Neuroanatomy
The brain has two cerebral hemispheres sitting on central
structures that emerge from the spinal cord and swell into bilateral
cell complexes dominated by the thalamus. Around and below the
thalamus are clusters of smaller nuclei that retain old strategies
of existence going back beyond reptiles. The surface of the cerebral
hemispheres is described as the cerebral cortex. The cortex is
folded into hills (gyri) and valleys (sulci) that increase its
surface area. The cortex is a thin layer, populated with cell
bodies of two types, neurons and glia. Much of the computing is
carried out by neurons and the glia act as nurturing, protective and
immune cells that may participate in computation but we do not know
how. Below the cortex is white matter, bundles of axons that are
insulated with myelin and carry signals to and from the cortex and
among areas of the cortex.
In human anatomy the body is seen as symmetrical about the
midline. To create place names, you use the term medial to refer to
the midline and lateral to refer to each side. Anterior is front and
posterior is toward the back. Superior is above and inferior or
basal is below. You can describe structures in the brain using
composite terms such as the anterolateral nucleus of the thalamus or
the posteromedial amygdala. Brain structures bear fanciful
names and their numerous interconnections are described in
polysyllabic terms that are only learned and used by specialists.
Neurology is focused on easier-to understand brain structures;
the long wires that carry information to and from the brain are the
best understood. Advanced neuroanatomical knowledge, required in
clinical neurology and neurosurgery, is not easy to learn. Skilled
neurologists can always bamboozle fellow MDs by deducing
the location of a brain lesion by knowing the wiring diagram well.
Most neurological diseases cannot be treated and neurologists
have tended to be intellectual diagnosticians rather than
therapists. Stroke is the most common neurological disease, caused
by eating the wrong food. Neurologists seldom want to be involved
with such banal matters as diet or modifying their patients eating
behaviors. They are more interested in the consequences of stroke
and for many years, the study of brain damage effects was the basis
of the neurological understanding of mind.
The long tract from the motor cortex to the muscles that it controls can be
interrupted by injury or disease at any point causing loss of muscle function;
there are several different clinical syndromes depending on the location of the
damage to the tract. Neurologists have little difficulty identifying a lesion of
the motor tract by doing a neurological exam and are often accurate in
determining the site of the lesion.
One useful distinction, for example, is that damage to the motor
neurons at and beyond the spinal cord causes a flaccid paralysis
with loss of reflexes. Damage to the motor system in the brain or
upper spinal cord causes a spastic paralysis with hyperactive
reflexes. Spastic means that the muscles are in a state of increased
contraction rather than no contraction. A neuroanatomy text states
that: ”corticospinal fibers arise from area 4 and 6 of the
precentral gyrus (motor cortex) and in areas 3,1,2 of the postcental
gyrus (sensory cortex). The (motor) fibers descend through the
internal capsule into the cerebral peduncle and through the basilar
part of the Pons. As they enter the medulla, they are grouped along
the ventral border of the pyramids; hence the name pyramidal tract.”
Neuroanatomy was a precocious subject for me that I started
informally in a neuropathology lab in Cleveland when I was a
premedical student. I wanted to explore the idea that the brain was
the organ of the mind. I was introduced to neurology and learned
about neurologists, their personal and professional struggles. When
I was confronted with a real human brain, the whole notion of
understanding the mind evaporated. The real task was to become
familiar with a strange organ that gave few clues about the way it
worked.
I was dissatisfied with the way neuroanatomy had developed. The
anatomy of the brain had been revealed piecemeal and named in a
complex way by people who understood little about how the brain
worked. The task of anatomy was originally to describe all the
physical features of the brain and function was left to other
disciplines and the future. Anatomists were often hostile to new
functional concepts such as the “limbic system’ mostly because it
was not an anatomic concept but a functional one. Early neurological
names and concepts of how the brain works now appear to be crude and
simplistic.
When a person thinks or behaves strangely, but has normal
reflexes and a normal CAT scan, a neurologist sends the patient to a
psychiatrist who seldom considers brain structures or function in
the description or understanding of the problem. If a stroke or
brain injury causes personality changes, emotional and cognitive
difficulties, neuropsychologists are often called in to document the
effects of loss of brain tissue. Other specialties assist brain
injured patients adapt to their disability.
There are textbooks full of syndromes, packages of dysfunction,
associated with damage to the brain. If the frontal lobes are
damaged, for example, the personality changes with loss of emotional
sensibility, loss of initiative and planning and inappropriate
behavior. If the occipital lobes in the back of the brain are
damaged, vision is impaired in a variety of ways from complete
blindness to curious gaps in visual perception, memory and
understanding. Damage to the temporal lobes, just in from the ears,
will result in memory deficits, damaged hearing and auditory
language perception; 92% of humans are right-handed and their
language storage tends to be concentrated in the left hemisphere. If
a stroke damages the left temporal lobe, the loss of language
ability is more severe than occurs with right-sided damage.
Left-handed people often have more bilateral language storage so
that a single hemisphere lesion causes less language disruption.
The brain has many black boxes that no one understands well.
Although you can name, describe the location of these boxes, their
cell structure and describe the wiring to and from other structures,
understanding what these modules do is still difficult, except in
the most general terms. Neuroanatomy names should be revised, based on a
more functional approach to identifying structures.
The surface of the cerebral hemispheres has been divided into
regions because of deep sulci. The frontal lobes, for example, are
not a coherent assembly of closely related functions but a dispersed
assortment of functions that relate to many different parts of the
brain. On the other hand, the occipital cortex in the back of the
brain is more specialized to visual functions, although as you move
forward in the occipital lobe, you encounter less specialized and
more interactive zones that bring visual information into
relationship with other kinds of information.
