Chapter 7
Somatic Sensation
I. A variety of receptors sensitive to one (or a few) stimulus types
provide sensory function of the skin and underlying tissues.
II. Information about somatic sensation enters both specifi c and
nonspecifi c ascending pathways. The specifi c pathways cross to
the opposite side of the brain.
III. The somatic sensations include touch, pressure, the senses of
posture and movement, temperature, and pain.
a. Rapidly adapting mechanoreceptors of the skin give rise to
sensations such as vibration, touch, and movement, whereas
slowly adapting ones give rise to the sensation of pressure.
b. Skin receptors with small receptive fi elds are involved in fi ne
spatial discrimination, whereas receptors with larger receptive
fi elds signal less spatially precise touch-pressure sensations.
c. A major receptor type responsible for the senses of posture
and kinesthesia is the muscle spindle stretch receptor.
d. Cold receptors are sensitive to decreasing temperature;
warmth receptors signal information about increasing
e. Tissue damage and immune cells release chemical agents that
stimulate specifi c receptors that give rise to the sensation of
f. Stimulation-produced analgesia, transcutaneous nerve
stimulation (TENS), and acupuncture control pain by
blocking transmission in the pain pathways.
I. Light is defi ned by its wavelength or frequency.
II. The light that falls on the retina is focused by the cornea and
a. Lens shape changes (accommodation) to permit viewing near
or distant objects so that they are focused on the retina.
b. Stiffening of the lens with aging interferes with
accommodation. Cataracts decrease the amount of light
transmitted through the lens.
c. An eyeball too long or too short relative to the focusing
power of the lens causes nearsighted or farsighted vision,
III. The photopigments of the rods and cones are made up of a
protein component (opsin) and a chromophore (retinal).
a. The rods and each of the three cone types have different
opsins, which make each of the four receptor types sensitive
to different ranges of light wavelengths.
b. When light falls upon the chromophore, the photic energy
causes the chromophore to change shape, which triggers
a cascade of events leading to hyperpolarization of the
photoreceptors and decreased neurotransmitter release from
them. When exposed to darkness, the rods and cones are
depolarized and therefore release more neurotransmitter
than in light.
IV. The rods and cones synapse on bipolar cells, which synapse on
ganglion cells.
a. Ganglion cell axons form the optic nerves, which lead into
the brain.
b. The optic nerve fi bers from the medial half of each retina
cross to the opposite side of the brain in the optic chiasm.
The fi bers from the optic nerves terminate in the lateral
geniculate nuclei of the thalamus, which send fi bers to the
visual cortex.
c. Visual information is also relayed to areas of the brain
dealing with biological rhythms.
V. Coding in the visual system occurs along parallel pathways,
in which different aspects of visual information, such as color,
form, movement, and depth, are kept separate from each
VI. The colors we perceive are related to the wavelength of
light. Different wavelengths excite one of the three cone
photopigments most strongly.
a. Certain ganglion cells are excited by input from one type of
cone cell and inhibited by input from a different cone type.
b. Our sensation of color depends on the output of the
various opponent color cells and the processing of this
output by brain areas involved in color vision.
VII. Six skeletal muscles control eye movement to scan the visual
fi eld for objects of interest, keep the fi xation point focused
on the fovea centralis despite movements of the object or the
head, and prevent adaptation of the photoreceptors.
I. Sound energy is transmitted by movements of pressure waves.
a. Sound wave frequency determines pitch.
b. Sound wave amplitude determines loudness.
II. The sequence of sound transmission is as follows:
a. Sound waves enter the external auditory canal and press
against the tympanic membrane, causing it to vibrate.
b. The vibrating membrane causes movement of the three
small middle-ear bones; the stapes vibrates against the oval
window membrane.
c. Movements of the oval window membrane set up pressure
waves in the fl uid-fi lled scala vestibuli, which cause
vibrations in the cochlear duct wall, setting up pressure
waves in the fl
uid there.
d. These pressure waves cause vibrations in the basilar
membrane, which is located on one side of the cochlear
e. As this membrane vibrates, the hair cells of the organ of
Corti move in relation to the tectorial membrane.
f. Movement of the hair cells’ stereocilia stimulates the hair
cells to release glutamate, which activates receptors on the
peripheral ends of the afferent nerve fi bers.
III. Separate parts of the basilar membrane vibrate maximally in
response to one particular sound frequency; high frequency is
detected near the oval window and low frequency toward the
far end of the cochlear duct.
Vestibular System
I. A vestibular apparatus lies in the temporal bone on each side
of the head and consists of three semicircular canals, a utricle,
and a saccule.
II. The semicircular canals detect angular acceleration during
rotation of the head, which causes bending of the stereocilia
on their hair cells.
III. Otoliths in the gelatinous substance of the utricle and saccule
move in response to changes in linear acceleration and the
position of the head relative to gravity, and stimulate the
stereocilia on the hair cells.
Chemical Senses
I. The receptors for taste lie in taste buds throughout the
mouth, principally on the tongue. Different types of taste
receptors operate by different mechanisms.
II. Olfactory receptors, which are part of the afferent olfactory
neurons, lie in the upper nasal cavity.
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