femme Neuroscience Notes

Some Topics

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 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 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.