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The Nervous System II:
The Spinal Cord, Spinal Nerves and
Reflexes
Anatomy & Physiology
Chapter 13
Functions of the Spinal Cord

conduction
◦ Information highway passing information between
the PNS and the brain
◦ bundles of fibers passing information up and down
spinal cord, connecting different levels of the
trunk with each other and with the brain

reflexes
◦ involuntary, stereotyped responses to stimuli
 withdrawal of hand from pain
◦ involves brain, spinal cord and peripheral nerves
Anatomy of The Spinal Cord

spinal cord – cylinder of nervous tissue that arises
from the brainstem at the foramen magnum of the
skull
◦ About as thick as your finger
◦ passes through the vertebral canal
◦ occupies the upper two-thirds of the vertebral
canal
 inferior margin ends at L1 or a little beyond
Surface Anatomy
◦ segment – part of the spinal cord supplied by each
pair of spinal nerves
◦ spinal cord divided into the cervical, thoracic,
lumbar, and sacral regions
◦ gives rise to 31 pair of spinal nerves
 first pair passes between the skull and first vertebra
 rest pass through intervertebral foramina
Anatomy of the Spinal Cord

Enlargements of the Spinal
Cord
Cervical spinal
nerves
Cervical
enlargement
◦ Caused by:
 Amount of gray matter in segment
Thoracic spinal
nerves
 Involvement with sensory and motor
nerves of limbs
◦ Cervical enlargement
Lumbar enlargement
Conus medullaris
Lumbar
spinal
nerves
Interior tip of
spinal cord
Cauda equina
 Nerves of shoulders and upper limbs
◦ Lumbar enlargement
 Nerves of pelvis and lower limbs
Sacral spinal
nerves
Filum terminale
Coccygeal
nerve (Co1)
Anatomy of the Spinal Cord
◦ The distal end
 Conus medullaris
 Thin, conical spinal cord below lumbar enlargement
 Filum terminale
 Thin thread of fibrous tissue at end of conus medullaris
 Attaches to coccygeal ligament
 Cauda equina
 Nerve roots extending below conus medullaris
Anatomy of The Spinal Cord
Cervical spinal
nerves
Cervical
enlargement
Posterior median sulcus
Thoracic spinal
nerves
Lumbar enlargement
Conus medullaris
Lumbar
spinal
nerves
Interior tip of spinal cord
Cauda equina
Sacral spinal
nerves
Filum terminale
Coccygeal
nerve (Co1)
The Spinal Meninges

Specialized membranes isolate spinal cord
from surroundings

Functions of the spinal meninges include:
◦ Protecting spinal cord
◦ Carrying blood supply
◦ Continuous with cranial meninges

Meningitis
◦ Viral or bacterial infection of meninges
Meninges of the Spinal Cord

meninges – three fibrous
connective tissue
membranes that enclose
the brain and spinal cord

separate soft tissue of
central nervous system
from bones of cranium
and vertebral canal

from superficial to deep:
◦ dura mater – outer layer
◦ arachnoid mater – middle
layer
◦ and pia mater – inner layer
Meninges:
Dura mater
Arachnoid mater
Pia mater
The Dura Mater

Tough and fibrous
◦ Cranially
 Fuses with periosteum of occipital bone
 Is continuous with cranial dura mater
◦ Caudally
 Tapers to dense cord of collagen fibers
 Joins filum terminale in coccygeal ligament

Epidural Space
 Between spinal dura mater and walls of vertebral canal
 Contains loose connective and adipose tissue
 Anesthetic injection site
The Arachnoid Mater
◦ Middle meningeal layer
◦ Subdural space
 Between arachnoid mater and dura mater
◦ Subarachnoid space
 Between arachnoid mater and pia mater
 Contains collagen/elastin fiber network
 Filled with cerebrospinal fluid (CSF)
◦ Cerebrospinal Fluid (CSF)
 Carries dissolved gases, nutrients, and wastes
 Lumbar puncture or spinal tap withdraws CSF
The Pia Mater

Is the innermost meningeal layer
Is a mesh of collagen and elastic fibers
Is bound to underlying neural tissue

Paired denticulate ligaments


 Extend from pia mater to dura mater
 Stabilize side-to-side movement

Blood vessels
 Along surface of spinal pia mater
 Within subarachnoid space
Spinal cord
Anterior median fissure
Pia mater
Denticulate
ligaments
Dorsal root
Ventral root, formed by
several “rootlets” from
one cervical segment
Arachnoid mater
(reflected)
Dura mater (reflected)
Spinal blood vessel
Gray Matter and White Matter
◦ White matter
 Is superficial
 Contains myelinated and unmyelinated axons (myelin
provides whitish appearance)
◦ Gray matter
 Surrounds central canal of spinal cord
 Contains neuron cell bodies, neuroglia, unmyelinated axons
 Has projections (anterior, posterior and lateral gray horns)
Organization of Gray Matter

The gray horns
 Dorsal (posterior) horns
 Contains cell bodies (nuclei) of sensory neurons
 Connect to peripheral receptors
 Ventral (anterior) horns
 Contains cell bodies (nuclei) of motor neurons
 Connect to peripheral effectors
 Lateral horns
 contain visceral motor (ANS) nuclei
 are in thoracic and lumbar segments
Gray matter:
Posterior horn
Gray commissure
Lateral horn
Anterior horn
Central canal
Posterior
median sulcus
White matter:
Posterior column
Lateral column
Anterior column
Posterior root of spinal nerve
Posterior root ganglion
Spinal nerve
Anterior median fissure
Meninges:
Pia mater
Arachnoid mater
Dura mater (dural sheath)
Anterior root
of spinal nerve
Organization of Gray Matter

Gray commissure
 Axons that cross from one side of cord to the other
before reaching gray matter
Spinal Nerve Roots

Two branches of spinal nerves
1. Ventral (anterior) root

Contains axons of motor neurons
2. Dorsal (posterior) root

◦
Contains axons of sensory neurons
Dorsal (posterior) root ganglia

Contain cell bodies of sensory neurons
Spinal Nerves

The Spinal Nerve
◦ Each side of spine
 Dorsal and ventral roots join
 To form a spinal nerve
◦ Mixed Nerves
 Carry both afferent (sensory) and efferent (motor)
fibers
Spinal Cord, Nerve roots and Nerves
Posterior
Meninges:
Dura mater (dural sheath)
Arachnoid mater
Pia mater
Spinous process of vertebra
Fat in epidural space
Subarachnoid space
Spinal cord
Denticulate ligament
Posterior root ganglion
Spinal nerve
Vertebral body
Anterior
Spinal Nerves

31 pairs of spinal nerves (mixed nerves)
◦ 8 cervical (C1 – C8) C1 between skull and atlas
 others exiting at intervertebral foramen
◦ 12 thoracic (T1 – T12)
◦ 5 lumbar (L1 – L5)
◦ 5 sacral (S1 – S5)
◦ 1 coccygeal (Co)
13-21
Organization of White Matter

Anterior, Posterior and Lateral Columns
◦ Tracts
 In white columns
 Bundles of axons
 Relay same information in same direction
 Ascending tracts
 Carry information to brain
 Descending tracts
 Carry motor commands to spinal cord
Damage to Spinal Cord

accidents damage the spinal cord of thousands of
people every year
◦ 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
◦ hemiplegia – paralysis of one side of the body only
13-23
Dermatomes
C2C3
NV
C2C3
C2
C3
A dermatome is a region
of the skin supplied by a
single spinal nerve.
T2
C6
L1
L2
C8
C7
T1
L3
L4
L5
C3
C4
C5
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
L1
L2
L4 L3
L5
C4
C5
T2
C6
T1
C7
SS
S2
4 3
L1
S5
C8
S1L5
L2 S2
L3
S1
L4
ANTERIOR
POSTERIOR
Peripheral Neuropathy

Regional loss of sensory or motor function

Due to trauma or compression
Shingles

chickenpox - common disease of early childhood
◦ caused by varicella-zoster virus
◦ produces itchy rash that clears up without complications

virus remains for life in the posterior root ganglia
◦ kept in check by the immune system

shingles (herpes zoster) – localized disease caused by the
virus traveling down the sensory nerves by fast axonal
transport when immune system is compromised
◦ common after age of 50
◦ painful trail of skin discoloration and fluid-filled vesicles along path of
nerve
◦ usually in chest and waist on one side of the body
◦ pain and itching
◦ childhood chicken pox vaccinations reduce the risk of shingles later
13-26
in life
Shingles
Poliomyelitis

Polio and ALS - diseases that cause destruction of
motor neurons and production of skeletal muscle
atrophy from lack of innervation

caused by the poliovirus

destroys motor neurons in brainstem and anterior
horn of spinal cord

signs of polio include muscle pain, weakness, and
loss of some reflexes
◦ followed by paralysis, muscular atrophy, and
respiratory arrest

virus spreads by fecal contamination of water
Amyotrophic Lateral Scerosis (ALS)

amyotrophic lateral sclerosis (ALS) – Lou
Gehrig disease
◦ destruction of motor neurons and muscular
atrophy
◦ also sclerosis (scarring) of lateral regions of the
spinal cord
◦ astrocytes fail to reabsorb the neurotransmitter
glutamate from the tissue fluid
 accumulate to toxic levels
◦ early signs – muscular weakness, difficulty speaking,
swallowing, and use of hands
◦ sensory and intellectual functions remain
unaffected
13-29
Spinal Nerves and Plexuses

Nerve Plexuses
◦ Complex, interwoven networks of nerve fibers
◦ Formed from blended fibers of ventral rami of
adjacent spinal nerves
◦ Control skeletal muscles of the neck and limbs
Spinal Nerves and Plexuses

The Four Major Plexuses of Ventral Rami
1. Cervical plexus
2. Brachial plexus
3. Lumbar plexus
4. Sacral plexus
Nerve Plexuses
Cervical
plexus
Brachial
plexus
C1
C2
C3
C4
C5
C6
C7
C8
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
Lesser occipital nerve
Great auricular nerve
Transverse cervical nerve
Supraclavicular nerve
Phrenic nerve
Axillary nerve
Musculocutaneous
nerve
Thoracic nerves
Nerve Plexuses
T12
L1
Lumbar
plexus
Sacral
plexus
Radial nerve
L2
L3
L4
L5
S1
S2
S3
S4
S5
Co1
Ulnar nerve
Median nerve
Iliohypogastric
nerve
Ilioinguinal
nerve
Lateral femoral
cutaneous nerve
Genitofemoral
nerve
Femoral nerve
Obturator nerve
Superior
Inferior
Gluteal nerves
Pudendal nerve
Saphenous nerve
Sciatic nerve
The Cervical Plexus

Includes ventral rami of spinal nerves C1–C5

Innervates neck, thoracic cavity,
diaphragmatic muscles

Major nerve
◦ Phrenic nerve (controls diaphragm)
The Brachial Plexus

Includes ventral rami of spinal nerves C5–T1

Innervates pectoral girdle and upper limbs

Major nerves
◦ Musculocutaneous nerve (lateral cord)
◦ Median nerve (lateral and medial cords)
◦ Ulnar nerve (medial cord)
◦ Axillary nerve (posterior cord)
◦ Radial nerve (posterior cord)
The Brachial Plexus
Dorsal scapular nerve
Trunks of
Brachial Plexus Suprascapular nerve
Spinal Nerves Forming
Brachial Plexus
Superior
Middle
Inferior
Musculocutaneous
nerve
Median nerve
Ulnar nerve
Radial nerve
Lateral antebrachial
cutaneous nerve
Superficial branch
of radial nerve
Deep radial nerve
Ulnar nerve
Median nerve
Palmar digital
nerves
Major nerves originating at the right brachial plexus, anterior view
C4
C5
C6
C7
C8
T1
The Lumbar Plexus

Includes ventral rami of spinal nerves T12–L4

Major nerves
◦ Genitofemoral nerve
◦ Lateral femoral cutaneous nerve
◦ Femoral nerve
The Sacral Plexus

Includes ventral rami of spinal nerves L4–S4

Major nerves
◦ Pudendal nerve
◦ Sciatic nerve

Two branches of the sciatic nerve
1. Fibular nerve
2. Tibial nerve
The Sacral Plexuses
Spinal Nerves
Forming the
Sacral Plexus
Lumbosacral trunk
L4 nerve
L5 nerve
Nerves of the
Sacral Plexus
S1 nerve
Superior gluteal
S2 nerve
Inferior gluteal
S3 nerve
Sciatic
Posterior femoral
cutaneous
S4 nerve
S5
Co1
Pudendal
Sacral plexus, anterior view
Reflexes

rapid, automatic responses to specific stimuli
◦ Show little variability
Preserve homeostasis by making rapid
adjustments in functions of organs or organ
systems
 In neural reflexes:

◦ Sensory fibers carry information from peripheral
receptors to integration center
◦ Motor fibers carry motor commands to peripheral
effectors

Reflex arc
◦ “Wiring” of a single reflex from receptor to
effector
Reflex Arc

Components of a reflex arc (neural path)
1. Receptor—site of stimulus action
2. Sensory neuron—transmits afferent impulses to
the CNS
3. Interneuron – (Integration center) within the
CNS
4. Motor neuron—conducts efferent impulses
from the integration center to an effector organ
5. Effector—muscle fiber or gland cell that
responds to the efferent impulses by contracting
or secreting
Stimulus
Reflex Arc
Skin
1 Receptor
Interneuron
2 Sensory neuron
3 Integration center
4 Motor neuron
5 Effector
Spinal cord
(in cross section)
Typical Reflex Arc
Numbers show the
sequence of impulses
through the spinal cord
(solid arrows).
Contraction of the
biceps brachii results in
flexion of the arm at
the elbow.
ZOOMING IN
• Is this a somatic or an
autonomic reflex arc? What
type of neuron is located
between the sensory and
motor neuron in the CNS?
The Flexor (Withdrawal) Reflexes
Sensory neuron
2 activates multiple
interneurons
+
+
+
+
+
+

flexor reflex – the quick
contraction of flexor muscles
resulting in the withdrawal of
a limb from an injurious
stimulus

requires contraction of the
flexors and relaxation of the
extensors in that limb

polysynaptic reflex arc –
pathway in which signals
travel over many synapses on
their way back to the muscle
+
+
5
Contralateral
motor neurons
to extensor
excited
3 Ipsilateral motor
neurons to flexor
excited
4 Ipsilateral flexor
contracts
+
+
6Contralateral
extensor
contracts
1 Stepping on glass
stimulates pain receptors
in right foot
withdrawal of right leg
(flexor reflex)
Extension of left leg
(crossed extension reflex)
A Flexor Reflex
Distribution within gray horns to other segments of the spinal cord
Painful
stimulus
Flexors
stimulated
Extensors
inhibited
KEY
Sensory neuron
(stimulated)
Excitatory
interneuron
Motor neuron
(stimulated)
Motor neuron
(inhibited)
Inhibitory
interneuron
The Babinski Reflexes
The plantar reflex
(negative Babinski
reflex), a curling of the
toes, is seen in healthy
adults.
The Babinski sign (positive
Babinski reflex) occurs in
the absence of descending
inhibition. It is normal in
infants, but pathological in
adults.
Medical Procedures Involving the
Spinal Cord

Lumbar puncture (spinal tap)
◦ Cerebrospinal fluid (CSF) removed for testing

Drug administration
◦ Anesthetic (an epidural)
◦ Pain medication
End of Presentation