HUMAN BIOLOGY

SenSory SyStemS 267

What are the parts of a sensory system?

• A sensory system has receptors for specific stimuli, such as
  light. It also has nerve pathways that conduct information from
  receptors to the brain, and brain regions that receive and
  process the information.


• The brain senses a stimulus based on which nerves carry the
  incoming signals, the frequency of nerve impulses along axons in
  the nerve, and the number of axons that are carrying the signals.

taKe-Home message

event has occurred? It assesses
which nerves are carrying nerve
impulses, the frequency of the
nerve impulses on each axon
in the nerve, and the number
of axons that responded to the
stimulus. Let’s consider the steps
involved in this processing.
First, specific sensory areas
of the brain can interpret action
potentials only in certain ways.
That is why you “see stars” when
your eye is poked, even in the
dark. The mechanical pressure
on photoreceptors in the eye trig-
gers signals that travel along the
optic nerve. The brain always
interprets signals from an optic
nerve as “light.” In fact, as you
will read in Section 14.2, the brain
has a detailed map of the sources
of different sensory stimuli.
Second, a strong signal makes
receptors fire nerve impulses
more often and longer than a
weak one does. So, while the
same receptor in your ear can
detect the sounds of a whisper
and a screech, the brain senses
the difference through varia-
tions in the signals each sound
produces.
Third, the stronger a stimulus, the more sensory recep-
tors respond. Gently tap a spot of skin on your arm and
you activate only a few touch recep tors. Press hard on the
same spot and you activate more. The increase translates
into nerve impulses in many sensory neurons at once.

B Stretched muscle stimulates

a stretch receptor (the ending of a

sensory neuron) that is adjacent to it.

C Message travels from stimulated

sensory neuron to motor neuron

and interneuron in spinal cord.

interneuron in spinal cord

sensory neuron

axon endings of motor

neuron terminating on

the same muscle

D Message is sent back to

the muscle, also to other

interneurons in the brain.

muscle spindle

motor neuron in spinal cord

A

E © Viorel Sima/Shutterstock.com

Figure 14.2 Signals from stretch receptors in muscles provide an example of a sensory pathway.
A–D The diagram above depicts the path of impulses from receptors called muscle spindles to the spinal
cord and brain. E A dancer’s complex moves depend on feedback from stretch receptors. (© Cengage Learning)

Your brain interprets the combined activity as an increase

in the intensity of the stimulus.

In some cases the frequency of nerve impulses (how

often they occur in a given period of time) slows or stops

even when the stimulus continues at constant strength. For

instance, after you put on a T-shirt, you quickly become

only dimly aware of its pressure against your skin. This

diminishing response to an ongoing stimulus is called

sensory adaptation.

Some mechano receptors adapt rapidly to a sustained

stimulus and only signal when it starts and stops. Other

receptors adapt slowly or not at all; they help the brain

monitor particular stimuli all the time.

The dancer in Figure 14.2 is holding his position in

response to signals from his skin, skeletal muscles, joints,

tendons, and ligaments. For example, how fast and how far

a muscle stretches depends on activation of stretch recep-

tors in muscle spindles (Figure 14.2B and Section 13.4). By

responding to changes in the length of muscles, his brain

helps him maintain his balance and posture.

In the rest of this chapter we explore examples of the

body’s sensory receptors. Receptors that are found at

more than one location in the body contribute to somatic

(“of the body”) sensations. Other receptors are restricted

to sense organs, such as the eyes or ears, and contribute

to what are called the “ special senses.”

chemoreceptor
Receptor that detects
dissolved chemicals.

encapsulated receptor
A sensory receptor enclosed
in epithelial or connective
tissue.

free nerve ending
Dendrite of a sensory
neuron.

mechanoreceptor
Receptor that detects
changes in pressure, posi-
tion, or acceleration.

nociceptor Receptor that
detects tissue damage.

osmoreceptor Receptor
that detects change in water
volume of a solution.

perception Understanding
the meaning of a sensation.

photoreceptor Receptor
that detects light.

sensation Awareness of a
stimulus.

sensory adaptation
Diminishing response to an
ongoing stimulus.

stimulus Energy that can
activate a sensory neuron.

thermoreceptor
Receptor that responds to
heat or cold.

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