Transcript Chapter 13

The Spinal Cord, spinal
nerves and reflexes
Portions of Chapters 13
Reflex arcs, figure 13.3
1.
Sensory receptor- distal end of sensory
neuron (dendrite) or associated sensory
structure
*responds to internal or external stimuli
2.
Sensory neuron- nerve impulses propagate
from sensory receptor  axon of sensory
neuron axon terminals located in gray
matter of brain or spinal cord
3.
Integrating center- one or more regions of CNS
gray matter
*single synapse- sensory & motor neuron= monosynaptic.
*More often the integration center consists of one or more
interneurons which may relay the impulse to other
interneurons or the motor neuron= polysynaptic
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Interneuron- axons extend only short distance contact
nearby neuron in CNS or ganglia, comprise vast
majority of neurons in the body
4.
5.
Motor neuron- impulses triggered by integration
center propagate out of CNS  motor neuron 
body part that responds:
Effector- if skeletal muscle= somatic reflex, if
smooth, cardiac muscle or gland=autonomic
(visercal) reflex
Reflexes in diagnosis
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Damage/diseasereflex absent or abnormal
Somatic- tapping or stroking:
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Autonomic- difficult stimulating visceral organs 
impractical
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Patellar- N= extend knee, A= Damage to sensory or motor
neuron, s.c. lumbar region
Pupillary light reflex- N= pupils of both eyes diameter if
either exposed to light, A= maybe damage or injury to
receptors in brainstem or midbrain
Plantar flexion reflex- gentle stroke of foot sole
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Babinski sign- toes fan rather than curl (curl= normal sign)
 Incomplete myelination in corticospinal tract
 Damage to descending motor pathway
Somatic spinal reflexes
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Stretch reflex- fig 13.14, Monosynaptic
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Contraction of skeletal muscle in response to stretching of
muscle
Elicit by tapping tendons: knee, wrist, elbow, ankle
Mechanism:
 Slight stretch stimulates musle spindles (see figure)
 Spindles generate one or more AP  posterior root
 Spinal cord integrates & activates neuron in ant gray horn
 If excitation strong enough, AP sent thru motor neuron axon
 Ach released, trigger muscle AP, muscle contracts to relieve
the stretch
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Flexor or withdrawal reflex- fig 13.16, polysynaptic
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withdrawing from painful stimulus (step on tack)
Mechanism:
 Stepping on tack stimulates dendrites- pain sensitive
neuron
 Sensory neuron generates AP  spinal cord
 Spinal cord integrates, activates interneurons that extend to
several spinal cord segments
 Interneurons activate motor neurons, generate AP
 Ach released, causes flexor muscles to contract producing
withdrawal
Spinal cord functions
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White matter- sensory and motor tracts
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Highway for conduction of sensory impulse to
brain and motor impulse to effector
Gray matter is site of integration
Spinal nerves & nerves that branch from
them connect CNS to sensory receptors,
muscles, & glands
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Cervical enlargement- extends from 4th
cervical to 1st thoracic
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nerves to & from upper limbs arise
Lumbar enlargement- 9th-12th thoracic
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nerves to & from lower limbs arise
inferior to this is a conical portion = conus
medullaris ending at the intervertebral disc
between 1st and 2nd lumbar, from this arises:
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filum terminale- extension of the pia mater
anchoring the spinal cord to coccyx
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The roots (pts of attachment of spinal nerve to
the spinal cord) are angled downward when
arising towards inferior portion of s.c.
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Cord appears segmented
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cauda equina-“horse tail,” roots angle down vertebral
canal from end of the spinal cord, look like hair
31 spinal nerves leave at regular intervals
where pairs of spinal nerves arise are spinal segments
no segments within cord dividing gray & white matter
Spinal nerves- paths of communication between
spinal cord & nerves innervating specific regions
Roots-2 bundles of axons connecting spinal
nerves to spinal cord:
A. Posterior or dorsal root- contains only
sensory fibers conducting impulses from
periphery to CNS
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-each has swelling = posterior or dorsal root
ganglion-contains cell bodies of sensory
neurons
B.
Anterior or ventral root- contain axons of
motor neurons conducting impulses from
CNS to effector organs & cells
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Gray matter of spinal cord- cell bodies of
neurons, neuroglia, unmyelinated axons, &
dendrites of interneurons & motor neurons
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in CNS- clusters of neuronal cell bodies form
functional groups = nuclei
Sensory nuclei
Motor nuclei
On each side of cord divided into groupshorns
White matter in cord- bundles of
myelinated and unmyelinated axons of
sensory, inter, and motor neurons
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Organized into columns…
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Gray commissure- center of butterfly connecting 2
lateral masses of gray matter
Central canal- small space in center gray
commissure
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extending entire length of spinal cord
Continuous with 4th ventricle
bundles extending long distance up/down= tracts
Sensory (ascending) tracts- axons that conduct
nerve impulses toward brain
Motor (descending) tracts- axons carry nerve
impulses down cord
Both continuous w/sensory & motor tracts in brain
Upper Motor Neurons
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Planning, initiating, directing sequences of voluntary
movements
Pyramidal (direct) pathway- convey nerve impulses
originating in cerebral cortex & are destined to
cause precise, voluntary movements of the skeletal
muscle
Extrapyramidal (indirect) pathway- convey nerve
impulses destined to program automatic movements
& help coordinate body movements with visual
stimuli, maintain skeletal tone, and posture &
equilibrium
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Corticospinal tract- white columns in the spinal cord,
convey nerve impulses from motor cortex to skeletal
muscle
Decussation- cross over to opposite side
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90% axons in large motor tracts to opposite side in
medullary pyramids
10% eventually do in the s.c.
Then, will synapse with interneuron or LMN
Right cerebral cortex controls left muscles
Left cerebral cortex controls right muscles
Spinal nerves, figure 13.2
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Cord appears segmented: 31 spinal nerves
leave at regular intervals
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areas where pairs of spinal nerves arise are
called spinal segment
Spinal nerves- paths of communication
between spinal cord and nerves innervating
specific regions of the body.
Structure of spinal nerve, 13.4
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typical spinal nerve has 2 connections to the cord:
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posterior root and anterior root
 unite at intervertebral foramen to form a spinal nerve
Posterior = sensory
Anterior = motor  mixed nerve
Each spinal and cranial nerve contains layers of
protective CT coverings
individual axons (un- & myelinated) are wrapped in
endoneurium
fascicles of axons are wrapped in perineurium
epineurium covers the entire nerve
5 Spinal nerve plexuses…
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Cervical- C1-C4, some C5
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Innervates: skin & muscles of head, neck,
shoulder, chest, diaphragm
Injuries to phrenic nerve- breathing would stop
Plexus (2)
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Brachial- C5-C8, T1
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Shoulders and upper limbs
Injuries to nerves emerging:
 Forceful pulling of head from shoulder, loss sensation on
lateral arm
 Radial nerve- improper deltoid injection. Wrist drop
indicates injury (inability to extend wrist & fingers)
 Median nerve- pronate forearm, proximal interphalangeal
joints (2-3), weakened wrist flexion, adduction & thumb
movements
 Ulnar nerve- abduct & adduct fingers, clawhand
 Long thoracic nerve- paralysis of serratus anterior  medial
border of scapula protrudes  wing like appearance
Plexuses (3)
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Lumbar- L1-L4
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Anterolateral abdominal wall, ext genitals, pt of
lower limbs
Injuries:
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Femoral nerve- (stab, gunshot wound for ex) inability
to extend leg, loss sensation in skin anteromedial
aspect of thigh
Obturator nerve- paralysis of adductor muscles of leg,
loss of sensation over medial aspect thigh, pressure
on by fetus head during pregnancy
Plexus (4)
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Sacral- L4-L5, S1-S4
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Supplies buttocks, perineum, lower limbs
*largest nerve in body (sciatic nerve) arises from
Injury- RESULTS in: footdrop- plantar flexed and
inverted
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Loss of function along anterolateral leg, dorsum of foot
& toes
Calcaneovalgus- dorsiflexion and eversion of foot due
to injury to tibial portion of sciatic
Loss of sensation to sole
Plexus (5)
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Coccygeal- S4-S5, coccygeal nerves
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Supplies skin in coccygeal region
Disorders: spinal cord
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Polio- poliomyelitis- cause by virus
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Fever; headache; stiff neck, back; deep muscle pain &
weak; loss of somatic reflexes
Can cause paralysis by destroying cell body of motor
neurons, esp. anterior horn & nuclei of cranial nerves
Can cause death from resp or heart failure
Vaccine virtually erradicated polio in US
Meningitis- inflammation of the meninges
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Can be due to infection by bacteria or virus
Bacterial- more serious, treated w/antibiotics (vaccine for)
 May be fatal if not treated promptly
Viral- no specific treatment, usually resolved in 1-2 weeks
Disorders: spinal cord (2)
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Sciatica- injury to sciatic nerve
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pain- buttocks, posterior, lateral aspect of leg
Due to slipped disc, dislocated hip, osteoarthritis
of lumbosacral spine, pressure from uterus (preg),
or improper gluteal intramuscular injection
Shingles- acute infection of PNS
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after chicken pox- virus retreats to posterior root
ganglion
If reactivated can send down sensory neurons to
skin