The brain has two cerebral hemispheres that sit on top of
central structures, bilateral cell complexes dominated by the thalamus that
emerge from the brain stem. Around and below the thalamus are clusters of
smaller nuclei that retain old strategies of existence going back beyond
reptiles. The main input (sensory) and output (motor) cables to the brain are in
the spinal cord that enlarges into the medulla (brain stem) after entering the
skull. A set of 12 cranial nerves that supply movement and sensation to the head
enter the brain directly, several through the medulla. The medulla contains
nuclei, clusters of neurons that are the first processors of sensory input and
last processors of motor output. The optic nerve is the second cranial nerve
that connects the eye to the brain via a specialized component of the thalamus,
the lateral geniculate body. The eye is the only brain structure that is visible
on the surface of the body. Eye movements involve 6 muscles that are controlled
by 3 other cranial nerves.
Seeing requires a close integration of sensory and
motor functions. Visual processing in the occipital cortex requires continuous
eye movements that scan the visual field, alter pupil dimensions and change eye
convergence, depending on the distance of objects of interest. Seeing is also
connected to hearing, to body movements and to instant response systems in the
brain that allow us to react to a perceived threat. The idea of separate sensory
and motor systems in the brain is not longer valid. To understand brain function
you have to think in terms of the integration of inputs and outputs. You have to
recognize that humans and all other animals move to sense and sense to move.
When you look at peripheral nerves, you do find more distinct separation of
sensory and motor functions.
The spinal cord has a central core of neurons
surrounded by columns of myelinated long axons. The nerve supply to the body is
segmental, an ancient design that appears in all animals. Below each vertebra in
the spine, a sensory nerve enters the spinal cord from behind and a motor nerve
exits from in front. The rear cord neurons are the first sensory processors
whose axons select and send incoming signals to the brain, mostly to the
thalamus. Neurons in the front of the cord are the motor output neurons that
send motor signals to muscle cells. Several spinal nerves converge through
complex plexuses to form nerves that enter the arms and legs, one set for each
arm and one set for each leg.
The nerve supply to the internal organs is a
complex mesh works of nerves that emerge from and converge toward the
sympathetic and parasympathetic regions of the brain. The vagus nerve, the 10th
cranial nerve, for example, is the main parasympathetic pathway, linking the
heart, lungs and digestive tract with the brain.
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 cell layer, populated by two kinds of cells, 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 between areas of the cortex.
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.
Brain structures bear fanciful names and their numerous interconnections are
described by polysyllabic terms that are unwieldy and 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 brain function.
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 chord 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 flaccid, 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
postcentral 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.”
When a person thinks or
behaves strangely, but has normal reflexes and a normal CAT scan, the
neurologist may make no effort to locate the lesion, but instead refers the
patient to a psychiatrist who may not even consider 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 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.
Language & Temporal Lobes
Damage to the temporal lobes 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 cell clusters or modules that no one understands well. Although
you can name and describe the location of these modules, 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.
When you do start to consider how the universe is represented in the tissues of the brain
and ask how beauty, music, ethics, and deep desires develop in neuronal
networks; you can only stop and wonder. Awe takes over and neuroanatomy seems
inadequate to explain the rich complexity of mental experience. We can say that
the universe is held somehow in the cerebral cortex, encoded by an elaborate set
of overlapping but different maps with meta-programs overseeing meaningful
connections among the maps.
- You are viewing the Brain Mind Center at Alpha Online.
Understanding the human brain is essential to become a well-informed, modern
Stephen Gislason MD is the author of
the Human Brain. 2018 edition. 238 Pages.
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Further reading: Neuroscience Notes, Intelligence and
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