Chapter 12

Principles of Human Anatomy and Physiology, 11e

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Chapter 12

Nervous Tissue

Lecture Outline

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INTRODUCTION The nervous system, along with

the endocrine system, helps to keep controlled conditions within limits that maintain health and helps to maintain homeostasis.

The nervous system is responsible for all our behaviors, memories, and movements.

The branch of medical science that deals with the normal functioning and disorders of the nervous system is called neurology.

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Chapter 12Nervous Tissue

Controls and integrates all body activities within limits that maintain life

Three basic functions sensing changes with sensory receptors

fullness of stomach or sun on your face interpreting and remembering those changes reacting to those changes with effectors

muscular contractions glandular secretions

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Major Structures of the Nervous System

Brain, cranial nerves, spinal cord, spinal nerves, ganglia, enteric plexuses and sensory receptors

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Structures of the Nervous System - Overview Twelve pairs of cranial nerves emerge from the base of

the brain through foramina of the skull. A nerve is a bundle of hundreds or thousands of axons,

each of which courses along a defined path and serves a specific region of the body.

The spinal cord connects to the brain through the foramen magnum of the skull and is encircled by the bones of the vertebral column. Thirty-one pairs of spinal nerves emerge from the spinal

cord, each serving a specific region of the body. Ganglia, located outside the brain and spinal cord, are

small masses of nervous tissue, containing primarily cell bodies of neurons.

Enteric plexuses help regulate the digestive system. Sensory receptors are either parts of neurons or

specialized cells that monitor changes in the internal or external environment.

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12 pair of cranial nerves

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Cranial Nerves

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Cranial Nerve Names

On Olfactory CN I

Old Optical CN II

Olympus’ Occulomotor CN III

Towering Trochlear CN IV

Top Trigeminal CN V

A Abducens CN VI

Fat Facial CN VII

Vivacous Vestibulocochlear CN VIII

Goat Glossopharyngeal CN IX

Victomized Vagus CN X

A Accessory CN XI

Hat Hypoglossal CN XII

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Functions of the Nervous Systems The sensory function of the nervous system is to

sense changes in the internal and external environment through sensory receptors. Sensory (afferent) neurons serve this function.

The integrative function is to analyze the sensory information, store some aspects, and make decisions regarding appropriate behaviors. Association or interneurons serve this function.

The motor function is to respond to stimuli by initiating action. Motor(efferent) neurons serve this function.

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Nervous System Divisions

Central nervous system (CNS) consists of the brain and spinal cord

Peripheral nervous system (PNS) consists of cranial and spinal nerves that contain

both sensory and motor fibers connects CNS to muscles, glands & all sensory

receptors

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Subdivisions of the PNS Somatic (voluntary) nervous system (SNS)

neurons from cutaneous and special sensory receptors to the CNS

motor neurons to skeletal muscle tissue Autonomic (involuntary) nervous systems

sensory neurons from visceral organs to CNS motor neurons to smooth & cardiac muscle and

glands sympathetic division (speeds up heart rate) parasympathetic division (slow down heart rate)

Enteric nervous system (ENS) involuntary sensory & motor neurons control GI

tract neurons function independently of ANS & CNS

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Organization of the Nervous System

CNS is brain and spinal cord PNS is everything else

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Enteric NS

The enteric nervous system (ENS) consists of neurons in enteric plexuses that extend the length of the GI tract. Many neurons of the enteric plexuses function

independently of the ANS and CNS. Sensory neurons of the ENS monitor chemical

changes within the GI tract and stretching of its walls, whereas enteric motor neurons govern contraction of GI tract organs, and activity of the GI tract endocrine cells.

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Neuronal Structure & Function

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Neurons

Functional unit of nervous system Have capacity to produce action potentials

electrical excitability Cell body

single nucleus with prominent nucleolus Nissl bodies (chromatophilic substance)

rough ER & free ribosomes for protein synthesis

neurofilaments give cell shape and support microtubules move material inside cell lipofuscin pigment clumps (harmless aging)

Cell processes = dendrites & axons

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Nucleus with Nucleolus

Parts of a Neuron

Axons or Dendrites

Cell body

Neuroglial cells

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Rat Neuron

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Cell membrane

The dendrites are the receiving or input portions of a neuron.

The axon conducts nerve impulses from the neuron to the dendrites or cell body of another neuron or to an effector organ of the body (muscle or gland).

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Dendrites

Conducts impulses towards the cell body

Typically short, highly branched & unmyelinated

Surfaces specialized for contact with other neurons

Contains neurofibrils & Nissl bodies

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Axons

Conduct impulses away from cell body

Long, thin cylindrical process of cell

Arises at axon hillock Impulses arise from initial

segment (trigger zone) Side branches (collaterals)

end in fine processes called axon terminals

Swollen tips called synaptic end bulbs contain vesicles filled with neurotransmitters Synaptic boutons

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Axonal Transport

Cell body is location for most protein synthesis neurotransmitters & repair proteins

Axonal transport system moves substances slow axonal flow

movement in one direction only -- away from cell body movement at 1-5 mm per day

fast axonal flow moves organelles & materials along surface of

microtubules at 200-400 mm per day transports in either direction for use or for recycling in cell body

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Axonal Transport & Disease

Fast axonal transport route by which toxins or pathogens reach neuron cell bodies tetanus (Clostridium tetani bacteria) disrupts motor neurons causing painful

muscle spasms Bacteria enter the body through a

laceration or puncture injury more serious if wound is in head or neck

because of shorter transit time

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Diversity in Neurons Both structural and functional features are used to

classify the various neurons in the body. On the basis of the number of processes extending

from the cell body (structure), neurons are classified as multipolar, biopolar, and unipolar (Figure 12.4).

Most neurons in the body are interneurons and are often named for the histologist who first described them or for an aspect of their shape or appearance. Examples are Purkinje cells (Figure 12.5a) or Renshaw cells (Figure 12.5b).

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Structural Classification of Neurons

Based on number of processes found on cell body multipolar = several dendrites & one axon

most common cell type bipolar neurons = one main dendrite & one axon

found in retina, inner ear & olfactory unipolar neurons = one process only(develops from a

bipolar) are always sensory neurons

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Functional Classification of Neurons

Sensory (afferent) neurons transport sensory information from skin, muscles,

joints, sense organs & viscera to CNS Motor (efferent) neurons

send motor nerve impulses to muscles & glands Interneurons (association) neurons

connect sensory to motor neurons 90% of neurons in the body

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Association or Interneurons

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Half of the volume of the CNS Smaller cells than neurons 50X more numerous Cells can divide

rapid mitosis in tumor formation (gliomas) 4 cell types in CNS

astrocytes, oligodendrocytes, microglia & ependymal 2 cell types in PNS

schwann and satellite cells

Neuroglial Cells

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Astrocytes Star-shaped cells Form blood-brain

barrier by covering blood capillaries

Metabolize neurotransmitters

Regulate K+ balance

Provide structural support

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Microglia

Small cells found near blood vessels

Phagocytic role -- clear away dead cells

Derived from cells that also gave rise to macrophages & monocytes

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Ependymal cells

Form epithelial membrane lining cerebral cavities &

central canal Produce

cerebrospinal fluid (CSF)

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Satellite Cells Flat cells

surrounding neuronal cell bodies in peripheral ganglia

Support neurons in the PNS ganglia

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Oligodendrocytes

Most common glial cell type

Each forms myelin sheath around more than one axons in CNS

Analogous to Schwann cells of PNS

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Myelination

A multilayered lipid and protein covering called the myelin sheath and produced by Schwann cells and oligodendrocytes surrounds the axons of most neurons (Figure 12.8a).

The sheath electrically insulates the axon and increases the speed of nerve impulse conduction.

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Schwann Cell

Cells encircling PNS axons Each cell produces part of the myelin sheath

surrounding an axon in the PNS

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Axon Coverings in PNS All axons surrounded by a lipid &

protein covering (myelin sheath) produced by Schwann cells

Neurilemma is cytoplasm & nucleusof Schwann cell gaps called nodes of Ranvier

Myelinated fibers appear white jelly-roll like wrappings made of

lipoprotein = myelin acts as electrical insulator speeds conduction of nerve

impulses Unmyelinated fibers

slow, small diameter fibers only surrounded by neurilemma

but no myelin sheath wrapping

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Myelination in PNS

Schwann cells myelinate (wrap around) axons in the PNS during fetal development

Schwann cell cytoplasm & nucleus forms outermost layer of neurolemma with inner portion being the myelin sheath

Tube guides growing axons that are repairing themselves

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Myelination in the CNS Oligodendrocytes myelinate axons in the CNS Broad, flat cell processes wrap about CNS axons,

but the cell bodies do not surround the axons No neurilemma is formed Little regrowth after injury is possible due to the

lack of a distinct tube or neurilemma

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Gray and White Matter

White matter = myelinated processes (white in color) Gray matter = nerve cell bodies, dendrites, axon terminals,

bundles of unmyelinated axons and neuroglia (gray color) In the spinal cord = gray matter forms an H-shaped inner

core surrounded by white matter In the brain = a thin outer shell of gray matter covers the

surface & is found in clusters called nuclei inside the CNS A nucleus is a mass of nerve cell bodies and dendrites

inside the CNS.

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Electrical Signals in Neurons Neurons are electrically excitable due to

the voltage difference across their membrane

Communicate with 2 types of electric signals action potentials that can travel long

distances graded potentials that are local

membrane changes only In living cells, a flow of ions occurs

through ion channels in the cell membrane

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Two Types of Ion Channels

Leakage (nongated) channels are always open nerve cells have more K+ than Na+ leakage

channels as a result, membrane permeability to K+ is higher explains resting membrane potential of -70mV in

nerve tissue Gated channels open and close in response to a

stimulus results in neuron excitability

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Ion Channels

Gated ion channels respond to voltage changes, ligands (chemicals), and mechanical pressure. Voltage-gated channels respond to a direct

change in the membrane potential (Figure 12.10a).

Ligand-gated channels respond to a specific chemical stimulus (Figure 12.10b).

Mechanically gated ion channels respond to mechanical vibration or pressure.

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Gated Ion Channels

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Resting Membrane Potential Negative ions along inside of cell membrane & positive ions

along outside potential energy difference at rest is -70 mV cell is “polarized”

Resting potential exists because concentration of ions different inside & outside

extracellular fluid rich in Na+ and Cl cytosol full of K+, organic phosphate & amino acids

membrane permeability differs for Na+ and K+ 50-100 greater permeability for K+ inward flow of Na+ can’t keep up with outward flow of K+ Na+/K+ pump removes Na+ as fast as it leaks in

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Graded Potentials

Small deviations from resting potential of -70mV hyperpolarization = membrane

has become more negative depolarization = membrane has

become more positive The signals are graded, meaning

they vary in amplitude (size), depending on the strength of the stimulus and localized.

Graded potentials occur most often in the dendrites and cell body of a neuron.

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How do Graded Potentials Arise? Source of stimuli

mechanical stimulation of membranes with mechanical gated ion channels (pressure)

chemical stimulation of membranes with ligand gated ion channels (neurotransmitter)

Graded/postsynaptic/receptor or generator potential ions flow through ion channels and change membrane

potential locally amount of change varies with strength of stimuli

Flow of current (ions) is local change only

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Generation of an Action Potential An action potential (AP) or impulse is a sequence of

rapidly occurring events that decrease and eventually reverse the membrane potential (depolarization) and then restore it to the resting state (repolarization). During an action potential, voltage-gated Na+ and K+

channels open in sequence (Figure 12.13). According to the all-or-none principle, if a

stimulus reaches threshold, the action potential is always the same. A stronger stimulus will not cause a larger impulse.

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Action Potential

Series of rapidly occurring events that change and then restore the membrane potential of a cell to its resting state

Ion channels open, Na+ rushes in (depolarization), K+ rushes out (repolarization)

All-or-none principal = with stimulation, either happens one specific way or not at all (lasts 1/1000 of a second)

Travels (spreads) over surface of cell without dying out

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Depolarizing Phase of Action Potential

Chemical or mechanical stimuluscaused a graded potential to reachat least (-55mV or threshold)

Voltage-gated Na+ channels open& Na+ rushes into cell in resting membrane, inactivation gate of sodium channel is

open & activation gate is closed (Na+ can not get in) when threshold (-55mV) is reached, both open & Na+ enters inactivation gate closes again in few ten-thousandths of second only a total of 20,000 Na+ actually enter the cell, but they

change the membrane potential considerably(up to +30mV) Positive feedback process

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Repolarizing Phase of Action Potential When threshold potential of

-55mV is reached, voltage-gated K+ channels open

K+ channel opening is muchslower than Na+ channelopening which caused depolarization

When K+ channels finally do open, the Na+ channels have already closed (Na+ inflow stops)

K+ outflow returns membrane potential to -70mV If enough K+ leaves the cell, it will reach a -90mV membrane

potential and enter the after-hyperpolarizing phase K+ channels close and the membrane potential returns to the

resting potential of -70mV

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Refractory Period of Action Potential

Period of time during whichneuron can not generateanother action potential

Absolute refractory period even very strong stimulus will

not begin another AP inactivated Na+ channels must return to the resting state

before they can be reopened large fibers have absolute refractory period of 0.4 msec

and up to 1000 impulses per second are possible Relative refractory period

a suprathreshold stimulus will be able to start an AP K+ channels are still open, but Na+ channels have closed

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The Action Potential: Summarized Resting membrane

potential is -70mV Depolarization is

the change from -70mV to +30 mV

Repolarization is the reversal from +30 mV back to -70 mV)

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Local Anesthetics

Local anesthetics and certain neurotoxins Prevent opening of voltage-gated Na+

channels Nerve impulses cannot pass the

anesthetized region

Examples: Novocaine and lidocaine

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Propagation of Action Potential

An action potential spreads (propagates) over the surface of the axon membrane as Na+ flows into the cell during

depolarization, the voltage of adjacent areas is effected and their voltage-gated Na+ channels open

self-propagating along the membrane The traveling action potential is called a nerve

impulse

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Continuous versus Saltatory Conduction

Continuous conduction (unmyelinated fibers) step-by-step depolarization of each portion of the

length of the axolemma Saltatory conduction

depolarization only at nodes of Ranvier where there is a high density of voltage-gated ion channels

current carried by ions flows through extracellular fluid from node to node

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Saltatory Conduction

Nerve impulse conduction in which the impulse jumps from node to node

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Speed of Impulse Propagation

The propagation speed of a nerve impulse is not related to stimulus strength. larger, myelinated fibers conduct impulses faster

due to size & saltatory conduction Fiber types

A fibers largest (5-20 microns & 130 m/sec) myelinated somatic sensory & motor to skeletal muscle

B fibers medium (2-3 microns & 15 m/sec) myelinated visceral sensory & autonomic preganglionic

C fibers smallest (.5-1.5 microns & 2 m/sec) unmyelinated sensory & autonomic motor

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Encoding of Stimulus Intensity

How do we differentiate a light touch from a firmer touch? frequency of impulses

firm pressure generates impulses at a higher frequency

number of sensory neurons activated firm pressure stimulates more neurons than

does a light touch

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Action Potentials in Nerve and Muscle

Entire muscle cell membrane versus only the axon of the neuron is involved

Resting membrane potential nerve is -70mV skeletal & cardiac muscle is closer to -90mV

Duration nerve impulse is 1/2 to 2 msec muscle action potential lasts 1-5 msec for skeletal

& 10-300msec for cardiac & smooth Fastest nerve conduction velocity is 18 times

faster than velocity over skeletal muscle fiber

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SIGNAL TRANSMISSION AT SYNAPSES A synapse is the functional junction between

one neuron and another or between a neuron and an effector such as a muscle or gland.

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Signal Transmission at Synapses

2 Types of synapses electrical

ionic current spreads to next cell through gap junctions faster, two-way transmission & capable of synchronizing

groups of neurons chemical

one-way information transfer from a presynaptic neuron to a postsynaptic neuron

axodendritic -- from axon to dendrite axosomatic -- from axon to cell body axoaxonic -- from axon to axon

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Chemical Synapses Action potential reaches end bulb

and voltage-gated Ca+ 2 channels open

Ca+2 flows inward triggering release of neurotransmitter

Neurotransmitter crosses synaptic cleft & binding to ligand-gated receptors the more neurotransmitter

released the greater the change in potential of the postsynaptic cell

Synaptic delay is 0.5 msec One-way information transfer

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Excitatory & Inhibitory Potentials

The effect of a neurotransmitter can be either excitatory or inhibitory a depolarizing postsynaptic potential is called an

EPSP it results from the opening of ligand-gated Na+ channels the postsynaptic cell is more likely to reach threshold

an inhibitory postsynaptic potential is called an IPSP it results from the opening of ligand-gated Cl- or K+

channels it causes the postsynaptic cell to become more negative

or hyperpolarized the postsynaptic cell is less likely to reach threshold

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Removal of Neurotransmitter Diffusion

move down concentration gradient

Enzymatic degradation acetylcholinesterase

Uptake by neurons or glia cells neurotransmitter

transporters Prozac = serotonin

reuptake inhibitor

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Three Possible Responses Small EPSP occurs

potential reaches -56 mV only An impulse is generated

threshold was reached membrane potential of at least -55 mV

IPSP occurs membrane hyperpolarized potential drops below -70 mV

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Origin GPs arise on dendrites and cell bodies APs arise only at trigger zone on axon hillock

Types of Channels AP is produced by voltage-gated ion channels GP is produced by ligand or mechanically-

gated channels Conduction

GPs are localized (not propagated) APs conduct over the surface of the axon

Comparison of Graded & Action Potentials

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Comparison of Graded & Action Potentials

Amplitude amplitude of the AP is constant (all-or-none) graded potentials vary depending upon stimulus

Duration The duration of the GP is as long as the stimulus

lasts Refractory period

The AP has a refractory period due to the nature of the voltage-gated channels, and the GP has none.

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Summation

If several presynaptic end bulbs release their neurotransmitter at about the same time, the combined effect may generate a nerve impulse due to summation

Summation may be spatial or temporal.

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Summation of effects of neurotransmitters released from several end bulbs onto one neuron

Spatial Summation

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Temporal Summation

Summation of effect of neurotransmitters released from 2 or more firings of the same end bulb in rapid succession onto a second neuron

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Summation The postsynaptic neuron is an integrator, receiving and

integrating signals, then responding. If the excitatory effect is greater than the inhibitory effect but

less that the threshold level of stimulation, the result is a subthreshold EPSP, making it easier to generate a nerve impulse.

If the excitatory effect is greater than the inhibitory effect and reaches or surpasses the threshold level of stimulation, the result is a threshold or suprathreshold EPSP and a nerve impulse.

If the inhibitory effect is greater than the excitatory effect, the membrane hyperpolarizes (IPSP) with failure to produce a nerve impulse.

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Neurotransmitters

Both excitatory and inhibitory neurotransmitters are present in the CNS and PNS; the same neurotransmitter may be excitatory in some locations and inhibitory in others.

Important neurotransmitters include acetylcholine, glutamate, aspartate, gamma aminobutyric acid, glycine, norepinephrine, epinephrine, and dopamine.

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Neurotransmitter Effects

Neurotransmitter effects can be modified synthesis can be stimulated or inhibited release can be blocked or enhanced removal can be stimulated or blocked receptor site can be blocked or activated

Agonist anything that enhances a transmitters effects

Antagonist anything that blocks the action of a neurotranmitter

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Small-Molecule Neurotransmitters

Acetylcholine (ACh) released by many PNS neurons & some CNS excitatory on NMJ but inhibitory at others inactivated by acetylcholinesterase

Amino Acids glutamate released by nearly all excitatory neurons

in the brain ---- inactivated by glutamate specific transporters

GABA is inhibitory neurotransmitter for 1/3 of all brain synapses (Valium is a GABA agonist -- enhancing its inhibitory effect)

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Biogenic Amines modified amino acids (tyrosine)

norepinephrine -- regulates mood, dreaming, awakening from deep sleep

dopamine -- regulating skeletal muscle tone serotonin -- control of mood, temperature

regulation, & induction of sleep removed from synapse & recycled or destroyed

by enzymes (monoamine oxidase or catechol-0-methyltransferase)

Small-Molecule Neurotransmitters

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ATP and other purines (ADP, AMP & adenosine) excitatory in both CNS & PNS released with other neurotransmitters (ACh & NE)

Gases (nitric oxide or NO) formed from amino acid arginine by an enzyme formed on demand and acts immediately

diffuses out of cell that produced it to affect neighboring cells

may play a role in memory & learning first recognized as vasodilator that helps lower

blood pressure

Small-Molecule Neurotransmitters

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Neuropeptides

3-40 amino acids linked by peptide bonds Substance P -- enhances our perception of

pain Pain relief

enkephalins -- pain-relieving effect by blocking the release of substance P (bind to morphine receptors in the central nervous system and have opioid properties)

acupuncture may produce loss of pain sensation because of release of opioids-like substances such as endorphins or dynorphins

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Strychnine Poisoning

In spinal cord, Renshaw cells normally release an inhibitory neurotransmitter (glycine) onto motor neurons preventing excessive muscle contraction

Strychnine binds to and blocks glycine receptors in the spinal cord

Massive tetanic contractions of all skeletal muscles are produced when the diaphragm contracts & remains

contracted, breathing can not occur

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Neuronal Circuits Neuronal pools are organized into circuits

(neural networks.) These include simple series, diverging, converging, reverberating, and parallel after-discharge circuits (Figure 12.18 a-d).

A neuronal network may contain thousands or even millions of neurons.

Neuronal circuits are involved in many important activities breathing short-term memory waking up

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Diverging -- single cell stimulates many others Converging -- one cell stimulated by many others Reverberating -- impulses from later cells repeatedly stimulate

early cells in the circuit (short-term memory) Parallel-after-discharge -- single cell stimulates a group of cells

that all stimulate a common postsynaptic cell (math problems)

Neuronal Circuits

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Regeneration & Repair

Plasticity maintained throughout life sprouting of new dendrites synthesis of new proteins changes in synaptic contacts with other

neurons Limited ability for regeneration (repair)

PNS can repair damaged dendrites or axons CNS no repairs are possible

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Damage and Repair in the Peripheral Nervous System

When there is damage to an axon, usually there are changes, called chromatolysis, which occur in the cell body of the affected cell; this causes swelling of the cell body and peaks between 10 and 20 days after injury.

By the third to fifth day, degeneration of the distal portion of the neuronal process and myelin sheath (Wallerian degeneration) occurs; afterward, macrophages phagocytize the remains.

Retrograde degeneration of the proximal portion of the fiber extends only to the first neurofibral node.

Regeneration follows chromatolysis; synthesis of RNA and protein accelerates, favoring rebuilding of the axon and often taking several months.

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Axons & dendrites may be repaired if neuron cell body remains intact schwann cells remain active and

form a tube scar tissue does not form too

rapidly Chromatolysis

24-48 hours after injury, Nissl bodies break up into fine granular masses

Repair within the PNS

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By 3-5 days, wallerian degeneration occurs

(breakdown of axon & myelin sheath distal to injury)

retrograde degeneration occurs back one node

Within several months, regeneration occurs neurolemma on each side of

injury repairs tube (schwann cell mitosis)

axonal buds grow down the tube to reconnect (1.5 mm per day)

Repair within the PNS

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Neurogenesis in the CNS Formation of new neurons from stem cells was not

thought to occur in humans 1992 a growth factor was found that stimulates

adult mice brain cells to multiply 1998 new neurons found to form within adult

human hippocampus (area important for learning) There is a lack of neurogenesis in other regions of

the brain and spinal cord. Factors preventing neurogenesis in CNS

inhibition by neuroglial cells, absence of growth stimulating factors, lack of neurolemmas, and rapid formation of scar tissue

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Multiple Sclerosis (MS)

Autoimmune disorder causing destruction of myelin sheaths in CNS sheaths becomes scars or plaques 1/2 million people in the United States appears between ages 20 and 40 females twice as often as males

Symptoms include muscular weakness, abnormal sensations or double vision

Remissions & relapses result in progressive, cumulative loss of function

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The second most common neurological disorder affects 1% of population

Characterized by short, recurrent attacks initiated by electrical discharges in the brain lights, noise, or smells may be sensed skeletal muscles may contract involuntarily loss of consciousness

Epilepsy has many causes, including; brain damage at birth, metabolic disturbances,

infections, toxins, vascular disturbances, head injuries, and tumors

Epilepsy

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end