The Structure of the Nervous System
The mammalian nervous system is divided into central and peripheral nervous systems.
The Peripheral Nervous System
Figure 3 The various components of the peripheral nervous system
The peripheral nervous system is divided into somatic and autonomic nervous systems (Figure 3). Where the somatic nervous system consists of cranial nerves (12 pairs) and spinal nerves (31 pairs) and is under the volitional control of the individual in maneuvering bodily muscles, the autonomic nervous system also running through these nerves lets the individual have little control over muscles and glands. Main divisions of the autonomic nervous system that control visceral structures are the sympathetic and parasympathetic nervous systems.
At an appropriate cue (say a fear-inducing object like a snake), the sympathetic division generally energizes many muscles (e.g., heart) and glands (e.g., adrenals), causing activity and release of hormones that lead the individual to negotiate the fear-causing snake with fight- or-flight responses. Whether the individual decides to fight the snake or run away from it, either action requires energy; in short, the sympathetic nervous system says “go, go, go.” The parasympathetic nervous system, on the other hand, curtails undue energy mobilization into muscles and glands and modulates the response by saying “stop, stop, stop.” This push–pull tandem system regulates fight-or-flight responses in all of us.
Figure 4 the central nervous system and its components
The Central Nervous System
The central nervous system is divided into a number of important parts (see Figure 4), including the spinal cord, each specialized to perform a set of specific functions. Telencephalon or cerebrum is a newer development in the evolution of the mammalian nervous system. In humans, it is about the size of a large napkin and when crumpled into the skull, it forms furrows called sulci (singular form, sulcus). The bulges between sulci are called gyri (singular form, gyrus). The cortex is divided into two hemispheres, and each hemisphere is further divided into four lobes (Figure 5a), which have specific functions. The division of these lobes is based on two delineating sulci: the central sulcus divides the hemisphere into frontal and parietal-occipital lobes and the lateral sulcus marks the temporal lobe, which lies below.
Figure 5a The lobes of the brain
Just in front of the central sulcus lies an area called the primary motor cortex (precentral gyrus), which connects to the muscles of the body, and on volitional command moves them. From mastication to movements in the genitalia, the body map is represented on this strip (Figure 5b).
Some body parts, like fingers, thumbs, and lips, occupy a greater representation on the strip than, say, the trunk. This disproportionate representation of the body on the primary motor cortex is called the magnification factor (Rolls & Cowey, 1970) and is seen in other motor and sensory areas.