Human Brain & Mind
  • Neuroanatomy

    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.

    Spinal Cord

    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.

    Motor Tracts

    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.”

    Mental Pathways

    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 citizen.
      Stephen Gislason MD is the author of the Human Brain. 2018 edition. 238 Pages.

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