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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4390_Библиотеки_им_академика_М_И_Перельмана
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44 MOTOR SPEECH DISORDERS: DIAGNOSIS AND TREATMENT
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will almost always affect these other tracts as well, with results that
complicate the clinical picture.
Extrapyramidal System
The extrapyramidal system is composed of a number of different,
interconnected descending motor pathways. The term extrapyra-
midal simply refers to the motor tracts that are not part of the
pyramidal system (“extra” to the pyramidal system). Many neurophysiologists do not use extrapyramidal when referring to these
additional motor tracts because they think it inadequately depicts
their varied functions. Some prefer to call it the indirect activation
system, believing that this phrase better describes the system’s
complex, multiple interconnections. Others simply omit any single collective name and describe each descending motor pathway
separately. Although recognizing these objections, this textbook
uses the term extrapyramidal for this system because it is still used
widely in clinical settings, and it does provide a broad, shorthand
way to describe these motor tracts.
Four descending pathways of the extrapyramidal system are
discussed here: the rubrospinal tract, reticulospinal tract, vestibulospinal tract, and tectospinal tract. Remember that each of these
is similar to the pyramidal system in that they are neural motor
pathways between the higher levels of the nervous system and the
cranial or spinal nerves. They are different in that they originate
in the brainstem, not in the cortex. They also are different in that
they have many connections with other regions of the brain as they
proceed to the peripheral nerves. For example, the cerebral cortex,
the basal ganglia, and the cerebellum all have neural connections
to the extrapyramidal system, through which these higher levels of
the brain are able to directly inuence the actions of the muscles
innervated by the extrapyramidal tracts.
The rubrospinal tract originates in a group of neurons in the
brainstem called the red nucleus (Figure 2–18). Its nerve bers
cross the midline shortly after leaving the red nucleus and continue
down to the spinal cord. Because numerous rubrospinal bers
are mixed with pyramidal bers and have synaptic connections in
many of the same areas, it is thought that this tract might assist the
pyramidal system in controlling voluntary movements.
The reticulospinal tract originates in the reticular formation,
which is a group of cells coursing through the midbrain, pons,
and medulla (Figure 2–19). The reticular formation has several
important functions. In addition to being part of the extrapyramidal motor system, it has controlling effects on an individual’s level
of consciousness, blood pressure, respiration, and attention. The
Primary Motor
s
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Cortex
Fibers from Cerebellar
Deep Nuclei via
Superior Cerebellar
Peduncle
Midbrain
To Pyramid
Medulla
Oblongata
Cervical Level of
Spinal Cord
Lumbar Level of
Spinal Cord
Red Nucleus
Medullary Reticular
Formation
Facial Nerve
Nucleus
Rubrospinal Tract
Excitatory Interneuron
Inhibitory Interneuron
To Extensor Muscle
To Flexor Muscles
Excitatory Interneuron
FIGURE 2–18. The rubrospinal tract—one of the tracts of the extrapyra-
midal system.
Inhibitory Interneuron
45
Occipital
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Parietal
Frontal
Orbitofrontal
Temporal
Facial Nerve Nucleus
Medial (anterior)
Reticulospinal Tract
Excitatory Interneuron
Medial Pontine
Reticular Formation
Trigeminal Motor
Nucleus
Pons
Medial Medullary
Reticular Formation
Medulla
Oblongata
Cervical Level of
Spinal Cord
Inhibitory Interneuron
Motor Neuron
FIGURE 2–19. The reticulospinal tract (reticulospinal and corticoreticular
pathways)—another tract of the extrapyramidal system.
46
Lumbar Level of
Spinal Cord
2. THE MOTOR SYSTEM 47
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bers of the reticulospinal tract receive afferent input from many
sources, including the motor and sensory cortices, the basal ganglia, the substantia nigra, and the red nucleus. Because of these
varied inputs, the reticulospinal tract has an important inuence
on the spinal nerves. This tract is believed to be especially important in maintaining upright posture and the body’s ability to turn
toward external stimuli. It also might allow for some voluntary,
gross motor movements, such as raising an arm or leg. Furthermore, the reticular formation and the reticulospinal tract contain
“built-in” reexive motor patterns that can operate without higher
nervous system input. For example, this tract enables some infants
born without a cerebrum to perform basic movements such as
sucking, stretching, and yawning.
The nal two extrapyramidal tracts have little to do with motor
speech production, so we mention them only briey. The vestibulospinal tract originates in the vestibular apparatus of the inner
ear, courses through the pons and medulla, and terminates in the
spinal cord. It helps the body maintain posture and balance. The
last tract is the tectospinal tract. Its bers originate in the midbrain and end in the cervical portion of the spinal cord. This tract
receives many afferent inputs from the eyes and the visual cortex
and, consequently, plays an important role in keeping the eyes and
the head oriented to external stimuli.
It is useful to think of the extrapyramidal system as operating
in parallel with the pyramidal system. This means that while the
pyramidal system is transmitting ne motor neural impulses to the
cranial and spinal nerves, the extrapyramidal system is simultaneously controlling the muscles that provide the postural support
needed to accomplish those ne motor movements. Taken as a
whole, the postural muscles innervated by the extrapyramidal system include those of the torso and the larger muscle groups of the
arms and legs. The extrapyramidal system’s inuence on the cranial
nerves and muscles of the speech mechanism is not completely
understood. It is known that neurons in the reticular formation (the
origin of the reticulospinal tract) have many synaptic connections
with the cranial nerves. Through those connections, the extrapyramidal system inuences the reexes, muscle tone, and probably some voluntary movements of the speech mechanism.
Cranial and Spinal Nerves
Before discussing cranial and spinal nerves, the difference between
upper and lower motor neurons needs to be examined. Various
authors have used different criteria for dening upper motor
48 MOTOR SPEECH DISORDERS: DIAGNOSIS AND TREATMENT
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neurons. For example, some say that upper motor neurons are only
those in the pyramidal system. Others state that the denition
should include all the motor neurons in the CNS. This textbook
uses this second denition. Upper motor neurons are all the
descending motor bers coursing through the CNS that eventually
make a synaptic connection to the motor neurons in the PNS. This
includes the two pathways of the pyramidal system (the corticospinal and corticobulbar tracts) and the pathways of the extrapyramidal system (the rubrospinal, reticulospinal, vestibulospinal,
and tectospinal tracts). To put it briey, upper motor neurons are
the motor bers within the CNS. Lower motor neurons, in con-
trast, are the motor neurons in the cranial and spinal nerves. From
a clinical standpoint, the distinction between these is important
because damage to upper motor neurons results in symptoms that
are usually quite different from those seen after damage to lower
motor neurons. In general, upper motor neuron damage results
in spasticity. The motor speech disorder associated with bilateral
upper motor neuron damage is called spastic dysarthria (Chapter 5). Lower motor neuron damage results in muscle paralysis or
paresis (weakness). Flaccid dysarthria (Chapter 4) is the result
of damage to the lower motor neurons in the cranial nerves that
innervate the muscles of speech production.
Cranial Nerve Nuclei
As stated previously, the cranial nerves are attached to the brainstem at points called the cranial nerve nuclei. Figure 2–20 shows
that a cranial nerve’s sensory and motor bers separately branch
out from the brainstem. The cell bodies of the sensory neurons
are gathered outside the brainstem in a bundle called a cranial
ganglion. The cell bodies of the lower motor neurons are grouped
inside the brainstem. It is in that area within the brainstem that
the lower motor neurons in the cranial nerves synapse with upper
motor neurons from the pyramidal and extrapyramidal systems.
If the complex interaction of neurotransmitters from upper motor
neurons reaches a certain excitatory threshold, the upper motor
neurons will transmit their motor impulses to the cranial nerve
motor neuron, which will then transmit its own neural impulse
directly to the muscle tissue it innervates.
Spinal Nerve Nuclei
The spinal nerves are attached to the spinal cord in a manner that is
roughly similar to the attachments of the cranial nerves and brain-
2. THE MOTOR SYSTEM 49
Primary motor cortex
the brainstem
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Upper motor neuron
of corticobulbar tract
Cross-section of
Interneuron
Sensory neuron
of a cranial nerve
Lower motor
neuron of a
cranial nerve
FIGURE 2–20. A typical neural pathway between the primary motor cor-
tex and the lower motor neuron of a cranial nerve. The cell bodies of motor
neurons in the cranial nerves are located within the brainstem at sites known
as cranial nerve nuclei.
stem (Figure 2–21). The sensory and motor bers branch from the
spinal cord separately. As with the sensory bers of the cranial nerves,
a spinal ganglion contains the cell bodies of the sensory neurons.
Spinal sensory bers attach to the spinal cord on its dorsal (back)
surface. A cross-sectional view of the spinal cord (Figures2–21 and
2–22) shows that the center of the spinal cord contains an H-shaped
region of gray matter. This spinal gray matter is composed of neuron cell bodies. The cell bodies of the lower motor neurons in the
spinal nerves are located in the ventral (front) horn of the spinal
gray matter. This is where the spinal nerves’ lower motor neurons
synapse with the pyramidal system, extrapyramidal system, and sensory neurons. From the ventral horn, the motor axons of the spinal nerves project out to the muscles they innervate (Figure 2–23).
Again, the interplay of neurotransmitters from the upper motor (and
50 MOTOR SPEECH DISORDERS: DIAGNOSIS AND TREATMENT
Spinal ganglion
Posterior
lateral sulcus
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median sulcus
Anterior
median
fissure
White matter
Dorsal root
Ventral root
Gray matter
Posterior
median fissure
(not shown)
Anterior
Anterior
median
fissure
Filaments of
dorsal root
Filaments of
ventral root
Spinal nerve
intermediate sulcus
Posterior
gray column
Central
canal
Posterior
Anterior
gray
column
FIGURE 2–21. Motor neurons in spinal nerves branch off from the anterior
portion of the spinal cord (inside the ventral roots).
also sensory) neurons at their synaptic connections with the spinal
lower motor neurons determines whether a neural impulse will be
transmitted to a muscle.
Neuromuscular Junction
Finally, the neural impulse arrives at the place where a muscle
actually contracts to cause a movement. The neuromuscular junc-
tion is the point where the axons of lower motor neurons make
synaptic connections with muscle cells (Figure 2–24). At the end
of a motor neuron axon, there are many small terminal branches
that synapse with the membrane of a muscle cell. Each of these
small branches makes a synaptic connection with only one muscle
cell. When the neural impulse traveling down the axon reaches
the terminal branch, the neurotransmitter acetylcholine is released
by the axon into the microscopic gap between the axon and the
muscle cell. This neurotransmitter binds to special receptors in the
membrane of the muscle cell. When enough acetylcholine is present in the muscle cell receptors, an electrochemical impulse occurs
throughout the muscle cell, which then causes the contraction of
the muscle ber.
Cortex
Brain
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From motor
output of
cerebrum
2. THE MOTOR SYSTEM 51
Upper Motor
Neuron
Ascending
Sensory Neuron
Gray matter
Synapse
Lower motor neuron
Spinal cord
Muscle
FIGURE 2–22. The cell bodies of motor neurons in the spinal nerves are
located in the ventral horns of the spinal cord gray matter. The cell bodies of
sensory neurons in the spinal nerves are located just outside the spinal cord.
With its numerous terminal branches, a single axon is able
to cause contractions in many individual muscle cells. The actual
number of muscle cells innervated by an axon varies according to
the amount of ne motor control needed by a body part. A single
axon may innervate many hundreds of individual muscle cells in
large muscle groups or just a few cells in the muscles that perform
intricate movements, depending on how much control of the movement is necessary. In the thigh, for example, a neural impulse from
one axon will cause the simultaneous contraction of many muscle
cells, which is appropriate, as the thigh is seldom called on to
make small, discrete movements. However, in the face or ngers,
Synaptic clef
Sensory Fibers
Skin
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Motor Fibers
Muscle
Dorsal Roots
Spinal Cord
Acetylcholine
Vertebra
Nerve
Ventral Roots
FIGURE 2–23. Spinal nerves transmit motor impulses to the skeletal mus-
cles and sensory impulses from the skin, joints, and other areas of the body.
Nucleus
Cell body
Bouton
Schwann cells
t
FIGURE 2–24. Lower motor neurons make synaptic connections with
muscle tissue at the neuromuscular junction.
52
Axon
Neuromuscular
junction
Muscle
one axon will control only a handful of muscle bers, resulting in
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greater cortical control over the contractions of those muscles. This
difference in the number of muscle bers innervated by a single
axon is called the innervation ratio.
As stated at the beginning of this chapter, understanding the
motor system is an important part of making the correct diagnosis
of a motor speech disorder, as well as developing an appropriate
treatment plan. If damage in any part of the motor system affects
the muscles of speech production, the result can be a motor speech
disorder. The particular motor speech disorder that occurs depends
on which part of the motor system is damaged. Each of the upcoming chapters of this textbook examines one of the dysarthrias and
apraxia of speech. Signicant parts of the chapters are devoted to
explaining how these disorders are linked to damage to specic
parts of the motor system.
Summary of the Motor System
2. THE MOTOR SYSTEM 53
n The motor system is an important and very complex compo-
n The motor system is composed of many parts, including the
n Damage at any level of the motor system can result in a move-
Study Questions
1. What is the cerebral cortex, and why is it important?
2. What are cranial nerve nuclei, and where are they located?
3. Describe the difference between tracts and nerves.
4. Describe the association cortex and its importance in formu-
nent of the nervous system. It is responsible for controlling all
volitional movements.
primary and association cortex, the basal ganglia, the cerebellum, the thalamus, the pyramidal and extrapyramidal tracts,
and the neuromuscular junction. Either directly or indirectly,
all of these components of the motor system communicate
with each other through highly complex pathways within the
nervous system.
ment disorder. When the damage affects the muscles of speech
production, the result can be a motor speech disorder.
lating a movement.
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