The Spinal Cord Basic Neuroscience James H. Baños, Ph.D.

Grey and White Matter

Grey and White Matter Grey Matter = Cell Body White Matter = Myelinated axon

Grey and White Matter Grey matter Cortex Nucleus (CNS) Ganglion (PNS) Exception: Basal Ganglia

Grey and White Matter White Matter Nerve (PNS) Tract (CNS) Fasciculus/Funiculus -- Group of fibers with common origin and destination Lemniscus -- Ribbon-like fiber tract Peduncle -- Massive group of fibers -- usually several tracts

Grey and White Matter Tracts are named with origin first, then destination Corticospinal tract -- cortex to spinal cord Mammilothalamic tract -- Mammilary bodies to thalamus Spinocerebellar tract -- Spinal cord to cerebellum Corticobulbar tract -- Cortex to brain stem

The Spinal Cord

General Organization Spinal cord is SMALL! 42-45 cm long 1 CM wide at widest point Does not extend all the way to the bottom of the spinal column Pattern of grey/white matter is reversed in the cord White matter tracts on outside Grey matter on the inside Staining reverses this!!!

General Organization White matter (tracts of axons) Grey Matter (cell bodies)

General Organization Spinal cord is segmented anatomically Input and output occurs in groups of rootlets arranged in a series longitudinally along the cord Dorsal rootlets -- Input -- carry sensory information Ventral rootlets -- Output -- motor neurons

General Organization Each set of rootlets forms a spinal nerve that innervates a corresponding segment of the body, called a dermatome

General Organization

There are 31 segments in the spinal cord: General Organization There are 31 segments in the spinal cord: 8 cervical (C1 - C8) 12 Thoracic (T1 - T12) 5 Lumbar (L1 - L5) 5 Sacral (S1 - S5) 1 Coccygeal

General Organization The spinal cord is housed within the vertebral column

General Organization Each cord segment has a corresponding vertebra of the same name (e.g., C3) Spinal nerves enter/exit underneath their corresponding vertebral segment

But wait! Something doesn’t add up! General Organization But wait! Something doesn’t add up! How can spinal nerves exit below their corresponding vertebral segment if the cord is only 42cm-45cm long? Answer: Spinal nerves extend down to the appropriate vertebral segment forming the cauda equina This means cord segments and vertebral segments don’t line up

General Organization

General Organization Cord is not of uniform thickness throughout its length. Why not?

Cord is not of uniform thickness throughout its length. Why not? General Organization Cord is not of uniform thickness throughout its length. Why not? Answer: Segments of the cord innervate parts of the body that differ in complexity There are fewer white matter tracts lower in the cord.

General Organization Cervical enlargement C5 - T1 Lumbar enlargement L2 - S3

The Spinal Cord in Cross Section

Cord Sections Segments of the spinal cord have a similar organization, but vary in appearance. Always know where you are in the cord (i.e., cervical, thoracic, lumbar, sacral)

Cord Sections -- Cervical Cervical cord is wide, flat, almost oval in appearance. Why?

Cord Sections -- Cervical Enlargement What’s different about the cervical enlargement . Why? Cervical Cervical Enlargement

Cord Section -- Thoracic Less White matter than cervical Rounder appearance Less prominent ventral horns than cervical enlargement

Less White matter than thoracic Rounder appearance Cord Section -- Lumbar Less White matter than thoracic Rounder appearance Larger ventral horns, especially in lumbar enlargement Lumbar Lumbar Enlargement

Cord Section -- Sacral Not much white matter Mostly grey, although not much of that either

Cross Sectional Organization Posterior intermediate sulcus Posterior median sulcus Tract of Lissauer Anterior white commissure Anterior median fissure

Grey Matter Laminar Laminae of Rexed

Grey Matter Posterior (dorsal) Horn Intermediate Grey Anterior (ventral) Horn

Grey Matter: Posterior Horn Mostly Interneurons Substantia gelatinosa Pain/temp proc Body of the posterior horn Sensory proc

Grey Matter: Intermediate Grey Clarke’s Column T1-L3 Balance/proprio. Intermediolateral Column Sympathetic neurons

Grey Matter: Anterior Horn Lower Motor Neurons

White Matter: The “Big Four” Pathways Corticospinal tract Dorsal Columns Spinothalamic tract Spinocerebellar tracts

The Big Four Corticospinal tract Dorsal columns/ medial lemniscus Voluntary motor Dorsal columns/ medial lemniscus Discriminative touch Conscious proprioception Spinocerebellar tract (dorsal and ventral) Unconscious proprioception Spinothalamic tract Pain/temperature

Corticospinal Tract Voluntary Motor

Corticospinal Tract First order neuron (upper motor neuron) originates in precentral gyrus Passes through internal capsule 90% decussates in caudal medulla Lateral corticospinal tract 10% undecussated Anterior corticospinal tract Synapses on second order neuron (lower motor neuron) in ventral gray of the cord Second order neuron innervates muscle

Motor Homunculus HAL

Motor Homonculus HAL: Arms Legs Head

Corticospinal Tract Spinal Cord Medulla Pons Midbrain

Corticospinal Tract Spinal Cord Medulla Pons Midbrain

Upper & Lower Motor Neurons Upper Motor Neuron Motor Cortex to Ventral Grey Horn Modulatory influence on stretch reflex arc Lower Motor Neuron Ventral Grey Horn to Neuromuscular Junction Efferent of stretch reflex arc Helps maintain tone Sensory Neuron Stretch receptors in muscle and tendons Afferent of basic stretch reflex arc Motor Ctx UMN Ventral Grey Horn LMN

Upper & Lower Motor Neurons Maintenance of Tone Input from stretch receptors causes lower motor neuron to supply tonic stimulation to the muscle The upper motor neuron modulates this -- will tend to “override” the tonic signal from the sensory neuron UMN LMN

Upper & Lower Motor Neurons Reflex Arc Afferent is sensory neuron detecting a sudden stretch Signal is strong and results in a strong response by the lower motor neuron Strong signal usually overcomes mild cortical input from the UMN UMN LMN

Upper & Lower Motor Neurons Upper Motor Neuron Signs Spastic paresis Hypertonia Hyperreflexia No muscle atrophy (until perhaps late in the course) Positive Babinski Why? Loss of voluntary UMN signal Loss of modulation of tone and reflexes by UMN -- the circuit runs unchecked Motor Ctx UMN Ventral Grey Horn LMN

Upper & Lower Motor Neurons Lower Motor Neuron Signs Flaccid paresis/paralysis Muscle fasciculations Hypotonia Hyporeflexia Muscle atrophy Negative Babinski Why? Loss of LMN for voluntary movement Loss of efferent component of reflex arc and tone pathway Motor Ctx UMN Ventral Grey Horn LMN

Babinski’s Sign In response to stimulation of the sole of the foot, the toes will usually curl downward. When UMN inhibition is removed, the toes will curl upward (Dorsiflexion). This is referred to as a positive Babinski or presence of Babinski’s sign.

Related Terms… Spasticity -- Increased muscle tone and increased reflex contraction (UMN) Clonus -- Rythmic contractions and relaxations seen when a spastic muscle is stretched (UMN)

Basics of Localization If all limbs are checked for upper and lower motor neuron signs, you can begin to localize lesions Left-right differences are also very important

Dorsal Column/ Medial Lemniscus Discriminative Touch Conscious Proprioception

Dorsal Columns/Medial Lemniscus First order neuron begins in receptor Enters cord at tract of Lissauer Legs run in fasciculus gracilis (medial dorsal) Arms run in fasciculus cuneatus (lateral dorsal) Synapse on nucleus gracilis and nucleus cuneatus (caudal medulla) 2nd order neuron decussates and runs from NG & NC to thalamus (as medial lemniscus) 3rd order neuron runs from thalamus to postcentral gyrus

Dorsal Columns/Medial Lemniscus Spinal Cord Medulla Pons Midbrain

Spinocerebellar Tracts Unconscious Proprioception

Dorsal (Posterior) Spinocerebellar Tract Involves Clark’s Column, a longitudinal gray matter body from about T1 to L3 Below Clark’s Column: Runs with f. gracilis, synapses in Clark’s Column, joins dorsal spinocerebellar tract Level of Clark’s Column Synapses in Clark’s Column, joins dorsal spinocerebellar tract Above Clark’s Column Runs with f. cuneatus, synapses in lateral cuneate nucleus (caudal medulla), projects to ipsilateral cerebellum

Dorsal (Posterior) Spinocerebellar Tract Spinal Cord Medulla Pons Midbrain To Cerebellum

Ventral (Anterior) Spinocerebellar Tract Supplements Dorsal Spinocerebellar Tract Information from more diverse array or receptors Originates from scattered cells in the intermediate grey caudal to L1 (which in turn have input from proprioceptive axons or their collaterals Crosses twice, to end up in ipsilateral cerebellum

Ventral (Anterior) Spinocerebellar Tract Spinal Cord Medulla Pons Midbrain

Spinothalamic Tract Pain and Temperature

Spinothalamic Tract First order neurons originate in pain receptors, enter cord at tract of Lissauer, and synapse in substantia gelatinosa or nucleus proprius Second order neurons cross at the anterior white commissure, rising 1 or 2 cord levels in the process, and form contralateral spinothalamic tract A third order neuron (not technically spinothalamic tract) projects to the cortex

Spinothalamic Tract

Spinothalamic Tract Spinal Cord Medulla Pons Midbrain

L1 L1 L2 L2 L3 L3 L4 L4 L5 L5

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