In this chapter... In studying how the nervous system sends its unique electrical signals, we will return to flow-down gradients and a variety of membrane transport methods. We’ll expand on the idea that gradients store energy, which is key to how nerves send signals so rapidly. We will also explore the coordination of physiological processes, including temperature regulation.

The function of the nervous system is so complex that it’s difficult to capture briefly. At the most basic level it brings information into the brain and spinal cord, which evaluate the information, and send signals out to the appropriate areas of the body to respond. For example, you might see a snake, which your brain evaluates as danger, so you jump away from the snake. Or maybe you have been working hard in the hot sun. Your brain notices that you are dehydrated and tired. In response, it signals you to take a break in the shade, get some water and eat something. It also sends a message to your kidneys to conserve water.

Why this is important…Everything we see, touch, think and feel is thanks to our nervous system. It is central to your identity as a person. It is also responsible for feelings of pleasure and pain, both of which are incredibly important to our life experience and to patient care.

Foundations

The nervous system has two major divisions:

Central nervous system – brain and spinal cord, which do all the processing or thinking
Peripheral nervous system – nerves and receptors, which send and receive signals

Diagram of human body with major nerves radiating from the brain and spinal cord.

FIGURE 10-1. Anatomy of the nervous system. Brain and spinal cord (yellow) are part of the central nervous system. Major nerves (blue) are part of the peripheral nervous system. ("Nervous System" by Medium69, Jmarchn is licensed under CC BY-SA 4.0)

Key terms

Neuron – nervous system cell that sends electrical signals

Neurotransmitter – chemicals released from the end of a neuron to signal other cells

Details

Signals travel along nerve cells, or more properly, neurons. Neurons are unusual cells which, like muscle cells, don’t follow the generic cell structure. The main part of the cell – the cell body – looks like a typical cell. It contains a nucleus and other organelles. But branching out from the cell body are many projections. Some projections (dendrites) bring signals into the cell body. Other projections (axons) carry signals away from the cell body. Axons can be up to about a meter long in humans. In larger organisms, they can be several meters long! (By the way, nerves are really a bundle of axons, so they are a collection of neurons.)

Diagram of a neuron with major parts labeled. It sends a neurotransmitter molecule out of the end, which binds to a receptor on a target cell.

FIGURE 10-2. Anatomy of a neuron. The neuron shown here is a motor neuron. (“Motor neuron” by J. Buegge is licensed under CC BY-SA 4.0)

The size of the neuron creates some challenges. The cell body contains the nucleus and all the protein-making apparatus (the endomembrane system). Proteins made in the cell body must be carried to the end of the projections.

Signals travel down the long axon of the neuron by rapid electrical changes. These signals can travel at speeds ranging from 1 to over 250 miles per hour, depending on the type of neuron.

Once the electrical signal reaches the end of the neuron it triggers the release of a neurotransmitter. Most neurotransmitters are modified amino acids. They sit in vesicles (Figure 10-2), waiting for an electrical signal. Once the signal arrives, they are released by exocytosis and stimulate receptors on the target cell.

Learning objectives

Name and identify the major organs of the nervous system
Describe the basic functions of the nervous system
Briefly explain how neurons send signals