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Marieb’s Human
Anatomy and Physiology
Ninth Edition
Marieb w Hoehn
Chapter 12
The Central Nervous System
Spinal Cord
Reflexes
Plexuses
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Lecture Overview
• The spinal cord
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Spinal cord structure
Spinal meninges
Cross-sectional anatomy of the spinal cord
Ascending and descending spinal tracts
Reflexes
Spinal Nerves
Nerve plexuses
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Spinal Cord Structure
• extends from the
foramen magnum to 2nd
lumbar vertebra
• cervical and lumbar
enlargements
• cauda equina (horse’s
tail) – thin nerve fibers
that exit at different
level than they arise
(note that spinal cord
does not extend into this
area of the lumbar
spine). Begins around
L2 and extends to S5.
Good area for lumbar
puncture and collection
of CSF.
Figure from: Saladin,
Anatomy & Physiology,
McGraw Hill, 2007
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Overview of the Spinal Cord
What type of vertebra is
shown above?
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Meninges of the Spinal Cord
Figure from: Marieb
Human Anatomy &
Physiology, Pearson 2013
- dura mater – outer, tough (anchoring dural folds)
- Subdural space – like interstitial fluid
- arachnoid mater – web-like
- Subarachnoid space – CSF
- pia mater – inner, delicate
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Organization of Spinal Cord
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
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Organization of Spinal Gray Matter
Figure from: Martini,
Anatomy & Physiology,
Prentice Hall, 2001
Posterior gray horn =
sensory
Lateral gray horn =
visceral motor
Anterior gray horn =
somatic motor
Anterior root
Posterior root
Gray matter = dendrites and unmyelinated axons
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Spinal Cord and Nerve Roots
Ventral root - axons of motor neurons whose cell bodies are in spinal cord
Dorsal root - axons of sensory neurons in the dorsal root ganglion
Dorsal root ganglion - cell bodies of sensory neurons
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Organization of Spinal White Matter
Figure from: Martini,
Anatomy & Physiology,
Prentice Hall, 2001
White matter = Myelinated axons
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Functions of the Spinal Cord
The spinal cord…
a. is a conduit for nerve impulses to and from the brain (nerve tracts)
b. is a center for spinal reflexes
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Tracts of the Spinal Cord
• Ascending tracts conduct sensory impulses to the brain
• Descending tracts conduct motor impulses from the brain to
motor neurons reaching muscles and glands
All the axons in a tract
share a common origin
and destination
Tracts are usually
named for their place
of origin (1st) and
termination (2nd)
Most axons cross over
during their travel.
What will this mean
clinically?
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Ascending Tracts
• fasciculus cuneatus/gracilis
- fine touch, pressure, body
movement
- cross (decussate) in
medulla
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• spinothalamic
- crude pain, temperature,
pressure, and touch
- cross in spinal cord
• spinocerebellar
- subconscious coordination of
muscle movements
(1st and 2nd order neurons)
- ipsilateral
Decussation
(crossing over)
2
1
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1st, 2nd, and 3rd Order Sensory Neurons
1st order neuron – from
receptor to the spinal cord
(cell bodies are located in the
dorsal root ganglion)
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2nd order neuron – from
spinal cord to thalamus
3rd order neuron – from
thalamus to sensory cerebral
cortex - terminate in the
cerebral cortex
Decussation
2
1
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Descending Tracts
• corticospinal (direct, pyramidal)
- voluntary movement of
skeletal muscles
- lateral cross in medulla
- contralateral
Upper motor
– begin in
precentral
gyrus of
cortex
• reticulospinal (indirect,
extrapyramidal)
- subconscious muscle tone,
sweat glands
- some lateral cross, anterior do
not cross
• rubrospinal (indirect,
extrapyramidal)
- subconscious regulation of
upper limb tone/movement
- cross in brain (less important in
humans)
Decussation
Lower
Upper MN – Cerebral cortex to spinal cord
Lower MN – Spinal cord to effector
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Somatic Reflex Arcs
Reflexes – automatic, subconscious, quick, stereotyped
responses to stimuli either within or outside the body
They occur in both the somatic
and autonomic divisions
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Knee-jerk (Stretch) Reflex
• helps maintain posture
Monosynaptic,
Ipsilateral
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Withdrawal Reflex
• protective
Polysynaptic,
Ipsilateral,
Intersegmental
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Crossed-Extensor Reflex
• flexor muscles contract
• flexor muscles on opposite side inhibited
• extensor muscles on opposite side contract for balance
Polysynaptic,
Contralaterial,
Intersegmental
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Spinal Nerves
• mixed nerves
• 31 pairs
• 8 cervical (C1 to C8)
• 12 thoracic (T1 to T12)
• 5 lumbar (L1 to L5)
• 5 sacral (S1 to S5)
• 1 coccygeal (Co)
THIRTY ONEderful flavors
of spinal nerves!
Below cervical spine, each
spinal nerve leaves inferior
to the same numbered
vertebra
Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007
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Spinal Nerves
These are ‘mixed’
nerves (sensory and
motor nerve fibers)
Ventral (anterior)
ramus leads to
formation of
plexuses
Spinal nerves are named according to the level of the spinal
cord from which they exit.
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Cervical Plexus
Nerve plexus – complex network formed by anterior (ventral) branches of
spinal nerves; fibers of various spinal nerves are sorted and recombined
Contains both
sensory and motor
fibers
Cervical Plexus
• C1-C4
• lies deep in the neck
• supplies muscles and
skin of the neck
• contributes to phrenic
nerve (diaphragm)
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
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Brachial Plexus
• C5-T1
• lies deep within shoulders
• supplies shoulder and upper
limbs
• musculocutaneous nerves
• supply muscles of anterior arms
and skin of forearms
• ulnar nerves
• supply muscles of forearms and
hands
• supply skin of hands
• radial nerves
• supply posterior muscles of
arms and skin of forearms and
hands
• axillary nerves
• supply muscles and skin of
superior, lateral, and posterior
arms
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Lumbosacral Plexus
• T12 – S5
• supplies pelvis and lower
limbs
• extend from lumbar
region into pelvic cavity
• obturator nerves
• supply adductors of
thighs
• femoral nerves
• supply muscles and skin
of thighs and legs
• sciatic nerves
• supply muscles and skin
of thighs, legs, and feet
May be separated into lumbar, sacral, pudendal, and coccygeal plexuses
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Classification of Nerve Fibers
SAME
Sensory = Afferent
Motor = Efferent
SOMAtic
- Skin
- BOnes
- Muscles
- Articulations
Table from: Saladin, Anatomy & Physiology, McGraw Hill, 2007
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