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Chapter 13Chapter 13

Lecture OutlineLecture Outline

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Spinal Cord, Spinal Nerves and Spinal Cord, Spinal Nerves and Somatic ReflexesSomatic Reflexes

Spinal cordSpinal cord Spinal nervesSpinal nerves Somatic reflexesSomatic reflexes

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Overview of Spinal CordOverview of Spinal Cord

Information highway between brain and bodyInformation highway between brain and body Extends through vertebral canal from foramen Extends through vertebral canal from foramen

magnum to L1magnum to L1 Each pair of spinal nerves receives sensory Each pair of spinal nerves receives sensory

information and issues motor signals to muscles information and issues motor signals to muscles and glandsand glands

Spinal cord is a component of the Central Spinal cord is a component of the Central Nervous System while the spinal nerves are part Nervous System while the spinal nerves are part of the Peripheral Nervous Systemof the Peripheral Nervous System

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Functions of the Spinal CordFunctions of the Spinal Cord ConductionConduction

bundles of fibers passing information up and down spinal bundles of fibers passing information up and down spinal cordcord

LocomotionLocomotion repetitive, coordinated actions of several muscle groupsrepetitive, coordinated actions of several muscle groups central pattern generators are pools of neurons providing central pattern generators are pools of neurons providing

control of flexors and extensors (walking)control of flexors and extensors (walking)

ReflexesReflexes involuntary, stereotyped responses to stimuli (remove involuntary, stereotyped responses to stimuli (remove

hand from hot stove)hand from hot stove) involves brain, spinal cord and peripheral nervesinvolves brain, spinal cord and peripheral nerves

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Gross Anatomy of Lower Spinal Gross Anatomy of Lower Spinal CordCord

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Anatomy of the Spinal CordAnatomy of the Spinal Cord Cylinder of nerve tissue within the vertebral canal Cylinder of nerve tissue within the vertebral canal

(thick as a finger)(thick as a finger) vertebral column grows faster so in an adult the spinal vertebral column grows faster so in an adult the spinal

cord only extends to L1cord only extends to L1 31 pairs of spinal nerves arise from cervical, 31 pairs of spinal nerves arise from cervical,

thoracic, lumbar and sacral regions of the cordthoracic, lumbar and sacral regions of the cord each cord segment gives rise to a pair of spinal nerveseach cord segment gives rise to a pair of spinal nerves

Cervical and lumbar enlargementsCervical and lumbar enlargements Medullary cone (conus medullaris) = tapered tip of Medullary cone (conus medullaris) = tapered tip of

cordcord Cauda equinae is L2 to S5 nerve roots resemble Cauda equinae is L2 to S5 nerve roots resemble

horse’s tailhorse’s tail

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Meninges of Vertebra and Spinal Meninges of Vertebra and Spinal CordCord

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Meninges of the Spinal CordMeninges of the Spinal Cord

3 Fibrous layers enclosing spinal cord3 Fibrous layers enclosing spinal cord Dura materDura mater

tough collagenous membrane surrounded by epidural tough collagenous membrane surrounded by epidural space filled with fat and blood vesselsspace filled with fat and blood vessels

• epidural anesthesia utilized during childbirthepidural anesthesia utilized during childbirth

Arachnoid materArachnoid mater layer of simple squamous epithelium lining dura mater layer of simple squamous epithelium lining dura mater

and loose mesh of fibers filled with CSFand loose mesh of fibers filled with CSF(creates subarachnoid space)(creates subarachnoid space)

Pia materPia mater delicate membrane adherent to spinal corddelicate membrane adherent to spinal cord filium terminale and denticulate ligaments anchor the cordfilium terminale and denticulate ligaments anchor the cord

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Spina BifidaSpina Bifida

Congenital defect in 1 baby out of 1000Congenital defect in 1 baby out of 1000 Failure of vertebral arch to close covering spinal cordFailure of vertebral arch to close covering spinal cord Folic acid (B vitamin) as part of a healthy diet for all Folic acid (B vitamin) as part of a healthy diet for all

women of childbearing age reduces riskwomen of childbearing age reduces risk

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Cross-Sectional Anatomy of the Spinal Cross-Sectional Anatomy of the Spinal CordCord

Central area of gray matter shaped like a butterfly and Central area of gray matter shaped like a butterfly and surrounded by white matter in 3 columnssurrounded by white matter in 3 columns

Gray matter = neuron cell bodies with little myelinGray matter = neuron cell bodies with little myelin White matter = myelinated axonsWhite matter = myelinated axons

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Gray Matter in the Spinal CordGray Matter in the Spinal Cord Pair of dorsal or posterior hornsPair of dorsal or posterior horns

dorsal root of spinal nerve is totally sensory fibersdorsal root of spinal nerve is totally sensory fibers

Pair of ventral or anterior hornsPair of ventral or anterior horns ventral root of spinal nerve is totally motor fibersventral root of spinal nerve is totally motor fibers

Connected by gray commissure punctured by a Connected by gray commissure punctured by a central canal continuous above with 4th ventriclecentral canal continuous above with 4th ventricle

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White Matter in the Spinal CordWhite Matter in the Spinal Cord White column = bundles of myelinated axons that carry White column = bundles of myelinated axons that carry

signals up and down to and from brainstemsignals up and down to and from brainstem 3 pairs of columns or funiculi3 pairs of columns or funiculi

dorsal, lateral, and anterior columnsdorsal, lateral, and anterior columns

Each column is filled with named tracts or fasciculi (fibers Each column is filled with named tracts or fasciculi (fibers with a similar origin, destination and function)with a similar origin, destination and function)

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Spinal TractsSpinal Tracts

Ascending and descending tract head up or down while Ascending and descending tract head up or down while decussation means that the fibers cross sidesdecussation means that the fibers cross sides

Contralateral means origin and destination are on opposite Contralateral means origin and destination are on opposite sides while ipsilateral means on same sidesides while ipsilateral means on same side

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Deep touch, Deep touch, visceral pain, vibration, and visceral pain, vibration, and proprioceptionproprioception

Fasciculus gracilis and Fasciculus gracilis and cuneatus carry signals from cuneatus carry signals from arm and leg arm and leg

Decussation of 2nd Decussation of 2nd order neuron in medullaorder neuron in medulla

3rd order neuron in 3rd order neuron in thalamus carries signal to thalamus carries signal to cerebral cortexcerebral cortex

Dorsal Column Ascending Dorsal Column Ascending PathwayPathway

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Spinothalamic PathwaySpinothalamic Pathway Pain, pressure, Pain, pressure,

temperature, light temperature, light touch, tickle and itchtouch, tickle and itch

Decussation of the Decussation of the second order neuron second order neuron occurs in spinal cordoccurs in spinal cord

Third order neurons Third order neurons arise in thalamus and arise in thalamus and continue to cerebral continue to cerebral cortexcortex

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Spinoreticular TractSpinoreticular Tract

Pain signals from tissue injuryPain signals from tissue injury Decussate in spinal cord and ascend with Decussate in spinal cord and ascend with

spinothalamic fibersspinothalamic fibers End in reticular formation (medulla and End in reticular formation (medulla and

pons)pons) 33rdrd and 4 and 4thth order neurons continue to order neurons continue to

thalamus and cerebral cortexthalamus and cerebral cortex

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Spinocerebellar PathwaySpinocerebellar Pathway

Proprioceptive signals from limbs and Proprioceptive signals from limbs and trunk travel up to the cerebellumtrunk travel up to the cerebellum

Second order nerves ascend in ipsilateral Second order nerves ascend in ipsilateral lateral columnlateral column

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Precise, coordinated Precise, coordinated limb movementslimb movements

Two neuron pathwayTwo neuron pathway upper motor neuron in upper motor neuron in

cerebral cortexcerebral cortex lower motor neuron in lower motor neuron in

spinal cordspinal cord Decussation in medulla Decussation in medulla

Corticospinal TractCorticospinal Tract

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Descending Motor TractsDescending Motor Tracts

Tectospinal tract (tectum of midbrain)Tectospinal tract (tectum of midbrain) reflex turning of head in response to sights and reflex turning of head in response to sights and

soundssounds

Reticulospinal tract (reticular formation)Reticulospinal tract (reticular formation) controls limb movements important to maintain controls limb movements important to maintain

posture and balanceposture and balance

Vestibulospinal tract (brainstem nuclei)Vestibulospinal tract (brainstem nuclei) postural muscle activity in response to inner ear postural muscle activity in response to inner ear

signalssignals

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Anatomy of a NerveAnatomy of a Nerve

A nerve is a bundle of nerve fibers (axons)A nerve is a bundle of nerve fibers (axons) Epineurium covers nerves, perineurium surrounds a Epineurium covers nerves, perineurium surrounds a

fascicle and endoneurium separates individual nerve fibersfascicle and endoneurium separates individual nerve fibers Blood vessels penetrate only to the perineuriumBlood vessels penetrate only to the perineurium

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Anatomy of Ganglia in the PNSAnatomy of Ganglia in the PNS

Cluster of neuron cell bodies in nerve in PNSCluster of neuron cell bodies in nerve in PNS Dorsal root ganglion is sensory cell bodies Dorsal root ganglion is sensory cell bodies

fibers pass through without synapsingfibers pass through without synapsing

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Spinal Nerve Roots and Spinal Nerve Roots and PlexusesPlexuses

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The Spinal NervesThe Spinal Nerves 31 pairs of spinal nerves (1st cervical above C1)31 pairs of spinal nerves (1st cervical above C1)

mixed nerves exiting at intervertebral foramenmixed nerves exiting at intervertebral foramen

Proximal branchesProximal branches dorsal root is sensory input to spinal corddorsal root is sensory input to spinal cord ventral root is motor output of spinal cordventral root is motor output of spinal cord cauda equina is roots from L2 to C0 of the cordcauda equina is roots from L2 to C0 of the cord

Distal branchesDistal branches dorsal ramus supplies dorsal body muscle and skindorsal ramus supplies dorsal body muscle and skin ventral ramus to ventral skin and muscles and limbsventral ramus to ventral skin and muscles and limbs meningeal branch to meninges, vertebrae and ligamentsmeningeal branch to meninges, vertebrae and ligaments

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Branches of a Spinal NerveBranches of a Spinal Nerve

Spinal nerves: 8 cervical, Spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal.sacral and 1 coccygeal.

Each has dorsal and Each has dorsal and ventral ramus.ventral ramus.

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Rami of Spinal NervesRami of Spinal Nerves

Notice the branching and merging of nerves Notice the branching and merging of nerves in this example of a plexusin this example of a plexus

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ShinglesShingles

Skin eruptions along path of nerveSkin eruptions along path of nerve Varicella-zosterVaricella-zoster virus (chicken pox) virus (chicken pox)

remains for life in dorsal root gangliaremains for life in dorsal root ganglia Occurs after age 50 if immune system is Occurs after age 50 if immune system is

compromisedcompromised No special treatmentNo special treatment

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Nerve PlexusesNerve Plexuses Ventral rami branch and anastomose repeatedly Ventral rami branch and anastomose repeatedly

to form 5 nerve plexusesto form 5 nerve plexuses cervical in the neck, C1 to C5 cervical in the neck, C1 to C5

• supplies neck and phrenic nerve to the diaphragmsupplies neck and phrenic nerve to the diaphragm brachial in the armpit, C5 to T1brachial in the armpit, C5 to T1

• supplies upper limb and some of shoulder and necksupplies upper limb and some of shoulder and neck lumbar in the low back, L1 to L4lumbar in the low back, L1 to L4

• supplies abdominal wall, anterior thigh and genitaliasupplies abdominal wall, anterior thigh and genitalia sacral in the pelvis, L4, L5 and S1 to S4sacral in the pelvis, L4, L5 and S1 to S4

• supplies remainder of lower trunk and lower limbsupplies remainder of lower trunk and lower limb coccygeal, S4, S5 and C0coccygeal, S4, S5 and C0

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The Brachial PlexusThe Brachial Plexus

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The Lumbar PlexusThe Lumbar Plexus

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The Sacral and Coccygeal The Sacral and Coccygeal PlexusesPlexuses

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Cutaneous Innervation and Cutaneous Innervation and DermatomesDermatomes

Each spinal nerve receive Each spinal nerve receive sensory input from a sensory input from a specific area of skin specific area of skin called dermatomecalled dermatome

Overlap at edges by 50%Overlap at edges by 50% a total loss of sensation a total loss of sensation

requires anesthesia of 3 requires anesthesia of 3 successive spinal nervessuccessive spinal nerves

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Nature of Somatic ReflexesNature of Somatic Reflexes Quick, involuntary, stereotyped reactions of glands Quick, involuntary, stereotyped reactions of glands

or muscle to sensory stimulation or muscle to sensory stimulation automatic responses to sensory input that occur without automatic responses to sensory input that occur without

our intent or often even our awarenessour intent or often even our awareness Functions by means of a somatic reflex arcFunctions by means of a somatic reflex arc

stimulation of somatic receptorsstimulation of somatic receptors afferent fibers carry signal to dorsal horn of spinal cordafferent fibers carry signal to dorsal horn of spinal cord one or more interneurons integrate the informationone or more interneurons integrate the information efferent fibers carry impulses to skeletal musclesefferent fibers carry impulses to skeletal muscles skeletal muscles respondskeletal muscles respond

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The Muscle SpindleThe Muscle Spindle

Sense organ (proprioceptor) that monitors length of Sense organ (proprioceptor) that monitors length of muscle and how fast muscles change in lengthmuscle and how fast muscles change in length

Composed of intrafusal muscle fibers, afferent fibers and Composed of intrafusal muscle fibers, afferent fibers and gamma motorneuronsgamma motorneurons

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The Stretch (Myotatic) ReflexThe Stretch (Myotatic) Reflex When a muscle is stretched, it contracts and When a muscle is stretched, it contracts and

maintains increased tonus (stretch reflex)maintains increased tonus (stretch reflex) helps maintain equilibrium and posturehelps maintain equilibrium and posture

• head starts to tip forward as you fall asleephead starts to tip forward as you fall asleep• muscles contract to raise the headmuscles contract to raise the head

stabilize joints by balancing tension in extensors and stabilize joints by balancing tension in extensors and flexors smoothing muscle actionsflexors smoothing muscle actions

Very sudden muscle stretch causes tendon reflexVery sudden muscle stretch causes tendon reflex knee-jerk (patellar) reflex is monosynaptic reflexknee-jerk (patellar) reflex is monosynaptic reflex testing somatic reflexes helps diagnose many diseasestesting somatic reflexes helps diagnose many diseases

Reciprocal inhibition prevents muscles from Reciprocal inhibition prevents muscles from working against each otherworking against each other

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The Patellar Tendon Reflex ArcThe Patellar Tendon Reflex Arc

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Flexor Withdrawal ReflexesFlexor Withdrawal Reflexes

Occurs during Occurs during withdrawal of foot withdrawal of foot from painfrom pain

Polysynaptic reflex Polysynaptic reflex arcarc

Neural circuitry in Neural circuitry in spinal cord controls spinal cord controls sequence and sequence and duration of muscle duration of muscle contractionscontractions

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Crossed Extensor ReflexesCrossed Extensor Reflexes Maintains balance Maintains balance

by extending other by extending other legleg

Intersegmental Intersegmental reflex extends up reflex extends up and down the and down the spinal cordspinal cord

Contralateral reflex Contralateral reflex arcs explained by arcs explained by pain at one foot pain at one foot causes muscle causes muscle contraction in other contraction in other legleg

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Golgi Tendon ReflexGolgi Tendon Reflex Proprioceptors in a Proprioceptors in a

tendon near its tendon near its junction with a muscle junction with a muscle -- 1mm long, -- 1mm long, encapsulated nerve encapsulated nerve bundlebundle

Excessive tension on Excessive tension on tendon inhibits motor tendon inhibits motor neuronneuron muscle contraction muscle contraction

decreaseddecreased Also functions when Also functions when

muscle contracts muscle contracts unevenlyunevenly