constant fine

tuning of response

Neuromodulators = chemicals that influence

synthesis, release, or degradation of

neurotransmitters thus altering normal

response of the synapse

Common Neurotransmitters:

1. Acetycholine- cholinergic synapses

-excititory

-direct effect

-skeletal neuromuscular junctions, many

CNS synapses, all neuron to neuron

PNS, all parasympathetic ANS

2. Norepinephrine- adrenergic synapses

-excititory

-second messengers

-many brain synapses, all sympathetic ANS

effector junctions

3. Dopamine

-excititory or inhibitory

-second messengers

-many brain synapses

-cocaine: inhibits removal = high

-Parkinsons disease: damage neurons =

ticks, jitters

4. Serotonin

-inhibitory

-direct or second messenger

-brain stem for emotion

-anti-depression/ anti-anxiety drugs

block uptake

5. Gamma aminobytyric acid (GABA)

-inhibitory

-direct effect

-brain: anxiety control, motor coordination

-alcohol: augments effects = loss of

coordination

Factors that disrupt neural function:

1. pH: normal = 7.4

@ pH 7.8 ! spontaneous action potentials

= convulsions

@ pH 7.0 ! no action potentials

= unresponsive

2. Ion concentrations

high extracellular [K+] ! depolarize

membranes = death, cardiac arrest

3. Temperature: normal = 37C

-higher: neurons more excitable

(fever = hallucinations)

-lower: neurons non-responsive

(hypothermia = lethargy, confusion)

4. Nutrients

-neurons: no reserves, use a lot of ATP

-require constant and abundant glucose

-glucose only

5. Oxygen

-aerobic respiration only for ATP

-no ATP = neuron damage/death


bio 130 chapter 12 notes

Documents

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peripheral nervous system

somaretrograde transport

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