Anatomy and Physiology I
Chapter 13
The Spinal Cord, Spinal Nerves,
and Somatic Reflexes
Spinal Cord Functions
• Conduction
– Bundles of nerve fibers
• conduct info up and down the cord
– Sensory info reaches brain
– Motor commands reach the effectors
• Locomotion
– Walking
– Central pattern generators- produce sequence of
walking
• Reflexes
– Involuntary, stereotyped responses to stimuli
Spinal Cord
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Cylinder of nervous tissue
Arises from brainstem
L1
31 pairs of spinal nerves
Divided into cervical,
thoracic, lumbar, and sacral
regions
The Meninges
• 3 fibrous connective tissue
membranes
– Dura mater
• Forms loose sleeve- dura sheath
• Epidural space- between
vertebrae and sheath
– Arachnoid mater
• Loose mesh of fibers
• Subarachnoid space- between
arachnoid membrane and
pia mater
– Filled with CSF
– Pia mater
• Delicate membrane
• Follows contours of spinal cord
Spinal Cord
• Consists of 2 kinds of nervous tissue
– Gray matter
• Dull color
• Little myelin
– White matter
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Bright, pearly white
Abundance of myelin
Surrounds gray matter
Composed of tracts
– Carry signals from one part of the CNS to another
Gray Matter
• Central core- butterfly,
H shaped
• 2 dorsal horns
• 2 ventral horns
• Gray commissureconnects R and L side
– Central canal
Gray Matter
• Branches into ventral and
dorsal roots
• Dorsal root- carries
sensory nerve fibers to
the spinal cord
– Enter posterior horn
• Ventral root- motor nerve
fibers exit and go to
skeletal muscles
Spinal Tracts
• Ascending tracts
– carry sensory information up the
spinal cord
to the brain
• Descending tracts
– conduct motor impulses down
the brain stem
and the spinal cord
• Muscles or organs
• Tracts cross- decussation
Spinal Nerves
• Spinal cord communicates with body
– Spinal nerves
• Nerve
– Composed of numerous nerve fibers (axons)
bound together by connective tissue
• PNS nerve fibers enclosed by Schwann cells
Spinal Nerves
• Endoneurium- thin layer
of connective tissue
– surrounds nerve fiber
• Fascicles- bundles of
nerve fibers
• Perineurium- sheath
– surrounds fascicle
• Epineurium- surrounds
several bundles of
fascicles
– Whole nerve
– Protects nerve
Peripheral Nerve Fibers
• Sensory fibers- afferent fibers
– Signals from sensory receptors to CNS
• Motor fibers- efferent fibers
– Signals from CNS to muscles and glands
Spinal Nerves
• Sensory nerves- afferent fibers
• Motor nerves- efferent fibers
• Mixed nerve- both afferent and efferent fibers
– Conducts signals in 2 directions
• Ganglion (knot)- cluster of neurosomas
outside the CNS
Spinal Nerves
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31 pairs
8 cervical
12 thoracic
5 lumbar
5 sacral
1 coccygeal
Nerve Plexus
• Nerves merge to form web-like nerve plexus
• Cervical plexus
• Brachial plexus
• Lumbar plexus
• Sacral plexus and Coccygeal plexus
Dermatomes
• Specific area of the skin the
sends sensory information to
the spinal cord
• Spinal nerves overlap
– Need to severe 3 successive
spinal nerves to lose sensation
in one dermatome area
• Spinal nerve damage
assessed with pin pricks
Reflexes
• Quick, involuntary, stereotyped reactions of glands or
muscles to stimulation
• 1. Require stimulation- not spontaneous reactions
• 2. Quick- minimum synaptic delay
• 3. Involuntary- difficult to suppress
– Occur w/out intent and awareness
• 4. Stereotyped- same way every time
Somatic Reflex
• Involuntary contraction of skeletal muscle
• Employs a reflex arc- signal travels along a pathway
– 1. Somatic receptors
• Skin, muscle, tendon
– 2. Afferent nerve fibers
• Carry info into dorsal horn of spinal cord
– 3.Integrating center
• interneurons
– 4. Efferent nerves
• Carry info from ventral horn out to the skeletal muscle
– 5. Skeletal muscles
• Effectors that carry out response
Muscle Spindle
• Stretch receptors imbedded in skeletal muscle
– Proprioceptors- sense organs specialized to monitor
body position and movement of body parts
• Function: inform brain of muscle length and
movement
– Brain sends motor commands back to muscles
The Stretch Reflex
• When muscle is stretched
– “fights back”
– Contracts
– Feels stiffer
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This helps maintain equilibrium and posture
Extension creates stretch reflex in flexors
Flexion creates stretch reflex in extensors
Stretch reflexes stabilize joints and coordinate
vigorous and precise movements
The Tendon Reflex
• Reflexive contraction of muscle when tendon is
tapped
• Depends on Reciprocal inhibition
– Prevents muscles from working against each other- inhibits
antagonist
The Withdrawal (Flexor) Reflex
• Quick contraction of flexor
muscles
– resulting in the withdrawal
of a limb from an injurious
stimulus
• Contraction of flexors and
relaxation of extensors
The Crossed Extension Reflex
• Contraction of extensor
muscles in the limp
opposite from the one
withdrawn
• Maintains balance,
prevents falling
• Shift in center of gravity