© 2012 pearson education, inc. an introduction to the nervous system organs of the nervous system...

An Introduction to the Nervous System

• Organs of the Nervous System

• Brain and spinal cord (Processing incoming info and

creating motor commands)

• Sensory receptors of sense organs (eyes, ears,

pressure, pain, chemicals, etc.)

• Nerves connect nervous system with other

systems( a nerve is a bundle of axons)

Anatomical Divisions of the Nervous System• The Central Nervous System (CNS)

• Consists of the spinal cord and brain

• Contains neural tissue, connective tissues, and blood vessels

• Functions of the CNS are to process and coordinate:

• Sensory data from inside and outside body

• Motor commands control activities of peripheral organs (e.g., skeletal muscles)

• Higher functions of brain intelligence, memory, learning, emotion

Anatomical Divisions of the Nervous System• The Peripheral Nervous System (PNS)

• Includes all neural tissue outside the CNS

• Functions of the PNS

• Deliver sensory information to the CNS

• Carry motor commands to peripheral tissues and systems

12-1 Divisions of the Nervous System

• The Peripheral Nervous System (PNS)

• Nerves (also called peripheral nerves)

• Bundles of axons with connective tissues and blood

vessels

• Carry sensory information and motor commands in PNS

• Cranial nerves — connect to brain

• Spinal nerves — attach to spinal cord

• Functional Divisions of the PNS

• Afferent division (means “entrance”)

• Carries sensory information

• From PNS sensory receptors to CNS

• Efferent division (means “exit”)

• Carries motor commands

• From CNS to PNS muscles and glands

• Functional Divisions of the PNS

• Receptors and effectors of afferent division

• Receptors

• Detect changes or respond to stimuli

• Neurons and specialized cells

• Complex sensory organs (e.g., eyes, ears)

• Effectors

• Respond to efferent signals

• Cells and organs

Functional Divisions of the PNS

• Somatic nervous system (SNS)

• Controls voluntary and involuntary (reflexes)

muscle skeletal contractions

• Autonomic nervous system (ANS)

• Controls subconscious actions, contractions of

smooth muscle and cardiac muscle, and

glandular secretions

• Sympathetic division has a stimulating effect

• Parasympathetic division has a relaxing effect

Figure 12-1a The Anatomy of a Multipolar Neuron

Dendrites

Perikaryon

NucleusCell body

TelodendriaAxon

This color-coded figureshows the four generalregions of a neuron.

Figure 12-1b The Anatomy of a Multipolar Neuron

Axolemma

Dendritic branches

Nissl bodies (RERand free ribosomes)

Mitochondrion

Axon hillock

Initial segmentof axon

Golgi apparatus

Neurofilament

Nucleus

Nucleolus

Dendrite

PRESYNAPTIC CELL

Telodendria

Synapticterminals

See Figure 12–2

POSTSYNAPTICCELL

An understanding of neuronfunction requires knowing itsstructural components.

12-2 Neurons

• The Structure of Neurons

• The synapse

• Presynaptic cell

• Neuron that sends message

• Postsynaptic cell

• Cell that receives message

• The synaptic cleft

• The small gap that separates the presynaptic

membrane and the postsynaptic membrane

• Neurotransmitters

• Are chemical messengers

• Are released at presynaptic membrane

• Affect receptors of postsynaptic membrane

• Are broken down by enzymes (like AChE)

• Are reassembled at synaptic terminal

12-2 Neurons

• Types of Synapses

• Neuromuscular junction

• Synapse between neuron and muscle

• Neuroglandular junction

• Synapse between neuron and gland

Figure 12-2 The Structure of a Typical Synapse

Telodendrion

Synaptic terminal

Mitochondrion

Synapticvesicles

Presynapticmembrane

Postsynapticmembrane

Synapticcleft

Endoplasmicreticulum

12-2 Neurons

• Functions of Sensory Neurons

• Monitor internal environment (visceral sensory neurons)

• Monitor effects of external environment (somatic sensory

neurons)

• Structures of Sensory Neurons

• Unipolar

• Cell bodies grouped in sensory ganglia

• Processes (afferent fibers) extend from sensory

receptors to CNS

12-2 Neurons

• Three Types of Sensory Receptors

1. Interoceptors

• Monitor internal systems (digestive, respiratory, cardiovascular, urinary, reproductive)

• Internal senses (taste, deep pressure, pain)

2. Exteroceptors

• External senses (touch, temperature, pressure)

• Distance senses (sight, smell, hearing)

3. Proprioceptors

• Monitor position and movement (skeletal muscles and joints)

12-2 Neurons• Motor Neurons

• Carry instructions from CNS to peripheral effectors

• Via efferent fibers (axons)

12-2 Neurons

• Motor Neurons

• Two major efferent systems

1. Somatic nervous system (SNS)

• Includes all somatic motor neurons that innervate skeletal

muscles

2. Autonomic (visceral) nervous system (ANS)

• Visceral motor neurons innervate all other peripheral (like

internal organs) effectors

• Smooth muscle, cardiac muscle, glands, adipose tissue

12-2 Neurons

• Interneurons

• Most are located in brain, spinal cord, and autonomic ganglia

• Between sensory and motor neurons

• Are responsible for:

• Distribution of sensory information

• Coordination of motor activity

• Are involved in higher functions

• Memory, planning, learning

12-6 Axon Diameter and Speed

• Information

• “Information” travels within the nervous system

• As propagated electrical signals (action potentials)

• The most important information (vision, balance,

motor commands)

• Is carried by large-diameter, myelinated axons

12-4 Transmembrane Potential

• Ion Movements and Electrical Signals

• All plasma (cell) membranes produce electrical

signals by ion movements

• Transmembrane potential is particularly important to

neurons

• Five Main Membrane Processes in Neural

Activities

1. Resting potential

• The transmembrane potential of resting cell

2. Graded potential

• Temporary, localized change in resting potential

• Caused by stimulus

Activities

3. Action potential

• Is an electrical impulse

• Produced by graded potential

• Propagates along surface of axon to synapse

Activities

4. Synaptic activity

• Releases neurotransmitters at presynaptic membrane

• Produces graded potentials in postsynaptic

membrane

5. Information processing

• Response (integration of stimuli) of postsynaptic cell

Figure 12-8 An Overview of Neural Activities

Presynaptic neuron

Restingpotential

stimulusproduces

Gradedpotential

mayproduce

Action potential

Figure 12-8 An Overview of Neural Activities

Action potential

triggers

Informationprocessing

Postsynaptic cell

• The Transmembrane Potential

• Three important concepts

1. The extracellular fluid (ECF) and intracellular fluid (cytosol) differ greatly in ionic composition

• Concentration gradient of ions (Na+, K+)

2. Cells have selectively permeable membranes

3. Membrane permeability varies by ion

A&P FLIX Resting Membrane Potential

Figure 12-9 The Resting Potential is the Transmembrane Potential of an Undisturbed Cell

CYTOSOL

Na+ leakchannel

EXTRACELLULAR FLUID

Sodium–potassiumexchange

Plasmamembrane

Protein

K+ leakchannel

ProteinProtein

• Changes in the Transmembrane Potential

• Transmembrane potential rises or falls

• In response to temporary changes in membrane

permeability

• Resulting from opening or closing specific membrane

channels

• Sodium and Potassium Channels

• Membrane permeability to Na+ and K+ determines

transmembrane potential

• They are either passive or active

• Passive Channels (Leak Channels)

• Are always open

• Permeability changes with conditions

• Active Channels (Gated Channels)

• Open and close in response to stimuli

• At resting potential, most gated channels are closed

• Three Classes of Gated Channels

1. Chemically gated channels

2. Voltage-gated channels

3. Mechanically gated channels

• Chemically Gated Channels

• Open in presence of specific chemicals (e.g., ACh) at a binding site

• Found on neuron cell body and dendrites

• Voltage-gated Channels

• Respond to changes in transmembrane potential

• Have activation gates (open) and inactivation gates (close)

• Characteristic of excitable membrane

• Found in neural axons, skeletal muscle sarcolemma, cardiac muscle

• Mechanically Gated Channels

• Respond to membrane distortion

• Found in sensory receptors (touch, pressure, vibration)

Figure 12-11a Gated Channels

Resting state

Presence of ACh

Bindingsite

GatedchannelChannel closed

Channel open

A chemically gated Na+ channel thatopens in response to the presence ofACh at a binding site.

Chemically gated channel

Figure 12-11b Gated Channels

Voltage-gated channel

Channel closed

Channel open

Channel inactivated

+30 mV

–60 mV

–70 mV

Inactivationgate

Figure 12-11c Gated Channels

Mechanically gated channel

Channel closed

Channel open

Channel closed

Appliedpressure

Pressureremoved

• Transmembrane Potential Exists Across Plasma

Membrane

• Because:

• Cytosol and extracellular fluid have different chemical/ionic

balance

• The plasma membrane is selectively permeable

• Transmembrane Potential

• Changes with plasma membrane permeability

• In response to chemical or physical stimuli

• Graded Potentials

• Also called local potentials

• Changes in transmembrane potential

• That cannot spread far from site of stimulation

• Any stimulus that opens a gated channel

• Produces a graded potential

• The resting state

• Opening sodium channel produces graded potential

• Resting membrane exposed to chemical

• Sodium channel opens

• Sodium ions enter the cell

• Transmembrane potential rises

• Depolarization occurs

Figure 12-12 Graded Potentials

Initialsegment

Resting State

–70 mV

EXTRA-CELLULARFLUID

CYTOSOL

Resting membrane with closed chemically gated sodium ion channels

• Depolarization

• A shift in transmembrane potential toward 0 mV

• Movement of Na+ through channel

• Produces local current

• Depolarizes nearby plasma membrane (graded potential)

• Change in potential is proportional to stimulus

Membrane exposed to chemical that opens the sodium ion channels

Stimulusapplied

Stimulation

–65 mV

Graded Potential

Spread of sodium ions inside plasma membrane produces a local currentthat depolarizes adjacent portions of the plasma membrane

–60 mV –65 mV –70 mVLocalcurrent

Local current

• Repolarization

• When the stimulus is removed, transmembrane

potential returns to normal

• Hyperpolarization

• Increasing the negativity of the resting potential

• Result of opening a potassium channel

• Opposite effect of opening a sodium channel

• Positive ions move out, not into cell

12-5 Action Potential

• Action Potentials

• Propagated changes in transmembrane potential

• Affect an entire excitable membrane

• Link graded potentials at cell body with motor end

plate actions

• Initiating Action Potential

• All-or-none principle

• If a stimulus exceeds threshold amount

• The action potential is the same

• No matter how large the stimulus

• Action potential is either triggered, or not

Figure 12-14 Generation of an Action Potential

Resting Potential

–70 mV

The axolemma contains both voltage-gated sodium channels and voltage-gated potassium channels that areclosed when the membrane is at theresting potential.

= Sodium ion

= Potassium ion

• Four Steps in the Generation of Action Potentials

• Step 1: Depolarization to threshold

• Step 2: Activation of Na channels

• Step 3: Inactivation of Na channels and activation

of K channels

• Step 4: Return to normal permeability

• Step 1: Depolarization to threshold

• Step 2: Activation of Na channels

• Rapid depolarization

• Na+ ions rush into cytoplasm

• Inner membrane changes from negative to positive

–60 mV

= Sodium ion

= Potassium ion

Localcurrent

Depolarization to Threshold

= Sodium ion

= Potassium ion

+10 mV

Activation of SodiumChannels and RapidDepolarization

• Step 3: Inactivation of Na channels and

activation of K channels

• At +30 mV

• Inactivation gates close (Na channel inactivation)

• K channels open

• Repolarization begins

= Sodium ion

= Potassium ion

+30 mV

Inactivation of SodiumChannels and Activationof Potassium Channels

• Step 4: Return to normal permeability

• K+ channels begin to close

• When membrane reaches normal resting potential (–70

• K+ channels finish closing

• Membrane is hyperpolarized to –90 mV

• Transmembrane potential returns to resting level

• Action potential is over

= Sodium ion

= Potassium ion

–90 mV

Closing of PotassiumChannels

Graded potential causes threshold

Sodium channels close, voltage-gated potassium channels open

Threshold

Restingpotential

D E P O L A R I Z A T I O N R E P O L A R I Z A T I O N

Time (msec)

Voltage-gated sodiumion channels open

All channels closed

Cannot respond Responds only to a largerthan normal stimulus

RELATIVE REFRACTORY PERIODABSOLUTE REFRACTORY PERIOD

• Propagation of Action Potentials

• Propagation

• Moves action potentials generated in axon hillock

• Along entire length of axon

• Two methods of propagating action potentials

1. Continuous propagation (unmyelinated axons)

2. Saltatory propagation (myelinated axons)

• Continuous Propagation

• Of action potentials along an unmyelinated axon

• Affects one segment of axon at a time

• Steps in propagation

• Step 1: Action potential in segment 1

• Depolarizes membrane to +30 mV

• Local current

• Step 2: Depolarizes second segment to threshold

• Second segment develops action potential

Figure 12-15 Continuous Propagation of an Action Potential along an Unmyelinated Axon

Actionpotential

Extracellular Fluid

Cell membrane Cytosol

As an action potential develops at the initial segment , thetransmembrane potential at this site depolarizes to +30 mV.

–70 mV+30 mV

–70 mV

As the sodium ions entering at spread away from theopen voltage-gated channels, a graded depolarizationquickly brings the membrane in segment to threshold.

Graded depolarization

current

–60 mV –70 mV

• Continuous Propagation

• Steps in propagation

• Step 3: First segment enters refractory period

• Step 4: Local current depolarizes next segment

• Cycle repeats

• Action potential travels in one direction (1 m/sec)

An action potential now occurs in segment whilesegment beings repolarization.

Repolarization(refractory) –70 mV+30 mV

As the sodium ions entering at segment spread laterally,a graded depolarization quickly brings the membrane insegment to threshold, and the cycle is repeated.

–60 mV

Local current

• Saltatory Propagation

• Action potential along myelinated axon

• Faster and uses less energy than continuous

propagation

• Myelin insulates axon, prevents continuous

propagation

• Local current “jumps” from node to node

• Depolarization occurs only at nodes

Figure 12-16 Saltatory Propagation along a Myelinated Axon

–70 mV+30 mV –70 mV

An action potentialhas occurred at the initial segment .

Extracellular Fluid

Myelinatedinternode

Plasma membrane Cytosol

–60 mV –70 mV

Localcurrent

A local currentproduces a gradeddepolarization thatbrings the axolemmaat the next node tothreshold.

–70 mV+30 mV

An action potentialdevelops at node .

Repolarization(refractory)

–60 mV

A local currentproduces a gradeddepolarization thatbrings the axolemmaat node to threshold.

Localcurrent

A&P FLIX Propagation of an Action Potential

12-6 Axon Diameter and Speed

• Axon Diameter and Propagation Speed

• Ion movement is related to cytoplasm concentration

• Axon diameter affects action potential speed

• The larger the diameter, the lower the resistance

12-7 Synapses

• Synaptic Activity

• Action potentials (nerve impulses)

• Are transmitted from presynaptic neuron

• To postsynaptic neuron (or other postsynaptic cell)

• Across a synapse

12-7 Synapses

• Two Types of Synapses

1. Electrical synapses

• Direct physical contact between cells

2. Chemical synapses

• Signal transmitted across a gap by chemical

neurotransmitters

12-7 Synapses

• Electrical Synapses

• Are locked together at gap junctions (connexons)

• Allow ions to pass between cells

• Produce continuous local current and action potential

propagation

• Are found in areas of brain, eye, ciliary ganglia

12-7 Synapses

• Chemical Synapses

• Are found in most synapses between neurons and all

synapses between neurons and other cells

• Cells not in direct contact

• Action potential may or may not be propagated to

postsynaptic cell, depending on:

• Amount of neurotransmitter released

• Sensitivity of postsynaptic cell

12-7 Synapses

• Two Classes of Neurotransmitters

1. Excitatory neurotransmitters

• Cause depolarization of postsynaptic membranes

• Promote action potentials

2. Inhibitory neurotransmitters

• Cause hyperpolarization of postsynaptic membranes

• Suppress action potentials

12-7 Synapses

• The Effect of a Neurotransmitter

• On a postsynaptic membrane

• Depends on the receptor

• Not on the neurotransmitter

• For example, acetylcholine (ACh)

• Usually promotes action potentials

• But inhibits cardiac neuromuscular junctions

12-7 Synapses

• Cholinergic Synapses

• Any synapse that releases ACh at:

1. All neuromuscular junctions with skeletal muscle

fibers

2. Many synapses in CNS

3. All neuron-to-neuron synapses in PNS

4. All neuromuscular and neuroglandular junctions of

ANS parasympathetic division

12-7 Synapses

• Events at a Cholinergic Synapse

1. Action potential arrives, depolarizes synaptic terminal

2. Calcium ions enter synaptic terminal, trigger

exocytosis of ACh

3. ACh binds to receptors, depolarizes postsynaptic

membrane

4. ACh removed by AChE

• AChE breaks ACh into acetate and choline

Figure 12-17 Events in the Functioning of a Cholinergic Synapse

POSTSYNAPTICNEURON

An action potential arrives anddepolarizes the synaptic terminal

Presynaptic neuron

Synaptic vesicles

Action potential

EXTRACELLULARFLUID

Synapticterminal

Initialsegment

Extracellular Ca2+ enters the synapticterminal, triggering the exocytosis of ACh

Ca2+Ca2+

Synaptic cleft

Chemically gatedsodium ion channels

Na+Na+ Na+Na+

ACh binds to receptors and depolarizesthe postsynaptic membrane

Initiation ofaction potentialif threshold isreached at theinitial segment

ACh is removed by AChE

Propagation ofaction potential

(if generated)

Table 12-4 Synaptic Activity

Mitochondrion

AcetylcholineSynapticvesicle

SYNAPTICTERMINAL

SYNAPTICCLEFT

AChreceptor

(AChE)

AcetylcholinesteraseCholine

Acetate

POSTSYNAPTICMEMBRANE

12-8 Neurotransmitters and Neuromodulators

• Other Neurotransmitters

• At least 50 neurotransmitters other than ACh,

including:

• Biogenic amines

• Amino acids

• Neuropeptides

• Dissolved gases

• Important Neurotransmitters

• Other than acetylcholine

• Norepinephrine (NE)

• Dopamine

• Serotonin

• Gamma aminobutyric acid (GABA)

• Norepinephrine (NE)

• Released by adrenergic synapses

• Excitatory and depolarizing effect

• Widely distributed in brain and portions of ANS

• Dopamine

• A CNS neurotransmitter

• May be excitatory or inhibitory

• Involved in Parkinson’s disease and cocaine use

• Serotonin

• A CNS neurotransmitter

• Affects attention and emotional states

• Gamma Aminobutyric Acid (GABA)

• Inhibitory effect

• Functions in CNS

• Not well understood

• Chemical Synapse

• The synaptic terminal releases a neurotransmitter that

binds to the postsynaptic plasma membrane

• Produces temporary, localized change in permeability

or function of postsynaptic cell

• Changes affect cell, depending on nature and number

of stimulated receptors

• Many Drugs

• Affect nervous system by stimulating receptors that

respond to neurotransmitters

• Can have complex effects on perception, motor

control, and emotional states

• Neuromodulators

• Other chemicals released by synaptic terminals

• Similar in function to neurotransmitters

• Characteristics of neuromodulators

• Effects are long term, slow to appear

• Responses involve multiple steps, intermediary compounds

• Affect presynaptic membrane, postsynaptic membrane, or

• Released alone or with a neurotransmitter

• Neuropeptides

• Neuromodulators that bind to receptors and activate

enzymes

• Opioids

• Neuromodulators in the CNS

• Bind to the same receptors as opium or morphine

• Relieve pain

12-9 Information Processing

• Information Processing

• At the simplest level (individual neurons)

• Many dendrites receive neurotransmitter messages

simultaneously

• Some excitatory, some inhibitory

• Net effect on axon hillock determines if action potential

is produced

• Postsynaptic Potentials

• Graded potentials developed in a postsynaptic cell

• In response to neurotransmitters

• Two Types of Postsynaptic Potentials

1. Excitatory postsynaptic potential (EPSP)

• Graded depolarization of postsynaptic membrane

2. Inhibitory postsynaptic potential (IPSP)

• Graded hyperpolarization of postsynaptic membrane

• Inhibition

• A neuron that receives many IPSPs

• Is inhibited from producing an action potential

• Because the stimulation needed to reach threshold is increased

• Summation

• To trigger an action potential

• One EPSP is not enough

• EPSPs (and IPSPs) combine through summation

1. Temporal summation

2. Spatial summation

• Temporal Summation

• Multiple times

• Rapid, repeated stimuli at one synapse

• Spatial Summation

• Multiple locations

• Many stimuli, arrive at multiple synapses

Figure 12-19a Temporal and Spatial Summation

First stimulus arrives

FIRSTSTIMULUS

Initialsegment

Second stimulus arrives and isadded to the first stimulus

SECONDSTIMULUS

Thresholdreached

Action potential is generated

ACTIONPOTENTIAL

PROPAGATION

Temporal Summation. Temporal summation occurs on a membrane that receives two depolarizing stimuli from thesame source in rapid succession. The effects of the second stimulus are added to those of the first.

Figure 12-19b Temporal and Spatial Summation

Two stimuli arrive simultaneously Action potential is generated

TWOSIMULTANEOUS

STIMULI

ACTIONPOTENTIAL

PROPAGATION

Thresholdreached

Spatial Summation. Spatial summation occurs when sources of stimulationarrive simultaneously, but at different locations. Local currents spread thedepolarizing effects, and areas of overlap experience the combined effects.

Figure 12-21a Presynaptic Inhibition and Presynaptic Facilitation

GABArelease

Actionpotentialarrives Inactivation of

calcium channels

4. Reducedeffect onpostsynapticmembrane

2. Less calciumenters

3. Lessneurotransmitterreleased

1. Actionpotentialarrives

Presynaptic inhibition

Figure 12-21b Presynaptic Inhibition and Presynaptic Facilitation

Actionpotentialarrives

Serotoninrelease Activation of

calcium channels

2. More calciumenters

1. Actionpotentialarrives 3. More

neurotransmitterreleased

4. Increasedeffect onpostsynapticmembrane

Presynaptic facilitation

© 2012 pearson education, inc. an introduction to the nervous system organs of the nervous system...

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