Sensory Receptors

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Chapter: Anatomy and Physiology for Health Professionals: Peripheral Nervous System and Reflex Activity

Sensory receptors are classified by what stimuli they detect, their location in the body, and the complexity of their structures:


Sensory Receptors

The sensory receptors of the PNS are specialized to respond to stimuli. Sensory receptors are diverse but sensitive to certain types of environmental changes. In most cases, a sensory receptor that is activated by enough stimuli results in graded potentials, which ­initiate nerve impulses along afferent fibers of the PNS. The brain can then process awareness of the stimulus (sensation) and interpret the meaning of the stimulus (perception).

 

Types of Sensory Receptors

Sensory receptors are classified by what stimuli they detect, their location in the body, and the complexity of their structures:

Chemoreceptors: These respond to chemicals in solution, including smelled or tasted molecules, changes in blood chemistry, and changes in inter-stitial fluid chemistry. Chemoreceptive neurons are also found in the carotid bodies in the neck and the aortic bodies between the primary branches of the aortic arch.

Mechanoreceptors: These respond to mechan-ical forces such as pressure, touch, stretching, and vibrations­. There are three classes: tactile receptors, baroreceptors, and proprioceptors­. Tactile receptors sense ­pressure, touch, and vibra-tion. Fine touch and pressure receptors allow us to sense sources of stimulation that include exact location, shape, size, movement, and texture. Crude touch and pressure receptors only allow generalized sensations. Baroreceptors detect pressure changes in blood vessel walls and in areas of the urinary, reproductive, and digestive tracts. Proprioceptors­ sense positions of skeletal muscles and joints and tension in the ligaments andtendons.

Nociceptors: These respond to stimuli that may be damaging, such as extreme heat or cold, excessive pressure, and inflammatory chemicals,­ resulting in pain. Various subtypes of chemo-receptors, mechanoreceptors, and thermore-ceptors may be stimulated by these stimuli. ­Nociceptors are common in the superficial skin, around blood vessel walls, inside joint capsules, and in the periostea of bones. Painful sensations are carried on two types of fibers called type A and type C fibers. The myelinated type A fibers carry fast pain sensations, such as from a vac-cination or a deep cut. The type C fibers carry slow pain, which is described as pain that feels aching or burning.

Photoreceptors: These receptors respond to light, for example, the receptors in the retinas of the eyes.

Thermoreceptors: These respond to tempera-ture changes and are free nerve endings in the der-mis, liver, skeletal muscles, and hypothalamus.Although not structurally different from each other, there are three to four more cold receptors to every warm receptor. Thermoreceptors are phasic receptors that quickly adapt to stable temperatures.


Classification by Receptor Structure

Most sensory receptors of the general senses are ­actually modified dendritic endings. They are located in most areas of the body and monitor the major-ity of different types of general sensory information (­FIGURE 13-2). Complex sense organs contain the receptors for the special senses, which include hearing,­ vision, equilibrium, smell, and taste. For example, the eyes contain sensory neurons and non-neural cells that make up their lenses, supporting­ walls, and related structures.



1. List the various types of receptors.

2. Explain the terms sensation and perception.

3. Identify the three classes of mechanoreceptors.

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