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The Spinal Cord & Peripheral
Nervous System
Peripheral Nerves
• There are 12 pairs of cranial nerves and
31 pairs of spinal nerves.
Spinal Nerves
• There are 31 pairs of spinal nerves.
• All are mixed nerves (motor & sensory).
Spinal Nerves
• There are 31 pairs of spinal nerves.
• All are mixed nerves.
• There are:
– 8 cervical spinal nerves (C8 comes out
between C7 and T1)
– 12 pairs of thoracic nerves,
– 5 pairs of lumbar spinal nerves
– and 5 pairs of sacral nerves.
Cervical plexus
Brachial plexus
Cervical
nerves
C1 – C8
Cervical
enlargement
Intercostal
nerves
Thoracic
nerves
T1 – T12
Lumbar
enlargement
Lumbar plexus
Sacral plexus
Cauda equina
Lumbar
nerves
L1 – L5
Sacral nerves
S1 – S5
Coccygeal nerve Co1
Cranial
dura mater
Terminus of
medulla
oblongata
of brain
Sectioned
pedicles of
cervical
vertebrae
Spinal nerve
rootlets
Dorsal
median sulcus
of spinal cord
(b) Cervical spinal cord.
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Anatomy of the spinal cord.
Epidural space
(contains fat)
Subdural space
Subarachnoid
space
(contains CSF)
Pia mater
Arachnoid
mater
Dura mater
Spinal
meninges
Bone of
vertebra
Dorsal root
ganglion
Body
of vertebra
(a) Cross section of spinal cord and vertebra
Copyright © 2010 Pearson Education, Inc.
Spinal Nerves
The spinal nerve consists of a dorsal root
and a ventral root.
The ventral root contains efferent, motor
neurons (voluntary and involuntary).
The dorsal root is, afferent, sensory fibers.
Spinal Nerves
The spinal nerve is only 1-2 cm long
coming out from the spine and the
branches into a dorsal ramus and a ventral
ramus.
Dorsal root (sensory)
Dorsal root ganglion
Dorsal horn (interneurons)
Somatic
sensory
neuron
Visceral
sensory
neuron
Visceral
motor
neuron
Spinal nerve
Somatic
motor neuron
Ventral root
(motor)
Ventral horn
(motor neurons)
Interneurons receiving input from somatic sensory neurons
Interneurons receiving input from visceral sensory neurons
Visceral motor (autonomic) neurons
Somatic motor neurons
Copyright © 2010 Pearson Education, Inc.
Ascending tracts
Fasciculus gracilis
Dorsal
white Fasciculus cuneatus
column
Dorsal
spinocerebellar
tract
Ventral
spinocerebellar
tract
Lateral
spinothalamic tract
Ventral spinothalamic
tract
Copyright © 2010 Pearson Education, Inc.
Descending tracts
Ventral white
commissure
Lateral
reticulospinal tract
Lateral
corticospinal tract
Rubrospinal
tract
Medial
reticulospinal
tract
Ventral corticospinal
tract
Vestibulospinal tract
Tectospinal tract
Innervations of Specific Body
Regions
Except for the spinal nerves T2-T12, all
other spinal nerves join into inter lacing
networks called nerve plexuses.
They are formed only by the ventral rami.
Dorsal ramus
Ventral ramus
Spinal nerve
Rami communicantes
Sympathetic trunk
ganglion
.
Intercostal nerve
Dorsal root
ganglion
Dorsal root
Ventral root
Branches of intercostal
nerve
• Lateral cutaneous
• Anterior cutaneous
Sternum
(b) Cross section of thorax showing the main roots and
branches of a spinal nerve.
Cervical Plexus
This is formed from the first 4 cervical
spinal nerves.
Cervical Plexus
This is formed from the first 4 cervical
spinal nerves.
The most important nerve coming from
this plexus is the phrenic nerve.
Cervical Plexus
This is formed from the first 4 cervical
spinal nerves.
The most important nerve coming from
this plexus is the phrenic nerve.
This is the chief motor nerve for the
diaphragm.
Ventral rami
Segmental
branches
Hypoglossal
nerve (XII)
Lesser occipital
nerve
Greater auricular
nerve
Transverse
cervical nerve
Ansa cervicalis
Ventral
rami:
C1
C2
C3
C4
Accessory nerve (XI)
Phrenic nerve
Supraclavicular
nerves
C5
Cervical Plexus
Damage to the spinal cord at C3- C5
could lead to respiratory arrest.
Brachial Plexus
It is formed from C5-C8 and T1 with 5
major roots supplying the muscles of the
muscles of the shoulder, thorax and
upper limb.
Roots (ventral rami):
C4
C5
Dorsal scapular
Nerve to
subclavius
Suprascapular
Cords
C6
Posterior
divisions
C7
Lateral
C8
Posterior
T1
Upper
Middle
Trunks
Lower
Long thoracic
Medial pectoral
Lateral pectoral
Medial
Axillary
Musculocutaneous
Radial
Upper subscapular
Median
Ulnar
Medial cutaneous
nerves of the arm
and forearm
Lower subscapular
Thoracodorsal
(a) Roots (rami C5 – T1), trunks, divisions, and cords
Anterior
divisions
Posterior
divisions
Trunks
Roots
Musculocutaneous
nerve
Axillary nerve
Biceps brachii
Coracobrachialis
Median nerve
Radial nerve
branches to triceps
Lateral
cord
Posterior
cord
Medial cord
Radial nerve
Ulnar nerve
(b) Cadaver photo
Lumbrosacral Plexus
It is formed from L1-L4 and lies within the
psoas major muscle.
The largest nerve of this plexus is the
femoral nerve.
The sacral plexus is formed from L4-S4
and lies just below the Lumbar Plexus
Ventral rami
Iliohypogastric
Ilioinguinal
Genitofemoral
Lateral femoral
cutaneous
Obturator
Femoral
Lumbosacral
trunk
Ventral
rami:
Iliohypogastric
L1
Ilioinguinal
Femoral
Lateral femoral
L2
cutaneous
Obturator
L3
Anterior femoral
cutaneous
Saphenous
L4
L5
(a) Ventral rami and major branches
of the lumbar plexus
(b) Distribution of the major nerves from
the lumbar plexus to the lower limb
Ventral rami
Ventral rami:
L4
Superior
gluteal
Lumbosacral
trunk
Inferior
gluteal
Common
fibular
Tibial
Posterior
femoral
cutaneous
Pudendal
Sciatic
L5
S1
S2
S3
S4
S5
Co1
(a) Ventral rami and major branches
of the sacral plexus
The largest nerve from this is the sciatic
nerve.
It is the largest and thickest nerve in the
body.
It supplies the entire lower limb.
Irritation to this nerve gives rise to
Sciatica.
Dermatomes
`Dermatomes are areas of the skin
innervated by the sensory nerves. These
are fairly uniform and follow the nerve
segments.
C2
C3
C4
C5
C6
C7
C8
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
C2
C3
C4
C5
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T2
C5
C6
C6
C7
L1
C8
L2
T12
S2
S3
T2
C5
C6
L1
C8
L2
S1
L4
S2
S3
S4
S5
C6
C7
C6
C7
C8
C8
L2
S2
S1
L1
L3
L5
L4
T11
T12
L1
L3
L5
C7
C6
S1 S2
L3
C5
L2
L5
L4
L3
L5
L5
L4
S1
(a) Anterior
view
S1
(b) Posterior
view
L4
L5
L4
L5
S1
Reflex Activity
• Reflexes can be inborn or learned.
– An inborn reflex is a predictable, rapid
response to a stimulus.
– Acquired reflexes result from repetition.
Reflex Arc
Stretch and Golgi Tendon Reflexes
The muscle spindles found in skeletal
muscles provide information on the length
of the muscle and the amount of tension
on it.
A common example of this is the patellar
reflex. This reflex causes the muscle to
contract in response to stretching.
The patellar (knee-jerk) reflex – a specific example of a stretch reflex
2
Quadriceps
(extensors)
1
3a
3b
3b
Patella
Muscle
spindle
Spinal cord
(L2 – L4)
Hamstrings
(flexors)
Patellar
ligament
1 Tapping the patellar ligament excites
muscle spindles in the quadriceps muscle.
2 Afferent impulses (blue) travel to the
spinal cord, where synapses occur with
motor neurons and interneurons.
3a The motor neurons (red) send
+ Excitatory synapse
– Inhibitory synapse
activating impulses to the quadriceps
causing it to contract, extending the
knee.
3b The interneurons (green) make
inhibitory synapses with ventral horn
neurons (purple) that prevent the
antagonist muscles (hamstrings) from
resisting the contraction of the
quadriceps.
Golgi tendon reflexes
Golgi tendon reflexes are the opposite and
respond by causing muscle relaxation.
This helps to prevent the muscle from over
contracting.
1 Quadriceps strongly
contracts. Golgi tendon
organs are activated.
2 Afferent fibers synapse
with interneurons in the
spinal cord.
Interneurons
Quadriceps
(extensors)
Spinal cord
Golgi
tendon
organ
Hamstrings
(flexors)
3a Efferent
+ Excitatory synapse
– Inhibitory synapse
impulses
to muscle with
stretched tendon are
damped. Muscle
relaxes, reducing
tension.
3b
Efferent
impulses to
antagonist
muscle cause
it to contract.
Flexor and Crossed-Extensor
Reflexes
• These are initiated by painful stimuli and
cause an automatic withdrawal of the
threatened body part. Placing your hand
on a hot stove is an example of this type of
reflex.
+ Excitatory synapse
– Inhibitory synapse
Interneurons
Efferent
fibers
Afferent
fiber
Efferent
fibers
Extensor
inhibited
Flexor
stimulated
Site of stimulus: a noxious
stimulus causes a flexor
reflex on the same side,
withdrawing that limb.
Arm
movements
Flexor
inhibited
Extensor
stimulated
Site of reciprocal
activation: At the
same time, the
extensor muscles
on the opposite
side are activated.
Superficial Reflexes
These are the result of gentle cutaneous
stimulation. Common examples are the
plantar reflex that tests the integrity of the
corticospinal tract.
Babinski's sign
This plantar reflex occurs when the
corticospinal tract is damaged.
In it the large toe dorsi flexes and the
smaller toes fan laterally.
Spinal Cord Injuries
Damage to the spinal cord results in
paralysis (loss of motor function) or
Paresthesia (sensory loss).
These are devastating injuries since the
cord will not heal.
Spinal Cord Injuries
The Autonomic Nervous System
The motor portion of the peripheral
nervous system is divided into
– the somatic (voluntary) motor system (SMS)
and
– the autonomic motor system (ANS).
Central nervous system (CNS)
Peripheral nervous system (PNS)
Figure 14.1 Place of the ANS
in the structural organization
of the nervous system.
Sensory (afferent)
division
Motor (efferent) division
Somatic nervous
system
Autonomic nervous
system (ANS)
Sympathetic
division
Parasympathetic
division
Comparison
The SMS stimulates skeletal muscle
whereas the ANS innervates cardiac and
smooth muscle as well as glands.
Comparison
In the SMS the motor neuron cell bodies
are located in the CNS and their axons
extend to the muscles they innervate.
It is a single neuron chain.
Comparison
The ANS uses a two neuron chain.
The first is the preganglionic neuron; its
cell body resides in the CNS or spinal
cord.
Comparison
The ANS uses a two neuron chain.
The first is the preganglionic neuron; its
cell body resides in the CNS or spinal
cord.
The second motor neuron outside of the
CNS.
This second neuron is called the
postganglionic neuron.
Cell bodies in central
nervous system
Peripheral nervous system
Neurotransmitter
at effector
Effector
organs
SOMATIC
NERVOUS
SYSTEM
Single neuron from CNS to effector organs
Effect
+
ACh
Stimulatory
Heavily myelinated axon
Skeletal muscle
Figure 14.2 Comparison of
somatic and autonomic
+
nervous systems.
NE
SYMPATHETIC
ACh
Unmyelinated
postganglionic axon
Lightly myelinated Ganglion
Epinephrine and
preganglionic axons
norepinephrine
ACh
Adrenal medulla
PARASYMPATHETIC
AUTONOMIC NERVOUS SYSTEM
Two-neuron chain from CNS to effector organs
Acetylcholine (ACh)
Blood vessel
ACh
ACh
Lightly myelinated
preganglionic axon
Norepinephrine (NE)
Ganglion
Unmyelinated
postganglionic
axon
Smooth muscle
(e.g., in gut),
glands, cardiac
muscle
Stimulatory
or inhibitory,
depending
on neurotransmitter
and
receptors
on effector
organs
ANS Divisions
The ANS is divided up into the
parasympathetic and sympathetic
divisions.
They innervate the same organs and
typically have opposite effects.
Parasympathetic Division
This system is associated with “resting and
digesting”.
When this system is active, the body’s
energy use is low, the body is relaxed and
the body can focus on digestive processes.
Sympathetic Division
This is often referred to as the “fight or flight”
system.
When this system is active, the body is
excited and the body is in a high energy
state.
Parasympathetic
Sympathetic
Eye
Brain
stem
Salivary
glands
Heart
Eye
Skin*
Cranial
Cervical
Sympathetic
ganglia
Salivary
glands
Figure 14.3 Overview of the
subdivisions of the ANS.
Lungs
Lungs
T1
Heart
Stomach
Stomach
Thoracic
Pancreas
Liver
and gallbladder
Pancreas
L1
Liver and
gallbladder
Adrenal
gland
Lumbar
Bladder
Bladder
Genitals
Genitals
Sacral
Parasympathetic (Craniosacral)
Division
• The cranial outflow includes Cranial
nerves III, VII, IX, and X.
• The sacral outflow innervates the colon
and pelvic organs.
Sympathetic (Thoracolumbar)
Division
It supplies the visceral organs but also
structures are the body such as the sweat
glands and peripheral arteries.
All cell bodies for the preganglionic
neurons arise in the spinal cord between
T1 and L12.
Visceral reflexes
The visceral reflex arc is similar to the
somatic reflex arc and contains the same
components except the motor neuron is
from the ANS.
Visceral reflexes
The visceral pain afferent fibers travel along
the same pathways as the somatic pain
receptors and gives rise to the so called
referred pain.
Heart
Lungs and
diaphragm
Liver
Gallbladder
Appendix
Heart
Liver
Stomach
Pancreas
Small intestine
Ovaries
Colon
Kidneys
Urinary
bladder
Ureters
ANS Physiology
• The major neurotransmitters released by
the ANS are acetylcholine (Ach) and
norepinephrine (NE).
ANS Physiology
Ac is released by:
– All ANS preganglionic axons
– All parasympathetic postganglionic axons at
the synapses with their effectors
Ach releasing fibers are called cholinergic
fibers
ANS Physiology
NE is released by postganglionic
sympathetic fibers at the synapses with
their effectors.
NE releasing fibers are called adrenergic
fibers.
Receptor Type
To further complicate the matter, the
effects of these neurotransmitters depend
on the type of receptor to which they
attach.
Cholinergic Receptors
Two types of receptors bind Ach and they
are named for the drugs that can bind to
them.
These are the nicotinic receptors
(preganglionic sympathetic)and the
muscarinic receptors (postganglionic
parasympathetic).
Effects of Drugs
The existence of various types of
receptors has led to the design of specific
drugs that can either inhibit or stimulate a
selected organ.
Atropine is an anticholinergic drug that
causes the pupil to dilate, dry up
respiratory secretions or prevent
salivation.
Disorders
• POTS
– Postural Orthostatic Tachycardia Syndrome