HUMAN BIOLOGY

266 Chapter 14

Sensory neuron

(to CNS)

Sensory neuron

Action potential

Action potential

Action potential

Stimulus

Stimulus

Stimulus

A Sensory receptor consisting of free nerve endings—dendrites of a sensory neuron

B Sense organ—sensory receptor involving nerve endings of an afferent neuron

enclosed in a specialized structure

C Sensory receptor formed by a cell that synapses with a sensory neuron

Sensory

receptor cell

Synapse

Free

nerve

endings

Nerve ending

sensory receptors and pathways

n    Sensory systems notify the brain and spinal cord of specific

changes inside and outside the body.

n    Links to Action potentials 13.2, Information pathways 13.4,

Sensory areas of the brain 13.8

In a sensory system, a stimulus activates receptors, which

then convert the stimulus to a nerve impulse—that is, an

action potential—that travels to the brain. There the action

potential may trigger a sensation or perception:

the brain produces a perception—an understanding of

what the sensation means.

There are three basic forms of sensory receptors (Figure

14.1). Those called free nerve endings are present in the

epidermis and many connective tissues. They detect touch,

pressure, heat, cold, or pain. An encapsulated receptor

is an organ in which a capsule of epithelial or connective

tissue encloses nerve endings of a sensory neuron. The

third basic type of sensory receptor is formed by a cell that

synapses with a sensory neuron.

We organize sensory receptors based on the type of

stimulus each detects. Photoreceptors detect visible light.

Chemoreceptors detect chemicals dissolved in the fluid

around them. Mechanoreceptors detect changes in pres-

sure, position, or acceleration. Thermoreceptors respond

to heat or cold. Receptors for pain, called nociceptors,

detect damage to tissues. Osmoreceptors detect changes

in water volume (solute concentration) in a body fluid.

Regardless of their differences, all sensory receptors con-

vert the stimulus to nerve impulses (action potentials).

Nerve impulses that move along sensory neurons are all

the same. So how does the brain know what sort of sensory

stimulus

energy

received

stimulus energy

converted to

nerve impulse

(action potential)

brain response

(sensation or

perception)

A stimulus (plural: stimuli) is a form of energy that

activates receptor endings of a sensory neuron. That

energy is converted to the electrochemical energy of action

potentials—the nerve impulses by which the brain receives

information and sends out commands in response. This

basic response in the brain is a sensation, which is con-

scious awareness of a stimulus. Higher-level processing in

F i g u r e 14.1 There are three main forms of sensory receptors. (© Cengage Learning)

14.1

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