23.8–23.14: Our senses detect and transmit stimuli.

A bat-eared fox stands alert in the long grass.
23.8: Sensory receptors are our windows to the world around us.

Did you ever wonder why animals tend to have only one head and why it’s always in the front of the body? When organisms became bilaterally symmetrical—that is, having mirror-image left and right sides—suddenly they had a “front” and a “back.” As such animals move through their environment, their “front” part encounters new things in the environment first; with the sensory equipment up front, the organism is able to decide what to do—eat or run—as quickly as possible (FIGURE 23-15), which may have conferred evolutionary benefits. And as the decisions to be made get increasingly complex, the circuitry required for the animal to “make up its mind” becomes more complex. In fact, only then does it really become necessary to have a mind. And so the brain and head (which always go hand in hand) are prominent structures.

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Question 23.4

Why do animals have only one head? Why is it in the front?

Figure 23.15: Eyes up front. Most of an animal’s sensory organs are located in its head. Shown: a lemur.

Our senses—sight, hearing, smell, taste, and touch—make us physically aware of the environment around us. They are our windows to the world. Each of our senses puts us in touch with and brings us information about a different and unique slice of the world, but the senses also have much in common (FIGURE 23-16).

The process by which all of our senses work is basically the same. A receptor—commonly a modified dendrite on a sensory neuron (in the eye, nose, tongue, ear, or skin)—is stimulated by some aspect of the outside world (light, odor, taste, sound, or touch). This outside stimulus causes a change in the neuron, causing the sensory neuron to either (1) fire an action potential itself, which shoots down the axon and ultimately reaches a part of the brain where the signal is perceived as a particular smell or sound, for example, or (2) alter its rate of neurotransmitter secretion so that it increases or reduces the rate of firing of action potentials in a neighboring neuron.

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How is it possible to experience variations in the intensity of sensation, when an action potential is an all-or-nothing event? As we’ve seen, it either occurs or doesn’t occur as the result of dendrite stimulation. After all, our experience of sensations is not “all or nothing.” We are able to feel gradations of intensity. A 10-pound bowling ball resting in your left hand will feel very heavy (and perhaps cooler and smoother) compared with a lightweight rubber ball in your right hand (FIGURE 23-17). How can the “fire” versus “don’t fire” event still produce such a range of sensations? The intensity of the sensation an individual feels is modulated by the number of action potentials per unit of time and the number of neurons stimulated. The rubber ball causes a small number to fire, and the bowling ball causes many more to fire.

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In the following sections, we present information about each of the five primary senses, exploring how the specific senses are variations on a theme, fine-tuned by natural selection to give an animal specific information about its environment to help it respond appropriately.

Figure 23.16: Making us aware of our environment: the senses. (Note that most of the body parts—such as our hands and tongue—may have more than one function.)
Figure 23.17: Heavy or light?

TAKE-HOME MESSAGE 23.8

The process by which all our senses work is basically the same. A modified dendrite on a sensory neuron is stimulated by some aspect of the outside world, causing the sensory neuron either to fire an action potential (which shoots down the axon and ultimately reaches a part of the brain where the signal is perceived as a particular smell or sound, for example) or to alter its rate of neurotransmitter secretion (which increases or reduces the rate of action potential firing in a neighboring neuron).

What portion of a nerve cell detects external stimuli such as light or odors?