ch. 9 power point

8/2/2019 Ch. 9 Power Point

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Chapter 9- The Nervous System

Introduction

-the nervous system is a collection

of cells that help to coordinate

all other body functions

i. also key in helping tomaintain homeostasis

ii. helps the body to respond to

changing conditions

-information is brought to the

brain and spinal cord from both

inside and outside of the body


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i. the brain and spinal cord thenstimulate muscles and glands inresponse to the information itreceived

-nervous tissue consists of masses ofnerve cells called neurons

i. neurons are specialized to reactto physical and chemical changes

in their surroundingsii. the neurons transmitinformation in the form ofelectrochemical changes

-these changes are callednerve impulses

iii. nerve impulses are transmittedto other neurons and cells outside

of the nervous system


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-structure of a typical neuron (fig.9.1)

i. have a rounded area called thecell body

ii. two types of extensions fromthe cell body: dendrites and axon

iii. dendrites

-may be many on eachneuron

-function is to receiveelectrochemical signals

iv. axons

-usually a single extensionfrom the cell body

-sends information to othercells and structures in the formof nerve impulses


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-nerves are bundles of axons

-nervous tissue also includes

neuroglial cells (left)

i. these cells provide physicalsupport, insulation and nutrients

for the neurons


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-the organs of the nervous system

can be divided into two groups

i. central nervous system (CNS)

-consists of the brain andspinal cord


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ii. the peripheral nervous

system (PNS)

-composed of peripheral

nerves that are connected tothe CNS

-the CNS and PNS carry out three

functions: sensory, integration

and motor

General Functions of the Nervous

System

-the sensory function of the

nervous system comes from

sensory receptors


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i. the receptors are at the ends of peripheral nerves

ii. the receptors collect information by detectingchanges inside and outside of the body

iii. the sensory receptors convert information into nerveimpulses

-the impulses are transmitted to peripheral nerves

-the peripheral nerves carry information to theCNS

iv. at the CNS, the signals are brought together

-the results of the signals include creating

sensations, committing the signal to memory orproducing thoughts that translate sensations intoperceptions


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v. we then make conscious and unconscious

decisions that involve motor functions (muscles)

-motor functions involve peripheral neurons that take

information from the CNS to structures called

effectors

i. effectors are structures outside of the nervous

system

-includes muscles and glands

-motor functions of the PNS are divided into two

categories

i. somatic nervous system- control conscious

movement


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-controls skeletal muscles

ii. autonomic nervous system- effectors are

involuntary

-includes the heart, smooth muscle in blood

vessels, and glands

-regardless of what division is being used, the overallgoal of the nervous system is to maintain

homeostasis

Neuroglial Cells

-neurons cannot exist without neuroglial cells

(neuroglia)


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i. neuroglia have many functions

-fill spaces between neurons

-provide structural framework-produce an electrical insulator called myelin

-perform phagocytosis

-neuroglia outnumber neuronsi. unlike neurons, they can divide


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-different types of neuroglial cells (fig. 9.3)

i. microglial cells- scattered throughout the CNS

-support neurons

-phagocytize bacterial cells and cellular debris


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ii. oligodendrocytes- line up

along nerve fibers

-they create an insulating layer

of myelin called myelin

sheaths-myelin wraps around the

axons of neurons within the

brain and spinal cord


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iii. astrocytes- star shaped cells

-found between neurons and

blood vessels

-provide nutritional support and

regulate the amount of nutrientsand ions within the nervous tissue

-can form scar tissue when there

is damage to the CNS


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iv. ependymal cells

-form a membrane that covers

specialized regions of various

brain parts-form the inner lining that

enclose spaces within the brain

and spinal cord


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v. the PNS also has neuroglial cells

called Schwann cells

-form myelin sheaths around axonsin the PNS


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-astrocytes and blood vessels in the

brain create the blood-brain barrier

i. protects the brain from manydifferent things

-chemical fluctuations

-prevents unwanted substancesfrom reaching the brain

ii. on the negative side, the blood-

brain barrier can prevent drugs

needed to treat various brain

conditions from reaching the brain

tissue


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Neurons-neuron structure

i. neurons vary in size and shape, but all have commonfeatures

ii. common features include:

-cell body

-dendrites that receive nerve impulses and conduct themto the cell body

-an axon that conducts impulses away from the cell body


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iii. dendrites are short and highly

branched

-they act as antennae that receiveinput from other structures


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iv. the axon arises from a slight bump of the cell body called the

axon hillock

-the axon arises as a single structure but can have side branches

i. these are called collaterals

-the end of the axon may branch into fine extensions

i. these extensions make contact with other cells


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v. larger axons of peripheral nerves areenclosed in myelin sheaths

-these sheaths are produced bySchwann cells

-looks like a band-aid wrappedaround a finger

-there are many layers of myelin-the myelin is not continuous, sothere are spaces between eachsheath

i. these spaces are called nodesof Ranvier (fig. 9.5)


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vi. axons with myelin sheaths are called myelinated fibers

-these neurons appear white

i. form the white matter in the CNS

vii. those that lack the sheaths are called unmyelinated

-form gray matter in the CNS

l ifi i f


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-classification of neurons

i. along with differences in size and shape, neurons

vary in length and size of the axons and dendrites

-also differ in the number of connections they

make with other neurons

ii. neurons can be classified into three groups basedon their structural differences (fig. 9.6)

l i l


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-multipolar neurons

i. have many processes coming

from their cell body

ii. many dendrites and one axon

iii. found within the brain and

spinal cord

-bipolar neuronsi. have two processes each

coming from one end of the cell

body

ii. one extension is the axon and

the other is a dendrite

iii. found in the eyes, nose and

ears

i l


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-unipolar neurons

i. have a single extension coming from thecell body

ii. the extension splits into two branches-one branch acts as the axon and theother as the dendrite

iii. the dendrite is called the peripheral

process-located near the peripheral body part

iv. the axon is called the central process

-enters into the brain and spinal cordv. some cell bodies of unipolar neuronsaggregate into masses called ganglia

-these are always located outside of the

CNS

iii l i f i


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iii. neurons also vary in function

-can carry impulses to the brain and spinal cord

-conduct impulses from neuron to neuron withinthe brain or spinal cord

-transmit impulses out of the brain and spinal

cord

i l ifi ti f b d


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iv. classification of neurons based

on function (fig. 9.7)

-sensory neurons (afferent

neurons) carry impulses from abody part into the brain or

spinal cord

i. these neurons have

sensory receptors on the tips

of the dendrites

-interneurons (association

neurons) are found entirelywithin the brain or spinal cord

i. these link up with other

neurons


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ii. transmit impulses from

one part of the brain or spinal

cord to another region-motor neurons (efferent

neurons) carry nerve impulses

out of the brain or spinal cord

to effectors

i. stimulate muscles to

contract and glands to release

secretions

Synapse


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Synapse

-nerve impulses travel along complexnerve pathways

i. the junction between two

communicating neurons is calleda synapse

ii. the neurons are not in physicalcontact with each other

-there is a small spacebetween neurons called thesynaptic cleft (fig. 9.8)

iii. the nerve carrying theimpulse to the synapse is called

the presynaptic neuron-the neuron that receives theinformation is called thepostsynaptic neuron

the proce of the


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-the process of the

impulse crossing the

synapse is called synaptic

transmission-synaptic transmission is

accomplished by chemicals

called neurotransmitters

i. the end of the axon has

extensions called synaptic

knobs (fig. 9.9)

-within the knob arevesicles with

neurotransmitter called

synaptic vesicles

ii when a nerve impulse


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ii. when a nerve impulse

reaches the synaptic knob,

some of the vesicles release

neurotransmitter (fig. 9.9)-the neurotransmitter

diffuses across the

synaptic cleft

-bind to receptors on the

postsynaptic membrane

-the action of the

neurotransmitter on thepostsynaptic neuron is either

excitatory (turns some

processes on ) or inhibitory

(turns some processes off)

S ti T i i


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Synaptic Transmission

-some neurotransmitters are excitatory

i. this means they will cause the postsynapticneuron to become excited enough to generate a

nerve impulse

-others are inhibitory

i. these will prevent the postsynaptic neuron from

being excited and preventing a nerve impulse

a single postsynaptic neuron


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-a single postsynaptic neuron

may receive input from

thousands of presynaptic

neuronsi. neurotransmitter released

from some of these

presynaptic neurons are

excitatory, while others areinhibitory

ii. neurotransmitters released

from some of thesepresynaptic neurons are

excitatory, while others are

inhibitory


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iii classes of neurotransmitters


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iii. classes of neurotransmitters

-acetylcholine- stimulates muscle contractions

-monoaminesi. includes epinephrine, norepinephrine,

dopamine and serotonin

-group of modified amino acidsi. includes glycine, glutamic acid and

GABA

-neuropeptidesi. small proteins composed of short chains

of amino acids

iv acetylcholine and norepinephrine are always


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iv. acetylcholine and norepinephrine are alwaysexcitatory

v. dopamine, GABA and glycine are always

inhibitoryvi. after the neurotransmitter is released into thesynaptic cleft, some will be destroyed by enzymes

-others will be taken back up into nearby neuronsor neuroglial cells

Impulse Processing-the way the nervous system processes and responds to

nerve impulses depends on the organization ofneurons and their axons in the brain and spinal cord

-neuronal pools


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neuronal pools

i. neurons within the CNS areorganized into neuronal pools

ii. these are groups of neurons that

make up hundreds of synapses witheach other

-the group works together toperform a common function

iii. each neuronal pool receivesinput from neurons

-the neurons may be part ofthis pool or other pools

-as a group the neuronal pool

will generate some type ofoutput

iv. neuronal pools can haveexcitatory or inhibitory effects onother neuronal pools or peripheral

effectors, such as skeletal muscle

facilitation


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-facilitation

i. a neuron or neuronal pool can receive both

excitatory and inhibitory input

ii. if the net effect is excitatory, but not enough to

cause the neuron or neuronal pool to elicit an

impulse, the neuron does move closer to having an

impulse

-when the nerves are more excited than they

were prior to stimulation, they are undergoing

facilitation

iii. with facilitation the stimulated neuron will

require less input to actually create an impulse


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-convergence

i. a single neuron may receive impulses from two or morepresynaptic neurons

-presynaptic neurons originate from different parts of thenervous system and converge on the same neuron areexhibiting convergence (fig. 9.16a)

ii. convergence allows impulses to have additive effects on apostsynaptic neuron

-the additive effects can be stimulatory or inhibitory


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-divergence

i. impulses leaving a neuron can exhibit divergence

-this means a single neuron passes impulses to manypostsynaptic neurons (fig. 9.16b)

ii. divergence can amplify an impulse (spread it to multiple

neurons at the same time)


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Types of Nerves

-sensory fibers (afferent fibers) bring information into the

CNS

i. motor fibers or efferent fibers carry impulses from the

CNS to muscles or glands

-most nerves are a combination of sensory and motor fibers

i. called mixed nerves

Nerve Pathways


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Nerve Pathways

-the route that nerve impulses

follow as they move around

through the nervous system arecalled nerve pathways

i. the simplest of these

pathways use only a few

neurons

-called a reflex arc

-responsible for involuntary

actions called reflexes

-reflex arc


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reflex arc

i. the reflex arc begins with areceptor at the end of a sensory(afferent) neuron

ii. the sensory neuron passesthe information to interneurons

-located entirely within the

CNS i. processinformation

ii. act as the reflex

centeriii. interneurons passinformation from the CNS tomuscles or glands (fig. 9.18)


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-reflex behavior

i. reflexes are automatic responses to change within or outside

of the body

ii. patellar reflex (knee-jerk reflex) (fig. 9.19)

-this is a simple reflex that involves only two neurons

i. a sensory neuron communicates with a motor unit


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-hitting the patellar ligament initiates this reflex

i. a group of muscles called the quadriceps femoris is pulledslightly

ii. this activates stretch receptors in the muscles

iii. the receptors cause a nerve impulse in a sensory neuron in

the spinal cordiv. in the spinal cord, the sensory neuron synapses with a motorneuron

v. the motor neuron travels back to the quadriceps femoris

vi. the muscles contract and the leg extends


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iii. withdrawal reflex (fig. 9.20)

-this reflex occurs when a person touches something painful

-the touch activates skin receptors that send sensory impulsesto the spinal cord

-in the spinal cord, the impulse is passed to an interneuron

-the interneuron processes the information and sends an

impulse out of the CNS via a motor neuron

-the motor neuron transmits the impulse to a


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the motor neuron transmits the impulse to a

muscle in the injured area

i. this causes the muscle to contract

-the injured body part withdrawals from the

painful stimulus

-the interneurons also send information to the

brain

i. this causes the person to perceive pain

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