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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