Spinal Cord Organization

January 12, 2011

Spinal Cord31 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 rootsensory

afferent

Ventral root motor

efferent

Cross Section of Spinal Cord

White matterperipheral

Gray mattercentral

Central canal

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

Central Gray Matter

Motor neurons

2nd order sensory neurons

Interneurons

Glial cellsMotor neuron = lower motor neuron

= anterior horn cell

= second order motor neuron

Organization of Gray Matter

Pain

Temperature

Organization of Gray Matter

Position sense

Vibration

Pressure

Touch

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

Organization of Gray Matter

Interomedio-lateral horn

Between T1 and L2 enlarged

Sympathetic neurons

(preganglionic)

Intermediolateral Horn

Sympathetic Innervation

Organization of Gray Matter

.

At S2, S3, and S4

Parasympathetic neurons

(preganglionic)

Intermediolateral Horn

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 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

Somatotopic Organization gamma reflex loop

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

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 MatterAbundant white matter high in the spinal cord

Enlargement of gray matter at the cervical level

Innervates arm muscles

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

Descending Motor Pathway

Upper motor neuron

1st order neuron

synapses on LMN

Crossed pathwayLateral corticospinal

Uncrossed pathwayAnterior corticospinal

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

Spinal Cord Tracts

movement

feedback

Unconscious position sense

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

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

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)

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

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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)

Blood Supply to Spinal Cord