Recall, it is divided into hornsDorsal, lateral (only in thoracic region), and ventralDorsal half – sensory roots and ganglia Ventral half – motor rootsBased on the type of neurons/cell bodies located in each horn, it is specialized further into 4 regionsSomatic sensory (SS) - axons of somatic sensory neuronsVisceral sensory (VS) - axons of visceral sensory neur.Visceral motor (VM) - cell bodies of visceral motor neuronsSomatic motor (SM) - cell bodies of somatic motor neurons

1. SPINAL CORD

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.

3. Meninges
• Connective tissue membranes
– 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

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
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–
Myelinated axons
Divided into three columns
(funiculi)
•
•
•
Ventral
Dorsal
lateral
– Each of these divided into sensory
or motor tracts

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) - axons 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
• 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
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Some UMN decussate in pyramids = Lateral corticospinal tracts
Others decussate at other levels of s.c. = Anterior corticospinal tracts
• LMN (anterior horn motor neurons)
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–
Exits spinal cord via anterior root
Activates skeletal muscles
• Regulates fast and fine (skilled) movements

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

14. Extrapyramidal Motor Tracts
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•
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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
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–
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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
4. LMN exits via ventral root
5. LMN axon stimulates skeletal
muscle

16. Extrapyramidal (Multineuronal)
Pathways
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•
•
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