SOME SELECTED DISEASES OF THE NERVOUS SYSTEM
Brain Damage: These areas cannot survive on the more limited rations of
oxygen and nutrients, and permanent damage results.
As a result of carbon monoxide poisoning, a twenty-two-year-old woman
sustained the pattern of brain damage we have described. The result was an
isolated speech system that functioned relatively well, in the absence of any
ability to not understand anything that was said to her, and her very minimal
spontaneous speech was of inappropriate and stereotyped expression (e.g., "Hi, daddy").
In short, she was much more compromised in language function than the conduction
aphasic~ because she could not really use language, although she could do
the one thing conduction aphasics fail to do: repeat sentences verbatim.
(Geschwind, Quadfasel, and Segarra, 2007)
We have described only a few of the many disorders of language, illustrating
particularly clear-cut cases. Most cases of disordered language involve
combinations of the deficits we have discussed, with one particular set
of symptoms being more prominent than the others. Furthermore, most
aphasias are accompanied by disorders outside of the area of language, such
as weakness or paralysis on the right side, resulting from brain damage to
areas near those that serve language.
DISORDERS OF MOVEMENT OF THE LEFT HAND
Disorders of movement offer particularly good illustrations of the nervous
system principles we have discussed, and ideal examples of the neurological
approach. In addition, paralysis, one symptom of damage to motor system
hardware, is one of the more common symptoms of software malfunction,
in hysteria. You will remember the case of lower body paralysis in Bear
following a traumatic event. This was a clear case of a conversion
reaction, that is, a software problem. Knowledge of the nervous system
is very helpful in differentiating hardware and software problems, that is,
those that are primarily organic in origin from those psychological in origin.
We have selected one set of symptoms, disorders of movement of the left
hand of a right-handed person, to accomplish this end. Our approach will be
to move from the most peripheral disorders (e.g., damage to the nerves innervating
the hand), through higher and higher levels of the nervous system.
We will see that, in general, the more peripheral the neural damage, the
more precisely we can relate structure and function of the nervous system to
the disorder. As we move 'up the neural hierarchy, however, we will note
that our understanding of the ways in which specific neurological damage
produces symptoms becomes less satisfactory. Eventually, we will deal with
symptoms that seem neurological but are in fact attributed to psychological
factors: the phenomena of hysteria and malingering. We will emphasize the
logic of neurological diagnosis, focusing on the determination of the site of
the lesion (as opposed to the agent of disease), based primarily on the
symptoms the patient presents.
By disorders of movement in the left hand, we refer to any abnormality,
from paralysis or weakness, to clumsiness or inability to perform complex
DISORDERS OF THE NERVOUS SYSTEM
sequences. A patient would qualify if he had such symptoms, whatever
other symptoms were exhibited in other parts of the body. From hundreds
of disorders of the left hand, we have selected a few on the grounds that they
are caused by neural damage (as opposed to arthritis and broken bones) and
because they illustrate the themes of this section.
The Basic Structure of the Control of Movement
At the lowest level of organization, movement is controlled by the motor
neurons (called lower motor neurons) with cell bodies in the spinal cord, and
with axons running out to the muscles in peripheral nerves.
The axons of the lower motor neurons are called the final common path,
because all muscle movements in the body are produced ultimately by activity
in these neurons. In the spinal cord, these neurons can be excited or
inhibited by input from receptors in the muscles, skin, and joints. These
connections form the reflex arcs. A second level of organization has to do
with the linkages between the brain and the lower motor neurons: the upper
motor neurons form these connections. Some of the upper motor neurons
that control the hand originate in the motor area of the cortex, cross to the
opposite side at the base of the brain, and descend in the
spinal cord to form synapses with the lower motor neurons. This pathway
from cortex to spinal cord is appropriately called the cortico-spinal tract. It
allows for particularly fine and rapid movements (because there are no
intervening synapses between the cortex and the lower motor neuron), and it
is the dominant form of control for the fine movements of the hand, especially
finger movements.
Brain Damage: These areas cannot survive on the more limited rations of
oxygen and nutrients, and permanent damage results.
As a result of carbon monoxide poisoning, a twenty-two-year-old woman
sustained the pattern of brain damage we have described. The result was an
isolated speech system that functioned relatively well, in the absence of any
ability to not understand anything that was said to her, and her very minimal
spontaneous speech was of inappropriate and stereotyped expression (e.g., "Hi, daddy").
In short, she was much more compromised in language function than the conduction
aphasic~ because she could not really use language, although she could do
the one thing conduction aphasics fail to do: repeat sentences verbatim.
(Geschwind, Quadfasel, and Segarra, 2007)
We have described only a few of the many disorders of language, illustrating
particularly clear-cut cases. Most cases of disordered language involve
combinations of the deficits we have discussed, with one particular set
of symptoms being more prominent than the others. Furthermore, most
aphasias are accompanied by disorders outside of the area of language, such
as weakness or paralysis on the right side, resulting from brain damage to
areas near those that serve language.
DISORDERS OF MOVEMENT OF THE LEFT HAND
Disorders of movement offer particularly good illustrations of the nervous
system principles we have discussed, and ideal examples of the neurological
approach. In addition, paralysis, one symptom of damage to motor system
hardware, is one of the more common symptoms of software malfunction,
in hysteria. You will remember the case of lower body paralysis in Bear
following a traumatic event. This was a clear case of a conversion
reaction, that is, a software problem. Knowledge of the nervous system
is very helpful in differentiating hardware and software problems, that is,
those that are primarily organic in origin from those psychological in origin.
We have selected one set of symptoms, disorders of movement of the left
hand of a right-handed person, to accomplish this end. Our approach will be
to move from the most peripheral disorders (e.g., damage to the nerves innervating
the hand), through higher and higher levels of the nervous system.
We will see that, in general, the more peripheral the neural damage, the
more precisely we can relate structure and function of the nervous system to
the disorder. As we move 'up the neural hierarchy, however, we will note
that our understanding of the ways in which specific neurological damage
produces symptoms becomes less satisfactory. Eventually, we will deal with
symptoms that seem neurological but are in fact attributed to psychological
factors: the phenomena of hysteria and malingering. We will emphasize the
logic of neurological diagnosis, focusing on the determination of the site of
the lesion (as opposed to the agent of disease), based primarily on the
symptoms the patient presents.
By disorders of movement in the left hand, we refer to any abnormality,
from paralysis or weakness, to clumsiness or inability to perform complex
DISORDERS OF THE NERVOUS SYSTEM
sequences. A patient would qualify if he had such symptoms, whatever
other symptoms were exhibited in other parts of the body. From hundreds
of disorders of the left hand, we have selected a few on the grounds that they
are caused by neural damage (as opposed to arthritis and broken bones) and
because they illustrate the themes of this section.
The Basic Structure of the Control of Movement
At the lowest level of organization, movement is controlled by the motor
neurons (called lower motor neurons) with cell bodies in the spinal cord, and
with axons running out to the muscles in peripheral nerves.
The axons of the lower motor neurons are called the final common path,
because all muscle movements in the body are produced ultimately by activity
in these neurons. In the spinal cord, these neurons can be excited or
inhibited by input from receptors in the muscles, skin, and joints. These
connections form the reflex arcs. A second level of organization has to do
with the linkages between the brain and the lower motor neurons: the upper
motor neurons form these connections. Some of the upper motor neurons
that control the hand originate in the motor area of the cortex, cross to the
opposite side at the base of the brain, and descend in the
spinal cord to form synapses with the lower motor neurons. This pathway
from cortex to spinal cord is appropriately called the cortico-spinal tract. It
allows for particularly fine and rapid movements (because there are no
intervening synapses between the cortex and the lower motor neuron), and it
is the dominant form of control for the fine movements of the hand, especially
finger movements.