spinal cord organization - the medical university of south...
TRANSCRIPT
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Spinal Cord Organization
January 12, 2011
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Spinal Cord31 segments
terminates at L1-L2
special components- conus medullaris- cauda equina
no input from the face
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Spinal Cord, Roots & Nerves
Dorsal root
Ventral root
Spinal nerve
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Dorsal Root Ganglion
Cell bodies of 1st order sensory neurons
Afferent
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Spinal Cord Cross Section
Dorsal rootsensory
afferent
Ventral root motor
efferent
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Cross Section of Spinal Cord
White matterperipheral
Gray mattercentral
Central canal
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Volume of Gray & White Matter
Gray matter enlarges at cervical and lumbar regions.
White matter increases at higher levels of the cord.
Cervical 5 Thoracic 7 Lumbar 4
Sacral 3 Coccygeal 1
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Central Gray Matter
Motor neurons
2nd order sensory neurons
Interneurons
Glial cellsMotor neuron = lower motor neuron
= anterior horn cell
= second order motor neuron
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Organization of Gray Matter
Pain
Temperature
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Organization of Gray Matter
Position sense
Vibration
Pressure
Touch
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Organization of Gray Matter
Position sense
from legsBetween T1 and L2 enlarged
Dorsal nucleus of Clarke
2nd order sensory neuron going to cerebellum
Neck & base of Posterior Horn
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Organization of Gray Matter
Interomedio-lateral horn
Between T1 and L2 enlarged
Sympathetic neurons
(preganglionic)
Intermediolateral Horn
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Sympathetic Innervation
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Organization of Gray Matter
.
At S2, S3, and S4
Parasympathetic neurons
(preganglionic)
Intermediolateral Horn
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Parasympathetic Innervation
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Organization of Gray Matter
Modulates motor activity via gamma motor neurons
Adjusts briskness of deep tendon reflexes
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Organization of Gray Matter
Nuclei of alpha and gamma motor neurons
Innervates muscle spindles and muscles
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Lower Motor Neurons
Alpha motor neurons
originate from Rexed lamina IX
ventral – dorsal organizationventral extensor muscles
dorsal flexor muscles
medial – lateral organizationmedial proximal muscles
lateral distal muscles
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Nature of Somatic Reflexes• Quick, involuntary, stereotyped reactions of glands
or muscle to sensory stimulation – automatic responses to sensory input that occur without
our intent or often even our awareness• Functions by means of a somatic reflex arc
– stimulation of somatic receptors– afferent fibers carry signal to dorsal horn of spinal cord– one or more interneurons integrate the information– efferent fibers carry impulses to skeletal muscles– skeletal muscles respond
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The Muscle Spindle
• Sense organ (proprioceptor) that monitors length of muscle and how fast muscles change in length
• Composed of intrafusal muscle fibers, afferent fibers and gamma motorneurons
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Stretch Reflex
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Somatotopic Organization gamma reflex loop
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Coordinating Role of Interneurons
flexor withdrawal reflexcrossed extensor reflex
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Flexor Withdrawal Reflexes
• Occurs during withdrawal of foot from pain
• Polysynaptic reflex arc
• Neural circuitry in spinal cord controls sequence and duration of muscle contractions
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Crossed Extensor Reflexes• Maintains balance by
extending other leg• Intersegmental reflex
extends up and down the spinal cord
• Contralateral reflex arcs explained by pain at one foot causes muscle contraction in other leg
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Breathing
Nuclei of alpha and gamma motor neurons
LMNs from spinal levels C3, C4 & C5 innervate the phrenic nerve
Controls diaphragm
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Control Functions
Nuclei of alpha and gamma motor neurons
At levels S1 - S4 LMNs form Onuf’s nucleus
Innervates anal and urethral spincters; needed for sexual function
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Sacral Cord
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Configuration of Gray MatterAbundant white matter high in the spinal cord
Enlargement of gray matter at the cervical level
Innervates arm muscles
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Configuration of Gray MatterEnlargement in thoracic region for sympathetic neurons
Enlargement in lumbar region for motor neurons to the legs
Minimal white matter at the lower cord
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Descending Motor Pathway
Upper motor neuron
1st order neuron
synapses on LMN
Crossed pathwayLateral corticospinal
Uncrossed pathwayAnterior corticospinal
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Somatotopic Organization
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White Matter in the Spinal Cord
• Divided into three funiculi (columns) – posterior, lateral, and anterior
• Each column (funiculus) contains several – Fiber tracts are either
• Ascending (sensory) • Descending (motor)
– Fiber tract names often reveal their origin and destination
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White Matter: Pathway Generalizations
• Tracts may decussate (cross-over)• Most consist of two or three neurons• Pathways are paired (one on each side of the
spinal cord or brain)• Contralateral means origin and destination
are on opposite sides while ipsilateral means on same side
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Spinal Cord Tracts
movement
feedback
Unconscious position sense
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Ascending & Descending Tracts• ↑ Sensory
– Gracile tract• Leg position & vibration
– Cuneate tract• Arm position & vibration
– Dorsal spinocerebellar tract• Strength & muscle speed
– Ventral spinocerebellar tract• Modulation; interneurons
– Lateral spinothalamic tract• Pain & temperature
– Anterior spinothalamic tract• Light touch
– Spinocervical thalamic tract• Kinesthetic movement &
discriminative touch
• ↓ Motor– Corticospinal tract
• Speed & agility– Reticulospinal tract
• Differential facilitation of motor neurons
– Rubrospinal tract• Fix movement errors
– Lateral vestibulospinal tract• Extensor & posture
– Medial vestibulospinal tract• Flexor & head position
– Tectospinal tract• Head turning
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Ascending Tracts
SensoryPosition Vibration
SensoryPain Temperature
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Ascending Tracts• Carry sensory signals up to the spinal cord• Typically uses 3 neurons
– 1st order neuron - detects stimulus and carries it to spinal cord
– 2nd order neuron - within s.c.; continues to the thalamus (the sensory relay station)
– 3rd order neuron - carries signal from thalamus to sensory region of cerebral cortex
• Most have names with prefix spino-
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• Carries sensations related to discriminative touch, visceral pain, vibration, and proprioception
• 1st order neuron - detects stimulus• Fasciculus gracilis
– Carries sensation from below T6 • Fasciculus cuneatus
– Carries sensation from T6 or higher• 2nd order neuron synapses with 1st
in medulla and decussates• 3rd order neuron synapses with 2nd
in thalamus and carries signal to cerebral cortex (postcentral gyrus)
• System is contralateral
Dorsal Column Ascending Pathway
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Spinothalamic Pathway
• Carries sensations of pain, pressure, temperature, light touch, tickle and itch
• Located in the anterior and lateral columns
• Decussation of the second order neuron occurs in spinal cord
• Third order neurons arise in thalamus and continue to cerebral cortex of the postcentral gyrus
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Spinoreticular Tract• Pain signals from tissue injury• Decussate in spinal cord and ascend with
spinothalamic fibers• End in reticular formation (medulla and
pons)• 3rd and 4th order neurons continue to
thalamus and cerebral cortex
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Spinocerebellar Pathway
• 1st order neurons originate in muscles and tendons
• 2nd order neurons ascend in ipsilateral lateral column– Terminate in cerebellum (a large motor control are
of the brain)• Transmit proprioceptive signals from limbs
and trunk
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Descending (Motor) Pathways• Descending tracts deliver efferent impulses
from the brain to the spinal cord, and are divided into two groups– Direct pathways equivalent to the pyramidal tracts– Indirect pathways, essentially all others
• Motor pathways involve two neurons– Upper motor neuron (UMN)
• Begins with soma in cerebral cortex or brainstem• Its axon terminates ON the LMN in anterior horn
– Lower motor neuron (LMN)• Soma in anterior horn; axon leads to muscle• aka ‘anterior horn motor neuron” (also, final common
pathway)
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The Direct (Pyramidal) System• Direct pathways originate with the pyramidal neurons in
the precentral gyri (aka, primary motor area). • Pyramidal neuron is the UMN; it forms the corticospinal
tract (cortico =cortex; spinal - s.c.)• UMN synapses in the anterior horn with LMN• LMN (anterior horn motor neurons) activates skeletal
muscles• The direct pathway regulates fast and fine (skilled)
movements• Lateral corticospinal tracts: UMN decussates in pyramids
of medulla• Anterior corticospinal tracts: UMN decussates at the spinal
cord level
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The Direct (Pyramidal)
System
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Indirect (Extrapyramidal) System
• Upper motor neuron (UMN) originates in nuclei deep in cerebrum (not in cerebral cortex); .e., in brain stem,
• UMN does not pass through the pyramids• LMN is an anterior horn motor neuron • This system includes the rubrospinal, vestibulospinal,
reticulospinal, and tectospinal tracts• These motor pathways are complex and multisynaptic
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Descending Motor TractsExtrapyramidal Tracts
• Tectospinal tract (tectum of midbrain)– reflex turning of head in response to sights and
sounds• Reticulospinal tract (reticular formation)
– controls limb movements important to maintain posture and balance
• Vestibulospinal tract (brainstem nuclei)– postural muscle activity in response to inner ear
signals• Rubrospinal tracts – originate in ‘red nucleus’ of
midbrain; control flexor muscles (see next slide)
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Indirect (Extrapyramidal)
System
b
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Spinal Cord Injury
Position
Pain
Brown – Sequard Syndrome
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Spinal Cord Trauma and Disorders• Severe damage to ventral root results in flaccid paralysis.
• Skeletal muscles cannot move either voluntarily or involuntarily• Without stimulation, muscles atrophy.
• When only UMN of primary motor cortex is damaged, spastic paraly-sis occurs.
• Spinal motor neurons remain intact, muscles continue to be stimulated irregularly by spinal reflex activity.
• Muscles remain healthy longer but their movements are no longersubject to voluntary control.
• Muscles commonly become permanently shortened. • Transection (cross sectioning) at any level results in total motor and
sensory loss in body regions inferior to site of damage.• If injury in cervical region, all four limbs affected (quadriplegia)• If injury between T1 and L1, only lower limbs affected
(paraplegia)
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Transverse vs Hemi Cord Syndrome
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Anterior vs Posterior Cord Syndromes
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Traumatic Spinal Cord Injury
DUI: $100 addt’l fine
South Carolina
Spinal Cord Injury
Research Board
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12-54
Spinal Cord Trauma and Disorders
Spinal shock - transient period of functional loss that follows the injury• Results in immediate depression of all reflex activity caudal to lesion.• Bowel and bladder reflexes stop, blood pressure falls, and all muscles
(somatic and visceral) below the injury are paralyzed and insensitive.• Neural function usually returns within a few hours following injury• If function does not resume within 48 hrs, paralysis is permanent.
• Amyotrophic Lateral Sclerosis (aka, Lou Gehrig’s disease)• Progressive destruction of anterior horn motor neurons and fibers of the
pyramidal tracts.• Lose ability to speak, swallow, breathe.• Death within 5 yrs• Cause unknown (90%); others have high glutamate levels
• Poliomyelitis• Virus destroys AHMN• Victims die from paralysis of respiratory muscles• Virus enters body in feces-contaminated water (public swimming pools)
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Blood Supply to Spinal Cord