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ANATOMY & PHYSIOLOGY
Ch 12 & 13
Spinal Cord and Spinal Nerves
1
Spinal Cord
• This part of the central nervous system
(CNS) is enclosed in the vertebral cavity
• It s protected by:
– Vertebrae
– Meninges –
• dura, arachnoid and pia
• covered in previous lecture
– Cerebrospinal Fluid (CSF)
• Epidural space – space between the
vertebrae and the dural sheath (dura
mater)
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Spinal Cord
• The spinal cord extends inferiorly
from the medulla oblongata to the
lumbar region
• Along its length there are two
notably thicker regions: The
Cervical and Lumbosacral
enlargements
– These are sites where the
nerves serving the limbs emerge
• Conus medullaris – the end of the
spinal cord in the lumbar region
• Cauda equina – the tail-like
collection of spinal nerve roots
inferior to the conus medullaris
• Spinal nerves (PNS) will be covered
below.
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Spinal Cord Gray Matter
• Gray matter consists of cell bodies, unmyelinated processes,
and neuroglia
• Gray commissure – connects masses of gray matter on the left
and right
• The central canal runs through the gray commissure
• Posterior (dorsal) horns – interneurons (sensory)
• Anterior (ventral) horns – some interneurons but mostly somatic
motor neurons
• Lateral horns – contain autonomic nerve fibers
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Gray Matter and Spinal Roots
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Cross Section of Spinal Cord
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Reflex Arc
• The reflex arc is the simplest network of cells capable of
receiving a stimulus and producing a response
• A reflex is an automatic response to a stimulus that occurs
without conscious thought.
• Components of a reflex arc
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–
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–
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Sensory receptor is stimulated and transmits an action potential to
Sensory neuron. (in the case of pain reception 2-in-1 cell) ToInterneurons. ToMotor neuron. To7
Effector organ which responds with a reflex
Spinal Nerves
• 31 pairs of spinal nerves supply all of the body except head
• Each nerve is named according to its nearby vertebra
– 8 cervical (C1-C8)
– 12 thoracic (T1-T12)
– 5 Lumbar (L1-L5)
– 5 Sacral (S1-S5)
– 1 Coccygeal (C0)
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Spinal Nerves
• Each spinal nerve is formed from two roots
– Dorsal root – sensory roots and ganglia
– Ventral root – motor roots
• A dorsal root and a ventral root fuse laterally (on the side) to
form spinal nerves
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Spinal Nerves: Roots
• Each spinal nerve arises from the spinal cord via two roots
• Each root is formed from a set of rootlets that attach to the
spinal cord
• Ventral roots arise from the ventral horn of the spinal cord and
contain motor (efferent) fibers
• Dorsal roots arise from sensory neurons in the dorsal root
ganglia and contain sensory (afferent) fibers
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Spinal Nerves: Roots
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Spinal Nerve Rami
• Spinal nerves are actually quite short.
• Two spinal nerve roots give rise to each spinal nerve.
– The dorsal root carries afferent (sensory) neurons.
– The ventral root carries efferent (motor) neurons.
– So each spinal nerve is a mixed nerve carrying both sensory
and motor neurons.
• Each spinal nerve, in turn, gives rise to two spinal nerve rami
(singular: ramus).
– Both the dorsal and ventral spinal nerve rami carry both
sensory and motor neurons.
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Spinal Nerve: Rami
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Ventral Rami Form Nerve Plexuses
• All ventral rami except T2-T12 form interlacing networks
called plexuses (i.e. spaghetti bowls)
• Plexuses are found in the:
–
–
–
–
–
Cervical region
Brachial region
Lumbar region
Sacral region
The last two will be dealt with as the Lumbosacral plexus
• Each resulting branch of a plexus contains fibers from
several spinal nerves
• Each muscle receives a nerve supply from more than
one spinal nerve
• Damage to one spinal segment cannot completely
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paralyze a muscle
Cervical Plexus
The cervical plexus is formed by
ventral rami of C1-C4
Most branches are cutaneous
nerves of the neck, ear, back
of head, and shoulders
The most important nerve of this
plexus is the phrenic nerve .
The phrenic nerve is the major
motor and sensory nerve of
the diaphragm.
As long as an injury to the spinal
cord occurs inferior to the
emergence of the phrenic
nerve the ability to breathe
independently may be spared.
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Brachial Plexus
• Formed by C5-C8 and T1
– In some individuals C4 and T2 may also contribute to this plexus.
• The brachial plexus gives rise to nerves supplying the arm.
• The five major branches
of this plexus are:
– Axillary
– Radial
– Musculocutaneous
– Ulnar
– Median
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Brachial Plexus for PT and OT
• Additional detail is provided for
those headed to PT and OT
programs:
• Five ventral rami (C5-T1) form
• three trunks that separate into
six divisions then form
• three cords that give rise to:
• Branches/nerves
– Axillary
– Radial
– Musculocutaneous
– Ulnar
– Median
– Smaller nerves, too.
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Lumbosacral Plexus
(Lumbar and Sacral Plexuses)
• Arises from L1-S4 and innervates the thigh, abdominal
wall, psoas muscle, the buttocks, lower limb, pelvic
structures, and the perineum
• The major nerves are the
– Femoral (Lumbar)
– Obturator (Lumbar)
– Sciatic (Sacral)
• Tibial
• Common fibular (peroneal)
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Practice Questions
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Which of the following is NOT a nerve
plexus?
A. The thoracic rami
B. The lumbosacral plexus
C. The cervical plexus
D. The brachial plexus
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Which of the following nerves does NOT
arise from the brachial plexus?
A. The musculocutaneous nerve
B. The radial nerve
C. The axillary nerve
D. The ulnar nerve
E. The phrenic nerve
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The muscle fiber or gland cell that responds
to the efferent impulses is known as the
A. Effector
B. Motor neuron
C. Integration center
D. Sensory neuron
E. Receptor
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The basic pattern of the CNS in the spinal
cord is a central cavity surrounded by a
gray matter core and a white matter layer
lying outside of that.
True
False
24
Although the pairs of spinal nerves
correspond to the number of vertebrae in
the each area, there are eight pairs of
cervical cranial nerves, but only seven
cervical vertebrae.
True
False
25
Each muscle in a limb receives its nerve
supply from one spinal nerve.
True
False
26
A sensory neuron transmits afferent
impulses to the CNS.
True
False
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