150
SECTION II
Physiology of Nerve & Muscle Cells
environment; and (4) proprioceptors, which provide informa-
tion about the position of the body in space at any given in-
stant. However, the conscious component of proprioception
(“body image”) is actually synthesized from information com-
ing not only from receptors in and around joints but also from
cutaneous touch and pressure receptors.
Other special terms are frequently used to identify sensory
receptors. The cutaneous receptors for touch and pressure are
mechanoreceptors.
Potentially harmful stimuli such as pain,
extreme heat, and extreme cold are said to be mediated by
nociceptors.
The term
chemoreceptor
is used to refer to
receptors stimulated by a change in the chemical composition
of the environment in which they are located. These include
receptors for taste and smell as well as visceral receptors such
as those sensitive to changes in the plasma level of O
2
, pH,
and osmolality.
Photoreceptors
are those in the rods and
cones in the retina that respond to light.
SENSE ORGANS
Sensory receptors can be specialized dendritic endings of af-
ferent nerve fibers, and they are often associated with nonneu-
ral cells that surround it, forming a
sense organ.
Touch and
pressure are sensed by four types of mechanoreceptors (Figure
8–1).
Meissner corpuscles
are dendrites encapsulated in con-
nective tissue and respond to changes in texture and slow vi-
brations.
Merkel cells
are expanded dendritic endings, and
they respond to sustained pressure and touch.
Ruffini corpus-
cles
are enlarged dendritic endings with elongated capsules,
and they respond to sustained pressure.
Pacinian corpuscles
consist of unmyelinated dendritic endings of a sensory nerve
fiber, 2
μ
m in diameter, encapsulated by concentric lamellae
of connective tissue that give the organ the appearance of a
cocktail onion. Theses receptors respond to deep pressure and
fast vibration.
The Na
+
channel BNC1 is closely associated with touch
receptors. This channel is one of the
degenerins,
so called
because when they are hyperexpressed, they cause the neu-
rons they are in to degenerate. However, it is not known if
BNC1 is part of the receptor complex or the neural fiber at the
point of initiation of the spike potential. The receptor may be
opened mechanically by pressure on the skin.
Some sensory receptors are not specialized organs but
rather are free nerve endings. Pain and temperature sensa-
tions arise from unmyelinated dendrites of sensory neurons
located around hair follicles throughout the glaborous and
hairy skin as well as deep tissue.TABLE 8–1
Principle sensory modalities.
Sensory System Modality Stimulus Energy Receptor Class Receptor Cell Types
Somatosensory Touch Tap, flutter 5–40 Hz Cutaneous mechanoreceptor Meissner corpuscles
Somatosensory Touch Motion Cutaneous mechanoreceptor Hair follicle receptors
Somatosensory Touch Deep pressure, vibration
60–300 HzCutaneous mechanoreceptor Pacinian corpusclesSomatosensory Touch Touch, pressure Cutaneous mechanoreceptor Merkel cells
Somatosensory Touch Sustained pressure Cutaneous mechanoreceptor Ruffini corpuscles
Somatosensory Proprioception Stretch Mechanoreceptor Muscle spindles
Somatosensory Proprioception Tension Mechanoreceptor Golgi tendon organ
Somatosensory Temperature Thermal Thermoreceptor Cold and warm receptors
Somatosensory Pain Chemical, thermal, and
mechanicalChemoreceptor, thermorecep-
tor, and mechanoreceptorPolymodal receptors or chemical,
thermal, and mechanical nociceptors
Somatosensory Itch Chemical Chemoreceptor Chemical nociceptor
Visual Vision Light Photoreceptor Rods, cones
Auditory Hearing Sound Mechanoreceptor Hair cells (cochlea)
Vestibular Balance Angular acceleration Mechanoreceptor Hair cells (semicircular canals)
Vestibular Balance Linear acceleration, gravity Mechanoreceptor Hair cells (otolith organs)
Olfactory Smell Chemical Chemoreceptor Olfactory sensory neuron
Gustatory Taste Chemical Chemoreceptor Taste buds