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).
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.
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.
Related Topics
TH 2019 - 2024 pharmacy180.com; Developed by Therithal info.