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34 MOTOR SPEECH DISORDERS: DIAGNOSIS AND TREATMENT
Basal ganglia
Substantia nigra
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Several other subcortical gray matter structures inuence the
basal ganglia. One of them is the substantia nigra (Figure 2–11).
It is connected to the striatum via its own neural circuit. The neural tract from the substantia nigra to the striatum contains a large
number of neurons that produce the neurotransmitter dopamine.
Many neurons of the striatum are dependent on dopamine for
proper functioning. If the levels of dopamine from the substantia
nigra are lowered in the striatum, the results include muscular
rigidity, tremor, gait disturbances, and difculty initiating movement. This decrease in dopamine in the striatum can occur either
as part of a disease process (e.g., Parkinson’s disease) or through
other means, such as an adverse effect of antipsychotic drugs that
block the production of dopamine. The motor speech disorder
associated with parkinsonism is called hypokinetic dysarthria,discussed in Chapter 8.
Another class of movement disorders, known as hyperkinetic
disorders, is also identied with damage to the basal ganglia. The
symptoms of these disorders can be dramatically different from
the tight, restricted movements seen in parkinsonian-type disorders. Huntington’s disease (also known as Huntington’s chorea) is
a good example of a hyperkinetic movement disorder. It is a fatal,
FIGURE 2–11. A coronal section of the brain showing the position of the
substantia nigra in relation to the basal ganglia.
2. THE MOTOR SYSTEM 35
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inherited disease that results in the progressive loss of neurons in
the striatum and other areas of the brain. The symptoms of the disorder include rapid, involuntary movements of the extremities, face,
and tongue. As the disease progresses, the movements increase in
intensity and could begin to affect the muscles of the torso. Dementia and behavioral problems eventually become evident as well. The
movements of the hands, arms, and legs in an individual with Huntington’s disease have sometimes been described as “dancelike”
and “graceful.” Although accurate to some extent, such descriptions
minimize the debilitating effects these involuntary movements have
on an individual’s voluntary movements. The motor speech disorder found in Huntington’s disease and similar disorders is called
hyperkinetic dysarthria, which is examined in Chapter 9.
Cerebellum
The cerebellum helps to regulate muscle tone, maintain balance,
and coordinate skilled motor movements. It is attached to the back
of the brainstem and lies just below the occipital lobe of the
cerebrum (Figure 2–12). Its name, which literally means “little cerebrum,” comes from early anatomists, who thought it was an additional, smaller brain. This erroneous conclusion is understandable
because the cerebrum and cerebellum have a similar outward
appearance. Like the cerebrum, the cerebellum has two hemispheres
that are divided by a longitudinal ssure. Its surface also contains
many convolutions and grooves—more, in fact, than are found
on the cerebrum. Because of these numerous convolutions, the
cerebellum has a surface area that is nearly 75% of that of the cerebral cortex.
Like the basal ganglia, the cerebellum also receives neural
impulses of intended motor movements from the association
cortex. In addition, it receives sensory input from the vestibular
labyrinth of the inner ear and from visual, tactile, auditory, and proprioceptive sensory receptors located throughout the body, all of
which give the cerebellum access to information about the body’s
balance, position, and posture. It is thought that the cerebellum
takes the preliminary motor impulses from the association cortex
and integrates them with the sensory information available to it.
The cerebellum adjusts and renes the motor impulses according
to the body’s immediate circumstances and sends these processed
motor signals to the primary motor cortex via the thalamus. However, not all of the motor output from the cerebellum goes to the
thalamus. The cerebellum also has efferent neural tracts that indirectly synapse with descending extrapyramidal tracts (discussed
below). Through these connections with the extrapyramidal motor
Thalamus
Cerebellum
INFERIOR VIEW
Primary
fissure
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Cerebral
aqueduct
Pons
4th ventricle
LATERAL VIEW
Superior cerebellar
peduncle
Nodulus of
vermis
4th ventricle
Anterior lobe
Vermis
Middle cerebellar
peduncle
Inferior
cerebellar
peduncle
Flocculus
Middle lobe
FIGURE 2–12. The cerebellum is attached to the posterior of the brainstem.
36
2. THE MOTOR SYSTEM 37
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neurons, the cerebellum has a relatively direct inuence on such
motor activities as walking and maintaining posture.
Because of its many afferent and efferent connections with
diverse parts of the nervous system, a cerebellum that is damaged
can result in a variety of disorders. One of these is ataxia, which
is a disturbance in the speed, range, and direction of movements.
The muscle groups near the shoulders and pelvis might be particularly affected. The gait of an individual with ataxia is widebased, lurching, and stumbling and is often described as having a
“drunken” character. Intention tremor is also found with lesions
of the cerebellar hemispheres. This type of tremor is observed only
during the performance of voluntary movements, such as reaching for a glass of water. It is not present while an individual is at
rest. Other disorders include involuntary oscillatory movements
of the eyes (nystagmus), increased or decreased muscle tone, and
disturbances of equilibrium. The motor speech disorder usually
associated with cerebellar lesions is ataxic dysarthria, which isdis
cussed in Chapter 7.
-
Thalamus
The thalamus is yet another important subcortical gray matter
structure. Located deep to the basal ganglia and to the lateral
sides of the third ventricle (Figure 2–13), the thalamus has been
described as the doorway through which subcortical systems of the
nervous system communicate with the cerebral cortex. It receives
neural inputs of planned motor movements from both the basal
ganglia and the cerebellum. Exactly what it does with these signals
is not precisely understood. What is known, however, is that the
thalamus has a vast amount of somatosensory information available to it. Practically every sensory impulse from the body passes
through the thalamus on its way to the cortex. It is believed that the
thalamus uses this sensory information to further rene the motor
impulses from the basal ganglia and cerebellum.
Primary Motor Cortex
The primary motor cortex receives the neural motor impulses that
have been processed, smoothed, and coordinated by the basal ganglia, the cerebellum, and thalamus. The neurons in the primary
motor cortex have axons that are among the longest in the body;
many extend all the way from the cortex to the lower portions of
the spinal cord. These axons make up much of the descending
38 MOTOR SPEECH DISORDERS: DIAGNOSIS AND TREATMENT
Cingulate gyrus
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Corpus callosum
Septal area
Fornix
Thalamus
Orbital cortex
Anterior temporal cortex
Amygalia
Entorhinal cortex
FIGURE 2–13. A lateral view of the thalamus in relation to the cerebrum.
motor tract called the pyramidal system. Direct electrical stimulation of the primary motor cortex has shown that its neurons are
arranged in an inverted body scheme (Figure 2–14). When neurons near the bottom of the precentral gyrus are stimulated with
an electrical probe, contractions occur in the muscles of the head
and neck. In contrast, muscle contractions are observed in the leg
and foot when neurons near the top of the gyrus are stimulated.
has revealed another important nding: Stimulation never elicits
a complex, coordinated motor movement. Only simple muscle contractions are observed. This implies that the primary motor cortexis not the designer of purposeful, sequenced movements. If it
were, electrical stimulation of its neurons would result in some
type of complex movement pattern. The principal role of the primary motor cortex is thought to be to take voluntary movement
patterns that are formulated elsewhere and transmit them to the
cranial or spinal nerves via the tract of motor neurons called the
pyramidal system.
cortex as just a relay station for incoming movement patterns. It
Hippocampus
Direct electrical stimulation of the primary motor cortex also
However, it is too simplistic to think of the primary motor
Hips
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Knees
Trunk
Shoulders
Elbows
Wrists
2. THE MOTOR SYSTEM 39
Hips
Tr unk
Arm
Hand
Face
Lateral aspect of
cerebral cortex to
show topographic
projection of motor
centers
Ankles
To es
Fingers
Thumbs
Neck
Brows
Eyelids
Nares
Lips
Tongue
Larynx
FIGURE 2–14. The neurons in the primary motor cortex are arranged in
an inverted body scheme, with the neurons at the bottom of the gyrus being
responsible for transmitting motor impulses to the neck and face muscles and
the neurons at the top transmitting the impulses to the leg and foot muscles.
also has the ability to integrate information from other cortical
areas into a planned movement. The premotor area and supple-
mentary motor area both provide additional input to the primary
motor cortex before a movement is initiated (Figure 2–15). These
two cortical areas are located immediately anterior to the primary
motor cortex, with the supplementary motor area extending over
the top of the cerebral hemisphere and down into the longitudinal ssure. The neural signals contributed by these two areas are
believed to exert further control over the nal motor signals sent
out by the primary motor cortex. The impulses from the premotor
area seem to be especially important in visually guided movements,
such as inserting a key in a lock. When this area is damaged, hand
movements are notably clumsy. The inuence of the supplementary
motor area is a bit less denite. Neural signals from this area appear
to facilitate the simultaneous use of both hands during complex
sequences of movements.
40 MOTOR SPEECH DISORDERS: DIAGNOSIS AND TREATMENT
Supplementary motor area
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Premotor area
FIGURE 2–15. The premotor and supplementary motor areas play impor-
tant roles in controlling and refining movements. The premotor area contributes to visually guided movements, and the supplementary motor area helps
coordinate complex movements that involve the use of both hands.
Descending Motor Tracts
The descending motor tracts are the neural pathways carrying
motor impulses that travel from the cortex to the brainstem and
spinal cord. They are divided into two categories: the pyramidal
system and extrapyramidal system. The functions of these two
systems can be generalized by saying that the pyramidal system
is responsible for carrying the impulses that control voluntary,
ne motor movements, and the extrapyramidal system transmits
impulses that control the postural support needed by those ne
motor movements. For example, when someone is typing on a keyboard, it is the pyramidal system that carries the motor impulses
that enable the person to make coordinated, independent nger
movements on the keys. The extrapyramidal system, in turn, carries
the impulses that keep the arms, shoulders, and back in a position
that permits the ngers to move over the keyboard. Another generalization about these two systems is that the pyramidal system
works at a conscious level, with the extrapyramidal system being
more unconscious and automatic in its functions. As with any generalization, these are not absolutely true in every respect, but they
do provide a good starting point for understanding the complex
2. THE MOTOR SYSTEM 41
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motor pathways that connect the higher centers of the brain to the
muscles.
Pyramidal System
In the pyramidal system, most of the nerve bers take a more or
less direct path from the primary motor cortex to the brainstem or
spinal cord, where they eventually synapse with cranial or spinal
nerves. In fact, the pyramidal tract is sometimes called the direct
activation system because of its relatively straight pathway from
the cortex to the cranial and spinal nerves. Incidentally, the name
pyramidal system comes from a point in the medulla (called the
pyramids) where these descending bers are compressed tightly
together. By whatever name, this motor pathway is a key component of the motor system. Its bers are divided into the corticospinal and corticobulbar tracts. The corticospinal tract is made up
of axons that descend down from the cortex, through the internal
capsule, the brainstem, and into the spinal cord (Figure 2–16). The
axons terminate in the spinal cord, where many of them synapse
with spinal nerves. The corticobulbar tract also is composed of
axons descending from the cortex, but its axons terminate in the
brainstem, where they synapse eventually with the cranial nerves.
The term bulbar is a reference to an old name for the medulla,
which once was known as the bulb. In summary, the pyramidal
system consists of motor neurons that make a mostly direct course
from the cortex to the spinal cord (corticospinal tract) or to the
brainstem (corticobulbar tract).
Most axons of the pyramidal system have cell bodies that are
located in the primary motor cortex. Some bers of this system,
however, also originate from the premotor cortex, the supplementary motor cortex, and the primary sensory cortex. Both corticospinal and corticobulbar bers descend close to each other through
the cerebrum. In the medulla, most corticospinal bers cross the
mid line at a point called the pyramidal decussation and continue
down the opposite side into the spinal cord. Beginning in the midbrain and continuing through the rest of the brainstem, the corticobulbar bers gradually separate from the corticospinal bers.
Unlike in the corticospinal tract, the bers in the corticobulbar
tract do not all cross the midline. They are distributed in a complex
bilateral pattern before they synapse with the cranial nerves (Figure 2–17), which results in bilateral cortical innervation for most
cranial nerves. This means, for example, that a stroke affecting the
corticobulbar bers in the left hemisphere will not paralyze most
muscles served by the cranial nerves, because both the right and
42 MOTOR SPEECH DISORDERS: DIAGNOSIS AND TREATMENT
t
Ventral root fibers
FRONTAL SECTION
Precentral gyrus
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CEREBRUM
MIDBRAIN
PONS
UPPER
MEDULLA
Pyramidal
decussation
LOWER
MEDULLA
Genu of
internal capsule
Corticospinal
tract
Pyramid
Lateral
corticospinal tract
To muscle
To muscle
To muscle
Anterior
corticospinal trac
FIGURE 2–16. The corticospinal tract of the pyramidal system provides a
more or less direct connection between the primary motor cortex (precentral
gyrus) and the spinal nerves.
left cranial nerves will still receive motor innervation from the
undamaged right hemisphere. The cranial nerves serving the muscles of the larynx, pharynx, palate, upper face, and jaw all receive
this bilateral innervation. Keep in mind, however, that the muscles
of the lower face and tongue primarily have unilateral innervation.
These two parts of the head can be notably affected by unilateral
cortical damage. Cranial nerve innervation of the head and neck
muscles is discussed in greater detail in Chapter 4.
The pyramidal system is rudimental in lower animals such
as mice and rats. In successively higher animals (dogs, monkeys,
humans), it becomes increasingly larger and more sophisticated.
PRECENTRAL GYRUS
nal capsule
III, oculomotor
V, trIgeminal
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IV, trochlear
2. THE MOTOR SYSTEM 43
OF CEREBRAL CORTEX
Inter
Nucleus of III,
oculomotor
Substantia nigra
Trochlear nucleus
PONS
PONS
VII, facial
VI, abducens
MEDULLA
X, vagus
XII, hypoglossal
MEDULLA
Motor nucleus of V
Dorsal motor
nucleus of X
Nucleus
ambiguus
XI, spinal
accessory
FIGURE 2–17. The corticobulbar tract of the pyramidal system connects
the primary motor cortex (precentral gyrus) with most of the cranial nerves.
Damage to these bers usually results in muscle weakness and
rapid fatigue. Patients with injuries to the pyramidal system also
report that increased mental concentration is needed to perform
motor tasks that were previously accomplished with ease. In the
motor speech system, unilateral damage to the pyramidal system
results in a loss of ne motor movement in the articulators, a
condition known as unilateral upper motor neuron dysarthria
(see Chapter 6). However, the symptoms of pyramidal tract damage must be interpreted cautiously because other neural tracts are
located close to the pyramidal bers as they course through the
cerebrum and brainstem. Damage that affects the pyramidal tract
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