1. Chapter 13: The Spinal Cord and Spinal Nerves

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Spinal Cord Anatomy
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
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4. Spinal Cord Anatomy

5. External Anatomy of the Spinal Cord
Two enlargements: cervical and lumbar
Conus medullaris
Filum terminale
Cauda equina
Posterior (dorsal root) & anterior(ventral) root
Posterior (dorsal root) ganglion
Spinal nerve
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6. External Anatomy of Spinal Cord

7. Internal Anatomy of the Spinal Cord
Anterior median fissure
Posterior median sulcus
Gray and white commissures
Central canal
Anterior, posterior & lateral gray horns
Anterior, posterior & lateral white columns
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8. Internal Anatomy of Spinal Cord

10. Spinal Nerves

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Spinal Nerves
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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12. Distribution of Spinal Nerves
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.
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Cervical Plexus
Formed by the anterior rami of C1-C5.
Phrenic nerves- important nerves from the cervical plexuses.
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Brachial plexus
Formed by the anterior rami of C5-C8 & T1.
Supplies the shoulders and upper limbs.
Roots → trunks → divisions → cords → nerves.
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Brachial plexus
Important nerves that arise from the brachial plexuses are
Axillary nerve
Musculocutaneous nerve
Radial nerve
Median nerve
Ulnar nerve
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16. Injuries to the Brachial Plexus
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.

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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.
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18. Injuries to the Brachial Plexus

19. Lumbar Plexus 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.

20. Sacral Plexus Formed by the anterior rami of L4-L5 and S1-S4.
Supplies the buttocks, perineum, and lower limbs.
Gives rise to the largest nerve in the body- the sciatic nerve.

21. Distribution of Nerves from the Lumbar and Sacral Plexuses
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Coccygeal Plexus
Formed by the anterior rami of S4-S5 and the coccygeal nerves.
Supplies a small area of skin in the coccygeal region.
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23. Dermatome
Dermatome is the area of the skin that provides sensory input to the CNS via one pair of spinal nerves or the trigeminal nerve.

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Reflex
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.
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Reflex Arc
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
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27. Reflex Arc 1 2 3 4 5 1 2 3 4 1 2 3 1 2 1 SENSORY NEURON
(axon conducts impulses from
receptor to integrating center)
SENSORY RECEPTOR
(responds to a stimulus
by producing a generator
or receptor potential)
INTEGRATING CENTER
(one or more regions within the CNS
that relay impulses from sensory to
motor neurons)
MOTOR NEURON
integrating center to effector)
EFFECTOR
(muscle or gland that
responds to motor
nerve impulses)
Interneuron 2 3 4 5 1
SENSORY NEURON
(axon conducts impulses from
receptor to integrating center)
SENSORY RECEPTOR
(responds to a stimulus
by producing a generator
or receptor potential)
INTEGRATING CENTER
(one or more regions within the CNS
that relay impulses from sensory to
motor neurons)
MOTOR NEURON
integrating center to effector)
Interneuron 2 3 4 1
SENSORY NEURON
(axon conducts impulses from
receptor to integrating center)
SENSORY RECEPTOR
(responds to a stimulus
by producing a generator
or receptor potential)
INTEGRATING CENTER
(one or more regions within the CNS
that relay impulses from sensory to
motor neurons)
Interneuron 2 3 1
SENSORY NEURON
(axon conducts impulses from
receptor to integrating center)
SENSORY RECEPTOR
(responds to a stimulus
by producing a generator
or receptor potential) 2 1
SENSORY RECEPTOR
(responds to a stimulus
by producing a generator
or receptor potential)

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The Stretch Reflex
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.
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29. Stretch Reflex

30. 1 2 3 4 5 1 2 3 4 1 2 3 1 2 1 Stretching stimulates
SENSORY RECEPTOR (muscle spindle)
SENSORY
NEURON
excited
MOTOR
EFFECTOR
(same muscle)
contracts and
relieves the
stretching
Antagonistic
muscles relax
Motor neuron to
antagonistic muscles
is inhibited
Within INTEGRATING
CENTER (spinal cord),
sensory neuron activates
motor neuron
Inhibitory
interneuron
To brain
Spinal
Nerve + – 2 3 4 5 1
Stretching stimulates
SENSORY RECEPTOR (muscle spindle)
SENSORY
NEURON
excited
MOTOR
Antagonistic
muscles relax
Motor neuron to
antagonistic muscles
is inhibited
Within INTEGRATING
CENTER (spinal cord),
sensory neuron activates
motor neuron
Inhibitory
interneuron
To brain
Spinal
Nerve + – 2 3 4 1
Stretching stimulates
SENSORY RECEPTOR (muscle spindle)
SENSORY
NEURON
excited
Within INTEGRATING
CENTER (spinal cord),
sensory neuron activates
motor neuron
Inhibitory
interneuron
To brain
Spinal
Nerve + – 2 3 1
Stretching stimulates
SENSORY RECEPTOR (muscle spindle)
SENSORY
NEURON
excited
To brain
Spinal
Nerve + 2 1
Stretching stimulates
SENSORY RECEPTOR (muscle spindle)
Antagonistic
muscles relax

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The Tendon Reflex
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.
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32. Tendon Reflex

33. 1 2 3 4 5 1 2 3 4 1 2 3 1 2 1 – EFFECTOR (muscle attached
to same tendon)
relaxes and
relieves excess
tension
Within INTEGRATING
CENTER (spinal cord),
sensory neuron activates
inhibitory interneuron
Excitatory
interneuron
Antagonistic
muscles
contract
Spinal
nerve
MOTOR NEURON
inhibited
Inhibitory
SENSORY
NEURON
excited +
To brain
Increased tension
stimulates
RECEPTOR (tendon)
Motor neuron to
antagonistic
muscles is excited 2 3 4 5 1 –
Within INTEGRATING
CENTER (spinal cord),
sensory neuron activates
inhibitory interneuron
Excitatory
interneuron
Antagonistic
muscles
contract
Spinal
nerve
MOTOR NEURON
inhibited
Inhibitory
SENSORY
NEURON
excited +
To brain
Increased tension
stimulates
RECEPTOR (tendon)
Motor neuron to
antagonistic
muscles is excited 2 3 4 1
Within INTEGRATING
CENTER (spinal cord),
sensory neuron activates
inhibitory interneuron
Excitatory
interneuron
Spinal
nerve
Inhibitory
SENSORY
NEURON
excited +
To brain
Increased tension
stimulates
RECEPTOR (tendon) 2 3 – 1
Spinal
nerve
SENSORY
NEURON
excited
To brain
Increased tension
stimulates
RECEPTOR (tendon) 2 + 1
Increased tension
stimulates
SENSORY
RECEPTOR (tendon)

34. Flexor (Withdrawal) Reflex
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.
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35. Flexor (Withdrawal) Reflex

36. 1 2 3 4 5 1 2 3 4 1 2 3 1 2 1 + Stepping on tack stimulates
SENSORY RECEPTOR (dendrites
of pain-sensitive neuron)
SENSORY
NEURON
excited
MOTOR
NEURONS
EFFECTORS
(flexor muscles)
contract and
withdraw leg
Within INTEGRATING CENTER
(spinal cord), sensory neuron
activates interneurons in several
spinal cord segments
Ascending
interneuron
Interneuron
Descending
Spinal
nerve 2 3 4 5 1 +
Stepping on tack stimulates
SENSORY RECEPTOR (dendrites
of pain-sensitive neuron)
SENSORY
NEURON
excited
MOTOR
NEURONS
Within INTEGRATING CENTER
(spinal cord), sensory neuron
activates interneurons in several
spinal cord segments
Ascending
interneuron
Interneuron
Descending
Spinal
nerve 2 3 4 1 +
Stepping on tack stimulates
SENSORY RECEPTOR (dendrites
of pain-sensitive neuron)
SENSORY
NEURON
excited
Within INTEGRATING CENTER
(spinal cord), sensory neuron
activates interneurons in several
spinal cord segments
Ascending
interneuron
Interneuron
Descending
Spinal
nerve 2 3 1 +
Stepping on tack stimulates
SENSORY RECEPTOR (dendrites
of pain-sensitive neuron)
SENSORY
NEURON
excited 2 1
Stepping on tack stimulates
SENSORY RECEPTOR (dendrites
of pain-sensitive neuron)

37. Crossed Extensor Reflex
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.
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38. Crossed Extensor Reflex
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39. 1 2 3 4 5 1 2 3 4 1 2 3 1 2 1 + Ascending interneurons EFFECTORS
(extensor muscles)
contract, and extend
left leg
SENSORY
NEURON
excited
Spinal
nerve
Within INTEGRATING CENTER
(spinal cord), sensory neuron
activates several interneurons
MOTOR
NEURONS
Descending
Withdrawal of right leg
(flexor reflex)
Flexor muscles
contract and with-
drawright leg
Stepping on a tack
stimulates SENSORY
RECEPTOR (dendrites of
pain-sensitive neuron) in
right foot
Extension of left leg
(crossed extensor reflex) 2 3 4 5
Interneurons
from other side 1 +
Ascending
interneurons
SENSORY
NEURON
excited
Spinal
nerve
Within INTEGRATING CENTER
(spinal cord), sensory neuron
activates several interneurons
MOTOR
NEURONS
Descending
Withdrawal of right leg
(flexor reflex)
Stepping on a tack
stimulates SENSORY
RECEPTOR (dendrites of
pain-sensitive neuron) in
right foot
Extension of left leg
(crossed extensor reflex) 2 3 4
Interneurons
from other side 1 +
Ascending
interneurons
SENSORY
NEURON
excited
Spinal
nerve
Within INTEGRATING CENTER
(spinal cord), sensory neuron
activates several interneurons
Descending
Withdrawal of right leg
(flexor reflex)
Stepping on a tack
stimulates SENSORY
RECEPTOR (dendrites of
pain-sensitive neuron) in
right foot 2 3
Interneurons
from other side 1
SENSORY
NEURON
excited
Withdrawal of right leg
(flexor reflex)
Stepping on a tack
stimulates SENSORY
RECEPTOR (dendrites of
pain-sensitive neuron) in
right foot 2 + 1
Withdrawal of right leg
(flexor reflex)
Stepping on a tack
stimulates SENSORY
RECEPTOR (dendrites of
pain-sensitive neuron) in
right foot

40. Figure 13-21a The Babinski Reflexes
The plantar reflex (negative Babinski reflex), a curling of the toes, is seen in healthy adults. 40

41. Figure 13-21b The Babinski Reflexes
The Babinski sign (positive Babinski reflex) occurs in the absence of descending inhibition. It is normal in infants, but pathological in adults. 41