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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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Chapter 2
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The Motor System
Components of the Motor System
Brain
Cerebrum
Brainstem
Cerebellum
Nervous System Cells
Types of Neurons
Other Nervous System Cells
Tracts and Nerves
Transmission of Neural Impulses
Summary of Motor System
Components
Structure and Function of the
Motor System
The Desire to Move
Primary and Association Cortices
Basal Ganglia and Cerebellum
Basal Ganglia
Cerebellum
Thalamus
Primary Motor Cortex
Descending Motor Tracts
Pyramidal System
Extrapyramidal System
Cranial and Spinal Nerves
Cranial Nerve Nuclei
Spinal Nerve Nuclei
Neuromuscular Junction
Summary of the Motor System
Study Questions
15
16 MOTOR SPEECH DISORDERS: DIAGNOSIS AND TREATMENT
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he parts of the nervous system that control voluntary
movement are known collectively as the motor system.
Understanding how this system works is an important
T
part of being an effective diagnostician of motor speech
disorders. Familiarity with the workings of upper and lower
motor neurons, the basal ganglia, the cerebellum, and the
pyramidal and the extrapyramidal systems is essential inmaking the correct diagnosis of a motor speech disorder and
in designing appropriate treatment plans. This chapter provides an overview of the motor system to lay the foundation
for the more specific investigations of the motor speech
disorders presented in later chapters.
The motor system is what allows thought to be turned
into movement, whether it is moving a hand, a leg, or the
tongue. By any measure, the motor system is extremely
complex. The nerve cells of the motor system are arranged
into many different pathways, with each pathway performing different functions. Some parts of the motor system
work at a conscious level and others at a subconscious
level. The system’s scope also is impressive. It ranges from
the very highest cognitive centers of the brain down to the
body’s simplest muscles. A properly functioning motor system allows movements of the
eyebrows, all at the same time and in a coordinated manner.
When a portion of it is damaged, though, the result can
be a debilitating movement disorder. The type of disorder
is dependent on the location and extent of the damage to
the motor system. For example, lesions in the basal ganglia
can result in involuntary movements that seriously interfere
with an individual’s voluntary attempts to speak, walk, or do
any number of other things. Because of this relationship
between the type of disorder and the site of damage, it is
important to understand the basics of the motor system.
fingers, vocal folds, feet, and
Components of the Motor System
The motor system is actually one of several subdivisions of the
nervous system. Consequently, any discussion of the motor system
is difcult, if not impossible, without a basic understanding of the
2. THE MOTOR SYSTEM 17
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a
a
e
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Brain
Spinal
cord
FIGURE 2–1. The central nervous system consists of the brain and spinal
cord. The peripheral nervous system contains the 12 cranial nerves and the
31 spinal nerves.
nervous system. Because of the link between these two systems,
the rst portion of this chapter reviews the fundamental structures
of the nervous system before moving on to the specics of the
motor system. The nervous system is organized into the central
and peripheral nervous systems (Figure 2–1). The central ner-
vous system (CNS) consists of the brain and the spinal cord. The
peripheral nervous system (PNS) is composed of 12 pairs of cra-
nial nerves and 31 pairs of spinal nerves. The cranial nerves are so
named because they project from parts of the CNS that are within
the cranium (i.e., inside the skull). They innervate many organs and
muscles of the head, neck, thorax, and abdomen. In contrast, the
spinal nerves branch from the spinal cord and innervate most of
the other muscles of the body, including the chest, arms, and legs.
Brain
The brain is the key component, and the most complex part, of
the nervous system. Almost all activity in the nervous system originates in or is ultimately processed by the brain. Voluntary motor
commands to the muscles originate in the brain. The brain also
18 MOTOR SPEECH DISORDERS: DIAGNOSIS AND TREATMENT
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receives sensory information from the body and controls the cognitive functions, including reasoning, memory, language, and problem solving.
Humans have large brains relative to their body size, as compared with most other animals. The normal adult brain weighs
about 2.5 to 3.5 lb. It has an amazingly complex number of interconnections among its various parts, as well as connections with
the other portions of the nervous system. The following sections
review the parts of the brain that are most relevant to understanding the motor system.
Cerebrum
The brain is divided into the cerebrum, brainstem, and cerebellum (Figure 2–2). The largest and most prominent of these is the
cerebrum. It is split into two hemispheres by the longitudinal ssure, which runs front to back along the middle of the brain. The
cerebrum is organized into four areas called lobes. The frontal
lobe is located on the anterior (front) portion of the cerebrum.
The temporal lobe lies on the lower sides of the cerebrum. The
parietal lobe is found on the upper sides of the cerebrum behind
the frontal lobe. Finally, the occipital lobe is on the rearmost portion of the cerebrum, behind both the parietal and temporal lobes.
The most obvious feature of the cerebrum is its deep convolutions.
Each convolution is called a gyrus (plural, gyri), and the groove
between the gyri is called a sulcus (plural, sulci).
Hypothalamus
FIGURE 2–2. The brain consists of the cerebrum, brainstem (midbrain,
pons, and medulla), and cerebellum.
Cerebrum
Thalamus
Midbrain
Medulla oblongata
Pons
Cerebellum
Spinal cord
2. THE MOTOR SYSTEM 19
Precentral gyrus
(motor strip)
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The gyri and sulci of the cerebrum create several signicant
landmarks that will be referred to frequently in this textbook (Figure 2–3). The rst of these is the lateral sulcus, certainly the most
prominent sulcus on the cerebrum. It runs horizontally along the
lateral sides of each hemisphere and separates the temporal lobe
from the frontal lobe. Another landmark is the central sulcus,
Superior frontal gyrus
Superior frontal sulcus
Middle frontal gyrus
Inferior frontal sulcus
Broca’s area
Inferior frontal gyrus
Frontal pole
Lateral (sylvian) fissure
Frontal lobe
Cerebral longitudinal
fissure
Frontal, frontoparietal,
and temporal opercula
Temporal lobe
Superior temporal gyrus
Superior temporal sulcus
Precentral sulcus
FP
F
T
Middle temporal gyrus
Parietal lobe
Occipital lobe
Primary visual
cortex
Central (rolandic) sulcus
Postcentral gyrus
Postcentral sulcus
Superior parietal lobule
Inferior temporal gyrus
Inferior temporal sulcus
Circular sulcus
of insula
Intraparietal sulcus
Inferior parietal lobule
Supramarginal gyrus
Angular gyrus
Parieto-occipital sulcus
Wernicke’s area
Calcarine sulcus
Occipital lobe
Occipital pole
Preoccipital notch
Precentral gyrus
FIGURE 2–3. Four of the most prominent landmarks on the lateral surface
of the brain are the lateral and central sulci and the precentral and postcentral gyri.
Supramarginal gyrus
Postcentral gyrus
Insula
Short gyri
Limen
Long gyrus
Central sulcus
of insula
20 MOTOR SPEECH DISORDERS: DIAGNOSIS AND TREATMENT
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probably the second most prominent sulcus on the cerebrum. It
is located near the center of the lateral sides of each hemisphere
(hence its name) and extends vertically from the very top of the
hemisphere down to the lateral sulcus. The central sulcus separates
the frontal lobe from the parietal lobe.
The gyrus immediately in front of the central sulcus is known
variously as the precentral gyrus, the primary motor cortex, or
the motor strip. The nerve cells located in this gyrus play a very
important role in controlling the voluntary movements of the body.
The gyrus just behind the central sulcus is called either the post-
central gyrus, the primary sensory cortex, or the sensory strip.
Here, the higher centers of the brain receive sensory information
from the body via the PNS and other portions of the CNS.
The surface of the cerebrum is called the cerebral cortex. Its
thickness varies between 2 mm and 5 mm, and it is composed of
six different layers of nervous system cells. In its entirety, the cortex
contains about 15 billion nerve cells (neurons). The cortex is gray
and is often described as being the “gray matter” of the brain. Only
about one third of the cortex is visible because of the cerebrum’s
many convolutions; the other two thirds are hidden between the
many sulci. If laid at, the total surface area of the cortex would
be approximately 340 square inches.
The cortex is one of the most important parts of the nervous
system. In this thin cortical layer of nerve cells, the higher cognitive
activities, such as language, motor planning, problem solving, and
much sensory perception, are performed. Because it is so thin, the
cortex makes up only a small percentage of the cerebrum’s total
size. Most of the cerebrum is actually composed of large groupings of white matter located below the cortex. This white matter
consists of nerve-cell axons that course to and from other parts of
the CNS. The white color is from the fatty myelin that covers the
axons. More information about axons and myelin is presented in
a subsequent section.
Brainstem
The brainstem is divided (from top to bottom) into the midbrain,
pons, and medulla (Figure 2–4). It sits between the cerebrum and
the spinal cord. The brainstem’s importance is threefold. First, it
acts as a passageway for the descending and ascending neural
tracts that travel between the cerebrum and spinal cord. Second, it
controls certain integrative and reexive actions, such as respiration, consciousness, and some functions of the cardiovascular system. Third, and probably most important with regard to the motor
speech system, it contains the places where the cranial nerves
Corona radiata
ANTERIOR
Inferior colliculus
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Lateral geniculate
body
Medial
geniculate
body
Superior colliculus
II optic nerve
III oculomotor nerve
V trigeminal nerve
VI abducens nerve
VII facial nerve
VIII vestibulocochlear
nerve
XII hypoglossal nerve
Superior colliculus
XI accessory nerve
Cerebral peduncle
Superior
Thalamus
Diencephalon
Diencephalon
Mid brain
Mid brain
Inferior colliculus
IV trochlear nerve
Superior cerebellar
peduncle
Middle cerebellar
peduncle
Inferior cerebellar
peduncle
IX glossopharyngeal
nerve
X vagus nerve
IV Trochlear nerve
MIDBRAIN
PONS
MEDULLA
Third ventricle
Pineal body
FIGURE 2–4. The brainstem is divided into the midbrain, pons, and medulla.
These figures also illustrate the roots of the cranial nerves.
Cerebellar
peduncles
Middle
Inferior
Pons
Pons
Medulla
Medulla
Facial
colliculus
Hypoglossal trigone
Vagal trigone
Obex
Dorsal median sulcus
Dorsal intermediate sulcus
Dorsal lateral sulcus
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22 MOTOR SPEECH DISORDERS: DIAGNOSIS AND TREATMENT
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project out from the CNS. It is the cranial nerves that convey motor
impulses from the CNS to the muscles of the larynx, face, tongue,
pharynx, and velum. The cranial nerves are attached to the brainstem at points called the cranial nerve nuclei.
Cerebellum
The cerebellum is shaped somewhat like the cerebrum, but it is much
smaller. It is attached to the back of the brainstem, where it makes
neural connections with the cerebral cortex and numerous other
parts of the CNS. The most important function of the cerebellum is
to coordinate voluntary movements, so that muscles will contract
with the correct amount of force and at the appropriate times. Cerebellar damage can cause signicant decits in the performance of
both gross and skilled motor actions. Movements such as walking,
writing, and speech can become uncertain and awkward when the
cerebellum is not functioning properly. The cerebellum is examined
in more detail later in this chapter and in Chapter 7.
Nervous System Cells
The nervous system contains many different types of cells. The most
important are the neurons (Figure 2–5), which transmit the electrochemical signals that control nearly every function of the body.
Estimates of the number of neurons in the human body range from
50 billion to 100 billion. Neurons have three primary components.
The rst is the cell body, which contains the nucleus responsible
for the cell’s vital metabolic functions. The cell bodies of neurons
are gray. When many cell bodies are grouped together, they cause
the distinctive grayish tint that is visible in many structures in the
CNS, such as the cortex and the central portion of the spinal cord.
Dendrites are the second component of neurons. These are the
many short processes that extend from the cell body. Dendrites
receive electrochemical impulses from other neurons or from sensory organs. The third component is the axon, the single long
extension from the cell body. Axons conduct neural impulses away
from the cell body and transfer the impulses to muscles, glands, or
other neurons. The end of an axon may have many small branches
called terminal ramications, or terminal boutons, which allow
one axon to communicate with many additional neurons. An axon
also may have longer branches called collaterals, further extending the inuence of a neuron to other parts of the nervous system.
Most axons are covered by myelin, a white, lipid-protein mem-
brane that covers the length of the axon. Myelin insulates a neu-
Dendrites
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Soma
2. THE MOTOR SYSTEM 23
Axonal
hillock
Axon
Myelin sheath
Node of Ranvier
Telodendria
End buttons
FIGURE 2–5. A neuron contains many dendrites, a cell body, and a single
axon.
ron’s electrochemical impulses from the surrounding tissues and
uids, which would otherwise degrade the strength of an impulse
as it travels the length of the axon. Myelin acts very much like the
insulation on household electrical wiring to prevent the leakage
of electrical energy.
Types of Neurons
Neurons are categorized by their shape and size. The cell body of
some neurons is in the middle of the axon, and in others it is to
the side. Some have very large cell bodies; others do not. Some
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