Download Chapter 9—Sensory Systems. I. Sensory receptors receive stimuli

Chapter 9—Sensory Systems.
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Sensory receptors receive stimuli (information on changes in the internal and external environment)
and relay this sensory information to the CNS. Fig. 9.1.
a. Sensation—processed sensory information that manifests as a conscious awareness of a stimulus.
b. Perception—understanding of what a sensation means or represents.
Classes of sensory receptors.
a. Mechanoreceptors.
i. Touch and pressure receptors—Example: certain free nerve endings and Pacinian
corpuscles in the skin that are stimulated by mechanical pressure against the body
surface.
ii. Baroreceptors—Example: carotid and aortic sinus, stimulated by blood pressure changes.
iii. Stretch receptors—Example: muscle spindle in skeletal muscle, stimulated by stretching.
iv. Auditory receptors—Example: hair cells inside cochlea, stimulated by vibration.
v. Balance receptors—Example: hair cells in equilibrium structures of inner ear stimulated
by fluid movement.
b. Thermoreceptors—Example: certain free nerve endings that respond to a change in temperature.
c. Nociceptors (pain receptors)—Example: certain free nerve endings stimulated by tissue damage.
d. Chemoreceptors.
i. Internal chemical sense receptors—Example: carotid and aortic bodies in the walls of
these blood vessels that respond to the concentration of dissolved gasses.
ii. Taste receptors—Example: taste cells that respond to chemicals dissolved in saliva.
iii. Smell (olfactory) receptors—Example: olfactory cells in the nose that respond to
dissolved chemicals.
e. Osmoreceptors—Example: osmoreceptors of the hypothalamus that respond to a change in
water volume (solute concentration) of fluid that bathes them.
f. Photoreceptors—Example: rods and cones of retina that respond to light.
The general senses and receptors near the body surface. Fig. 9.2.
a. Touch, pressure, vibration, cold, warmth, and pain receptors.
b. Free nerve endings—thinly myelinated or naked dendrites of sensory neurons.
i. Different types function as mechanoreceptors, thermoreceptors, and nociceptors.
ii. Merkel disk (cell)—light touch receptor.
iii. All adapt slowly to stimulation.
c. Encapsulated receptors—surrounded by epithelial or CT.
i. Meissner’s corpuscle—light touch receptors.
ii. Bulb of Krause—thermoreceptor activated by temperatures below 20° C (68° F); below
10° C (50° F) a painful sensation of freezing occurs.
iii. Ruffini corpuscles—sensitive to steady touching and pressure.
iv. Pacinian corpuscle—sensitive to pressure, coarse touch, vibration.
Body and limb position: muscle sense—limb motions and body position is detected by
mechanoreceptors in skeletal muscles, joints, tendons, ligaments, and skin. (Stretch receptors of
muscle spindles, and Golgi tendon organs which measure muscle tension).
Pain—the perception of injury.
a. Many nociceptors are free nerve endings within the skin and tissues.
i. They trigger the perception of pain in the thalamus, but the type and intensity of pain are
interpreted in the cerebral cortex.
b. Somatic pain—skin, skeletal muscles, joints, and tendons.
c. Visceral pain—associated with the organs and abnormal conditions concerning those organs.
d. Referred pain—when sensations of pain from some internal organs are projected to part of the
skin surface. (Heart attack). Figure 9.3.
Vision.
a. Eye structure—made up of three layers (tunics). Table 9.1, Figure 9.4.
i. Fibrous tunic—outer layer of the eyeball.
1. Sclera—white part; tough and fibrous.
2. Cornea—clear structure through which light first passes as it enters the eye.
ii. Vascular tunic.
1. Choroid—contains blood vessels, and is darkly pigmented.
2. Ciliary body—smooth muscle that helps to adjusts lens shape, and hold it in place
via suspensory ligaments.
3. Iris—adjusts the amount of light entering the eye.
4. Pupil—the “hole” in the iris.
iii. Sensory tunic. (Neural tissue).
1. Retina—contains the photoreceptors and the pigmented epithelium.
2. Fovea—area of high visual acuity.
3. Optic nerve (blind spot)—sends sensory information to the thalamus and visual
cortex of the brain.
iv. Other structures.
1. Lens—focuses light on the retina by changing shape (ciliary body).
2. Aqueous humor—fluid in the anterior chamber of the eye, which maintains
pressure and supports the structure of the eye, as well as bringing in nutrients and
removing wastes.
3. Vitreous body—supports the lens and the eyeball.
b. Focusing mechanisms. Fig. 9.5.
i. The cornea and lens bend incoming light; this causes the incoming light rays to converge
on the retina upside down and reversed. The brain compensates for this.
ii. Accommodation—the focusing of light onto the retina, by the action of ciliary body
muscles changing the shape of the lens.
1. Contraction of the ciliary body reduces tension on the suspensory ligaments,
which causes the lens to bulge; this causes the focal point to move from a location
behind the retina to a point on the retina. This occurs when viewing a close
object.
2. Relaxation of the ciliary body increases tension on the suspensory ligaments and
causes the lens to flatten; this causes the focal point to move from a location in
front of the retina to a point on the retina. This occurs when viewing distant
objects.
iii. Table 9.2 summarizes some common focusing problems of the eye. You will not be
tested on this material, but it is something that applies to many of us in our lives…
c. Organization of the retina. Fig. 9.7.
i. Photoreceptors:
1. Cones—detect bright light; contribute to daytime vision and color vision.
2. Rods—very sensitive to light; contribute to night vision and black and white
vision only.
3. [Fovea and the concentrations of rods vs. cones].
ii. The retina is organized in distinct layers; as light enters the eye, it passes through the
retina in this order: Fig. 9.7b.
1. Optic nerve fibers—axons that converge at the optic nerve and exit the eye on
their way to the thalamus and the visual cortex of the brain.
2. “Ganglion cell” layer—multipolar neurons and neuroglia (site of synaptic
integration).
3. Bipolar interneuron layer (synaptic integration).
4. Cell bodies of cones and rods.
5. Photoreceptive portion of cones and rods (“pigment layer” in figure).
6. Pigmented epithelium.
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VIII.
iii. Synaptic integration allows for the input from about 125 million photoreceptors to
converge on about 1 million ganglion cells.
iv. [Eye structure and evolution vs. intelligent design].
d. Neuronal responses to light.
i. The outer segments of cones and rods contain many membranous disks, which contain
“visual pigments.” Fig. 9.8.
ii. In rods, the visual pigment is rhodopsin.
1. Rhodopsin consists of a protein (opsin) to which the pigment retinal is bound.
(Retinal is synthesized from vitamin A).
iii. Cones contain the same retinal pigment as rods, but it is attached to other forms of opsin,
which respond to blue, green, or red light wavelengths.
1. The brain interprets color according to how strongly each type of cone is
stimulated.
iv. When a photon of light strikes a visual pigment, it causes a conformational change in the
opsin molecule, causing a series of reactions to occur that then causes the photoreceptor
cell to reduce its release of neurotransmitter; this signals the adjacent bipolar cell that a
photon has been absorbed.
v. Signals moving down the optic nerves cross at the optic chiasma, and as a result, signals
from the left field of view from each eye reaches the visual cortex on the right occipital
lobe, and vice versa. This allows for 3-D imaging. Sensory information from the eyes is
also passed to reflex centers in the brain and brainstem. Fig. 9.7c.
vi. A lack or reduced number of a particular type of cone cell causes color blindness—
certain colors cannot be distinguished from each other. Fig. 9.9.
Hearing.
a. Results from stimulation of vibration sensitive mechanoreceptors in the inner ear.
b. Sound travels like a wave in the air, with the amplitude of the wave corresponding to “loudness”
and the frequency of the wave to “pitch.” Fig. 9.10.
c. [Anatomy of the ear and its function]. Table 9.3 and Figs. 9.11, 9.12, 9.13.
d. Louder sounds produce greater movement of the basilar membrane, which stimulates more hair
cells within the Organ of Corti at that location.
e. Pitch stimulates the Organ of Corti at different points along its length.
i. Higher pitches (frequencies) stimulate the Organ of Corti at points closer to the oval
window, where it is narrow and stiff.
ii. Lower pitches (frequencies) stimulate the Organ of Corti farther away from the oval
window, where it is wider and more delicate.
f. Hearing loss and ear infections. You will not be tested on this material, but it is interesting
reading…
Balance (equilibrium).
a. Involves a sense of the “natural” position of the body.
b. Balance relies on input from the eyes, skin, joints, skeletal muscles, and the vestibular apparatus,
which is found in the inner ear. It is a fluid filled series of canals and sacs. Fig. 9.14.
c. The vestibular apparatus is made up of the following:
i. Three semicircular canals, corresponding to the three planes of space.
1. They detect rotational acceleration/movement.
2. At the base of each semicircular canal is an enlarged region (ampulla) which
contains a sensory structure called the crista ampullaris; the crista ampullaris is
covered with a structure called the cupula (a gelatinous mass overlying the
sensory hair cells of the crista ampullaris).
3. Fluid flowing past the cupula causes it to deform, bending the hair cells, which
causes action potentials to fire.
4. [Spinning in circles…].
ii. The vestibule: two fluid filled sacs = utricle and saccule.
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X.
1. These contain otolithic organs, with hair cells projecting into a jelly-like
membrane containing otoliths.
2. These organs detect linear acceleration, and the head’s changing orientation
relative to gravity.
3. These movements cause the otolithic organ to deform, bending the underlying
hair cells, which causes action potentials to fire.
d. Signals from the vestibular apparatus pass through reflex centers in the brainstem, and the
brain/cerebellum integrates this information with signals from the eyes, skin, joints, and the
muscles, and then issues appropriate commands for balance to be maintained.
Smell (olfaction)—a chemical sense that is activated when a chemical is dissolved in fluid that
bathes these receptors. Sensory information passes from the receptors to the thalamus, cerebral
cortex, and the limbic system. Fig. 9.15.
a. Olfactory receptors are found in the upper portions of the nasal cavity, and make up an
“olfactory epithelium.”
b. They detect dissolved water soluble substances, and volatile substances.
c. [Anatomy and connection to olfactory bulbs].
d. Humans have about 10 million olfactory receptors, and in addition to smell, we can detect
pheromones—signaling molecules for social interactions.
Taste (gustation)—a chemical sense that is activated when a chemical is dissolved in a fluid (saliva)
that bathes these receptors. Sensory information passes from the receptors to the thalamus, cerebral
cortex, and the limbic system. Fig. 9.16.
a. Taste receptors (cells) are found in the taste buds, which are found on the papillae of the tongue,
inner surface of the cheeks, roof of the mouth, and throat.
b. Taste receptors can distinguish five basic taste classes: sweet, salty, sour, bitter, and umami.
c. Chemicals must dissolve in saliva before the taste receptors can bind them and detect an
associated taste.
d. “Flavors” are a combination of the basic taste classes, plus the sensory input from the nose.
(Ability to taste when you have a cold…).
Study suggestions for this chapter: In the textbook at the end of the chapter, the sections entitled 1)
Highlighting the Concepts, 2) Recognizing Key Terms, and 3) Reviewing the Concepts are all good for you to
gauge your comprehension and focus your study efforts.