Concept 35.1: The Endocrine and Nervous Systems Play Distinct, Interacting Roles

All the cells in an animal’s body need to act together harmoniously. During muscular exercise such as a game of tennis, for example, cells in the breathing muscles and heart need to work harder to ensure that the cells of the exercising skeletal muscles receive O₂ at an accelerated rate. Cells in the liver also need to contribute, by mobilizing stored glucose for use by the exercising muscles. When we survey all the tissues of the body, we quickly recognize that various cells are specialized to produce forces, store nutrients, make gametes, or secrete digestive juices. Some cells must also be specialized to carry out control and coordination throughout the body.

We saw in Chapter 34 that nerve cells are one type of cell specialized for control and coordination. Endocrine cells are the second principal type of cell specialized in this way. We will define endocrine cells and hormones in a formal way at the beginning of Concept 35.2. Here it’s sufficient to say simply that endocrine cells secrete hormones into the blood. Nerve and endocrine cells work together to ensure that an animal functions in ways that are harmonious and coordinated, rather than clashing and disjointed.

Nerve and endocrine cells need to communicate with other cells to carry out their functions of control and coordination. Most such intercellular communication takes place by means of chemical signals that are released from a nerve cell (neuron) or an endocrine cell and travel to another cell, called the target cell (FIGURE 35.1). The signal molecules bind to receptors on the target cell, triggering a response in it. These responses can affect an animal’s function, anatomy, and behavior.

Figure 35.1: Chemical Signaling by Nerve and Endocrine Cells Both nerve cells (neurons) and endocrine cells release chemical signaling molecules (red dots) that affect target cells. (A) A nerve cell makes synaptic contact with a target cell at the end of its axon. Action potentials in the axon trigger release of neurotransmitter molecules at the synapse. These molecules then bind with neurotransmitter receptor molecules in the postsynaptic cell, initiating a response in the postsynaptic cell. (B) An endocrine cell releases hormone molecules into the blood. These molecules circulate throughout the body and bind with hormone receptor molecules on target cells, initiating responses.

The nervous and endocrine systems work in different ways

Most tissues and organs in an animal’s body are affected by both the nervous and endocrine systems. The heart, for example, is subject to both nervous and endocrine controls. Why have two systems evolved rather than just one? The answer is undoubtedly complicated. One reason is that the nervous and endocrine systems work in different ways. The two systems are specialized to carry out different types of control and coordination.

As we saw in Chapter 34, neurons typically operate in ways similar to wires in a telephone system or computer network. A neuron is a cell specially adapted to generate brief signals—the nerve impulses, or action potentials—that travel at high speeds along its axon (see Figure 35.1A). The axon makes functional contacts—synapses—with only certain target cells, and only those cells are affected by the neuron. Each axon terminal releases a chemical signal—neurotransmitter—that affects the cell with which it has synaptic contact. This target cell responds quickly to the signal, and the signal ends rapidly because the neurotransmitter molecules released into a synapse are immediately either broken down or taken up into cells.

We see that nervous control has two essential features: it is fast and addressed. Neuronal signals are fast in that they travel rapidly and begin and end abruptly. Like a letter or an e-mail, they are addressed because they are delivered to highly defined target cells.

Unlike nerve cells, endocrine cells release their chemical signals—the hormones—into the blood. These chemical signals are then carried throughout the body by the circulation of the blood, potentially reaching all the cells present in most or all tissues and organs (see Figure 35.1B).

Endocrine control has two essential features, both of which distinguish it from nervous control: it is slow and broadcast. Individual hormonal signals are relatively slow because they operate on much longer time scales than individual neuronal signals. Initiation of hormonal effects requires at least several seconds or minutes because a hormone, once it is released into the blood, must circulate to target tissues and diffuse to effective concentrations within the tissues before it can elicit responses. After a hormone has entered the blood, it may act on target cells for a substantial amount of time—minutes, hours, or even days—before its blood concentration is reduced to ineffective levels by metabolic destruction or excretion. A final reason that endocrine signals can be slow is that they sometimes act on target cells by altering gene transcription and protein synthesis—processes that require at least many minutes to have effects on cell function.

Endocrine control is said to be broadcast because after a hormone is released into the blood, all cells in the body are potentially exposed to it. Those that respond are the ones that express a receptor protein for the hormone. That is, hormone action has specificity, but its specificity depends not on addressed delivery of the chemical signal but on which cells have receptor molecules for the signal. In any one tissue, it is common for all cells to express receptor molecules for a particular hormone, meaning all cells in the tissue respond to a single release of the hormone. Moreover, cells in more than one tissue may respond, perhaps with different cell types responding in different ways. In principle, hormones may exert either limited or widespread effects, but in practice they commonly affect at least an entire tissue, and often multiple tissues.

Nervous systems and endocrine systems tend to control different processes

Lines of communication in the nervous system are capable of much finer control—in both time and space—than is possible for endocrine systems. Not surprisingly, the two systems tend to be used to control different functions in the body. The nervous system controls predominantly the fine, rapid movements of discrete skeletal muscles. The endocrine system typically controls more widespread, prolonged activities such as developmental or metabolic changes.

Consider, for example, the tennis player in our opening photo. Tennis requires rapid body movements that anticipate ball movements and an opponent’s possible responses. It also requires specific control of multiple skeletal muscles in split-second time. These functions could only be coordinated by the nervous system. In contrast, the coordination of adolescent development requires adjusting the activities of many tissues over a prolonged period. In principle, the nervous system could carry out a coordination task of this sort. To do so, however, the nervous system would require tens of thousands of discrete axons between integrating centers (such as the brain) and target cells, and would need to send trains of impulses along all these axons for years. In contrast, endocrine glands can accomplish this task with greater economy, by secreting a relatively small number of long-lasting chemicals into the blood.

Most tissues in an animal’s body are under dual control of the nervous and endocrine systems, as noted earlier. The tennis player’s skeletal muscles illustrate this dual control. A single muscle, such as the biceps, typically contains thousands of muscle cells innervated by more than 100 motor neurons. The nervous system can selectively activate a few, many, or all of the motor neurons to control rapidly and precisely the amount of force the player’s biceps generates. And it can separately control each of the other skeletal muscles in his body. Simultaneously—as days and months go by—testosterone and other hormones secreted steadily by endocrine glands during adolescence facilitate widespread muscle growth and development.

The nervous and endocrine systems work together

In spite of the differences between the nervous and endocrine systems, these two systems do not operate in mutually exclusive ways. On the contrary, they work together closely, and each system can affect the other.

The nervous system exerts control over the endocrine system in many well known ways. The brain initiates endocrine development of the gonads during puberty in mammals, for example. Moreover, throughout life the brain helps control the secretion of gonadal hormones, thyroid hormones, and other hormones. We will discuss these controls in Concepts 35.3 and 35.4.

Conversely, the endocrine system sometimes exerts control over the nervous system. Sex hormones from the testes and ovaries affect brain development during mammalian puberty, for example. Studies on rats, hamsters, and other research animals indicate that the sex hormones have an organizing effect on the development of certain neural circuits in the brain so that the circuits develop some different specific properties in females and males.

Chemical signaling operates over a broad range of distances

Animals use chemical signaling over a very broad range of spatial scales (FIGURE 35.2). In this concept we have discussed chemical signaling on two spatial scales—the two different scales of distance seen in nervous signaling and in endocrine signaling. However, chemical signaling also takes place on even shorter and even longer scales of distance.

Figure 35.2: Chemical Signaling Occurs over a Broad Range of Spatial Scales (A) Cells communicate with neighboring cells in a tissue via molecules called paracrines. (B) Neurons have long processes (axons), at the ends of which they release molecules called neurotransmitters to communicate across synapses with target cells. (C) Endocrine cells secrete molecules called hormones that can travel long distances by circulating in the blood. (D) Animals give off molecules called pheromones that can communicate specific information (such as their sex) over very long distances to other members of their species.

Many chemical signals diffuse from cell to cell in a tissue without entering the blood. Two such types of signals are paracrines and autocrines. Paracrines are chemicals that are secreted by one cell and affect the functions of other, neighboring cells in a tissue by binding to receptors in or on the neighboring cells (see Figure 35.2A). Autocrines are chemicals that are secreted by a cell into the surrounding intercellular fluids and then diffuse to receptors on that very same cell and affect its functions (see Figure 5.10).

Neurotransmitters and hormones work at intermediate distances. Neurotransmitters resemble paracrines in that they move just short distances from cell to cell by diffusion, because only the width of a synaptic cleft separates a presynaptic neuron from its target cell. However, the secretion of neurotransmitters is controlled from farther away by electrical signals that travel the length of the presynaptic cell (see Figure 35.2B). Hormones are carried to the farthest reaches of an animal’s body by the circulation of the blood (see Figure 35.2C).

Pheromones are chemical signals that an individual animal releases into its external environment and that exert specific effects (e.g., behavioral effects) on other individuals of the same species (see Figure 35.2D). Some pheromones travel hundreds of meters before reaching their targets.

Now that we’ve compared and contrasted the nervous and endocrine systems, let’s turn our spotlight on the endocrine system.