CENTRAL NERVOUS SYSTEM NEURON: THE BASIC FUNCTIONAL UNIT
The central nervous system contains more than 100 billion neurons. Figure 1 shows a typical neuron of a type found in the brain motor cortex. Incoming signals enter this neuron through synapses located mostly on the neuronal dendrites, but also on the cell body. For different types of neurons, there may be only a few hundred or as many as 200,000 such synaptic connections from input fibers. Conversely, the output signal travels by way of a single axon leaving the neuron. Then, this axon may have many separate branches to other parts of the nervous system or peripheral body.
Fig1. Structure of a large neuron in the brain showing its important functional parts. (Modified from Guyton AC: Basic Neuroscience: Anatomy and Physiology. Philadelphia: WB Saunders, 1987.)
A special feature of most synapses is that the signal normally passes only in the forward direction, from the axon of a preceding neuron to dendrites on cell mem branes of subsequent neurons. This feature forces the signal to travel in required directions to perform specific nervous functions.
SENSORY PART OF THE NERVOUS SYSTEM—SENSORY RECEPTORS
Most activities of the nervous system are initiated by sensory experiences that excite sensory receptors, whether visual receptors in the eyes, auditory receptors in the ears, tactile receptors on the surface of the body, or other kinds of receptors. These sensory experiences can either cause immediate reactions from the brain, or memories of the experiences can be stored in the brain for minutes, weeks, or years and determine bodily reactions at some future date.
Figure 2 shows the somatic portion of the sensory system, which transmits sensory information from the receptors of the entire body surface and from some deep structures. This information enters the central nervous system through peripheral nerves and is con ducted immediately to multiple sensory areas in (1) the spinal cord at all levels; (2) the reticular substance of the medulla, pons, and mesencephalon of the brain; (3) the cerebellum; (4) the thalamus; and (5) areas of the cerebral cortex.
Fig2. Somatosensory axis of the nervous system.
MOTOR PART OF THE NERVOUS SYSTEM—EFFECTORS
The most important eventual role of the nervous system is to control the various bodily activities. This task is achieved by controlling (1) contraction of appropriate skeletal muscles throughout the body, (2) contraction of smooth muscle in the internal organs, and (3) secretion of active chemical substances by both exocrine and endocrine glands in many parts of the body. These activities are collectively called motor functions of the nervous system, and the muscles and glands are called effectors because they are the actual anatomical structures that perform the functions dictated by the nerve signals.
Figure 3 shows the “skeletal” motor nerve axis of the nervous system for controlling skeletal muscle contraction. Operating parallel to this axis is another system, called the autonomic nervous system, for control ling smooth muscles, glands, and other internal bodily systems.
Fig3. Skeletal motor nerve axis of the nervous system.
Note in Figure 3 that the skeletal muscles can be controlled from many levels of the central nervous system, including (1) the spinal cord; (2) the reticular substance of the medulla, pons, and mesencephalon; (3) the basal ganglia; (4) the cerebellum; and (5) the motor cortex. Each of these areas plays its own specific role. The lower regions are concerned primarily with automatic, instantaneous muscle responses to sensory stimuli, and the higher regions are concerned with deliberate complex muscle movements controlled by the thought processes of the brain.
PROCESSING OF INFORMATION—“INTEGRATIVE” FUNCTION OF THE NERVOUS SYSTEM
One of the most important functions of the nervous system is to process incoming information in such a way that appropriate mental and motor responses will occur. More than 99 percent of all sensory information is dis carded by the brain as irrelevant and unimportant. For instance, one is ordinarily unaware of the parts of the body that are in contact with clothing, as well as of the seat pressure when sitting. Likewise, attention is drawn only to an occasional object in one’s field of vision, and even the perpetual noise of our surroundings is usually relegated to the subconscious.
However, when important sensory information excites the mind, it is immediately channeled into proper integrative and motor regions of the brain to cause desired responses. This channeling and processing of information is called the integrative function of the nervous system. Thus, if a person places a hand on a hot stove, the desired instantaneous response is to lift the hand. Other associated responses follow, such as moving the entire body away from the stove and perhaps even shouting with pain.
ROLE OF SYNAPSES IN PROCESSING INFORMATION
The synapse is the junction point from one neuron to the next. Later in this chapter, we discuss the details of synaptic function. However, it is important to point out here that synapses determine the directions that the nervous signals will spread through the nervous system. Some synapses transmit signals from one neuron to the next with ease, whereas others transmit signals only with difficulty. Also, facilitatory and inhibitory signals from other areas in the nervous system can control synaptic transmission, sometimes opening the synapses for transmission and at other times closing them. In addition, some postsynaptic neurons respond with large numbers of output impulses, and others respond with only a few. Thus, the synapses perform a selective action, often blocking weak signals while allowing strong signals to pass, but at other times selecting and amplifying certain weak signals and often channeling these signals in many directions rather than in only one direction.
STORAGE OF INFORMATION—MEMORY
Only a small fraction of even the most important sensory information usually causes immediate motor response. However, much of the information is stored for future control of motor activities and for use in the thinking processes. Most storage occurs in the cerebral cortex, but even the basal regions of the brain and the spinal cord can store small amounts of information.
The storage of information is the process we call memory, and this, too, is a function of the synapses. Each time certain types of sensory signals pass through sequences of synapses, these synapses become more capable of transmitting the same type of signal the next time, a process called facilitation. After the sensory signals have passed through the synapses a large number of times, the synapses become so facilitated that signals generated within the brain itself can also cause transmission of impulses through the same sequences of synapses, even when the sensory input is not excited. This process gives the person a perception of experiencing the original sensations, although the perceptions are only memories of the sensations.
The precise mechanisms by which long-term facilitation of synapses occurs in the memory process are still uncertain, but what is known about this and other details of the sensory memory process.
Once memories have been stored in the nervous system, they become part of the brain processing mechanism for future “thinking.” That is, the thinking processes of the brain compare new sensory experiences with stored memories; the memories then help to select the important new sensory information and to channel this into appropriate memory storage areas for future use or into motor areas to cause immediate bodily responses.
