Download Spinal Cord Organization

Spinal Cord Organization
January 12, 2011
Spinal Cord
31 segments
terminates at L1-L2
special components
- conus medullaris
- cauda equina
no input from the face
Spinal Cord, Roots & Nerves
Dorsal root
Ventral root
Spinal nerve
Dorsal Root Ganglion
Cell bodies of 1st order sensory neurons
Afferent
Spinal Cord Cross Section
Dorsal root
sensory
afferent
Ventral root
motor
efferent
Cross Section of Spinal Cord
White matter
peripheral
Gray matter
central
Central canal
Volume of Gray & White Matter
Cervical 5
Thoracic 7
Sacral 3
Lumbar 4
Coccygeal 1
Gray matter enlarges at cervical and lumbar regions.
White matter increases at higher levels of the cord.
Central Gray Matter
Motor neurons
2nd order
sensory neurons
Interneurons
Motor neuron = lower motor neuron
= anterior horn cell
= second order motor neuron
Glial cells
Organization of Gray Matter
Pain
Temperature
Organization of Gray Matter
Position sense
Vibration
Pressure
Touch
Organization of Gray Matter
Position sense
Neck & base of
Posterior Horn
from legs
Between T1 and
L2 enlarged
Dorsal nucleus
of Clarke
2nd order sensory
neuron going to
cerebellum
Organization of Gray Matter
Intermediolateral Horn
Interomediolateral horn
Between T1 and
L2 enlarged
Sympathetic
neurons
(preganglionic)
Sympathetic Innervation
Organization of Gray Matter
Intermediolateral Horn
.
At S2, S3, and S4
Parasympathetic
neurons
(preganglionic)
Parasympathetic Innervation
Organization of Gray Matter
Modulates
motor activity
via gamma
motor neurons
Adjusts briskness
of deep tendon
reflexes
Organization of Gray Matter
Nuclei of alpha
and gamma
motor neurons
Innervates muscle
spindles and
muscles
Lower Motor Neurons
Alpha motor neurons
originate from Rexed lamina IX
ventral – dorsal organization
ventral Æ extensor muscles
dorsal Æ flexor muscles
medial – lateral organization
medial Æ proximal muscles
lateral Æ distal muscles
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
12-19
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
12-20
gamma motorneurons
Stretch Reflex
12-21
Somatotopic Organization
gamma reflex loop
Coordinating Role of Interneurons
flexor withdrawal reflex
crossed extensor reflex
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
12-24
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
12-25
Breathing
Nuclei of alpha
and gamma
motor neurons
LMNs from spinal
levels C3, C4 &
C5 innervate the
phrenic nerve
Controls
diaphragm
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
Sacral Cord
Configuration of Gray Matter
Abundant white
matter high in the
spinal cord
Enlargement of
gray matter at the
cervical level
Innervates arm
muscles
Configuration of Gray Matter
Enlargement in
thoracic region for
sympathetic
neurons
Enlargement in
lumbar region for
motor neurons to
the legs
Minimal white
matter at the lower
cord
Descending Motor Pathway
Upper motor neuron
1st order neuron
synapses on LMN
Crossed pathway
Lateral corticospinal
Uncrossed pathway
Anterior corticospinal
Somatotopic Organization
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
12-33
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
12-34
Spinal Cord Tracts
Unconscious position sense
movement
feedback
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
Ascending Tracts
Sensory
Position
Vibration
Sensory
Pain
Temperature
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 spino12-38
Dorsal Column Ascending Pathway
•
•
•
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
12-39
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
12-40
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
12-41
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
12-42
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)
12-43
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
12-44
The Direct
(Pyramidal)
System
12-45
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
12-46
Descending Motor Tracts
Extrapyramidal 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)
12-47
Indirect
(Extrapyramidal)
System
12-48 b
Spinal Cord Injury
Position
Pain
Brown – Sequard Syndrome
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 paralysis 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 longer
subject 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)
12-50
Transverse vs Hemi Cord Syndrome
Anterior vs Posterior Cord Syndromes
Traumatic Spinal Cord Injury
DUI: $100 addt’l fine
South Carolina
Spinal Cord Injury
Research Board
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)
12-54
Blood Supply to Spinal Cord