CHAPTER 8
Properties of Sensory Receptors 155organ to cortex. Therefore, when the nerve pathways from a
particular sense organ are stimulated, the sensation evoked is
that for which the receptor is specialized no matter how or
where along the pathway the activity is initiated. This princi-
ple, first enunciated by Müller in 1835, has been given the
rather cumbersome name of the
law of specific nerve ener-
gies.
For example, if the sensory nerve from a Pacinian cor-
puscle in the hand is stimulated by pressure at the elbow or by
irritation from a tumor in the brachial plexus, the sensation
evoked is touch. Similarly, if a fine enough electrode could be
inserted into the appropriate fibers of the dorsal columns of
the spinal cord, the thalamus, or the postcentral gyrus of the
cerebral cortex, the sensation produced by stimulation would
be touch. The general principle of specific nerve energies re-
mains one of the cornerstones of sensory physiology.
LAW OF PROJECTION
No matter where a particular sensory pathway is stimulated
along its course to the cortex, the conscious sensation pro-
duced is referred to the location of the receptor. This principle
is called the
law of projection.
Cortical stimulation experi-
ments during neurosurgical procedures on conscious patients
illustrate this phenomenon. For example, when the cortical re-
ceiving area for impulses from the left hand is stimulated, the
patient reports sensation in the left hand, not in the head.
RECRUITMENT OF SENSORY UNITS
As the strength of a stimulus is increased, it tends to spread over
a large area and generally not only activates the sense organs im-
mediately in contact with it but also “recruits” those in the sur-
rounding area. Furthermore, weak stimuli activate the receptors
with the lowest thresholds, and stronger stimuli also activate
those with higher thresholds. Some of the receptors activated
are part of the same sensory unit, and impulse frequency in the
unit therefore increases. Because of overlap and interdigitation
of one unit with another, however, receptors of other units are
also stimulated, and consequently more units fire. In this way,
more afferent pathways are activated, which is interpreted in the
brain as an increase in intensity of the sensation.NEUROLOGICAL EXAM
The sensory component of a neurological exam includes an
assessment of various sensory modalities including touch,
proprioception, vibratory sense, and pain. Clinical Box 8–2
describes the test for vibratory sensibility. Cortical sensory
function can be tested by placing familiar objects in a patient’s
hands and asking him or her to identify it with the eyes closed
(see Clinical Box 8–3).CHAPTER SUMMARY
■
Sensory receptors are commonly classified as mechanorecep-
tors, nociceptors, chemoreceptors, or photoreceptors.
■
Touch and pressure are sensed by four types of mechanoreceptors:
Meissner’s corpuscles (respond to changes in texture and slow vi-
brations), Merkel’s cells (respond to sustained pressure and
touch), Ruffini corpuscles (respond to sustained pressure), and Pa-
cinian corpuscles (respond to deep pressure and fast vibrations).
■
Nociceptors and thermoreceptors are free nerve endings on un-
myelinated or lightly myelinated fibers in hairy and glaborous
skin and deep tissues.
■
The generator or receptor potential is the nonpropagated depo-
larizing potential recorded in a sensory organ after an adequateCLINICAL BOX 8–2
Vibratory Sensibility
Vibratory sensibility
is tested by applying a vibrating (128-
Hz) tuning fork to the skin on the fingertip, tip of the toe, or
bony prominences of the toes. The normal response is a
“buzzing” sensation. The sensation is most marked over
bones. The term
pallesthesia
is also used to describe this
ability to feel mechanical vibrations. The receptors involved
are the receptors for touch, especially
Pacinian corpuscles,
but a time factor is also necessary. A pattern of rhythmic
pressure stimuli is interpreted as vibration. The impulses re-
sponsible for the vibrating sensation are carried in the
dor-
sal columns.
Degeneration of this part of the spinal cord oc-
curs in poorly controlled diabetes, pernicious anemia,
vitamin B
12
deficiencies, or early tabes dorsalis. Elevation of
the threshold for vibratory stimuli is an early symptom of this
degeneration. Vibratory sensation and proprioception are
closely related; when one is diminished, so is the other.CLINICAL BOX 8–3
Stereognosis
Stereognosis
is the perception of the form and nature of an
object without looking at it
.
Normal persons can readily
identify objects such as keys and coins of various denomina-
tions. This ability depends on relatively intact touch and
pressure sensation and is compromised when the dorsal col-
umns are damaged. The inability to identify an object by
touch is called
tactile agnosia.
It also has a large cortical
component; impaired stereognosis is an early sign of dam-
age to the cerebral cortex and sometimes occurs in the ab-
sence of any detectable defect in touch and pressure sensa-
tion when there is a lesion in the parietal lobe posterior to
the postcentral gyrus. Stereoagnosia can also be expressed
by the failure to identify an object by sight
(visual agnosia),
the inability to identify sounds or words
(auditory agnosia)
or color
(color agnosia),
or the inability to identify the loca-
tion or position of an extremity
(position agnosia).