Uniting the Senses for Action

The term "spatial" here refers to "the space that it occupies" and does not refer to "space flight." The fact is that no matter where we find ourselves - on Earth, Mars or orbiting somewhere in outer space - there will always be "space" around us and it is the job of our vestibular system, brain, eyes, muscles, and our touch receptors to judge our position in space at all times. This not only includes a general awareness of position, but it also includes how the different parts of our body are positioned relative to other parts of the body. Even further, these organs help us develop and maintain the awareness of our own body in relation to the pull of gravity.

Figure 6.

Let's consider the case of a person walking on a tightwire (Figure 6). This acrobat's subconscious mind can make extraordinarily delicate interpretations of signals from the vestibular organs and instantly send appropriate orders to his responsive muscles. The eyes, the ears, and the proprioceptors of touch contribute instantly and constantly to the vestibular information and keep him informed of the position of his entire body at every moment; none of these senses alone would be enough to keep the acrobat balanced. In fact, receptors lodged in the muscles, joints, ligaments or tendons fire off signals to the brain any time a muscle contracts or a joint moves or is subjected to added pressure and tension. While the sensory neurons are bringing in information from all parts of the body to the brain, motor neurons are at the same time carrying directions from the brain to the muscles, where signals travel from a nerve through the synapse, and directly to the muscle fiber. These signals cause the muscles to move. Let's examine how this works.

The fibers of motor neurons terminate in tiny flat plates, called motor end plates, which lie in close contact with individual muscle fibers (refer back to Figure 3). This is called the neuromuscular junction. Here the nerve ending is separated from the muscle only by a tiny gap called the synaptic cleft. This gap would be large enough to actually stop the motor signals in their tracks except for a special chemical reaction that takes place at the gap. As the impulse reaches the motor end plate, the nerve ending releases a chemical transmitter known as acetylcholine, which initiates events in the muscle cell. With the muscle's contraction, another substance, an enzyme called cholinesterase, begins breaking down the accumulated acetylcholine and clearing it away so that the next arriving nerve impulse can set the cycle in motion again. It is this rechargeable chemical cycle that efficiently bridges the gap between motor nerve ending and muscle cell so that muscle action can be stimulated.

Figure 7.

Determining which muscles contract and which relax in the continuous adjustments required to maintain our body's balance and keep us upright here on Earth is a function of the "lower centers" of the brain, the brain stem (including the reticular formation) and the cerebellum. From Figure 7, you can see that the brain stem receives input from four major sources and transmits output to three major regulators of body movement and perception. The brain stem and the cerebellum also participate in the control of our heart rate, respiration, and digestion. In addition, when appropriate signals from our gut receptors, various chemical signals from the brain, and unfamiliar and confusing sensory signals are received and integrated, the brain stem reticular formation is the site in the brain that can trigger vomiting. Other areas of the brain are also very involved with our senses, perceptions, and responses to movement. Somewhat "higher centers" of the brain regulate such complex motor activities as walking, running, and reaching for objects as well as body temperature and appetite. The "highest center" of the brain, the cerebral cortex, consists of distinctive areas called the sensory cortex and the motor cortex (Figure 6). The cerebral cortex gives us our awareness of sight and sound, as well as our delicate sensations of weight, texture and form. It also initiates emotional responses such as fear and anger, and enables us to experience pleasure or pain. With all of its importance, the nervous system is well cared for by our bodies. Let's discuss what it takes to keep our nerves alive.