1. AP 150
Chapter 12
Spinal Cord Anatomy

2. Spinal Cord Runs through the vertebral canal
Extends from foramen magnum to second lumbar vertebra
Regions
Cervical
Thoracic
Lumbar
Sacral
Coccygeal
Gives rise to 31 pairs of spinal nerves
All are mixed nerves
Not uniform in diameter
Cervical enlargement: supplies upper limbs
Lumbar enlargement: supplies lower limbs
Conus medullaris- tapered inferior end
Ends between L1 and L2
Cauda equina - origin of spinal nerves extending inferiorly from conus medullaris.
Spinal Cord

3. Meninges Connective tissue membranes Spaces
Dura mater: outermost layer; continuous with epineurium of the spinal nerves
Arachnoid mater: thin and wispy
Pia mater: bound tightly to surface
Forms the filum terminale
anchors spinal cord to coccyx
Forms the denticulate ligaments that attach the spinal cord to the dura
Spaces
Epidural: external to the dura
Anesthestics injected here
Fat-fill
Subdural space: serous fluid
Subarachnoid: between pia and arachnoid
Filled with CSF
Meninges

4. Cross Section of Spinal Cord
Anterior median fissure and posterior median sulcus
deep clefts partially separating left and right halves
Gray matter: neuron cell bodies, dendrites, axons
Divided into horns
Posterior (dorsal) horn
Anterior (ventral) horn
Lateral horn
White matter
Myelinated axons
Divided into three columns (funiculi)
Ventral
Dorsal
lateral
Each of these divided into sensory or motor tracts
Cross Section of Spinal Cord

5. Cross section of Spinal Cord
Commissures: connections between left and right halves
Gray with central canal in the center
White
Roots
Spinal nerves arise as rootlets then combine to form dorsal and ventral roots
Dorsal and ventral roots merge laterally and form the spinal nerve

6. Organization of Spinal Cord Gray Matter
Recall, it is divided into horns
Dorsal, lateral (only in thoracic region), and ventral
Dorsal half – sensory roots and ganglia
Ventral half – motor roots
Based on the type of neurons/cell bodies located in each horn, it is specialized further into 4 regions
Somatic sensory (SS) - axons of somatic sensory neurons
Visceral sensory (VS) - neurons of visceral sensory neur.
Visceral motor (VM) - cell bodies of visceral motor neurons
Somatic motor (SM) - cell bodies of somatic motor neurons

7. Gray Matter: Organization
Figure 12.31

8. White Matter in the Spinal Cord
Divided into three funiculi (columns) – posterior, lateral, and anterior
Columns contain 3 different types of fibers (Ascend., Descend., Trans.)
Fibers run in three directions
Ascending fibers - compose the sensory tracts
Descending fibers - compose the motor tracts
Commissural (transverse) fibers - connect opposite sides of cord

9. White Matter Fiber Tract Generalizations
Pathways decussate (most)
Most consist of a chain of two or three neurons
Most exhibit somatotopy (precise spatial relationships)
All pathways are paired
one on each side of the spinal cord

10. White Matter: Pathway Generalizations

11. Descending (Motor) Pathways
Descending tracts deliver motor instructions from the brain to the spinal cord
Divided into two groups
Pyramidal, or corticospinal, tracts
Indirect pathways, essentially all others
Motor pathways involve two neurons
Upper motor neuron (UMN)
Lower motor neuron (LMN)
aka ‘anterior horn motor neuron” (also, final common pathway)

12. Pyramidal (Corticospinal) Tracts
Originate in the precentral gyrus of brain (aka, primary motor area)
I.e., cell body of the UMN located in precentral gyrus
Pyramidal neuron is the UMN
Its axon forms the corticospinal tract
UMN synapses in the anterior horn with LMN
Some UMN decussate in pyramids = Lateral corticospinal tracts
Others decussate at other levels of s.c. = Anterior corticospinal tracts
LMN (anterior horn motor neurons)
Exits spinal cord via anterior root
Activates skeletal muscles
Regulates fast and fine (skilled) movements

13. Corticospinal tracts Location of UMN cell body in cerebral cortex
Decussation of UMN axon in pyramids or at level of exit of LMN
Synapse of UMN and LMN occurs in anterior horn of s.c.
LMN axon exits via anterior root

14. Extrapyramidal Motor Tracts
Includes all motor pathways not part of the pyramidal system
Upper motor neuron (UMN) originates in nuclei deep in cerebrum (not in cerebral cortex)
UMN does not pass through the pyramids!
LMN is an anterior horn motor neuron
This system includes
Rubrospinal
Vestibulospinal
Reticulospinal
Tectospinal tracts
Regulate:
Axial muscles that maintain balance and posture
Muscles controlling coarse movements of the proximal portions of limbs
Head, neck, and eye movement

15. Extrapyramidal Tract
Note: 1. UMN cell body location 2. UMN axon decussates in pons 3. Synapse between UMN and LMN
occurs in anterior horn of sc
3. LMN exits via ventral root
4. LMN axon stimulates skeletal
muscle

16. Extrapyramidal (Multineuronal) Pathways
Reticulospinal tracts – originates at reticular formation of brain; maintain balance
Rubrospinal tracts – originate in ‘red nucleus’ of midbrain; control flexor muscles
Tectospinal tracts - originate in superior colliculi and mediate head and eye movements towards visual targets (flash of light)

17. Main Ascending Pathways
The central processes of first-order neurons branch diffusely as they enter the spinal cord and medulla
Some branches take part in spinal cord reflexes
Others synapse with second-order neurons in the cord and medullary nuclei

18. Three Ascending Pathways
The nonspecific and specific ascending pathways send impulses to the sensory cortex
These pathways are responsible for discriminative touch (2 pt. discrimination) and conscious proprioception (body position sense).
The spinocerebellar tracts send impulses to the cerebellum and do not contribute to sensory perception

19. Nonspecific Ascending Pathway
Include the lateral and anterior spinothalamic tracts
Lateral: transmits impulses concerned with pain and temp. to opposite side of brain
Anterior: transmits impulses concerned with crude touch and pressure to opposite side of brain
1st order neuron: sensory neuron
2nd order neuron: interneurons of dorsal horn; synapse with 3rd order neuron in thalamus
3rd order neuron: carry impulse from thalamus to postcentral gyrus

20. Specific and Posterior Spinocerebellar Tracts
Dorsal Column Tract
1. AKA Medial lemniscal pathway
2. Fibers run only in dorsal column
3. Transmit impulses from receptors in
skin and joints
4. Detect discriminative touch and
body position sense =proprioception
1st O.N.- a sensory neuron
synapses with 2nd O.N. in nucleus gracilis and nucleus cuneatus of medulla
2nd O.N.- an interneuron
decussate and ascend to thalamus where it synapses with 3rd O.N.
3rd-order (thalamic neurons)
transmits impulse to somato-
sensory cortex (postcentral gyrus)
Spinocerebellar Tract
Transmit info. about trunk and lower limb muscles and tendons to cerebellum
No conscious sensation

21. Spinal Cord Trauma and Disorders
Severe damage to ventral root results in flaccid paralysis (limp and unresponsive)
Skeletal muscles cannot move either voluntarily or involuntarily
Without stimulation, muscles atrophy.
When only UMN of primary motor cortex is damaged
spastic paralysis occurs - muscles affected by persistent spasms and
exaggerated tendon reflexes
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)

22. 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 anterior horn motor neurons
Victims die from paralysis of respiratory muscles
Virus enters body in feces-contaminated water (public swimming pools)