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38
CHAPTER
Alterations in
Neuromuscular Function
Spinal Cord Injury
Injury to the Vertebral Column
Types and Classification of Spinal Cord Injury
Primary and Secondary Injuries
Classification
Acute Spinal Cord Injury
Disruption of Functional Abilities
Somatosensory and Skeletal Muscle Function
Autonomic Nervous System Function
Other Functions
The Organization and Control of Motor Function
The Motor Unit
The Motor Cortex
Spinal Reflexes
Myotatic or Stretch Reflex
Crossed-Extensor Reflex
Disorders of Muscle Tone and Movement
Disorders of Muscle Tone
Disorders of Muscle Movement
Skeletal Muscle and Peripheral Nerve Disorders
Skeletal Muscle Disorders
Muscle Atrophy
Muscular Dystrophy
Neuromuscular Junction Disorders
Myasthenia Gravis
Peripheral Nerve Disorders
Peripheral Nerve Injury and Repair
Peripheral Neuropathies
Back Pain and Intervertebral Disk Injury
Back Pain
Herniated Intervertebral Disk
Basal Ganglia and Cerebellum Disorders
The Basal Ganglia
Movement Disorders
Parkinson’s Disease
The Cerebellum
Cerebellar Dysfunction
Upper and Lower Motoneuron Disorders
Amyotrophic Lateral Sclerosis
Clinical Course
Multiple Sclerosis
Clinical Course
Diagnosis
Treatment
E
ffective motor function requires that muscles move and
that the mechanics of their movement be programmed in
a manner that provides for smooth and coordinated move-
ment. In some cases, purposeless and disruptive movements
can be almost as disabling as relative or complete absence of
movement.
THE ORGANIZATION AND CONTROL
OF MOTOR FUNCTION
As with other parts of the nervous system, the motor systems
are organized in functional hierarchy, with each concerned
with levels of function (Fig. 38-1). The highest level of func-
tion, which occurs at the level of the frontal cortex, is concerned
with the purpose and planning of the movement.
1
The lowest
level of the hierarchy occurs at the level of the spinal cord,
which contains the basic reflex circuitry needed to coordinate
the function of the motor units involved in the planned move-
ment. Several anatomically distinct pathways project in paral-
lel to the spinal cord from the higher motor centers. Above the
spinal cord is the brain stem, and above the brain stem is the
cerebellum and basal ganglia, structures that modulate the ac-
tions of the brain stem systems. Overseeing these supraspinal
structures are the motor centers in the cerebral cortex.
The Motor Unit
The major effects of the elaborate processing of movement in-
formation that takes place in the brain has to do with con-
traction of skeletal muscles. The neurons that control skeletal
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Chapter 38: Alterations in Neuromuscular Function
■
FIGURE 38-1
■
The motor control system. The final common
pathway transmits all central nervous system commands to the
skeletal muscles. This path is influenced by sensory input from the
muscle spindles and tendon organs (
dashed lines
) and descending
signals from the cerebral cortex and brain stem. The cerebellum
and basal ganglia influence the motor function indirectly, using
brain stem and cortical pathways.
muscle contraction are referred to as
motoneurons
or sometimes
as
alpha motoneurons
. A motor unit consists of one motoneuron
and the group of muscle fibers it innervates in a skeletal mus-
cle. The motoneurons supplying a motor unit are located in the
ventral horn of the spinal cord and are called
lower motoneurons
(LMNs) (Fig. 38-2). The synapse between a LMN and the mus-
cle fibers of a motor unit is called the
neuromuscular junction
.
Upper motoneurons (UMNs), which exert control over LMNs,
project from the motor strip in the cerebral cortex to the ven-
tral horn of the spinal cord and are fully contained within the
central nervous system (CNS).
Axons of the LMNs exit the spinal cord at each segment to
innervate skeletal muscle cells, including those of the limbs,
back, abdomen, and chest. Each LMN undergoes multiple
branching, making it possible for a single LMN to innervate 10
to 2000 muscle cells. In general, large muscles—those contain-
ing hundreds or thousands of muscle cells and providing gross
motor movement—have large motor units. This sharply con-
trasts with those that control the hand, tongue, and eye move-
ments, for which the motor units are small and permit very dis-
crete control.
In addition to the output from LMNs that innervate the
motor unit, the body uses information from a vast array of sen-
sory input to ensure the generation of correct patterns of mus-
cle activity. Much of this information goes to spinal cord re-
flexes that control muscle tone and coordinate the movement
of the extensor and flexor muscles used in walking and other
motor activities.
The Motor Cortex
Delicate, skillful, intentional movement of distal and especially
flexor muscles of the limbs and the speech apparatus is initiated
and controlled from the motor cortex located in the posterior
part of the frontal lobe. It consists of the primary, premotor, and
supplementary motor cortex
2
(Fig. 38-3). These areas receive in-
formation from the thalamus and the somesthetic (sensory)
cortex and indirectly from the cerebellum and basal ganglia.
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Unit Ten: Alterations in the Nervous System
Supplementary
(6,8)
KEY CONCEPTS
MOTOR SYSTEMS
Motor (4)
Premotor (6)
Somatosensory
(3,1,2)
Motor systems require upper motoneurons (UMNs)
that project from the motor cortex to the brain stem
or spinal cord, where they directly or indirectly in-
nervate the lower motoneurons (LMNs) of the con-
tracting muscles; sensory feedback from the involved
muscles that is continuously relayed to the cerebel-
lum, basal ganglia, and sensory cortex; and a func-
tioning neuromuscular junction that links nervous
system activity with muscle contraction.
■
Primary
visual
(17)
Input from the basal ganglia and cerebellum pro-
vides the background for the more crude,
supportive movement patterns.
■
Premotor
cortex
Motor
cortex
(4)
Somatosensory
cortex
(3,1,2)
(8)
(6)
Frontal
eye fields
(part of
area 8)
The efficiency of the movement by the motor system
depends on a background of muscle tone provided
by the stretch reflex and vestibular system input to
maintain stable postural support.
■
The
primary motor cortex
(area 4), also called the
motor strip
,
is located on the rostral surface and adjacent portions of the
central sulcus. The primary motor cortex controls discrete mus-
cle movement sequences and is the first level of descending
control for precise movements. The neurons in the primary
motor cortex are arranged in a somatotopic array or distorted
map of the body called the
motor homunculus
(Fig. 38-4).
3
The
body parts that require the greatest dexterity have the largest
cortical areas devoted to them. More than one half of the pri-
mary motor cortex is concerned with controlling the muscles
of the hands, of facial expression, and of speech.
2
The premotor cortex (areas 6 and 8), which is located just
anterior to the primary motor cortex, sends some fibers into the
corticospinal tract but mainly innervates the primary motor
strip. A movement pattern to accomplish a particular objective,
such as throwing a ball or picking up a fork, is programmed by
the prefrontal association cortex and associated thalamic nu-
clei. The
supplementary motor cortex
, which contains representa-
tions of all parts of the body, is located on the medial surface
of the hemisphere (areas 6 and 8) in the premotor region. It is
intimately involved in the performance of complex, skillful
movements that involve both sides of the body.
The primary motor cortex contains many layers of pyramid-
shaped output neurons that project to the same side of the cor-
tex (
i.e.
, premotor and somesthetic areas), project to the oppo-
site side of the cortex, or descend to subcortical structures such
as the basal ganglia and thalamus. The large pyramidal cells lo-
cated in the fifth layer project to the brain stem and spinal cord.
The axons of these UMNs project through the subcortical white
matter and internal capsule to the deep surface of the brain
stem, through the ventral bulge of the pons, and to the ventral
surface of the medulla, where they form a ridge or pyramid (see
Fig. 38-1). At the junction between the medulla and cervical
spinal cord, 80% or more of the UMN axons cross the midline
to form the lateral corticospinal tract in the lateral white mat-
ter of the spinal cord. This tract extends throughout the spinal
Broca's
area
(45,44)
Primary
visual
cortex
(17)
Primary
auditory
cortex
(44)
Vestibular
cortex
■
FIGURE 38-3
■
Primary motor cortex. (
Top
) The location of the
primary, premotor, and supplementary cortex on the medial sur-
face of the brain. (
Bottom
) The location of the primary and pre-
motor cortex on the lateral surface of the brain. (Courtesy of
Carole Russell Hilmer, C.M.I.)
cord, with roughly 50% of the fibers terminating in the cervical
segments, 20% in the thoracic segments, and 30% in the lum-
bosacral segments. Most of the remaining uncrossed fibers
travel down the ventral column of the cord, mainly to cervical
levels, where they cross and innervate contralateral LMNs.
By convention, motor tracts have been classified as belong-
ing to one of two motor systems: the pyramidal and extrapyra-
midal systems. According to this classification system, the pyra-
midal system consists of the motor pathways originating in
the motor cortex and terminating in the corticobulbar fibers
in the brain stem and the corticospinal fibers in the spinal cord.
The corticospinal fibers traverse the ventral surface of the me-
dulla in a bundle called the
pyramid
before decussating or cross-
ing to the opposite side of the brain at the medulla-spinal cord
junction, thus the name
pyramidal system
. Other fibers from the
cortex and basal ganglia also project to the brain stem reticular
formation and reticulospinal systems, following a more ancient
pathway to LMNs of proximal and extensor muscles. These
fibers do not decussate in the pyramids, thus the name
extra-
pyramidal system
. Disorders of the pyramidal tracts (
e.g.
, stroke)
are characterized by spasticity and paralysis and those affecting
699
Chapter 38: Alterations in Neuromuscular Function
caused by dysregulated function of the stretch reflex are seen in
persons with conditions such as spinal cord injury and stroke.
The myotatic reflex uses specialized afferent sensory end-
ings in skeletal muscles and tendons to relay information re-
garding the sense of body position, movement, and muscle
tone to the CNS. Information from these sensory afferents is
relayed to the cerebellum and cerebral cortex and is experi-
enced as the sense of body movement and position (
proprio-
ception
). To provide this information, the muscles and their
tendons are supplied with two types of stretch receptors: mus-
cle spindle receptors and Golgi tendon organs (Fig. 38-5A).
The muscle spindles, which are distributed throughout the
belly of a muscle, provide information about muscle length
and rate of stretch. The
Golgi tendon organs
are found in mus-
cle tendons and transmit information about muscle tension or
force of contraction at the junction of the muscle and the ten-
don that attaches to bone. A likely role of the tendon organs is
to equalize the contractile forces of the separate muscle groups,
spreading the load over all the fibers to prevent the local mus-
cle damage that might occur when small numbers of fibers are
overloaded.
The muscle spindles consist of a group of specialized minia-
ture skeletal muscle fibers called
intrafusal fibers
that are encased
in a connective tissue capsule and attached to muscle fibers
(
i.e.
, extrafusal fibers) of a skeletal muscle (Fig. 38-5A). An af-
ferent sensory neuron, which spirals around the intrafusal fi-
bers, transmits information to the spinal cord. The extrafusal
fibers and the intrafusal fibers are innervated by motoneurons
that reside in the ventral horns of the spinal cord. Extrafusal
fibers are innervated by large alpha motoneurons that produce
contraction of the muscle. The intrafusal fibers are innervated
by gamma motoneurons that adjust the length of the intrafusal
fibers to match that of the extrafusal fibers.
The intrafusal muscle fibers function as stretch receptors.
When a skeletal muscle is stretched, the spindle and its intra-
fusal fibers are stretched, resulting in increased firing of its af-
ferent fibers. The increased firing of the afferent neurons synap-
tically depolarizes the alpha motoneuron fibers. This causes the
extrafusal muscle fibers to contract, thereby shortening the mus-
cle. The knee-jerk reflex that occurs when the knee is tapped
with a reflex hammer tests for the intactness of the myotatic re-
flex arc in the quadriceps muscle (Fig. 38-5B).
Axons of the spindle afferent neurons enter the spinal cord
through the several branches of the dorsal root. Some branches
end in the segment of entry, and others ascend in the dorsal
column of the cord to the medulla of the brain stem. Segmental
branches make connections, along with other branches, that
pass directly to the anterior gray matter of the spinal cord and
establish monosynaptic contact with each of the LMNs that
have motor units in the muscle containing the spindle receptor.
This produces an opposing muscle contraction. Another seg-
mental branch of the same afferent neuron innervates an inter-
nuncial neuron that is inhibitory to motor units of antagonis-
tic muscle groups. Inhibition of these muscle units helps in
opposing muscle stretch. Branches of the afferent axon also as-
cend into the dorsal horn of the adjacent segments, influencing
intersegmental reflex function. Ascending fibers from the stretch
reflex ultimately provide information about muscle length to
the cerebellum and cerebral cortex.
The role of afferent spindle fibers is to inform the CNS of the
status of muscle length. When a skeletal muscle lengthens or
the extrapyramidal tracts (
e.g.
, Parkinson’s disease) by invol-
untary movements, muscle rigidity, and immobility without
paralysis. This classification is no longer used extensively. As in-
creased knowledge regarding motor pathways has emerged, it
has become evident that the extrapyramidal and pyramidal sys-
tems are extensively interconnected and cooperate in the con-
trol of movement.
1
Spinal Reflexes
Reflexes are coordinated, involuntary motor responses initiated
by a stimulus applied to peripheral receptors. Some reflexes,
such as the flexion-withdrawal reflex, initiate movements to
avoid hazardous situations; whereas others, such as the stretch
or crossed-extensor reflex, serve to integrate motor movements
so they function in a coordinated manner. The anatomic basis
of a reflex consists of (1) an afferent neuron, (2) the connection
or synapse with CNS interneurons that communicate with the
effector neuron, and (3) the effector neuron that innervates a
muscle. Reflexes are essentially “wired into” the CNS so that
they are always ready to function; with training, most reflexes
can be modulated to become parts of more complicated move-
ments. A reflex may involve neurons in a single cord segment
(
i.e.
, segmental reflexes), several or many segments (
i.e.
, inter-
segmental reflexes), or structures in the brain (
i.e.
, supra-
segmental reflexes).
Myotatic or Stretch Reflex
The myotatic (“myo” from the Greek for “muscle,” “tatic” from
the Greek for “stretch”) or stretch reflex controls muscle tone
and helps maintain posture. Stretch reflexes can be evoked in
many muscles throughout the body and are routinely tested
(
e.g.
, knee-jerk reflex) during the clinical examination for the
diagnosis of neurologic conditions. Disorders of muscle tone
700
Unit Ten: Alterations in the Nervous System
simultaneously activate both alpha and gamma motoneurons
so that the sensitivity of the spindle fibers is coordinated with
muscle movement.
Central control over the gamma LMN mechanism permits
increases or decreases in muscle tone in anticipation of changes
in the muscle force required to oppose ongoing conditions,
such as when weight is about to be lifted. The CNS, through its
coordinated control of the muscle’s alpha LMNs and the spin-
dle’s gamma LMNs, can suppress the stretch reflex. This occurs
during centrally programmed movements, such as pitching a
baseball, permitting the muscle to produce its greatest range of
motion. Without this programmed adjustability of the stretch
reflex, any movement is immediately opposed and prevented.
Crossed-Extensor Reflex
The crossed-extensor reflex, in which the limb on one side of
the body extends as the limb on the other side relaxes, pro-
vides the basis for postural stability during walking (Fig. 38-6).
4
For example, when the crossed-extensor reflex produces relax-
ation of antigravity muscles (with flexion) of one leg as we
walk, the contralateral component produces contraction and
extension of the opposite leg. Intersegmental connections of
the crossed-extensor reflex between the lumbar and cervical
spinal segments also accounts for the swinging of the arms that
accompanies walking.
Disorders of Muscle Tone and Movement
Disorders of Muscle Tone
In the muscles that are supporting body weight, the stretch re-
flex operates continuously, producing a continuous resistance
to passive stretch called
muscle tone
. Muscle tone is evidenced
by the resistance to passive movement around a joint. Disorders
of skeletal muscle tone are characteristic of many nervous sys-
tem pathologies. Any interruption of the myotatic reflex circuit
by peripheral nerve injury, pathology of the neuromuscular
junction and of skeletal muscle fibers, damage to the corti-
cospinal system, or injury to the spinal cord or spinal nerve
root results in disturbance of muscle tone. Muscle tone may be
■
FIGURE 38-5
■
(
A
) Spinal cord innervation of the muscle spin-
dles. Cell bodies from both the alpha motoneuron that innervates
the extrafusal muscle fiber and the gamma motoneuron that in-
nervates the intrafusal fiber reside in the ventral horn of the same
segment of the spinal cord. (
B
) The knee-jerk reflex. Stretching of
the extrafusal fiber by tapping with a reflex hammer leads to
lengthening of the intrafusal fiber and increased firing of the type
Ia afferent fiber. Impulses from the Ia fiber enter the dorsal horn
of the spinal cord and make monosynaptic contact with the ven-
tral horn alpha motoneuron supplying the extrafusal fibers in the
quadriceps muscle. The resultant contraction (shortening) of the
quadriceps muscle is responsible for the knee-jerk response.
shortens against tension, a feedback mechanism needs to be
available for readjustment such that the spindle apparatus re-
mains sensitive to moment-to-moment changes in muscle
stretch, even while changes in muscle length are occurring. This
is accomplished by the gamma motoneurons that adjust spin-
dle fiber length to match the length of the extrafusal muscle
fiber. Descending fibers of motor pathways synapse with and
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