The brain

The brain is covered by a thin layer of nerve cell bodies called the cerebral cortex. Two-fifths of it consist of no more than supporting glial cells. Yet it contains from 10 to 100 billion neurons with complex interlinking sensory, motor, and association pathways. Signals selectively transmitted through these routes enable us to eat, walk, lift loads, thread a needle, speak, love, hate, dream, think, remember, and make decisions. Computers may calculate faster than the human brain, but cannot match the versatility of this most amazing of all the body’s mechanisms.

The brain has three major parts. Each of these parts has its own shape and function, and all contribute to the amazing total of human mental capabilities. The principal areas of the brain are the cerebrum, the cerebellum, and the brain stem.

The brain is composed of nerve tissue differentiated into a number of areas and structures, many of which have specialized functions. The cerebrum is divided into the left and right cerebral hemispheres, which appear to have separate but related areas of responsibility—the left being primarily concerned with speech and logical thought, the right with three-dimensional shapes and subjective judgment. Conscious movement is controlled by the frontal lobes; the parietal lobe monitors position and sensation; the occipital and temporal lobes are associated with sight and hearing, respectively. The cerebellum exerts fine control over muscular movements; the pons and medulla of the brain stem are centers for the regulation of essential body mechanisms, such as blood pressure, heart rate, and respiration.

The cerebrum (outer forebrain)

Seven-tenths of the cells of the entire nervous system lie in the cerebral hemispheres, the two large connected swellings at the front end of the forebrain. Deep wrinkles enormously enlarge the surface area of the cerebral cortex the thin outer layer of gray matter that is, apparently, the seat of human intelligence. No other animal species has so much brain space allocated to this fragile layer. The human cerebral cortex contains about half a billion nerve cells with 620 miles (1,000 kilometers) of connecting fibers for every cubic half-inch.

Deep fissures in each convoluted hemisphere help to divide it into lobes, and special areas of cortex in different lobes deal with specific kinds of mental activity. Those areas receiving sensory input and dispatching motor signals are known as the sensory cortex and the motor cortex, also sometimes referred to collectively as the primary cortex. Primary cortex in the occipital lobes at the back of the brain receives signals from the eyes. Sensory strips of cortex down each side of the brain where frontal and parietal lobes meet receive input from the tongue, lips, face, head, hands, trunk, arms, legs, feet, and other areas. A strip of motor cortex running down the frontal lobe near the sensory strip triggers movement in specific muscles.

From primary cortex, signals flow to association cortex in the frontal, parietal, and temporal lobes. Here, received sensations may be associated with conceptual thought.

Prefrontal areas at the very front of the brain help to control personality and intellect. Speaking and understanding speech depend heavily on areas in three lobes of the left cerebral hemisphere. Visual recognition seems to reside in the right side of the brain.

A cross section through the skull, made with a CAT (computerized axial tomography) scan, shows the horizontal relationship between the eyes, the nasal tissues, and the lower parts of the brain—the two temporal lobes and the cerebellum.

The cerebrum (inner forebrain)

Distinctive groups of clustered cells form special structures deep inside the brain above the brain stem and around the ventricles cavities filled with cerebrospinal fluid. Innermost of these structures are the thalamus and hypothalamus. The thalamus, a large double structure, is a major sensory coordinator. The thalamus processes information from ears, eyes, mouth, and skin as it passes to the higher centers of the brain. Below the thalamus, the tiny hypothalamus controls thirst, hunger, sweating, shivering, and other processes essential to life.

Four neuron clusters, collectively called basal ganglia, crown the thalamus. Relaying information from cerebral hemispheres to brain stem and cerebellum, they help to regulate the body’s movements. They also share a structure, the amygdaloid (“almondlike”) body, with the so-called limbic system, a wishbonelike “mini-brain” encircling the brain stem and concerned with emotions and memory. The human limbic system resembles that of a primitive mammal, and, for this reason, it is sometimes called the “old” mammalian brain in contrast to the “new” brain, the cerebrum.

The cerebral cortex, the outer layer of the cerebrum, contains closely packed neurons with many interconnecting dendrites.

Brain stem

This 3-inch (7.5-centimeter) long mass of nervous tissue forms the upper, clublike end of the spinal cord. The brain’s evolutionary core, or root the brain stem carries sensory and motor nerve tracts and houses neurons controlling basic body processes. The brain stem has three sections: from the bottom up, the medulla, pons, and midbrain.

Neurons in the medulla control the automatic actions of the heart and lungs, and serve as relay stations for five cranial nerves. They also act as the grand pathway for hundreds of millions of sensory and motor nerves connecting brain and spinal cord. Most of these cross over in the medulla, so that the brain’s left side controls the right side of the body and vice versa.
Above the medulla, the pons and midbrain serve as further relay stations. Running through all three is a thicket of neurons called the reticular formation, which receives signals from sense receptors and controls consciousness.

The limbic system forms two symmetrical loops, only one of which is illustrated, between the brain stem and the cerebral hemisphere. It is concerned principally with memory and emotion. The septum pellucidum is associated with pleasure, the amygdaloid body with aggression, the cingulate gyrus and hippocampus with memory. The thalamus and mamillary body appear to act as organizers. The fornix and anterior commissure link parts of the limbic system with each other and with other parts of the brain.

Cerebellum

The cerebellum, or “little brain,” accounts for 11 per cent of the brain’s total weight. Bulging from the back of the pons, it lies tucked beneath the rear of the much larger cerebral hemispheres. The cerebellum monitors information from muscles, tendons, joints, and the inner ear and acts to adjust and coordinate muscle movements on instructions from the cerebrum. Every action, from drinking a glass of water to walking or playing the piano, proceeds smoothly primarily because of the unobtrusive work done by the cerebellum, which acts like an automatic pilot.

Brain and mind

Most scientists believe that “mind” is just the product of the brain, not an independent entity, as some philosophers have held. Consciousness, perception, attention, memory, thought, judgment, emotion, personality, dreams, and hallucinations have all been shown to depend, at least partly, upon the function of some specific region of the brain. Yet much about the working of the brain remains mysterious. For instance, how do some mystics manage to achieve apparently voluntary control of automatic body mechanisms for example, reducing their oxygen needs at times below the level normally needed for survival? How do techniques like hypnosis and autogenic training help certain people to cope with chronic pain? Study of mind and the brain remains one of the most challenging and from a mechanistic point of view most mysterious areas of investigation into the nature of the human animal.