Sensory Physiology 311Summary
B. The receptive field of a cutaneous sensory neuron is the area
of skin that, when stimulated, produces responses in the
neuron.
1. The receptive fields are smaller where the skin has a
greater density of cutaneous receptors.
2. The two-point touch threshold test reveals that the
fingertips and tip of the tongue have a greater density of
touch receptors, and thus a greater sensory acuity, than
other areas of the body.
C. Lateral inhibition acts to sharpen a sensation by inhibiting
the activity of sensory neurons coming from areas of the
skin around the area that is most greatly stimulated.10.3 Taste and Smell 274
A. The sense of taste is mediated by taste buds.
1. There are four well-established modalities of taste
(salty, sour, sweet, and bitter); a fifth, called umami,
which is stimulated by glutamate, is now also
recognized.
2. Salty and sour taste are produced by the movement
of sodium and hydrogen ions, respectively, through
membrane channels; sweet and bitter tastes are
produced by binding of molecules to protein receptors
that are coupled to G-proteins.
B. The olfactory receptors are neurons that synapse within the
olfactory bulb of the brain.
1. Odorant molecules bind to membrane protein receptors.
There may be as many as 1,000 different receptor
proteins responsible for the ability to detect as many as
10,000 different odors.
2. Binding of an odorant molecule to its receptor causes
the dissociation of large numbers of G-protein subunits.
The effect is thereby amplified, which may contribute to
the extreme sensitivity of the sense of smell.10.4 Vestibular Apparatus and Equilibrium 278
A. The structures for equilibrium and hearing are located in the
inner ear, within the membranous labyrinth.
1. The structure involved in equilibrium, known as the
vestibular apparatus, consists of the otolith organs
(utricle and saccule) and the semicircular canals.
2. The utricle and saccule provide information about linear
acceleration, whereas the semicircular canals provide
information about angular acceleration.
3. The sensory receptors for equilibrium are hair cells that
support numerous stereocilia and one kinocilium.
a. When the stereocilia are bent in the direction
of the kinocilium, the cell membrane becomes
depolarized.
b. When the stereocilia are bent in the opposite
direction, the membrane becomes hyperpolarized.
B. The stereocilia of the hair cells in the utricle and saccule
project into the endolymph of the membranous labyrinth and
are embedded in a gelatinous otolithic membrane.10.1 Characteristics of Sensory Receptors 267
A. Sensory receptors may be categorized on the basis of their
structure, the stimulus energy they transduce, or the nature
of their response.
- Receptors may be dendritic nerve endings, specialized
neurons, or specialized epithelial cells associated with
sensory nerve endings. - Receptors may be chemoreceptors, photoreceptors,
thermoreceptors, mechanoreceptors, or nociceptors.
a. Proprioceptors include receptors in the muscles,
tendons, and joints.
b. The senses of sight, hearing, taste, olfaction, and
equilibrium are grouped as special senses. - Receptors vary in the duration of their firing in response
to a constant stimulus.
a. Tonic receptors continue to fire as long as the
stimulus is maintained; they monitor the presence
and intensity of a stimulus.
b. Phasic receptors respond to stimulus changes; they
do not respond to a sustained stimulus, and this
partly accounts for sensory adaptation.
B. According to the law of specific nerve energies, each sen-
sory receptor responds with lowest threshold to only one
modality of sensation. - That stimulus modality is called the adequate stimulus.
- Stimulation of the sensory nerve from a receptor by
any means is interpreted in the brain as the adequate
stimulus modality of that receptor.
C. Generator potentials are graded changes (usually depolariza-
tions) in the membrane potential of the dendritic endings of
sensory neurons. - The magnitude of the potential change of the generator
potential is directly proportional to the strength of the
stimulus applied to the receptor. - After the generator potential reaches a threshold value,
increases in the magnitude of the depolarization result
in increased frequency of action potential production in
the sensory neuron.
10.2 Cutaneous Sensations 270
A. Somatesthetic information—from cutaneous receptors and
proprioceptors—is carried by third-order neurons to the
postcentral gyrus of the cerebrum.
- Proprioception and pressure sensations ascend on the
ipsilateral side of the spinal cord, synapse in the medulla
and cross to the contralateral side, and then ascend in
the medial lemniscus to the thalamus; neurons in the
thalamus, in turn, project to the postcentral gyrus. - Sensory neurons from other cutaneous receptors
synapse and cross to the contralateral side in the spinal
cord and ascend in the lateral and ventral spinothalamic
tracts to the thalamus; neurons in the thalamus then
project to the postcentral gyrus.