Spinal Cord, Spinal Nerves, and
Somatic Reflexes
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Spinal cord
• spinal cord
Vertebra (cut)
Spinal nerve
Spinal nerve rootlets
• spinal nerves
Posterior median sulcus
Subarachnoid space
Epidural space
• somatic reflexes
Posterior root ganglion
Rib
Arachnoid mater
Dura mater
Functions of the Spinal Cord
• conduction
– bundles of fibers passing information up and down spinal cord, connecting
different levels of the trunk with each other and with the brain
• locomotion
– walking involves repetitive, coordinated actions of several muscle groups
– central pattern generators are pools of neurons providing control of flexors
and extensors that cause alternating movements of the lower limbs
• reflexes
– involuntary, stereotyped responses to stimuli
• withdrawal of hand from pain
– involves brain, spinal cord and peripheral nerves
(b)
Figure 13.1b
13-1
Damage to Spinal Cord
The Spinal Cord & Spinal Nerves
• accidents damage the spinal cord of thousands of
people every year
–
–
–
–
13-2
paraplegia - paralysis of lower limbs
quadriplegia – paralysis of all four limbs
respiratory paralysis, loss of sensation or motor control
disorders of bladder, bowel and sexual function
• damage to spinal cord from strokes or other brain
injuries
• Together with brain forms the CNS
• Functions
– spinal cord reflexes
– integration (summation of inhibitory and
excitatory) nerve impulses
– highway for upward and downward
travel of sensory and motor information
– hemiplegia – paralysis of one side of the body only
13-3
18-4
Structures Covering the Spinal Cord
Spinal Cord Protection
Protection: vertebral column, meninges,
cerebrospinal fluid, and vertebral ligaments.
18-5
18-6
1
External Anatomy of Spinal Cord
• Flattened cylinder
• 16-18 Inches long &
3/4 inch diameter
• In adult ends at L2
• Cervical enlargement
– upper limbs
• Lumbar enlargement
– lower limbs
18-7
Inferior End of Spinal Cord
18-8
Inferior End of Spinal Cord
• Conus medullaris
– cone-shaped
end of spinal
cord
• Filum terminale
– extension of pia
mater
– Anchors SC to
the coccyx
Caudae equinae
dorsal & ventral
roots of lowest
spinal nerves
• Spinal segment
– area of cord
from which each
pair of spinal
nerves arises
18-9
Spinal Cord & Spinal Nerves
• Spinal nerves begin as roots
• Dorsal or posterior root is incoming sensory
fibers
– dorsal root ganglion (swelling) = cell bodies of
sensory nerves
18-11
• Ventral or anterior root = outgoing motor fibers
18-10
Gray Matter of the Spinal Cord
• Gray matter is shaped like the letter H or a
butterfly
– contains neuron cell bodies, unmyelinated
axons & dendrites
– paired dorsal and ventral gray horns
• Central canal continuous with 4th ventricle of
18-12
brain
2
White Matter of the Spinal Cord
• White matter covers gray matter
• Anterior, Lateral and Posterior White Columns
contain axons that form ascending & descending
tracts
18-13
18-14
Location of Tracts inside Cord
Tracts of the Spinal Cord
• Function of tracts
– highway for sensory & motor information
– sensory tracts ascend
– motor tracts descend
• Many tracts decussate
- contralateral (origin/destination differ)
- ipsilateral (origin/dest. Same side)
• Naming of tracts
– indicates position & direction of signal
– example = anterior spinothalamic tract
• impulses travel from spinal cord towards
18-15
brain (thalamus)
18-16
Function of Spinal Tracts
Somesthetic cortex
(postcentral gyrus)
• Spinothalamic tract
– pain, temperature, deep pressure & crude touch
• Gracile fasciculus
– proprioception, discriminative touch, two-point
discrimination, pressure and vibration
• corticospinal & corticobulbar tracts
– precise, voluntary movements
• Rubrospinal & vestibulospinal
– programming automatic movements, posture &
muscle tone, equilibrium & coordination of
visual reflexes
18-17
Somesthetic cortex
(postcentral gyrus)
Third-order
neuron
Third-order
neuron
Thalamus
Thalamus
Cerebrum
Cerebrum
Midbrain
Medial
lemniscus
Midbrain
Second-order
neuron
Gracile
nucleus
Medulla
First-order
neuron
Spinothalamic
tract
Second-order
neuron
Cuneate
nucleus
Medial
lemniscus
Medulla
Gracile fasciculus
Cuneate fasciculus
Spinal cord
Spinal cord
First-order
neuron
Anterolateral system
Receptors for pain, heat, and cold
(a)
Receptors for body movement, limb positions,
fine touch discrimination, and pressure
Figure 13.5a
3
Nerves & Connective Tissue Coverings
Spinal Nerves
• 31 Pairs of spinal nerves
• All are mixed nerves!
• Named & numbered by cord
level of their origin
– 8 pairs of cervical nerves
(C1 to C8)
– 12 pairs of thoracic nerves
(T1 to T12)
– 5 pairs of lumbar nerves
(L1 to L5)
– 5 pairs of sacral nerves
(S1 to S5)
– 1 pair of coccygeal nerves
• Endoneurium = wrap each nerve fiber
• Perineurium = surrounds group of nerve
fibers forming a fascicle
• Epineurium = covers entire nerve
18-19
Branching of Spinal Nerve
18-20
Branching of Spinal Nerve
Ventral rami
form plexus:
supply
anterior trunk
& limbs
meningeal branches
supply
meninges,
vertebrae &
BV
• Spinal nerves
formed from d
& v roots
• Spinal nerves
branch into d
& v rami
– dorsal rami
supply skin
& muscles
of back
18-21
A Nerve Plexus
Rami of Spinal Nerves
Posterior and anterior rootlets
of spinal nerve
Spinal nerve
Posterior ramus
Anterior ramus
Posterior root
Communicating
rami
Posterior root ganglion
Intercostal nerve
Anterior root
Sympathetic
chain ganglion
Spinal nerve
Thoracic cavity
Anterior ramus
of spinal nerve
Sympathetic chain
ganglion
18-22
• Joining of ventral rami
of spinal nerves to
form nerve networks
(plexuses)
• Found in neck, arm,
low back & sacral
regions
• T7 to T12 supply
abdominal wall as well
Lateral
cutaneous nerve
Posterior ramus
of spinal nerve
Intercostal
muscles
Communicating rami
Anterior
cutaneous nerve
(a) Anterolateral view
(b) Cross section
Figure 13.13
13-23
18-24
4
Phrenic Nerve
Cervical Plexus
• Ventral rami of spinal
nerves (C1 to C5)
• Supplies parts of
head, neck, &
shoulders
• Phrenic nerve (C3-C5)
supplies diaphragm
• Damage to cord
above C3 causes
respiratory arrest
18-25
Brachial Plexus
18-26
Branches off Brachial Plexus
• Ventral rami from
C5 to T1
• Supplies shoulder
& upper limb
18-27
Lumbar Plexus
18-28
Branches of Lumbar Plexus
• Ventral rami of L1 to L4
• Supplies abdominal
wall, external genitals &
parts of thigh
• Femoral nerve injury:
inability to extend leg &
loss of sensation in
thigh
18-29
• Notice: Femoral and Obturator nerves
• Found anterior and medial to hip joint
18-30
5
Branches of Sacral Plexus
Sacral Plexus
• Ventral rami of L4-L5 & S1S4
• Anterior to the sacrum
• Supplies buttocks,
perineum & part of lower
limb
• Sciatic nerve = L4 to S3
supplies post thigh & all
below knee
18-31
• Note: Sciatic nerve origins
• Common
Peroneal nerve
and Tibial
nerve behind
the knee
• Sciatic Nerve
- may be sign
of herniated
disc
Sciatic Nerve Branches
18-33
Dermatomes:
C4
C3
C5
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
C6
C5
C8
T1
T11
T12
L1
L2
C7
L3
S2
S3
L4
Cervical nerves
Thoracic nerves
L5
Lumbar nerves
S1
18-34
Spinal Reflexes
C2
• Dermatome:
– area of skin that
sends sensory info
to one spinal nerve
– overlap up to 50%
– sensory anesthesia
requires 3 spinal
nerves to be
blocked
• Skin on face supplied
by Cranial Nerve V
18-32
Sacral nerves
18-35
• Quick stereotyped, involuntary response of
glands/muscles to stimuli
• Integration center for spinal reflexes is gray
matter of spinal cord
• Examples
– somatic reflexes result in skeletal muscle
contraction
– autonomic (visceral) reflexes involve smooth
& cardiac muscle and glands.
• heart rate, respiration, digestion, urination,
& more
18-36
6
Somatic reflex employs a Reflex Arc
Muscle Spindle
•
Bone
Figure 13.20
Peripheral nerve
(motor and sensory
nerve fibers)
Tendon
Muscle spindle
Secondary
afferent fiber
Skeletal muscle
Extrafusal
muscle fibers
Connective tissue
sheath (cut open)
Intrafusal
muscle fibers:
Nuclear chain
fiber
Nuclear bag
fiber
Motor nerve fibers:
Gamma
Alpha
Specific nerve
impulse pathway
• 5 components of
reflex arc
1. receptor
2. sensory neuron
3. integrating
center
4. motor neuron
5. effector
Sensory nerve fibers:
Primary
Secondary
13-37
18-38
The Flexor (Withdrawal) Reflex
The Crossed extension Reflex
2 Sensory neuron
activates multiple
interneurons
The Tendon Reflex
• tendon organs –
proprioceptors in a tendon
near its junction with a
muscle
+
+
+
+
+
+
+
+
5
Contralateral
motor neurons
to extensor
excited
3 Ipsilateral motor
neurons to flexor
excited
• tendon reflex – in response
to excessive tension on the
tendon
– inhibits muscle from
contracting strongly
– moderates muscle contraction
4 Ipsilateral flexor
contracts
+
+
Nerve fibers
Tendon organ
Tendon bundles
6 Contralateral
extensor
contracts
Muscle fibers
Figure 13.23
1 Stepping on glass
stimulates pain receptors
in right foot
ithdrawal of right leg
(flexor reflex)
Extension of left leg
(crossed extension reflex)
13-39
13-40
7