Nervous system overview

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The Nervous SystemThe Nervous System


Learning ObjectivesLearning Objectives Describe the structures and functions of the neurons and neuroglia of Describe the structures and functions of the neurons and neuroglia of

the cerebrum, the cerebellum, the diencephalon, and the brain stem.the cerebrum, the cerebellum, the diencephalon, and the brain stem. Differentiate between white matter and gray matter.Differentiate between white matter and gray matter. Describe the functions of afferent and efferent nerves.Describe the functions of afferent and efferent nerves. List the components of the central nervous system and the peripheral List the components of the central nervous system and the peripheral

nervous system.nervous system. Differentiate between the autonomic and somatic nervous systems.Differentiate between the autonomic and somatic nervous systems. Describe the process of depolarization and repolarization of neurons.Describe the process of depolarization and repolarization of neurons. List the excitatory and inhibitory neurotransmitters and describe their List the excitatory and inhibitory neurotransmitters and describe their

role in conduction of nerve impulses.role in conduction of nerve impulses. Describe the connective tissue layers surrounding the brain and spinal Describe the connective tissue layers surrounding the brain and spinal

cord.cord. Explain the function of the cerebrospinal fluid.Explain the function of the cerebrospinal fluid. List the cranial nerves and describe their functions.List the cranial nerves and describe their functions. Differentiate between the sympathetic and parasympathetic nervous Differentiate between the sympathetic and parasympathetic nervous

systems and between autonomic and somatic reflexes.systems and between autonomic and somatic reflexes. Describe the components of a reflex arc and explain the role of each.Describe the components of a reflex arc and explain the role of each. Describe the stretch reflex, withdrawal reflex, crossed extensor reflex, Describe the stretch reflex, withdrawal reflex, crossed extensor reflex,

palpebral reflex, and pupillary light reflex.palpebral reflex, and pupillary light reflex.


NeuronsNeurons

Basic functional units of the nervous Basic functional units of the nervous systemsystem

High requirement for oxygenHigh requirement for oxygen Cannot reproduce but can regenerate cell Cannot reproduce but can regenerate cell

processes if the cell body remains intactprocesses if the cell body remains intact Neuroglia (glial cells) - provide structural Neuroglia (glial cells) - provide structural

and functional support and protection to and functional support and protection to neuronsneurons


Neuron StructureNeuron Structure

Central cell body (soma or Central cell body (soma or perikaryon) perikaryon)

Cell processesCell processes Dendrites – receive stimuliDendrites – receive stimuli Axons - conducts nerve Axons - conducts nerve

impulses away impulses away


DendritesDendrites

Receive stimuli from other Receive stimuli from other neurons and conduct the neurons and conduct the stimuli to the cell bodystimuli to the cell body

May serve as sensory May serve as sensory receptors for heat, cold, receptors for heat, cold, touch, pressure, stretch, or touch, pressure, stretch, or other physical changes from other physical changes from inside or outside the bodyinside or outside the body

Short, numerous, Short, numerous, multibranchedmultibranched


AxonsAxons

Conduct nerve impulse away Conduct nerve impulse away from cell toward another from cell toward another neuron or an effector cell neuron or an effector cell

Single, long process; may be Single, long process; may be covered with myelincovered with myelin

White matter: tissue White matter: tissue containing myelinated axonscontaining myelinated axons


AxonsAxons Myelinated axons conduct Myelinated axons conduct

impulses faster than impulses faster than unmyelinated onesunmyelinated ones

Myelin sheath: cell Myelin sheath: cell membrane of glial cells membrane of glial cells tightly wrapped around the tightly wrapped around the axonaxon Oligodendrocytes in the brain Oligodendrocytes in the brain

and spinal cord and spinal cord Schwann cells in the nerves Schwann cells in the nerves

outside the brain and spinal outside the brain and spinal cordcord


AxonsAxons Multiple Schwann cells Multiple Schwann cells

or oligodendrocytes or oligodendrocytes cover the entire length cover the entire length of the axon of the axon

Nodes of Ranvier:Nodes of Ranvier: gaps gaps between adjacent glial between adjacent glial cellscells


Organization of Nervous SystemOrganization of Nervous System

AnatomicalAnatomical Central nervous system (CNS)Central nervous system (CNS)

Brain and spinal cordBrain and spinal cord Peripheral nervous system (PNS)Peripheral nervous system (PNS)

Extends outward from the central axis toward the Extends outward from the central axis toward the periphery of the bodyperiphery of the body

Cranial nerves originate directly from the brainCranial nerves originate directly from the brain Spinal nerves emerge from the spinal cordSpinal nerves emerge from the spinal cord



Direction of ImpulsesDirection of Impulses Afferent nerves - conduct impulses toward CNSAfferent nerves - conduct impulses toward CNS

Also called sensory nerves - conduct sensations from Also called sensory nerves - conduct sensations from sensory receptors in the skin and other locations in the sensory receptors in the skin and other locations in the body to the CNSbody to the CNS

Efferent nerves - conduct impulses away from Efferent nerves - conduct impulses away from CNSCNS Also called motor nerves - cause skeletal muscle Also called motor nerves - cause skeletal muscle

contraction and movementcontraction and movement Cranial and spinal nerves in the PNS and nerve Cranial and spinal nerves in the PNS and nerve

tracts (bundles of axons) in the CNS may carry tracts (bundles of axons) in the CNS may carry nerve fibers that are sensory, motor, or bothnerve fibers that are sensory, motor, or both



Function: Autonomic vs. SomaticFunction: Autonomic vs. Somatic Somatic nervous system - actions under Somatic nervous system - actions under

conscious, or voluntary, controlconscious, or voluntary, control Autonomic nervous system - controls and Autonomic nervous system - controls and

coordinates automatic functionscoordinates automatic functions Example: slowing of the heart rate in response to an Example: slowing of the heart rate in response to an

increased blood pressureincreased blood pressure


Neuron FunctionNeuron Function

Resting state - when a neuron is not being Resting state - when a neuron is not being stimulatedstimulated

Resting membrane potential - difference in Resting membrane potential - difference in electrical charge across neuronal electrical charge across neuronal membranemembrane Results from differences in distribution of Results from differences in distribution of

positive and negative charges from sodium, positive and negative charges from sodium, potassium, proteins, and other charged ions on potassium, proteins, and other charged ions on either side of the neuronal membrane either side of the neuronal membrane


Neuron FunctionNeuron Function Sodium-potassium Sodium-potassium

pump: specialized pump: specialized molecule that helps molecule that helps maintain cell resting statemaintain cell resting state Pumps (NaPumps (Na++) from inside of ) from inside of

neuron to the outside neuron to the outside Pump (KPump (K++) from outside of ) from outside of

neuron to the insideneuron to the inside


DepolarizationDepolarization

Neuron receives Neuron receives external stimulusexternal stimulus

Sodium channel Sodium channel opens on neuron opens on neuron cell membranecell membrane

Sodium ions flow Sodium ions flow into cell by passive into cell by passive diffusiondiffusion


DepolarizationDepolarization

Action potential Action potential During depolarization, During depolarization,

inside of the neuron inside of the neuron goes from a negatively goes from a negatively charged resting charged resting membrane potential to membrane potential to a net positive charge a net positive charge due to inflow of sodium due to inflow of sodium ionsions

Creates large change in Creates large change in electrical charge from electrical charge from negative to positivenegative to positive


RepolarizationRepolarization Sodium channels Sodium channels

closeclose KK++ channels open channels open KK++ diffuses out of diffuses out of

the cell the cell Resting state Resting state

restoredrestored


RepolarizationRepolarization

As repolarization As repolarization ends, sodium-ends, sodium-potassium pump potassium pump moves sodium and moves sodium and potassium ions potassium ions back to their back to their original sidesoriginal sides

Resting state Resting state restoredrestored


Threshold StimulusThreshold Stimulus Stimulus must be sufficient to make the neuron Stimulus must be sufficient to make the neuron

respond and cause complete depolarizationrespond and cause complete depolarization ““All-or-nothing principle” - neuron depolarizes to All-or-nothing principle” - neuron depolarizes to

its maximum strength or not at allits maximum strength or not at all Conduction of the action potential - spreading Conduction of the action potential - spreading

wave of opening sodium channels in sufficient wave of opening sodium channels in sufficient numbers to allow sodium influx and depolarizationnumbers to allow sodium influx and depolarization Wave of depolarization or nerve impulseWave of depolarization or nerve impulse


Refractory PeriodRefractory Period

Time period during which a neuron is Time period during which a neuron is insensitive to additional stimuliinsensitive to additional stimuli Cell is still in depolarization/early repolarizationCell is still in depolarization/early repolarization

Absolute refractory period - during sodium Absolute refractory period - during sodium influx and early potassium outflowinflux and early potassium outflow

Relative refractory period - during end of Relative refractory period - during end of the repolarization periodthe repolarization period May be possible to stimulate another May be possible to stimulate another

depolarization if stimulus is very largedepolarization if stimulus is very large


Saltatory ConductionSaltatory Conduction

Rapid means of conducting an action Rapid means of conducting an action potential potential

Depolarization in myelinated axons can Depolarization in myelinated axons can only take place at the nodes of Ranvier only take place at the nodes of Ranvier


Synaptic TransmissionSynaptic Transmission

Synapse - junction between two neurons Synapse - junction between two neurons or a neuron and a target cellor a neuron and a target cell

Synaptic cleft - gap between adjacent Synaptic cleft - gap between adjacent neuronsneurons

Presynaptic neuron - neuron bringing the Presynaptic neuron - neuron bringing the depolarization wave to the synapsedepolarization wave to the synapse Releases neurotransmitterReleases neurotransmitter

Postsynaptic neuron - contains receptors Postsynaptic neuron - contains receptors for the neurotransmitterfor the neurotransmitter



Telodendron - branched structure on Telodendron - branched structure on presynaptic neuronpresynaptic neuron Terminal bouton - slightly enlarged bulb on each Terminal bouton - slightly enlarged bulb on each

end of telodendron (synaptic end bulb, synaptic end of telodendron (synaptic end bulb, synaptic knob)knob)

Vesicles in the knob contain the neurotransmitterVesicles in the knob contain the neurotransmitter When depolarization wave reaches synaptic When depolarization wave reaches synaptic

knob, vesicles fuse with the knob's cellular knob, vesicles fuse with the knob's cellular membrane and dump neurotransmitter into membrane and dump neurotransmitter into the synaptic cleftthe synaptic cleft



Neurotransmitters Neurotransmitters diffuse across the diffuse across the synaptic cleft synaptic cleft toward the toward the postsynaptic postsynaptic membranemembrane

Receptors on the Receptors on the postsynaptic postsynaptic membrane bind the membrane bind the neurotransmitterneurotransmitter


Types of NeurotransmittersTypes of Neurotransmitters

Excitatory neurotransmittersExcitatory neurotransmitters Usually cause an influx of sodium so that the Usually cause an influx of sodium so that the

postsynaptic membrane moves toward postsynaptic membrane moves toward thresholdthreshold

Inhibitory neurotransmittersInhibitory neurotransmitters Move the charge within the postsynaptic cell Move the charge within the postsynaptic cell

farther away from thresholdfarther away from threshold


Types of NeurotransmittersTypes of Neurotransmitters

Acetylcholine Acetylcholine Can be either excitatory or inhibitory depending Can be either excitatory or inhibitory depending

on its location in the bodyon its location in the body CatecholaminesCatecholamines

Norepinephrine and epinephrine - associated Norepinephrine and epinephrine - associated with "fight or flight" reactions of the sympathetic with "fight or flight" reactions of the sympathetic nervous systemnervous system

Dopamine - involved with autonomic functions Dopamine - involved with autonomic functions and muscle controland muscle control

Gamma-aminobutyric acid (GABA) and Gamma-aminobutyric acid (GABA) and glycine - inhibitory glycine - inhibitory


Recycling the NeurotransmitterRecycling the Neurotransmitter

Acetylcholinesterase - found on Acetylcholinesterase - found on postsynaptic membrane; breaks down postsynaptic membrane; breaks down acetylcholine acetylcholine

Monoamine oxidase (MAO) - breaks down Monoamine oxidase (MAO) - breaks down norepinephrinenorepinephrine

Catechol-O-methyl transferase (COMT) - Catechol-O-methyl transferase (COMT) - breaks down norepinephrine that is not breaks down norepinephrine that is not reabsorbedreabsorbed


Central Nervous SystemCentral Nervous System BrainBrain

CerebrumCerebrum CerebellumCerebellum Diencephalon Diencephalon Brain stemBrain stem

Spinal CordSpinal Cord


CerebrumCerebrum

Gray matter - cerebral cortex; outer layer Gray matter - cerebral cortex; outer layer of the brainof the brain

White matter - fibers beneath the cortex White matter - fibers beneath the cortex and corpus callosum (fibers that connect and corpus callosum (fibers that connect the two halves of the cerebral cortex)the two halves of the cerebral cortex)

Area of the brain responsible for higher-Area of the brain responsible for higher-order behaviors (learning, intelligence, order behaviors (learning, intelligence, awareness, etc.)awareness, etc.)


CerebrumCerebrum

Gyri (gyrus): folds Gyri (gyrus): folds in cerebral in cerebral hemisphereshemispheres

Fissures: deep Fissures: deep grooves separating grooves separating the gyrithe gyri


CerebrumCerebrum

Sulci (sulcus): shallow Sulci (sulcus): shallow grooves separating the grooves separating the gyrigyri Divides the cerebral Divides the cerebral

hemispheres into lobeshemispheres into lobes Longitudinal fissure:Longitudinal fissure:

prominent groove that prominent groove that divides the cerebrum divides the cerebrum into right and left into right and left cerebral hemispherescerebral hemispheres


CerebellumCerebellum Located just caudal Located just caudal

to the cerebrumto the cerebrum Area of the brain Area of the brain

responsible for responsible for coordinated coordinated movement, balance, movement, balance, posture, and posture, and complex reflexescomplex reflexes


DiencephalonDiencephalon

Passageway between brain stem and Passageway between brain stem and cerebrumcerebrum

Structures associated with the Structures associated with the diencephalon:diencephalon: Thalamus - acts as a relay station for Thalamus - acts as a relay station for

regulating sensory inputs to the cerebrumregulating sensory inputs to the cerebrum Hypothalamus - interface between the nervous Hypothalamus - interface between the nervous

system and the endocrine systemsystem and the endocrine system Pituitary - endocrine “master gland”Pituitary - endocrine “master gland”


Brain StemBrain Stem

Connection between the rest of the brain Connection between the rest of the brain and the spinal cord and the spinal cord

Composed of the medulla oblongata, the Composed of the medulla oblongata, the pons, and the midbrain pons, and the midbrain

Area of the brain responsible for basic Area of the brain responsible for basic support functions of the bodysupport functions of the body

Many of the cranial nerves originate from Many of the cranial nerves originate from this area of the brain this area of the brain


MeningesMeninges

Connective tissue layers that surround Connective tissue layers that surround brain and spinal cord brain and spinal cord

Contain blood vessels, fluid, and fatContain blood vessels, fluid, and fat Supply nutrients and oxygen to the superficial Supply nutrients and oxygen to the superficial

tissues of the brain and spinal cordtissues of the brain and spinal cord Provide some cushioning and distribution of Provide some cushioning and distribution of

nutrients for the CNSnutrients for the CNS


MeningesMeninges Three layers:Three layers:1.1. Dura mater - tough, Dura mater - tough,

fibrous fibrous 2.2. Arachnoid - delicate, Arachnoid - delicate,

spiderweb-like spiderweb-like 3.3. Pia mater - very thin; Pia mater - very thin;

lies directly on surface lies directly on surface of brain and spinal of brain and spinal cordcord


Cerebrospinal FluidCerebrospinal Fluid

Fluid between layers of the meninges and Fluid between layers of the meninges and in canals and ventricles inside the brain in canals and ventricles inside the brain and central canal of spinal cord and central canal of spinal cord

Provides cushioning functionProvides cushioning function May play role in regulation of autonomic May play role in regulation of autonomic

functions such as respiration and vomitingfunctions such as respiration and vomiting


Blood-brain BarrierBlood-brain Barrier

Separates the capillaries in the brain from Separates the capillaries in the brain from the nervous tissue the nervous tissue

Capillary walls in the brain have no Capillary walls in the brain have no fenestrations;fenestrations; covered by cell membranes covered by cell membranes of glial cells of glial cells

Prevents many drugs, proteins, ions, and Prevents many drugs, proteins, ions, and other molecules from readily passing from other molecules from readily passing from the blood into the brainthe blood into the brain


Cranial NervesCranial Nerves 12 nerve pairs in PNS that originate directly from 12 nerve pairs in PNS that originate directly from

the brain the brain Numbered in Roman numerals from I through XII Numbered in Roman numerals from I through XII

(1 through 12) (1 through 12) Each nerve may contain axons of motor neurons, Each nerve may contain axons of motor neurons,

axons of sensory neurons, or combinations of axons of sensory neurons, or combinations of bothboth


Spinal CordSpinal Cord Medulla: central Medulla: central

part of spinal cordpart of spinal cord Composed of gray Composed of gray

matter matter Central canal – Central canal –

center of medullacenter of medulla


Spinal CordSpinal Cord Cortex: outer part Cortex: outer part

of spinal cordof spinal cord White matterWhite matter Surrounds the gray Surrounds the gray

mattermatter



Dorsal and ventral Dorsal and ventral nerve roots emerge nerve roots emerge from between each from between each pair of adjacent pair of adjacent vertebraevertebrae Dorsal nerve roots Dorsal nerve roots

contain sensory contain sensory fibers fibers

Ventral nerve roots Ventral nerve roots contain motor fiberscontain motor fibers



Dorsal horns: neurons in gray matter that Dorsal horns: neurons in gray matter that forward sensory nerve impulses to brain or forward sensory nerve impulses to brain or other parts of spinal cordother parts of spinal cord

Ventral horns: neurons in gray matter that Ventral horns: neurons in gray matter that forward motor (efferent) nerve impulses to forward motor (efferent) nerve impulses to the spinal nervesthe spinal nerves


Autonomic Nervous SystemAutonomic Nervous System

Controls automatic functions at the Controls automatic functions at the subconscious levelsubconscious level

Sympathetic nervous system - nerves Sympathetic nervous system - nerves emerge from thoracic and lumbar emerge from thoracic and lumbar vertebral regions (thoracolumbar system)vertebral regions (thoracolumbar system)

Parasympathetic nervous system - nerves Parasympathetic nervous system - nerves emerge from the brain and the sacral emerge from the brain and the sacral vertebral regions (cranial-sacral)vertebral regions (cranial-sacral)


Autonomic Nervous SystemAutonomic Nervous System


Sympathetic/Parasympathetic SystemsSympathetic/Parasympathetic Systems

Efferent motor nerves are composed of a Efferent motor nerves are composed of a sequence of two neuronssequence of two neurons Preganglionic neuron - cell body in the brain or Preganglionic neuron - cell body in the brain or

spinal cord and axon extended out to an spinal cord and axon extended out to an autonomic ganglionautonomic ganglion• Synapses with one or more additional neurons Synapses with one or more additional neurons

(postganglionic neuron) that are then connected to (postganglionic neuron) that are then connected to the target organ the target organ

Sympathetic ganglion chain - series of Sympathetic ganglion chain - series of autonomic ganglia outside thoracolumbar autonomic ganglia outside thoracolumbar area of spinal columnarea of spinal column


Sympathetic SystemSympathetic System Sympathetic preganglionic neuron may:Sympathetic preganglionic neuron may:

Synapse with a neuron within the ganglion chainSynapse with a neuron within the ganglion chain Pass through the ganglionic chain and synapse Pass through the ganglionic chain and synapse

with a neuron located beyond the sympathetic with a neuron located beyond the sympathetic chainchain

Usually synapses with many postganglionic Usually synapses with many postganglionic neurons in the sympathetic chain or ganglions neurons in the sympathetic chain or ganglions outside the sympathetic chainoutside the sympathetic chain

Sympathetic postganglionic neuron is much Sympathetic postganglionic neuron is much longer than its corresponding preganglionic longer than its corresponding preganglionic neuronneuron


Parasympathetic SystemParasympathetic System Parasympathetic preganglionic neuron Parasympathetic preganglionic neuron

travels directly from the CNS to its target travels directly from the CNS to its target organorgan Synapses with a short postganglionic neuron in Synapses with a short postganglionic neuron in

the target organ the target organ Parasympathetic preganglionic neuron is Parasympathetic preganglionic neuron is

relatively long compared with the very short relatively long compared with the very short postganglionic neuronpostganglionic neuron


Table 13-3. Effects of Sympathetic and Parasympathetic Nervous Systems

Sympathetic System Parasympathetic System Effect Effect

Heart rate Increases DecreasesForce of heart contraction Increases No significant effectDiameter of bronchioles Increases (dilates) Decreases (constricts)Diameter of pupil Increases (dilates) Decreases (constricts)Gastrointestinal motility, secretions, and blood flow Decreases IncreasesDiameter of skin blood vessels Decreases No significant effectDiameter of muscle blood vessels Increases No significant effectDiameter of blood vessels to kidney Decreases No significant effect


Neurotransmitters and ReceptorsNeurotransmitters and Receptors

Primary neurotransmitter of sympathetic Primary neurotransmitter of sympathetic nervous system is norepinephrinenervous system is norepinephrine Adrenergic neurons Adrenergic neurons - - neurons that release neurons that release

norepinephrinenorepinephrine Epinephrine and norepinephrine also released Epinephrine and norepinephrine also released

from adrenal medullafrom adrenal medulla


Neurotransmitters and ReceptorsNeurotransmitters and Receptors

Blood vessels in skin, GI tract, and skeletal Blood vessels in skin, GI tract, and skeletal muscle have adrenergic (catecholamine) muscle have adrenergic (catecholamine) receptors to epinephrine or norepinephrinereceptors to epinephrine or norepinephrine1.1. AlphaAlpha11- adrenergic receptors - cause vasoconstriction - adrenergic receptors - cause vasoconstriction

of the skin, GI tract, and kidney of the skin, GI tract, and kidney 2.2. BetaBeta11-adrenergic receptors - increase heart rate and -adrenergic receptors - increase heart rate and

force of contraction force of contraction 3.3. BetaBeta22-adrenergic receptors - cause bronchodilation-adrenergic receptors - cause bronchodilation


Neurotransmitters and ReceptorsNeurotransmitters and Receptors Primary neurotransmitter of parasympathetic Primary neurotransmitter of parasympathetic

nervous system is acetylcholinenervous system is acetylcholine Cholinergic neurons - neurons that release acetylcholineCholinergic neurons - neurons that release acetylcholine

Nicotinic acetylcholine receptors - on Nicotinic acetylcholine receptors - on postganglionic neurons of sympathetic and postganglionic neurons of sympathetic and parasympathetic systems and between motor parasympathetic systems and between motor neurons and muscleneurons and muscle

Muscarinic acetylcholine receptors - on target Muscarinic acetylcholine receptors - on target organs and tissues supplied by the postganglionic organs and tissues supplied by the postganglionic neuron of the parasympathetic nervous systemneuron of the parasympathetic nervous system


Summary:Summary:


ReflexesReflexes

Somatic reflexes - involve contraction of Somatic reflexes - involve contraction of skeletal musclesskeletal muscles

Autonomic reflexes - regulate smooth Autonomic reflexes - regulate smooth muscle, cardiac muscle, and endocrine muscle, cardiac muscle, and endocrine glandsglands

Contralateral reflex - starts on one side of Contralateral reflex - starts on one side of body and travels to opposite sidebody and travels to opposite side

Ipsilateral reflex - stimulus and response Ipsilateral reflex - stimulus and response are on the same side of the bodyare on the same side of the body


Reflex ArcReflex Arc

Sensory receptor sends an action potential Sensory receptor sends an action potential along the sensory neuron to the gray matter along the sensory neuron to the gray matter of the spinal cord or brain stemof the spinal cord or brain stem

Sensory neuron synapses with other Sensory neuron synapses with other neurons; incoming sensory impulse neurons; incoming sensory impulse integrated with other impulses from other integrated with other impulses from other sensory neuronssensory neurons

Integrated response of the reflex is sent out Integrated response of the reflex is sent out by the motor neuron, which ends at the by the motor neuron, which ends at the target organtarget organ


Stretch ReflexStretch Reflex Monosynaptic Monosynaptic

reflex arcreflex arc Involves a sensory Involves a sensory

neuron and a motor neuron and a motor neuron neuron

Only one synapse Only one synapse between them between them without any without any interneuronsinterneurons


Withdrawal ReflexWithdrawal Reflex

Also called flexor Also called flexor reflexreflex

Several Several interneuron interneuron synapses synapses

Results in Results in contraction or contraction or flexing of musclesflexing of muscles


Crossed Extensor ReflexCrossed Extensor Reflex

Contralateral reflex Contralateral reflex If withdrawal reflex If withdrawal reflex

initiated, afferent initiated, afferent sensory neuron sensory neuron synapse with synapse with interneuronsinterneurons Causes contraction Causes contraction

of opposite of opposite extensor musclesextensor muscles


CNS Moderation of ReflexesCNS Moderation of Reflexes

Upper CNS normally produces an Upper CNS normally produces an inhibitory effect on the reflex arcsinhibitory effect on the reflex arcs

With injury, intact reflex arcs caudal to the With injury, intact reflex arcs caudal to the spinal cord trauma become hyperreflexivespinal cord trauma become hyperreflexive

Trauma where reflex arc enters or leaves Trauma where reflex arc enters or leaves the spinal cord, or damage to the sensory the spinal cord, or damage to the sensory nerve or motor nerve of the reflex, results nerve or motor nerve of the reflex, results in either hyporeflexive or absent reflex in either hyporeflexive or absent reflex arcsarcs


Other Clinically Significant Reflexes Other Clinically Significant Reflexes

Palpebral reflex arc: light tap on the medial Palpebral reflex arc: light tap on the medial canthus of the eye produces a blink of the canthus of the eye produces a blink of the eyelids. eyelids.

Pupillary light reflex (PLR): normal Pupillary light reflex (PLR): normal response to shining light in the eye of an response to shining light in the eye of an animal is for the iris in both eyes to animal is for the iris in both eyes to constrictconstrict Shining the light in one eye causes a Shining the light in one eye causes a

constriction in both eyesconstriction in both eyes

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Nervous system overview

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