Organization of the Nervous System, Basic Functions of Synapses, and Neurotransmitters

Central Nervous System Neuron: The Basic Functional Unit

The central nervous system is estimated to contain 80 to 100 billion neurons. Figure 46-1 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. In contrast, the output signal travels via a single axon leaving the neuron. Then, this axon may have many separate branches to other parts of the nervous system or peripheral body.

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 membranes 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 types 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 46-2 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 conducted 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.

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 46-3 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 controlling smooth muscles, glands, and other internal bodily systems; this system is discussed in Chapter 61 .

Note in Figure 46-3 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 has 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 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% of all sensory information is discarded by the brain as irrelevant and unimportant. For example, one is ordinarily unaware of the parts of the body that are in contact with clothing, as well as 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 note 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 , which is also 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 whereby 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 are discussed in Chapter 58 .

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 select the important new sensory information and channel this into appropriate memory storage areas for future use or into motor areas to cause immediate bodily responses.

Spinal Cord Level

We often think of the spinal cord as being only a conduit for signals from the periphery of the body to the brain or in the opposite direction from the brain back to the body. This supposition is far from the truth. Even after the spinal cord has been cut in the high neck region, many highly organized spinal cord functions still occur. For example, neuronal circuits in the cord can cause (1) walking movements; (2) reflexes that withdraw portions of the body away from painful objects; (3) reflexes that stiffen the legs to support the body against gravity; and (4) reflexes that control local blood vessels, gastrointestinal movements, or urinary excretion. In fact, the upper levels of the nervous system often operate not by sending signals directly to the periphery of the body, but by sending signals to the control centers of the cord, simply “commanding” the cord centers to perform their functions.

Lower Brain or Subcortical Level

Many, if not most, of what we call subconscious activities of the body are controlled in the lower areas of the brain—that is, in the medulla, pons, mesencephalon, hypothalamus, thalamus, cerebellum, and basal ganglia. For example, subconscious control of arterial pressure and respiration is achieved mainly in the medulla and pons. Control of equilibrium is a combined function of the older portions of the cerebellum and the reticular substance of the medulla, pons, and mesencephalon. Feeding reflexes, such as salivation and licking the lips in response to the taste of food, are controlled by areas in the medulla, pons, mesencephalon, amygdala, and hypothalamus. In addition, many emotional patterns, such as anger, excitement, sexual response, reaction to pain, and reaction to pleasure, can still occur after destruction of much of the cerebral cortex.

Higher Brain or Cortical Level

After the preceding account of the many nervous system functions that occur at the cord and lower brain levels, one may ask, “what is left for the cerebral cortex to do?” The answer to this question is complex, but it begins with the fact that the cerebral cortex is an extremely large memory storehouse. The cortex never functions alone but always in association with lower centers of the nervous system.

Without the cerebral cortex, the functions of the lower brain centers are often imprecise. The vast storehouse of cortical information usually converts these functions to determinative and precise operations.

Finally, the cerebral cortex is essential for most of our thought processes, but it cannot function by itself. In fact, it is the lower brain centers, not the cortex, that initiate wakefulness in the cerebral cortex, thereby opening its bank of memories to the thinking machinery of the brain. Thus, each portion of the nervous system performs specific functions, but it is the cortex that opens a world of stored information for use by the mind.

Types of Synapses—Chemical and Electrical

There are two major types of synapses ( Figure 46-5 )—(1) chemical and (2) electrical.

Most of the synapses used for signal transmission in the central nervous system of the human being are chemical synapses. In these synapses, the first neuron secretes at its nerve ending synapse a chemical substance called a neurotransmitter (often called a transmitter substance ), and this transmitter in turn acts on receptor proteins in the membrane of the next neuron to excite the neuron, inhibit it, or modify its sensitivity in some other way ( Video 46-1 ). More than 50 important neurotransmitters have been discovered thus far. Some of the best known are acetylcholine, norepinephrine, epinephrine, histamine, gamma-aminobutyric acid (GABA), glycine, serotonin, and glutamate.

In electrical synapses , the cytoplasms of adjacent cells are directly connected by clusters of ion channels called gap junctions that allow free movement of ions from the interior of one cell to the interior of the next cell. Such junctions were discussed in Chapter 4 , and it is by way of gap junctions and other similar junctions that action potentials are transmitted from one smooth muscle fiber to the next in visceral smooth muscle ( Chapter 8 ) and from one cardiac muscle cell to the next in cardiac muscle ( Chapter 9 ).

Although most synapses in the brain are chemical, electrical and chemical synapses may coexist and interact in the central nervous system. The bidirectional transmission of electrical synapses permits them to help coordinate the activities of large groups of interconnected neurons. For example, electrical synapses are useful in detecting the coincidence of simultaneous subthreshold depolarizations within a group of interconnected neurons; this enables increased neuronal sensitivity and promotes synchronous firing of a group of interconnected neurons.

Physiologic Anatomy of the Synapse

Figure 46-6 shows a typical anterior motor neuron in the anterior horn of the spinal cord. It is composed of three major parts—the soma , which is the main body of the neuron, a single axon , which extends from the soma into a peripheral nerve that leaves the spinal cord, and dendrites , which are great numbers of branching projections of the soma that extend as much as 1 millimeter into the surrounding areas of the cord.

As many as 10,000 to 200,000 minute synaptic knobs called presynaptic terminals lie on the surfaces of the dendrites and soma of the motor neuron, with about 80% to 95% of them on the dendrites and only 5% to 20% on the soma. These presynaptic terminals are the ends of nerve fibrils that originate from many other neurons. Many of these presynaptic terminals are excitatory —that is, they secrete a neurotransmitter that excites the postsynaptic neuron. However, other presynaptic terminals are inhibitory —that is, they secrete a neurotransmitter that inhibits the postsynaptic neuron.

Neurons in other parts of the cord and brain differ from the anterior motor neuron in (1) the size of the cell body; (2) the length, size, and number of dendrites, ranging in length from almost zero to many centimeters; (3) the length and size of the axon; and (4) the number of presynaptic terminals, which may range from only a few to as many as 200,000. These differences make neurons in various parts of the nervous system react differently to incoming synaptic signals and, therefore, perform many different functions.