Nervous System and Nervous Tissue

Chapter 11

Functions of the Nervous System

1. Sensory input2. Integration3. Motor output

Divisions of the Nervous System

• Central nervous system (CNS) – Brain and spinal cord– Integration and command center

• Peripheral nervous system (PNS)– Paired spinal and cranial nerves carry messages to

and from the CNS

Peripheral Nervous System (PNS)

1. Sensory (afferent) division• Somatic afferent fibers—convey impulses from

skin, skeletal muscles, and joints • Visceral afferent fibers—convey impulses from

visceral organs 2. Motor (efferent) division • Transmits impulses from the CNS to effector

organs

Motor Division of PNS1. Somatic (voluntary) nervous system– Conscious control of skeletal muscles

2. Autonomic (involuntary) nervous system (ANS)– Visceral motor nerve fibers– Regulates smooth muscle, cardiac muscle, and

glands– Two functional subdivisions• Sympathetic• Parasympathetic

Figure 11.2

Central nervous system (CNS)

Brain and spinal cordIntegrative and control centers

Peripheral nervous system (PNS)

Cranial nerves and spinal nervesCommunication lines between theCNS and the rest of the body

Parasympatheticdivision

Conserves energyPromotes house-keeping functionsduring rest

Motor (efferent) division

Motor nerve fibersConducts impulses from the CNSto effectors (muscles and glands)

Sensory (afferent) divisionSomatic and visceral sensorynerve fibersConducts impulses fromreceptors to the CNS

Somatic nervoussystem

Somatic motor(voluntary)Conducts impulsesfrom the CNS toskeletal muscles

Sympathetic divisionMobilizes bodysystems during activity

Autonomic nervoussystem (ANS)

Visceral motor(involuntary)Conducts impulsesfrom the CNS tocardiac muscles,smooth muscles,and glands

StructureFunctionSensory (afferent)division of PNS Motor (efferent) division of PNS

Somatic sensoryfiber

Visceral sensory fiber

Motor fiber of somatic nervous system

Skin

StomachSkeletalmuscle

Heart

BladderParasympathetic motor fiber of ANS

Sympathetic motor fiber of ANS

Histology of Nervous Tissue

1. Neurons—excitable cells that transmit electrical signals

2. Neruoglia – glial cells• Astrocytes (CNS)• Microglia (CNS)• Ependymal cells (CNS)• Oligodendrocytes (CNS)• Satellite cells (PNS)• Schwann cells (PNS)

Astrocytes• Most abundant, versatile, and highly branched glial

cells• Cling to neurons, synaptic endings, and capillaries• Support and brace neurons• Help determine capillary permeability• Guide migration of young

neurons• Control the chemical

environment• Participate in information

processing in the brain

Microglia

• Small, ovoid cells with thorny processes• Migrate toward injured neurons• Phagocytize microorganisms and neuronal

debris

Ependymal Cells

• Range in shape from squamous to columnar• May be ciliated– Line the central cavities of the brain and spinal

column• Separate the CNS

interstitial fluid from the cerebrospinal fluid in the cavities

Oligodendrocytes

• Branched cells• Processes wrap CNS nerve fibers, forming

insulating myelin sheaths

Satellite Cells and Schwann Cells

• Satellite cells– Surround neuron cell bodies in the PNS

• Schwann cells (neurolemmocytes)– Surround peripheral nerve fibers and form myelin

sheaths– Vital to regeneration of damaged peripheral nerve

fibers

Neurons (Nerve Cells)

• Special characteristics:– Long-lived ( 100 years or more)– Amitotic—with few exceptions– High metabolic rate—depends on continuous

supply of oxygen and glucose– Plasma membrane functions in:• Electrical signaling • Cell-to-cell interactions during development

Cell Body (Perikaryon or Soma)

• Biosynthetic center of a neuron• Spherical nucleus with nucleolus• Well-developed Golgi apparatus• Rough ER called Nissl bodies (chromatophilic

substance)• Network of neurofibrils (neurofilaments) • Axon hillock—cone-shaped area from which axon

arises• Clusters of cell bodies are called nuclei in the

CNS, ganglia in the PNS

Processes

• Dendrites and axons• Bundles of processes are called – Tracts in the CNS– Nerves in the PNS

Dendrites

• Short, tapering, and diffusely branched • Receptive (input) region of a neuron• Convey electrical signals toward the cell body

as graded potentials

The Axon• One axon per cell arising from the axon hillock• Long axon = nerve fiber• Occasional branches = axon collaterals• Numerous terminal branches • Knoblike axon terminals (synaptic knobs or

boutons) – Secretory region of neuron– Release neurotransmitters to excite or inhibit

other cells

Axons: Function

• Conducting region of a neuron• Generates and transmits nerve impulses (action

potentials) away from the cell body• Molecules and organelles are moved along

axons by motor molecules in two directions:– Anterograde—toward axonal terminal

• Examples: mitochondria, membrane components, enzymes

– Retrograde—toward the cell body • Examples: organelles to be degraded, signal molecules,

viruses, and bacterial toxins

Figure 11.4b

Dendrites(receptive regions)

Cell body(biosynthetic centerand receptive region)

Nucleolus

Nucleus

Nissl bodies

Axon(impulse generatingand conducting region)

Axon hillock

NeurilemmaTerminalbranches

Node of Ranvier

Impulsedirection

Schwann cell(one inter-node)

Axonterminals(secretoryregion)

(b)

Myelin Sheath

• Segmented protein-lipoid sheath around most long or large-diameter axons

• It functions to:– Protect and electrically insulate the axon– Increase speed of nerve impulse transmission

Myelin Sheaths in the PNS

• Schwann cells wraps many times around the axon – Myelin sheath—concentric layers of Schwann cell

membrane • Outer collar of perinuclear cytoplasm—

peripheral bulge of Schwann cell cytoplasm• Nodes of Ranvier – gaps between adjacent Schwann cells– Sites where axon collaterals can emerge

Myelin Sheaths in the CNS

• Formed by processes of oligodendrocytes, NOT the whole cells

• Nodes of Ranvier are present• No OCPC• Thinnest fibers are unmyelinated

White Matter and Gray Matter

• White matter– Dense collections of myelinated fibers

• Gray matter– Mostly neuron cell bodies and unmyelinated fibers

Table 11.1 (1 of 3)

Structural Classification of Neurons

Functional Classification of Neurons

1. Sensory (afferent)• Transmit impulses from sensory receptors toward the

CNS

2. Motor (efferent)• Carry impulses from the CNS to effectors

3. Interneurons (association neurons)• Shuttle signals through CNS pathways; most are

entirely within the CNS

Neuron Function

• Neurons are highly irritable• Respond to adequate stimulus by generating

an action potential (nerve impulse) • Impulse is always the same regardless of

stimulus (Action potential)

Role of Membrane Ion Channels

1. Leakage (nongated) channels—always open2. Gated channels (three types):

– Chemically gated (ligand-gated) channels—open with binding of a specific neurotransmitter

– Voltage-gated channels—open and close in response to changes in membrane potential

– Mechanically gated channels—open and close in response to physical deformation of receptors

Resting Membrane Potential (Vr)

• Potential difference across the membrane of a resting cell– Approximately –70 mV in neurons

Membrane Potentials That Act as Signals

• Membrane potential changes when:1. Concentrations of ions across the membrane

change2. Permeability of membrane to ions changes

• Changes in membrane potential are signals used to receive, integrate and send information

Membrane Potentials That Act as Signals

• Two types of signals– Graded potentials • Incoming short-distance signals

– Action potentials • Long-distance signals of axons (outgoing)

Changes in Membrane Potential

• Depolarization– A reduction in

membrane potential (toward zero)

• Hyperpolarization– An increase in membrane

potential (away from zero)

Graded Potentials

• Short-lived, localized changes in membrane potential• Depolarizations or hyperpolarizations• Graded potential spreads as local currents change

the membrane potential of adjacent regions

Figure 11.10c

Distance (a few mm)

–70Resting potential

Active area(site of initialdepolarization)

(c) Decay of membrane potential with distance: Because current is lost through the “leaky” plasma membrane, the voltage declines with distance from the stimulus (the voltage is decremental ). Consequently, graded potentials are short-distance signals.

Mem

bra

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ote

nti

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V)

Action Potential (AP)

• Brief reversal of membrane potential with a total amplitude of ~100 mV

• Occurs in muscle cells and axons of neurons• Does not decrease in magnitude over distance• Principal means of long-distance neural

communication

Actionpotential

1 2 3

4

Resting state Depolarization Repolarization

Hyperpolarization

The big picture

1 1

2

3

4

Time (ms)

ThresholdMem

bra

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V)

Figure 11.11 (1 of 5)

Actionpotential

Time (ms)

1 1

2

3

4

Na+ permeability

K+ permeability

The AP is caused by permeability changes inthe plasma membrane

Mem

bra

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V)

Rela

tive m

em

bra

ne p

erm

eab

ility

Figure 11.11 (2 of 5)

Coding for Stimulus Intensity

• All action potentials are alike and are independent of stimulus intensity

• Strong stimuli can generate action potentials more often than weaker stimuli

• The CNS determines stimulus intensity by the frequency of impulses

Figure 11.13

Threshold

Actionpotentials

Stimulus

Time (ms)

Figure 11.14

Stimulus

Absolute refractoryperiod

Relative refractoryperiod

Time (ms)

Depolarization(Na+ enters)

Repolarization(K+ leaves)

After-hyperpolarization

Conduction Velocity

• Conduction velocities of neurons vary widely • Effect of axon diameter– Larger = faster

• Effect of myelination– Myelination = faster

Multiple Sclerosis (MS)• Autoimmune disease that mainly affects young adults• Symptoms: visual disturbances, weakness, loss of

muscular control, speech disturbances, and urinary incontinence

• Myelin sheaths in the CNS become nonfunctional scleroses

• Shunting and short-circuiting of nerve impulses occurs• Impulse conduction slows and eventually ceases

The Synapse

• A junction that mediates information transfer from one neuron:– To another neuron– To an effector cell

• Electrical or Chemical• Presynaptic neuron—conducts impulses

toward the synapse• Postsynaptic neuron—transmits impulses away

from the synapse

Figure 11.16

Dendrites

Cell body

Axon

Axodendriticsynapses

Axosomaticsynapses

Cell body (soma) ofpostsynaptic neuron

Axon

(b)

Axoaxonic synapses

Axosomaticsynapses

(a)

Electrical Synapses

• Less common than chemical synapses– Neurons are electrically coupled (joined by gap

junctions)– Communication = very rapid• may be unidirectional or bidirectional

– Important in:• Embryonic nervous tissue• Some brain regions

Chemical Synapses

• Specialized for the release and reception of neurotransmitters

• Typically composed of two parts – Axon terminal of the presynaptic neuron, which

contains synaptic vesicles – Receptor region on the postsynaptic neuron

Figure 11.17

Action potentialarrives at axon terminal.

Voltage-gated Ca2+

channels open and Ca2+

enters the axon terminal.

Ca2+ entry causesneurotransmitter-containing synapticvesicles to release theircontents by exocytosis.

Chemical synapsestransmit signals fromone neuron to anotherusing neurotransmitters.

Ca2+

Synapticvesicles

Axonterminal

Mitochondrion

Postsynapticneuron

Presynapticneuron

Presynapticneuron

Synapticcleft

Ca2+

Ca2+

Ca2+

Neurotransmitterdiffuses across the synapticcleft and binds to specificreceptors on thepostsynaptic membrane.

Binding of neurotransmitteropens ion channels, resulting ingraded potentials.

Neurotransmitter effects areterminated by reuptake throughtransport proteins, enzymaticdegradation, or diffusion awayfrom the synapse.

Ion movement

Graded potentialReuptake

Enzymaticdegradation

Diffusion awayfrom synapse

Postsynapticneuron

1

2

3

4

5

6

Postsynaptic Potentials

• Types of postsynaptic potentials – EPSP—excitatory postsynaptic potentials – IPSP—inhibitory postsynaptic potentials

Excitatory Synapses and EPSPs

• Neurotransmitter binding opens chemically gated channels

• Allows simultaneous flow of Na+ and K+ in opposite directions

• Na+ influx greater than K+ efflux net depolarization called EPSP (not AP)

• EPSP help trigger AP if EPSP is of threshold strength– Can spread to axon hillock, trigger opening of voltage-

gated channels, and cause AP to be generated

© 2013 Pearson Education, Inc.

Figure 11.18a Postsynaptic potentials can be excitatory or inhibitory.

An EPSP is a localdepolarization of the postsynaptic membranethat brings the neuroncloser to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing Na+ and K+ to pass through simultaneously.

Threshold

Stimulus

+30

0

–55

–70

Time (ms)10 20 30

Mem

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V)

Excitatory postsynaptic potential (EPSP)

Inhibitory Synapses and IPSPs

• Reduces postsynaptic neuron's ability to produce an action potential– Makes membrane more permeable to K+ or Cl–

• If K+ channels open, it moves out of cell• If Cl- channels open, it moves into cell

– Neurotransmitter hyperpolarizes cell• Inner surface of membrane becomes more negative• AP less likely to be generated

© 2013 Pearson Education, Inc.

Figure 11.18b Postsynaptic potentials can be excitatory or inhibitory.

Threshold

Stimulus

+30

0

–55

–70

Time (ms)10 20 30

Mem

bra

ne p

ote

nti

al (m

V) An IPSP is a localhyperpolarization of the postsynaptic membranethat drives the neuronaway from AP threshold. Neurotransmitter binding opens K+ or Cl– channels.

Inhibitory postsynaptic potential (IPSP)

Synaptic Integration: Summation

• A single EPSP cannot induce an AP• EPSPs and IPSPs can summate to influence

postsynaptic neuron• Most neurons receive both excitatory and

inhibitory inputs from thousands of other neurons– Only if EPSP's predominate and bring to threshold

AP

Two Types of Summation

• Temporal summation– One + presynaptic neurons transmit rapid-fire

impulses• Spatial summation– Postsynaptic neuron stimulated simultaneously by

large number of terminals at same time

Neurotransmitters

• Most neurons make two or more neurotransmitters, which are released at different stimulation frequencies

• 50 or more neurotransmitters have been identified

• Classified by chemical structure and by function

Chemical Classes of Neurotransmitters

• Acetylcholine (Ach)– Released at neuromuscular junctions and some

ANS neurons• Biogenic amines include:– Broadly distributed in the brain– Play roles in emotional behaviors and the biological

clock• Catecholamines

– Dopamine, norepinephrine (NE), and epinephrine• Indolamines

– Serotonin and histamine

Chemical Classes of Neurotransmitters

• Amino acids include:– GABA—Gamma ()-aminobutyric acid – Glycine– Glutamate

• Peptides (neuropeptides) include:• Substance P

– Mediator of pain signals

• Endorphins– Act as natural opiates; reduce pain perception

• Gut-brain peptides– Somatostatin and cholecystokinin

Chemical Classes of Neurotransmitters

• Purines such as ATP:– Act in both the CNS and PNS– Produce fast or slow responses– Induce Ca2+ influx in astrocytes– Provoke pain sensation

Chemical Classes of Neurotransmitters

• Gases and lipids– Nitric oxide (NO)• Synthesized on demand • Involved in learning and memory

– Carbon monoxide (CO) is a regulator of cGMP in the brain

– Endocannabinoids• Lipid soluble; synthesized on demand from membrane

lipids• Involved in learning and memory

Functional Classification of Neurotransmitters

• Neurotransmitter effects may be excitatory (depolarizing) and/or inhibitory (hyperpolarizing)– Determined by the receptor type of the

postsynaptic neuron – Acetylcholine• Excitatory at neuromuscular junctions in

skeletal muscle• Inhibitory in cardiac muscle

Neurotransmitter Actions

• Direct action – Neurotransmitter binds to channel-linked receptor and

opens ion channels– Promotes rapid responses

• Examples: ACh and amino acids

• Indirect action – Neurotransmitter binds to a G protein-linked receptor

and acts through an intracellular second messenger– Promotes long-lasting effects

• Examples: biogenic amines, neuropeptides, and dissolved gases

Neural Integration: Neuronal Pools

• Functional groups of neurons that:– Integrate incoming information– Forward the processed information to other

destinations• Simple neuronal pool– Single presynaptic fiber branches and synapses

with several neurons in the pool– Discharge zone—neurons most closely associated

with the incoming fiber– Facilitated zone—neurons farther away from

incoming fiber

Figure 11.21

Presynaptic(input) fiber

Facilitated zone Discharge zone Facilitated zone

Types of Circuits in Neuronal Pools

• Diverging circuit• Converging• Reverberating• Parallel after-discharge

Patterns of Neural Processing

• Serial processing– Input travels along one pathway to a specific

destination– Works in an all-or-none manner to produce a

specific response– Example: Reflexes• rapid, automatic responses to stimuli that always cause

the same response• Reflex arcs (pathways) have five essential components:

receptor, sensory neuron, CNS integration center, motor neuron, and effector

Figure 11.23

1

2

3

4

5

Receptor

Sensory neuron

Integration center

Motor neuron

Effector

Stimulus

ResponseSpinal cord (CNS)

Interneuron

Patterns of Neural Processing

• Parallel processing– Input travels along several pathways– One stimulus promotes numerous responses– Important for higher-level mental functioning

• Example: a smell may remind one of the odor and associated experiences