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Chapter 13: The Spinal Cord and Spinal Nerves
Chapter 13:
The Spinal Cord and Spinal
Nerves
Copyright 2009, John Wiley & Sons, Inc.
Spinal Cord Anatomy
Protective structures:
Vertebral column and the meninges protect
the spinal cord and provide physical stability.
a. Dura mater, b. Arachnoid, c. Pia mater
Epidural space, subdural space and
subarachnoid space
Spinal Cord Anatomy
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
Copyright 2009, John Wiley & Sons, Inc.
External Anatomy of Spinal Cord
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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Internal Anatomy of Spinal Cord
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
Copyright 2009, John Wiley & Sons, Inc.
Spinal Nerves
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.
Copyright 2009, John Wiley & Sons, Inc.
Cervical Plexus
Formed by the anterior
rami of C1-C5.
Phrenic nerves-
important nerves from
the cervical plexuses.
Copyright 2009, John Wiley & Sons, Inc.
Formed by the anterior
rami of C1-C5.
Phrenic nerves-
important nerves from
the cervical plexuses.
Brachial plexus
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
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.
P.472-473:
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
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
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.
Copyright 2009, John Wiley & Sons, Inc.
Distribution of Nerves from the
Lumbar and Sacral Plexuses
Copyright 2009, John Wiley & Sons, Inc.
Coccygeal Plexus
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
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
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
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
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.
1 SENSORY RECEPTOR
(responds to a stimulus
by producing a generator
or receptor potential)
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 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)
INTEGRATING CENTER
(one or more regions within the CNS
that relay impulses from sensory to
motor neurons) MOTOR NEURON
(axon conducts impulses from
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) MOTOR NEURON
(axon conducts impulses from
integrating center to effector)
EFFECTOR
(muscle or gland that
responds to motor
nerve impulses)
Interneuron
2
3
4 5
Reflex Arc
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.
Copyright 2009, John Wiley & Sons, Inc.
1 Stretching stimulates
SENSORY RECEPTOR
(muscle spindle)
Antagonistic muscles relax
1 Stretching stimulates
SENSORY RECEPTOR
(muscle spindle)
SENSORY NEURON excited
To brain
Spinal Nerve
+
+
2 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
MOTOR NEURON excited
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
MOTOR NEURON excited
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
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.
Copyright 2009, John Wiley & Sons, Inc.
1
Increased tension stimulates SENSORY RECEPTOR (tendon)
1
Spinal nerve
SENSORY NEURON excited
To brain
Increased tension stimulates SENSORY RECEPTOR (tendon)
2 + +
1
Within INTEGRATING CENTER (spinal cord), sensory neuron activates inhibitory interneuron
Excitatory interneuron
Spinal nerve
Inhibitory interneuron
SENSORY NEURON excited
+
To brain
Increased tension stimulates SENSORY RECEPTOR (tendon)
+ + 2
3
–
+ 1
–
Within INTEGRATING CENTER (spinal cord), sensory neuron activates inhibitory interneuron
Excitatory interneuron
Antagonistic muscles contract
Spinal nerve
MOTOR NEURON inhibited
Inhibitory interneuron
SENSORY NEURON excited
+
To brain
Increased tension stimulates SENSORY RECEPTOR (tendon)
Motor neuron to antagonistic muscles is excited
+
+
+
+ 2
3
4
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 interneuron
SENSORY NEURON excited
+
To brain
Increased tension stimulates SENSORY RECEPTOR (tendon)
Motor neuron to antagonistic muscles is excited
+
+
+ 2
3
4 5
+
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.
Copyright 2009, John Wiley & Sons, Inc.
1 Stepping on tack stimulates SENSORY RECEPTOR (dendrites of pain-sensitive neuron)
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)
SENSORY NEURON excited
Within INTEGRATING CENTER (spinal cord), sensory neuron activates interneurons in several spinal cord segments
Ascending interneuron
Interneuron
Descending interneuron
Spinal nerve
+
+
+
+
+
+
2
3
1
+
Stepping on tack stimulates SENSORY RECEPTOR (dendrites of pain-sensitive neuron)
SENSORY NEURON excited
MOTOR NEURONS excited
MOTOR NEURON excited
Within INTEGRATING CENTER (spinal cord), sensory neuron activates interneurons in several spinal cord segments
Ascending interneuron
Interneuron
Descending interneuron
Spinal nerve
+
+
+
+
+
+
+
+
+
2
3
4
4
1
+
Stepping on tack stimulates SENSORY RECEPTOR (dendrites of pain-sensitive neuron)
SENSORY NEURON excited
MOTOR NEURONS excited
MOTOR NEURON excited
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 interneuron
Spinal nerve
+
+
+
+
+
+
+
+
+
2
3
4
5
4
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.
Copyright 2009, John Wiley & Sons, Inc.
1
Withdrawal of right leg
(flexor reflex)
Stepping on a tack
stimulates SENSORY
RECEPTOR (dendrites of
pain-sensitive neuron) in
right foot
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
+
Ascending
interneurons
SENSORY
NEURON
excited
Spinal
nerve
Within INTEGRATING CENTER
(spinal cord), sensory neuron
activates several interneurons
Descending
interneurons
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
+
Ascending
interneurons
SENSORY
NEURON
excited
Spinal
nerve
Within INTEGRATING CENTER
(spinal cord), sensory neuron
activates several interneurons
MOTOR
NEURONS
excited
Descending
interneurons
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)
MOTOR
NEURONS
excited
+
+ +
2
3
4
4
+
+ +
+ +
+ +
+
+
+
Interneurons
from other side
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
excited
Descending
interneurons
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)
MOTOR
NEURONS
excited
+
+ +
2
3
4
4
5
+
+ +
+ +
+ +
+
+
+
Interneurons
from other side
+
+
+
Copyright 2009, John Wiley & Sons, Inc.
End of Chapter 13
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