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Chapter 13:

The Spinal Cord, Spinal Nerves,
and Spinal Reflexes
Introduction
The nervous system has input pathways that rout sensations, and processing centers that
prioritize and distribute information
...


13-1
The brain and spinal cord make up the central nervous system
(CNS), and the cranial nerves and spinal nerves make up the
peripheral nervous system (PNS)

Reflexes) are quick, automatic responses triggered by specific stimuli
...


Somatic nervous system= conscious and subconscious control of skeletal muscles
Autonomic nervous system= control of visceral functions

The ANS has processing centers in the:
• Brain
• Spinal cord
• Peripheral nervous system

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Chapter 13:

13-2
The spinal is surrounded by 3 meninges and carries sensory and
motor information
Gross Anatomy of the Spinal Cord
The adult spinal cord is approximately 45cm (18in
...
55in
...


Posterior median sulcus) the dorsal surface of the spinal cord where it has a shallow
longitudinal groove
...

The amount of gray matter is greatest in segments of the spinal cord dedicated to the sensory
and motor control of the limbs
...


Cervical enlargement) supplies nerves to the shoulder and upper limbs
...

Conus medullaris) inferior to the lumbar enlargement, where the spinal cord becomes tapered
and conical
...

The entire spinal cord can be divided into 31 segments on the basis of the origins of the spinal
nerves
...
It is located near the spinal
cord
...

The axons of the neurons form the dorsal roots, which bring sensory information into the spinal
cord
...


Spinal nerve) distal to each root ganglion, the sensory and motor nerves are bound together into
this single nerve
...

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Chapter 13:

Cauda equine) when seen in gross dissection, the filum terminale and the long ventral and dorsal
roots resemble a horse’s tail
...

The spinal meninges consists of 3 layers:

dura mater
The arachnoid mater
The pia mater

1
...

3
...
(the cranial meninges have the same 3 layers)
Meningitis) inflammation of the meninges (meningeal membranes)
...


The Dura Mater
Dura matter) the layer that forms the outer most covering of the spinal cord
...

┌─Between the dura mater and the walls of the vertebral canal lies the…
...

Spinal dura mater does not have extensive, firm connections to the surrounding vertebrae
...
It continues along the sacral canal,
ultimately blending into the periosteum of the coccyx
...


Arachnoid mater) the middle meningeal layer
...
It is filled with CSF which
acts as a shock absorber and a diffusion medium
...


The Pia Mater
Pia mater) the inner most meningeal layer
...


Denticulate ligaments) (along the length of the spinal cord (originate on the sides of the spinal
cord))
...


13-3
Gray matter integrates information and initiates commands, and
white matter carries information from place to place
Together, the anterior median fissure and the posterior median sulcus divide the spinal cord into
left and right sides
...


Horns) are the areas of gray matter on each side of the spinal cord
...

Sensory nuclei) receive and relay sensory information from peripheral receptors
...

A frontal section along the length of the central canal for the spinal cord separates the sensory
(posterior, or dorsal) nuclei from the motor (anterior, or ventral) nuclei
...

Anterior gray horns) contain somatic motor nuclei
...


Gray commissures) (a joining together)
...

On each side of the spinal cord, in medial to lateral sequences, are somatic motor nuclei that
control:
1
...

3
...


Muscles that position the pectoral girdle
Muscles that move the arm
Muscles that move the forearm and hand
Muscles that move the hand and fingers

Organization of White Matter
The white matter on each side of the spinal cord can be divided into 3 regions called columns:

•

Posterior white columns: lies between the posterior gray horns and the posterior
median sulcus
...
They are interconnected by the anterior white commissure, a region where axons cross
from 1 side of the spinal cord to other
...


Tract (or fasciculus)) a bundle of axons in the CNS that is somewhat uniform in diameter,
myelination, and propagation speed
...


Short tracts carry sensory or motor signals
...


Ascending tracts) (afferent)
...

Descending tracts) (efferent)
...


13-4
Spinal nerves from plexuses that are named according to their
level of emergence from the vertebral canal
Anatomy of Spinal Nerves
Every segment of the spinal cord is connected to a pair of spinal nerves
...
These
layers, best seen in sectional view, are comparable to those associated with skeletal muscles
...
Arteries and veins
penetrate the epineurium and branch within the perineurium
...
Capillaries leaving
the perineurium branch into the endoneurium
...


Endoneurium) (innermost layer) extend from the perineurium and surround individual axons
...

Each pair of spinal nerves supplies its own dermatome
...


Nerve Plexuses
Nerve plexus) as they converge, the ventral rami of adjacent spinal nerves blend their fibers,
producing a series of compound nerve trunks
...

These 4 major plexuses are:
1
...
brachial plexus
3
...
sacral plexus
These nerves arising at these plexuses contain sensory as well as motor fibers because these
nerves fuse from the fusion of ventral rami
...
The branches innervate the
muscles of the neck and extend into the thoracic cavity, where they control the diaphragmatic muscles
...
The left and right phrenic nerve supply the
diaphragm, a key respiratory muscle
...


The Brachial Plexus
Brachial plexus) innervates the pectoral girdle and upper limb, with contributions from the
ventral rami of spinal nerves C5-T1
...

The nerves that form this plexus originate from trunks and cords
...

Cords) are smaller branches that originate at trunks
...
Hence, we have superior, middle, and inferior trunks, and lateral, medial,
and posterior cords
...


Ulnar nerve) the other major nerve of the medial cord
...


The Lumbar Plexus and Sacral Plexus
The lumbar plexus and the sacral plexus arise from the lumbar and sacral segments of spinal
the spinal cord, respectively
...
The lumbar plexus contains axons from the ventral rami of spinal nerves T 12-L1
...


2major nerves arise at this plexus:
•

Sciatic nerve: passes posterior to the femur, deep to the long head of the biceps femoris muscle
...
The sural nerve, formed by branches of the fibular nerve, is a sensory nerve innervating the lateral
portion of the foot, and a section of this may be used for nerve grafts
...

Sensory neurons: deliver information to the CNS
Motor neurons: distribute commands to peripheral effectors, such as skeletal muscles
...


The billions of interneurons of the CNS are organized into a much smaller number of neuronal
pools
...
A neuronal pool may be scattered,
involving neurons in several regions of the brain, or localized, with neurons restricted to one specific
location in the brain or spinal cord
...
Divergence: is the spread of info from 1 neuron to several neurons, from 1 pool to multiple
pools
...


2
...
Several patterns of
activity in the presynaptic neurons can therefore have the same effect on the postsynaptic neuron
...
Serial processing: info is relayed in a stepwise fashion, from 1 neuron to another or from 1
neuronal pool to the next
...


4
...
Divergence must take place before parallel processing can occur
...


5
...
Reverberation is like a feedback
loop involving neurons
...

The simplest steps occur within the PNS and the spinal cord
...

Reflexes preserve homeostasis by making rapid adjustments in the function of organs or organ
systems
...

The 5 steps in a simple neural reflex are:
1
...


2
...
This info reaches the spinal cord by way of the dorsal root
...
However, step 2 may involve different cells
...
Information Processing in the CNS: info processing of a sensory innervation is performed and the
motor neuron performs the info processing
...
the Activation of a Motor Neuron: the axons of the stimulated motor neurons carry action potentials
into the periphery
...
the Response of a Peripheral Effector: the release of neurotransmitters by motor neurons at axon
terminals then leads to a response by the peripheral effector
...


Classification of Reflexes
Reflexes are classified on the basis of:
1
...

3
...


their development
the nature of the resulting motor response
the complexity of the neural circuit involved
the site of information processing

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Chapter 13:

Development of Reflexes
Innate reflexes) (ingrained) result from the connections that form between neurons during
development
...


Acquired reflexes) are more complex learned reflexes
...

Visceral reflexes) or autonomic reflexes
...
The
movements are neither delicate nor precise
...
Polysynaptic reflexes are more
complicated and have at least 1 interneuron between the sensory neuron and the motor neuron, they
have a longer delay between stimulus and response
...


13-7
Spinal reflexes vary in complexity
Spinal reflexes range in complexity from simple monosynaptic reflexes involving a single segment of
the spinal cord to polysynaptic reflexes that involve many segments
...
Many segments interact
to produce a coordinated, highly variable motor response
...
These
reflexes control the most rapid, stereotyped (preexisting, mechanically repetitive) motor responses of
the nervous system to specific stimuli
...
Automatically regulates skeletal muscle
length
...

•
•

•

When a physician your patellar tendon with a reflex hammer, receptors in the
quadriceps muscle are stretched
...

This leads to a reflexive contraction of the stretched muscle that extends the knee in
a brief kick

To Summarize
The stimulus (increases muscle length) activates a sensory neuron, which triggers an
immediate motor response (contraction of the stretched muscle) that counteracts the
stimulus
...


Muscle Spindles
Muscle spindles) the sensory receptors involved in the stretch reflex
...

↓

Intrafusal muscle fibers) a bundle of small, specialized skeletal fibers
...
These fibers are responsible for the
resting muscle tone and, at greater levels of stimulation, for the contraction of the entire muscle
...
Their axons are called
gamma efferents
...
These are reflexes that help us maintain a normal
upright posture
...

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Chapter 13:

Polysynaptic Reflexes
These reflexes can produce far more complicated responses than can monosynaptic reflexes
...


Tendon reflex) monitors the external tension produced during a muscular contraction and
prevents tearing or breaking of the tendons
...

As a result, a skeletal muscle generally cannot develop enough tension to break its tendon
...

Flexor reflex) a representative withdrawal reflex, affects the muscles of a limb
...
(When 1 set of motor neurons is stimulated, those neurons that control
antagonistic muscles are inhibited)
...

The crossed extensor reflex involves a contralateral reflex arc, because the motor response on
the side opposite the stimulus
...


Continued on next page

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Chapter 13:
All polysynaptic reflexes share the following basic characteristics:
•

They Involve Pools of Interneurons: processing takes place in pools of interneurons before motor neurons
are activated
...


•

They Involve Reciprocal Inhibition: this coordinates muscle contractions and reduces resistance to
movement
...


•

Several Reflexes May Cooperate to Produce a Coordinated, Controlled Response: as a reflex
movement gets under way, antagonistic muscles are inhibited
...
However, higher centers can have a profound effect on the performance of a reflex
...


Voluntary Movements and Reflex Motor Patterns
Spinal reflexes produce consistent, stereotyped motor neuron patterns that are triggered by
specific external stimuli
...
(A single command triggers a complex, predetermined sequence of events) (such as
initiating walking and then the body takes over the repetitional movement)
...


Reinforcement) an enhancement of spinal reflexes, when many of the excitatory synapses are
chronically active, the postsynaptic neuron can enter a state of generalized facilitation
...

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Chapter 13:

Related Clinical Terms
areflexia: absence of reflexes
Brown-Sequard syndrome: loss of sensation and motor function that results from unilateral spinal cord
lesions
...

Paraplegia: paralysis of the lower limbs
quadriplegia: paralysis involving loss of sensation and motor control of the upper and lower limbs
spinal shock: term applied to all phenomena surrounding physiological or anatomical transection of the spinal cord
that results in temporary loss or depression of all or most spinal reflex activity inferior to the level of
injury

tabes dorsalis: slow progressive degeneration of the myelin layer of the sensory neurons of the spinal cord that
occurs in the tertiary (3rd) phase of syphilis

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