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Chapter 13:
The Spinal Cord and Spinal
Nerves
Spinal Cord Anatomy
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Protective structures:
Vertebral column and the meninges provide
protect the spinal cord and provide physical
stability.
a. Dura mater, b. Arachnoid, c. Pia mater
Epidural space, subdural space and
subarachnoid space
Copyright 2009, John Wiley & Sons, Inc.
Spinal Cord Anatomy
External Anatomy of the Spinal Cord
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Two enlargements: cervical and lumbar
Conus medullaris
Filum terminale
Cauda equina
Posterior (dorsal root) & anterior(ventral) root
Posterior (dorsal root) ganglion
Spinal nerve
Copyright 2009, John Wiley & Sons, Inc.
External Anatomy of Spinal Cord
Internal Anatomy of the Spinal Cord
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Anterior median fissure
Posterior median sulcus
Gray and white commissures
Central canal
Anterior, posterior & lateral gray horns
Anterior, posterior & lateral white columns
Copyright 2009, John Wiley & Sons, Inc.
Internal Anatomy of Spinal Cord
Spinal Nerves
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31 pairs; mixed nerves.
Cervical (C1-C8), thoracic (T1-T12), lumbar
(L1-L5), sacral (S1-S5) and coccygeal.
Connective tissue coverings of spinal nerves:
 Epineurium, perineurium and endoneurium:
 Fascicles
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Spinal Nerves
Distribution of Spinal Nerves
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Spinal nerves branch and their braches are
called rami:
Posterior (dorsal) ramus
Anterior (ventral) ramus
Plexuses: a network of axons
Anterior rami except T1-T11 form plexuses.
Copyright 2009, John Wiley & Sons, Inc.
Cervical Plexus
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Formed by the anterior
rami of C1-C5.
Phrenic nervesimportant nerves from
the cervical plexuses.
Copyright 2009, John Wiley & Sons, Inc.
Brachial plexus
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Formed by the anterior
rami of C5-C8 & T1.
Supplies the shoulders
and upper limbs.
Roots → trunks →
divisions → cords →
nerves.
Copyright 2009, John Wiley & Sons, Inc.
Brachial plexus continued
Important nerves that
arise from the brachial
plexuses are
 Axillary nerve
 Musculocutaneous
nerve
 Radial nerve
 Median nerve
 Ulnar nerve
Copyright 2009, John Wiley & Sons, Inc.
Injuries to the Brachial Plexus
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Erb-Duchenne palsy
(waiter’s tip)- loss of
sensation along the
lateral side of the arm.
Wrist drop- inability to
extend the wrist and
fingers.
Copyright 2009, John Wiley & Sons, Inc.
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Median nerve palsy- numbness, tingling and
pain in the palm and fingers.
Ulnar nerve palsy- inability to abduct or
adduct fingers
Winged scapula- the arm cannot be abducted
beyond the horizontal position.
Copyright 2009, John Wiley & Sons, Inc.
Injuries to the Brachial Plexus
Lumbar Plexus
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Formed by the anterior
rami of L1-L4.
Supplies the
anterolateral abdominal
wall, external genitals,
and part of the lower
limbs.
Femoral nerves,
obturator nerves.
Copyright 2009, John Wiley & Sons, Inc.
Sacral Plexus
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Formed by the anterior
rami of L4-L5 and S1S4.
Supplies the buttocks,
perineum, and lower
limbs.
Gives rise to the largest
nerve in the body- the
sciatic nerve.
Copyright 2009, John Wiley & Sons, Inc.
Distribution of Nerves from the
Lumbar and Sacral Plexuses
Copyright 2009, John Wiley & Sons, Inc.
Coccygeal Plexus
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Formed by the anterior
rami of S4-S5 and the
coccygeal nerves.
Supplies a small area
of skin in the coccygeal
region.
Copyright 2009, John Wiley & Sons, Inc.
Dermatome
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Dermatome is the
area of the skin that
provides sensory
input to the CNS via
one pair of spinal
nerves or the
trigeminal nerve.
Copyright 2009, John Wiley & Sons, Inc.
Sensory and Motor Tracts
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The name of the tract
often indicates its
location in the white
matter and where it
begins and ends.
The white matter
contains both sensory
and motor tracts.
Copyright 2009, John Wiley & Sons, Inc.
Reflex
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A reflex is an automatic, sudden, involuntary
response to a stimulus.
When the integration takes place in the spinal
cord, the reflex is a spinal reflex.
Copyright 2009, John Wiley & Sons, Inc.
Reflex Arc
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The pathway followed by nerve impulses that
produce a reflex is a reflex arc.
A reflex arc includes:
a. sensory receptor
b. sensory neuron
c. integrating center
d. motor neuron
e. effector
Copyright 2009, John Wiley & Sons, Inc.
Reflex Arc
2 SENSORY NEURON
(axon conducts impulses from
receptor to integrating center)
1 SENSORY RECEPTOR
(responds to a stimulus
by producing a generator
or receptor potential)
Interneuron
3 INTEGRATING CENTER
(one or more regions within the CNS
that relay impulses from sensory to
motor neurons)
4 MOTOR NEURON
(axon conducts impulses from
integrating center to effector)
5 EFFECTOR
(muscle or gland that
responds to motor
nerve impulses)
The Stretch Reflex
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Causes contraction of a skeletal muscle in
response to stretching of the muscle.
Monosynaptic reflex.
Patellar or knee-jerk reflex: Stretching of a
muscle →activation of muscle spindles
→sensory neuron →spinal cord→motor
neuron → muscle contraction.
Ipsilateral.
Copyright 2009, John Wiley & Sons, Inc.
Stretch Reflex
To brain
1 Stretching stimulates
SENSORY RECEPTOR
(muscle spindle)
+
2 SENSORY
NEURON
excited
EFFECTOR
+
5 (same muscle)
contracts and
relieves the
stretching
–
4 MOTOR
NEURON
excited
Spinal
Nerve
+
3 Within INTEGRATING
CENTER (spinal cord),
sensory neuron activates
motor neuron
Antagonistic
muscles relax
Motor neuron to
antagonistic muscles
is inhibited
Inhibitory
interneuron
The Tendon Reflex
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Polysynaptic reflex.
Control muscle tension by causing muscle
relaxation when muscle tension is great.
Sensory receptors- Golgi tendon organs.
↑ Tension applied to the tendon → tendon
organ stimulation → nerve impulse → spinal
cord →motor neuron causes muscle
relaxation and relieves tension.
Copyright 2009, John Wiley & Sons, Inc.
Tendon Reflex
To brain
Inhibitory
interneuron
5 EFFECTOR
(muscle attached
to same tendon)
relaxes and
relieves excess
tension
4 MOTOR NEURON
inhibited
+
++
2 SENSORY
NEURON
excited
–
Increased tension
stimulates
1 SENSORY
RECEPTOR (tendon)
+
Spinal
nerve
3 Within INTEGRATING
+
Antagonistic
muscles
contract
CENTER (spinal cord),
sensory neuron activates
inhibitory interneuron
Motor neuron to
antagonistic
muscles is excited
Excitatory
interneuron
Flexor (Withdrawal) Reflex
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Polysynaptic reflex
Ipsilateral.
Stepping on a tack (stimulus) → nerve
impulse → activation of the interneuron →
activation of the motor neuron →muscle
contraction →withdrawal of the leg.
Copyright 2009, John Wiley & Sons, Inc.
Flexor (Withdrawal) Reflex
+
Spinal
nerve
+
4 MOTOR
NEURON
excited
Ascending
interneuron
+
+
Interneuron
+
+
5 EFFECTORS
(flexor muscles)
contract and
withdraw leg
Descending
interneuron
+
+
4 MOTOR
NEURONS
excited
+
+
3 Within INTEGRATING CENTER
2 SENSORY
NEURON
excited
(spinal cord), sensory neuron
activates interneurons in several
spinal cord segments
1 Stepping on tack stimulates
SENSORY RECEPTOR (dendrites
of pain-sensitive neuron)
Crossed Extensor Reflex
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Polysynaptic reflex.
Contralateral reflex.
Contraction of muscles that extend joints in
the opposite limb in response to a painful
stimulus.
Stepping on a tack (stimulus) → nerve
impulse →activation of several interneurons
→ activation of the motor neurons → muscle
contraction causing flexion of the leg stepping
on a tack & extension on the opposite side.
Copyright 2009, John Wiley & Sons, Inc.
Crossed Extensor Reflex
Copyright 2009, John Wiley & Sons, Inc.
+
+
+
Spinal
nerve
+
Ascending
interneurons
4 MOTOR
NEURONS
excited
5 EFFECTORS
(extensor muscles)
contract, and extend
left leg
+
+
Flexor muscles
contract and withdrawright leg
Interneurons
from other side
+
+
+
Descending
interneurons
+
+
+
+
+
+
4 MOTOR
NEURONS
excited
+
+
3 Within INTEGRATING CENTER
2 SENSORY
(spinal cord), sensory neuron
activates several interneurons
NEURON
excited
1 Stepping on a tack
Withdrawal of right leg
(flexor reflex)
stimulates SENSORY
RECEPTOR (dendrites of
pain-sensitive neuron) in
right foot
Extension of left leg
(crossed extensor reflex)
End of Chapter 13
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