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Visual Anatomy & Physiology
First Edition
Martini & Ober
Chapter 11
Spinal Cord, Spinal Nerves, and Spinal Reflexes
Lecture 19
1
Divisions of the Nervous System
Figure from: Hole’s Human A&P, 12th edition, 2010
You are
here
CNS
PNS
2
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
3
Meninges of the Spinal Cord
Figures from: Saladin,
Anatomy & Physiology,
McGraw Hill, 2007
Space between the
dura mater and the
vertebral body is
called the epidural
space
4
Cross Section of Spinal Cord
Figure from: Hole’s Human A&P, 12th edition, 2010
The spinal cord…
• is a center for spinal reflexes
• aids in locomotion
• is a conduit for nerve impulses to and from
the brain
5
Organization of Spinal Gray Matter
You should know the
major areas of gray
matter of within the
spinal cord:
Posterior = sensory
Lateral = visceral
motor
Anterior = somatic
motor
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
6
Organization of Spinal White Matter
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
7
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?
8
Figure from: Hole’s Human A&P, 12th edition, 2010
Ascending Tracts
Figure from: Hole’s Human A&P, 12th edition, 2010
• fasciculus cuneatus/gracilis
- fine touch, pressure, body
movement
- cross (decussate) in
medulla
3
• 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)
3
2nd order neuron – from
spinal cord to thalamus
3rd order neuron – from
thalamus to sensory cerebral
cortex - terminate in the
cerebral cortex
2
1
Decussation
10
Figure from: Hole’s Human A&P, 12th edition, 2010
Descending Tracts
• corticospinal (direct, pyramidal)
- voluntary movement of
skeletal muscles
- lateral cross in medulla
- contralateral
Figure from: Hole’s Human A&P, 12th edition, 2010
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
11
Peripheral Nervous System
Figure from: Hole’s Human A&P, 12th edition, 2010
You are
here
CNS
PNS
12
Figure from: Hole’s Human A&P, 12th edition, 2010
Bundle of
nerve fibers
(axons)
Structure of a
Peripheral Nerve
Epineurium –
surrounds entire
nerve
Perineurium –
surrounds a bundle
of nerve fibers =
fascicle
Endoneurium –
surrounds each
axon (nerve fiber)
Similar to the
naming of the CT
around muscle!!
13
Classification of Nerve Fibers
SAME
Sensory = Afferent
Motor = Efferent
SOMAtic
- Skin
- BOnes
- Muscles
- Articulations
Table from: Saladin, Anatomy & Physiology, McGraw Hill, 2007
14
Spinal Nerves
Figure from: Hole’s Human A&P, 12th edition, 2010
Nerves = spinal nerves
These are ‘mixed’
nerves (sensory and
motor nerve fibers)
Spinal nerves are named according to the level of the spinal
cord from which they exit.
15
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 – Somatic Motor Fibers
Ventral root - axons
of motor neurons
whose cell bodies are
in anterior horn of
spinal cord
*
*
Ventral ramus – supply
ventrolateral body
surface, body wall, and
limbs
Dorsal ramus – skin
and skeletal muscles of
the back
Figure from: Martini, Fundamentals of Anatomy & Physiology, Pearson Education, 2004
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Spinal Nerves – Somatic Sensory Fibers
Dorsal root
- axons of sensory
neurons whose cell
bodies are in the
dorsal root ganglion
*
*
Dorsal root ganglion
- cell bodies of
sensory neurons
Figure from: Martini, Fundamentals of Anatomy & Physiology, Pearson Education, 2004
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Were you paying attention?
1
2
10
9
3
4
11
8
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Dermatomes
• specific areas of
skin that the
sensory (general
somatic afferent)
nerve fibers of a
particular spinal
nerve innervate
The
Dermatome
Map
20
Figure from: Hole’s Human A&P, 12th edition, 2010
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
21
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
22
Figure from: Hole’s Human A&P, 12th edition, 2010
Lumbosacral Plexus
Figure from: Hole’s Human A&P, 12th edition, 2010
• 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, and coccygeal plexuses
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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
What form of feedback
would a reflex be?
Figure from: Hole’s Human A&P, 12th edition, 2010
24
Knee-jerk Reflex (Ipsilateral)
• helps maintain posture
Monosynaptic
Figure from: Hole’s Human A&P, 12th edition, 2010
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Withdrawal Reflex (Ipsilateral)
• protective
Polysynaptic
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Figure from: Hole’s Human A&P, 12th edition, 2010
Crossed-Extensor Reflex (Contralateral)
• flexor muscles contract
• flexor muscles on opposite side inhibited
• extensor muscles on opposite side contract for balance
Polysynaptic
Figure from: Hole’s Human A&P, 12th edition, 2010
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Review
• The spinal cord is a
– Center for spinal reflexes
– Conduit for nerve impulses to and from the
brain
• Reflexes are automatic, subconscious
responses to stimuli
• Some spinal reflexes include
– Knee-jerk
– Withdrawal
– Cross-extensor reflex
28
Review
Spinal cord contains
nerve tracts
Ascending = sensory
Descending = motor
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Review
• The peripheral nervous system (PNS) consists
of
– Cranial nerves
– Spinal nerves
• The PNS can be divided into two systems
– Sensory
– Motor
• Somatic
• Autonomic
Sensory afferent
SAME
(…because these sound the same)
Motor efferent
30
Review
• Nerves are bundles of axons surrounded by
several layers of CT
• Nerves can be classified by
– Whether they are special (smell, sight, taste,
equilibrium, and hearing) or general (everything
else except special)
– Whether they are part of the somatic or visceral NS
– The types of impulses they conduct
• Sensory (afferent)
• Motor (efferent)
• both (mixed)
31
Review
• There are 31 pairs of spinal nerves
– 8 C, 12 T, 5 L, 5 S, 1 Co
• A spinal nerve is a mixed nerve formed by the
junction of nerves from the
– Dorsal root (sensory)
– Ventral root (motor)
• Somatic
• Autonomic
Doris got kicked in the behind and
screamed
The motor is in the front
(anterior, ventral) of the car
• A dermatome is an area of skin that the
sensory nerve fibers of a particular spinal
nerve innervate
33
Review
• A Nerve plexus is a complex network of
nerves
– formed by anterior branches of spinal nerves
– fibers of various spinal nerves are sorted and
recombined
– There are 3 nerve plexuses
• Cervical (neck); C1-C4
• Brachial (shoulder and upper limbs); C5-T1
• Lumbosacral (pelvis and lower limbs); T12-S5
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