Organization of the Nervous System
A. Central nervous system (CNS):
a. Brain (intracranial part).
b. Spinal cord (extracranial part).
B. Peripheral nervous system (PNS):
1. Somatic (cranio-spinal) NS:
a. Cranial nerves: 12 pairs
attached to the brain.
b. Spinal nerves: 31 attached to
the spinal cord.
2. Autonomic NS:
a. Sympathetic NS.
b. Parasympathetic NS
Spinal Cord
•
•
•
Runs through the vertebral canal
Extends from foramen magnum to
second lumbar vertebra
Regions
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•
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.
• Connective tissue membranes
Meninges
– 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
Cross
Section of
Spinal Cord
•
Anterior median fissure and
posterior median sulcus
– deep clefts partially separating left and
right halves
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•
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
•
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
Blood supply of the spinal cord
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
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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
Gray Matter: Organization
Figure 12.31
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
White Matter: Pathway
Generalizations
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)
• Also known as ‘anterior horn motor neuron”
(also, final common pathway)
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
Corticospinal
tracts
1.
2.
3.
4.
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
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
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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
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 S.C
3. LMN exits via ventral root
4. LMN axon stimulates skeletal
muscle
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)
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
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
Nonspecific Ascending Pathway
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•
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•
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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
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 somatosensory cortex (postcentral gyrus)
Spinocerebellar Tract
• Transmit info. about trunk and lower
limb muscles and tendons to cerebellum
• No conscious sensation
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)