The dynamic system of controls that make up the nervous system is composed primarily of nerve cells, or neurons, that are woven together as tissue interspersed with other cells, called glia. The brain and the spinal cord serve as the system's main base of operations. From these organs, an extraordinary network of nerves reaches out to every part of the body. The brain, for all of its importance and glory, is really a modest looking structure. It is soft gray, and wrinkled, and about the size and shape of an average acorn squash. It weighs about three pounds and it has a four-inch-long, one-inch-wide central core, or brain stem, extending upward from the spinal cord (Figure 1). On either side of this core, and behind it, lies a mass of nervous tissue, the cerebellum. Draped over the brain stem and cerebellum is the cerebrum. Its outer layer, the cerebral cortex, is folded and convoluted to fit into the six-by-eight-inch vault of the skull; if flattened out, the cortex would cover more than two square feet in area. A deep crease (fissure) down the middle of the brain divides the cerebrum into two hemispheres connected by two bands that are much lighter in color than the gray exterior tissue of the brain. Here, as elsewhere in the nervous system, the two colors represent a significant difference:
What gives these fibers their color, as well as a generally waxy look, is a fatty coating, myelin, which not only insulates the fiber but also makes it a speedier conductor. The spinal cord, which is about 18 inches long in an average sized adult, resembles a cable tapered at both ends; at its widest, it is a little more than half an inch across. Like the brain, the spinal cord contains both gray and white matter; the gray nerve-cell bodies form a column in the center of the cord, wrapped by the white bundles of nerve fibers (Figure 1). Surrounding the cord on the outside is a perforated tunnel of bone formed by the rings of the vertebrae - the body's backbone.
The network of nerves that threads through the body is rooted both in the brain and in the spinal cord. Through the entire extent of the brain stem, between the brain and the spinal cord, are areas of neurons collectively known as the reticular formation. Many of the nerve fiber tracts that travel between the brain and spinal cord pass through the reticular formation. Some of the signals that enter the reticular formation are being sent by the balance organs of the body, particularly from the vestibular apparatus in the inner ear, the cerebellum, and both the sensory and motor regions of the cerebral cortex. The reticular formation seems to serve, then, as one of our body's integration sites to help interpret the multiple sensory and motor nerve signals that are constantly being sent throughout our system to the spinal cord and then up to the brain.
From the brain, nerve fibers converge to form 12 pairs of cranial nerves serving the head, eyes, ears, throat, and some organs in the chest and abdomen. From the spinal cord, 31 pairs of spinal nerves pass out through openings between the vertebrae at various levels (Figure 2). Although they are not named individually, they are grouped according to the level of the vertebrae from which they arise, and each nerve is numbered in sequence in the following way:
From the upper part of the spinal cord, the cervical and thoracic nerves branch and rebranch to form nerve trunks leading to the upper torso, arms and hands. From the lower part of the cord, the lumbar, sacral, and coccygeal nerves branch and rebranch to form nerve trunks leading to the pelvis, thighs, legs and feet. In an adult, the spinal cord does not extend the entire length of the vertebral column, but it ends at the level between the first and second lumbar vertebrae. It is at this point that the lumbar, sacral, and coccygeal nerves descend to their exits beyond the end of the cord. These descending nerves form a structure called the cauda equina, which is so named because it is shaped somewhat like a horse's tail.
The nerves reach every millimeter of skin surface, every muscle, every blood vessel, every bone - every part of the body from tip to toe. Each nerve carries electrochemical impulses that are triggered by some stimulus and that travel at rates of anywhere from 1 to about 400 feet per second. The speed of the impulse depends on the size and type of the particular nerve fiber involved and the thickness of the coating of myelin around it. Certain fibers which lack myelin are comparatively slow transmitters.
The basic unit of the nervous system, the neuron, has one structural characteristic that distinguishes it from all other cell types. Thin fibers that look like tiny tendrils (Figure 3) extend from the central part of each neuron. The central part of the neuron is the actual cell body, or Coma, and the fibers extending out from the soma are either called axons or dendrites depending on their function, but not all neurons have both. Let's distinguish between these two:
Depending upon their location and function, neurons may be anywhere from a fraction of an inch to as much as three or four feet long (the longest reaches from the base of the brain to the big toe). It is via their fibers that the neurons perform their unique function of transmitting electrochemical signals to and from the brain and spinal cord. Upon receiving a signal, one neuron sends it on to another neuron lying adjacent to it across a kind of bridge that "sparks" the impulse across to the other side through electrical and chemical means. This sparking bridge is called a synapse. We will discuss this more, as it applies to muscle stimulation, later in this section. Sensory signals that produce sensations of sight, sound, pain, pressure, touch, heat or cold go from various parts of the body to the brain either directly or through the spinal cord. Back from the brain or the spinal cord go "orders" from motor signals to muscles in the fingers, toes, heart, intestines, and elsewhere. In this way, all of our parts work together to allow us to function in the world around us. There are three general classes of neurons, grouped according to their function:
The sensory neurons keep the brain informed of what is happening to and in the body through a wide variety of sensory pick-up units called receptors. Some of these, called tactile or cutaneous receptors, are located at or near the skin surface and are especially sensitive to pain, touch, or pressure on the skin. There are special thermoreceptors that detect changes in body surface temperatures. The proprioceptors are the body's primary mechanical receptors, located in muscles, joints, and tendons, and they are sensitive to changes in muscle tension and movement. Other receptors on the tongue and in the upper parts of the nose respond to flavors and odors.
In the retina of the eye, special photoreceptors shaped like rods respond to light and shadow of different intensities and wavelengths, while receptors shaped like cones respond to color. Other receptors are embedded deep in the digestive tract, in the walls of the intestines; when the intestines are confronted by indigestible food or by gas, these receptors transmit signals of pain, which are then interpreted as cramps. Sensory receptors in the ear respond to vibrations caused by sound waves bouncing off the eardrum. Other sensory receptors in the ear react to and interpret gravitational information and initiate movement of our body here on Earth. How do we remain standing despite the perpetual pull of gravity? Why can you whirl around suddenly without falling down? The vestibular organs (Figure 4), also called the vestibular apparatus, in the inner ear help maintain equilibrium by sending the brain information about the motion and position of the head The vestibular organs consist of three membranous semi-circular canals (SCCs), and two large sacs, the utricle and saccule. All the vestibular organs share a common type of receptor cell, the hair cell. Let's examine the structure and function of the vestibular organs a little more closely.
The three semicircular canals (SSCs) within the vestibular organ of each ear contain fluid and hair receptor cells encased inside a fragile membrane called the cupula (Figure 5). The cupula is located in a widened area of each canal called the ampulla. When you move your head, the fluid in the ampulla lags behind, pushing the cupula a very tiny bit which causes the hairs to also bend a very tiny bit. The bending hairs stimulate the hair cells, which in turn trigger sensory impulses in the vestibular nerve going to the brain to "report" the movement. Hair cells are amazingly sensitive. For example, a cupula movement of even a thousandth of an inch is detected by the brain as a big stimulus.
Two other vestibular organs are located in membranous sacs called the utricle and the saccule. On the inside walls of both the utricle and the saccule is a bed (a macula) of several thousand hair cells covered by small flat piles of calcium carbonate crystals which look like sand, imbedded in a gel-like substance (Figure 5b). The crystals are called otoliths, a word which literally means "ear stones." In fact, the utricle and the saccule are often called the otolith organs.
When a person's head is in the normal erect position, the hair cells in the utricle lie approximately in a horizontal plane. When the head is tilted to one side, the stones want to slide "downhill." This moves the gel just enough to bend the sensory hairs. The bending hairs stimulate the hair cells, which in turn send a signal to the brain about the amount of head tilt. The stones also move if the person is accelerated forward and back, or side to side. Similarly, the hair cells in the saccule are oriented in somewhat of a vertical position when the head is erect. When a person tilts their head, or is accelerated up and down (as in an elevator), or moved forward and back, the otoliths move and a signal is sent to the brain. The signals from the otoliths in the saccule and the utricle complement each other and give us an integrated signal about our movement. The otolith organs are primarily responsible for detecting any degree of linear motion of the head.