Chapter 13: Spinal Cord and Spinal Nerves
General Organization
PNS
 neuron cell bodies are located in

bundled axons are called
CNS
 collection of neuron cell bodies with a common function is called a
o nucleus is

brain surface is covered by a thick layer of grey matter called

white matter of CNS is made up of
, which is a group of
is a bundle of
that share a common function and location.

Centers and tracts link the brain with the rest of the body
, which
o Sensory pathways
o Motor pathways
Gross Anatomy
 18” long, .55” wide, runs down vertebral foramen to

Posterior median sulcus –

Anterior median fissure –

Grey matter –
o
Dominated by cell bodies of neurons, nuroglia and unmyelinated axons
 Sensory nuclei
vs.
Motor nuclei
o
Projections are called horns:
 Posterior grey horns contain
o


Anterior grey horns contain somatic motor nuclei

Lateral grey horns (only located in thoracic and lumbar areas) contain
Grey commissures connect one side of the cord to the other
White matter
o
Contains a large number of
o
Divided into three columns

Posterior white columns


Tracts (or fascicles) – bundles of axons in the CNS with
 Ascending tracts carry
 Descending tracts convey motor commands
o

Anterior white columns
Lateral white columns
M.S. –
o Myelin degeneration within the white matter of lateral & posterior columns

Cervical enlargement (greater area of grey matter)
o Expanded area of the spinal cord that supplies nerves to

Lumbar enlargement
o Innervated structures of the

Inferior to lumbar enlargement the spinal cord tapers (conus medullaris)
o Filum terminale (slender strand of fibrous tissues) provides longitudinal support
of the

Dorsal roots – axons of

Cell bodies of these neurons form

Ventral roots – axons of

Distal to the dorsal root ganglion, each sensory and motor roots are bound into

Mixed nerves

31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal
Spinal Meninges
 layers of specialized membranes

provides physical stability and shock absorption in the vertebral canal
Three layers
1. Dura Mater – tough fibrous outer covering
 Superiorly fuses with

Inferiorly fuses with fibers of the filum terminale to form the

Supported laterally by loose connective tissue and adipose tissue in the epidural
space

Anesthetics are injected into this space
2. Arachnoid – middle layer
 Loose fitting epithelium

A delicate network of collagen and elastic fibers makes up the

Space between trabeculae and pia mater is the
o Contains
o Spinal Tap – (lumbar puncture)
3. Pia Mater – innermost layer
 Meshwork of elastic and collagen fibers firmly attached to

Paired denticulate ligaments extend from pia mater through arachnoid to
the dura mater to
Meningitis –

Disrupts CSF and circulation, damages nuroglia and neurons
Spinal nerves: 31 pairs spinal nerves
- Supply all parts of the body except the head and some areas of the neck
- All are mixed nerves
- Named for their point of issue from the spinal cord
Connective tissue coverings: epineurium, perineurium, endoneurium
Connected to spinal cord & formed by 2 roots
(note: roots are medial to spinal nerves and each root is strictly sensory or motor)
dorsal:
ventral:
Branches:
(note: rami lie distal to spinal nerves and carry both sensory & motor fibers)
dorsal ramus - posterior: supply skin and muscles of back
ventral ramus - anterior, longer
-supply lateral and anterior aspects of thoracic area
Rami communicantes
- white ramus – myelinated axons of preganglionic visceral moter nerves
-
grey ramus – unmyelinated axons of postganglionic fibers
These rami also carry sensory fibers
 Dermatomes – specific region of the skin
Peripheral Neuropathies (palsies) – loss of sensory or motor function
 Location of dermatome loss
Shingles – viral infection within dorsal root ganglia
 Causes a painful rash at the area of distribution of the affected sensory nerves
Ventral Rami in cervical, lumbar, and sacral areas form plexuses (networks): give rise to
nerves supplying skin, muscles, and joints of upper and lower extremities.
Nerve Plexuses:
Cervical Plexus: supplies head, neck, shoulder, & chest
-phrenic nerve innervates
Brachial Plexus: supplies axilla, skin & muscles of upper extremity
-Roots, Trunks, Cords
Axillary
Musculotaneous
Median
Ulnar
Radial
Lumbar Plexus: supplies lower abdomen, anterior and medial portions of lower extremity
 femoral nerve
Sacral Plexus: innervates lower back, pelvis, posterior surface of thigh, leg, and foot
 sciatic nerve
1. tibial
2. common fibular
Functional Organization
 _________ sensory neurons, __________ motor neurons, ________ interneurons
Interneurons of the CNS are organized into smaller neuronal pools

Functional groups of

Range between a

Interpret, plan, and coordinate
5 Circuit patterns
1. Divergence- one neuron spreads into several
2. Convergence- several neurons synapse at the same time at a
3. Serial Processing- one neuron stimulates the next and so on
4. Parallel- several neurons process the information at the same time as a result of
divergence →
5. Reverberation- A collateral from one neuron extends back toward the surface of
the source of input further stimulates presynaptic neurons →
Reflexes
 Rapid, automatic responses to
 Usually produces
Reflex Arc- Usually removes or opposes stimulus
1. Arrival of stimulus- specialized cells or dendrites are activated by physical or
chemical changes
2. Activation of a sensory neuron- formation and propagation of an action potential
along one maybe two sensory neurons.
3. Information Processing- one to several interneurons starts with a
neurotransmitter arrives at the post synaptic membrane of an interneuron to
produce an EPSP →
4. Activation of a motor neuron-stimulated motor neurons carry impulse out ventral
root to periphery
5. Response of peripheral effectors- release of neurotransmitter by a motor neuron
leads to an action of
Types of Reflexes
Innate vs Acquired
Spinal vs Cranial
Somatic vs visceral
Monosynaptic vs Polysynaptic
Monosynaptic reflexes- very little delay
1. Stretch reflex- regulation of skeletal muscle length

A stretch resulting in

Patellar reflex- knee jerk: stretching of the patellar tendon results in
2. Muscle spindles- sensory receptors on intrafusal muscle fibers that

gamma efferents are able to adjust to the sensitivity of muscle spindle

postural reflex-
Polysynaptic reflexes

more complicated and involve either

several reflexes produce a

involve pools of

involve reciprocal inhibition
1. Tendon reflex- monitors external tension produced during muscle contractions

stimulates IPSP’s
2. Withdrawal reflex- moves affected parts

a combination of the flexor reflex and reciprocal inhibition
3. Crossed extensor reflex

compliments the
Inhibitory effects
1. Positive Babinski- fans toes in an infant
i. Response disappears as
2. Negative Babinski (plantar reflex) –
i. Inhibitory response