I. Introduction
A. Introduction
1. The general senses are those with receptors widely distributed throughout the body, including the skin, various organs and joints.
2. The special senses have more specialized receptors and are confined to structures in the head, such as the eyes and ears.
3. Sensory receptors collect information from the environment and send impulses along sensory fibers to the brain.
4. The cerebral cortex forms perceptions
II. Receptors, Sensations and Perception
A. Receptor Types
1. Five types of sensory receptors are chemoreceptors, pain receptors, thermoreceptors, mechanoreceptors, and photoreceptors.
2. Chemoreceptors respond to changes in chemical concentrations.
3. Pain receptors respond to tissue damage.
4. Thermoreceptors respond to temperature changes.
5. Mechanoreceptors respond to mechanical forces.
6. Proprioceptors sense changes in the tension of muscles and tendons.
7. Baroreceptors detect changes in changes in blood pressure.
8. Stretch receptors respond to stretch.
9. Photoreceptors respond to light energy.
B. Sensory Impulses
1. Sensory receptors can be ends of neurons or other kinds of cells located close to them.
2. Stimulation of sensory receptors causes local changes in their membrane potential, generating a graded electric current that reflects the intensity of stimulation.
3. If a receptor is a neuron and the change in membrane potential reaches threshold, an action potential is generated.
4. If the receptor is another type of cell, its receptor potential must be transferred to a neuron to trigger an action potential.
C. Sensations
1. A sensation is a feeling that occurs when the brain interprets sensory impulses.
2. Sensations depend on which region of the cerebral cortex receives the impulse.
3. Projection is a process in which the cerebral cortex projects a sensation back to its apparent source.
D. Sensory Adaptation
1. Sensory adaptation is the ability to ignore unimportant stimuli.
2. An example of sensory adaptation is background nose in a room.
III. General Senses
A. Introduction
1. General senses are those whose sensory receptors are associated with the skin, muscles, joints, and viscera.
2. Three groups of somatic senses are exteroceptive senses, proprioceptive senses, and visceroceptive senses.
3. Exteroceptive senses include senses of touch, pressure, temperature, and pain.
4. Proprioceptive senses include senses associated with changes in muscles and tendons and in body position.
5. Visceroceptive senses include senses associate with changes in viscera.
B. Touch and Pressure Senses
1. Three kinds of touch and pressure receptors are free nerve endings, Meissner’s corpuscles, and Pacinian corpuscles.
2. Free nerve endings are located in epithelial tissues and are responsible for the sensation of itching.
3. Meissner’s corpuscles are located in hairless portions of skin and are involved in fine touch, as in distinguishing between two points on the skin.
4. Pacinian corpuscles are located in deeper subcutaneous tissues of the hands, feet, penis, clitoris, urethra, breasts, and tendons and ligament and are associated with heavier pressure, stretch, and vibrations.
C. Temperature Senses
1. Two types of temperature receptors are warm and cold receptors.
2. Warm receptors respond to temperatures between 25oC and 45oC.
3. Cold receptors respond to temperatures between 10oC and 20oC.
4. Temperatures above 45oC and below 10oC activate pain receptors.
D. Sense of Pain
1. Introduction
a. Pain receptors consist of free nerve endings.
b. Pain receptors are distributed widely throughout the skin and internal tissues, except in the nervous tissue of the brain.
c. Pain receptors can be stimulated by damaged tissue.
d. Pain receptors adapt very little, if at all.
2. Visceral Pain
a. Visceral pain receptors respond differently to stimulation than those of surface tissues.
b. Pain in visceral organs result from stimulation of mechanoreceptors and from decreased blood flow accompanied by lower tissue oxygen levels and accumulation of pain-stimulating chemicals.
c. Referred pain is a phenomenon is which visceral pain may feel as if it is coming from some part of the body other than the part being stimulated.
d. Referred pain may come from common nerve pathways that sensory impulses coming both from skin areas and from internal organs use.
e. During a heart attack, the cerebral cortex may incorrectly interpret the source of the impulses as coming from the left arm.
3. Pain Nerve Pathways
a. Two main types of pain fibers are acute pain fibers and chronic pain fibers.
b. Acute pain fibers are thin, myelinated nerve fibers and conduct impulses rapidly at velocities up to 30 meters per second.
c. Acute pain fibers are associated with the sensation of sharp pain.
d. Chronic pain fibers are thin, unmyelinated nerve fibers that conduct impulses more slowly.
e. Impulses from chronic pain fibers cause dull, aching pain
sensations.
f. Acute pain is usually sensed as being from a local area of skin and chronic pain is likely to be felt in deeper tissues as well as the skin.
g. Pain impulses that originate from tissues of the head reach the brain on sensory fibers of fifth, seventh, ninth, and tenth cranial nerves.
h. All other pain impulses travel on sensory fibers of spinal nerves and they pass into the spinal cord by way of dorsal roots.
i. Upon reaching the spinal cord, pain impulses enter the gray matter of the posterior horn, where they are processed.
j. Within the brain, most pain fibers terminate in the reticular formation and from there are conducted on fibers to the thalamus, hypothalamus, and cerebral cortex.
4. Regulation of Pain Impulses
a. Awareness of pain occurs when pain impulses reach the level of the thalamus.
b. The cerebral cortex judges the intensity of pair and locates its source.
c. Enkephalins and serotonin can suppress pain impulses.
d. Endorphins are natural pain controlling substances in the pituitary gland and hypothalamus.
E. Proprioception
a. Proprioceptors are mechanoreceptors that send information to the spinal cord and brain concerning the lengths and tensions of skeletal muscles.
b. Two main kinds of stretch receptors are muscle spindles and Golgi tendon organs.
c. Muscle spindles are located in skeletal muscles near their junctions with tendons and function to detect stretch.
d. Golgi tendon organs are located in tendons close to their attachments to muscles and function to detect increased tension.
e. The stretch reflex is an action that opposed the lengthening of a muscle and helps maintain the desired position of a limb in spite of gravitational or other forces tending to move it.
IV. Special Senses
A. Introduction
1. Examples of special senses are sight, smell, hearing, and taste.
2. Special senses are those whose sensory receptors are within sensory organs of the head.
B. Sense of Smell
1. Olfactory Receptors
a. Olfactory receptors are used to sense smell and are chemoreceptors.
b. Taste is a combination of smell and taste sensations.
2. Olfactory Organs
a. Olfactory organs contain the olfactory receptors.
b. Olfactory organs are located in the upper pars of the nasal cavity, the superior nasal conchae and a portion of the nasal septum.
c. The olfactory receptor cells are bipolar neurons surrounded by columnar epithelial cells.
d. Cilia of olfactory receptor cells project into the nasal cavity.
e. Smell impulses are generated when odorants enter the nasal cavity, dissolve in fluids, and bind to receptors proteins on cilia that are part of the cell membranes of the olfactory receptor cells.
3. Olfactory Nerve Pathways
a. Once olfactory receptors are stimulated, nerve impulses travel along their axons to synapse with neurons in the olfactory bulbs.
b. The olfactory bulbs function to analyze the sensory impulses.
c. From olfactory bulbs, impulses travel to olfactory tracts.
d. From olfactory tracts, impulses travel to portions of the limbic system and cerebral cortex.
e. The limbic system functions to put an emotion with the smell information.
f. The olfactory cortex is located in the temporal lobes and interprets the smell sensations.
4. Olfactory Stimulation
a. The olfactory code is a particular combination used by the brain to determine the smell sensation.
b. The intensity of a smell drops about 50% within a second because olfactory receptors adapt quickly.
c. The olfactory receptor neurons are the only damaged neurons that are regularly replaced.
C. Sense of Taste
1. Introduction
a. Taste buds are special organs of taste.
b. Papillae of the tongue are tiny elevations.
c. Taste buds are located on papillae of the tongue.
2. Taste Receptors
a. Taste cells are modified epithelial cells that function as receptors.
b. A taste pore is an opening in a taste bud.
c. Taste hairs are tiny projections from the surface of taste cells.
d. The mechanism of tasting probably involves combination of chemicals binding specific receptors on taste hair surfaces, altering membrane polarization, and thereby generating sensory impulses on nearby nerve fibers.
3. Taste Sensations
a. The five primary taste sensations are sweet, sour, salty, bitter, and umami.
b. Spicy foods activate pain receptors.
c. Responsiveness to a sweet stimulus peaks at the tip of the tongue.
d. Responsiveness to a sour stimulus is greatest at the margins of the tongue.
e. Receptors that are responsive to salt are widely distributed.
f. Sweet receptors are usually stimulated by carbohydrates.
g. Acids stimulate sour receptors.
h. Salt receptors are stimulated by ionized inorganic salts.
i. Bitter receptors are stimulated by a variety of chemicals.
j. Taste receptors, like olfactory receptors, undergo adaptation.
4. Taste Nerve Pathways
a. The three cranial nerves that carry taste sensations are the facial, glossopharyngeal, and the vagus nerves.
b. Cranial nerves conduct taste sensations to the medulla oblongata.
c. From the medulla oblongata, taste sensations go to the thalamus and the gustatory cortex.
d. The gustatory cortex is located in the parietal lobes.
D. Sense of Hearing
1. Introduction
a. The organ of hearing is the ear.
b. The three parts of the ear are external, middle, and inner.
c. The ear also provides the sense of equilibrium.
2. External Ear
a. The auricle of the ear is an outer, funnel-like structure and functions to collect sounds waves.
b. The external auditory meatus is a canal that extends from the auricle to the tympanic membrane and functions to deliver sounds waves to the tympanic membrane.
c. The external auditory meatus ends with the tympanic membrane.
d. Ceruminous glands line the external auditory meatus and secretes cerumen.
e. The tympanic membrane is semitransparent membrane and moves back and forth in response to sound waves.
3. Middle Ear
a. The middle ear is an air filled space in the temporal bone.
b. The middle ear contains three auditory ossicles.
c. The three auditory ossicles are the malleus, incus, and stapes.
d. The malleus is attached to the tympanic membrane.
e. The stapes covers the oval window.
f. The oval window is an opening in the wall of the tympanic cavity.
g. Vibration of the stapes moves a fluid within the inner ear.
h. Vibrations in the inner ear stimulate hearing receptors.
i. The tensor tympani is a small skeletal muscle that is anchored to the malleus and wall of the auditory tube.
j. The stapedius is a small skeletal muscle that is attached to the posterior side of the stapes and the inner wall of the tympanic cavity.
k. The tympanic reflex is a reflex that causes the ear ossicles to become more rigid.
l. The tympanic reflex reduces the effectiveness in transmitting vibrations to the inner ear.
4. Auditory Tube
a. The auditory tube connects the nasopharynx to the middle ear cavity.
b. The auditory tube functions to maintain equal pressure on both side of the eardrum.
5. Inner Ear
a. The inner ear is a complex system of intercommunicating chambers and tubes.
b. The osseous labyrinth is a bony canal in the temporal bone.
c. The membranous labyrinth is a tube that lies within the osseous labyrinth and has a similar shape.
d. Perilymph is located in osseous labyrinth.
e. Endolymph is located in membranous labyrinth.
f. The three parts of the labyrinths are the cochlea, semicircular canals, and vestibule.
g. The cochlea functions in hearing.
h. The semicircular canals provide a sense of equilibrium.
i. The vestibule is a bony chamber between the semicircular canals and the cochlea.
k. The cochlea is shaped like the shell of a snail.
l. The scala vestibuli is the upper compartment of the bony labyrinth of the cochlea.
m. The scala tympani is the lower compartment of the bony labyrinth of the cochlea.
n. The round window is a membrane-covered opening of the inner ear.
o. The cochlear duct is a portion of the membranous labyrinth within the cochlear that lies between the two bony compartments and is filled with endolymph.
p. The vestibular membrane is the membrane that separates the cochlear duct from the scala vestibuli.
q. The basilar membrane is the membrane that separates the cochlear duct from the scala tympani.
r. The organ of Corti is located on the upper surface of the basilar membranes and stretches from the apex to the base of the cochlea. It contains hearing receptor cells.
s. The tectorial membrane is a membrane above the hearing receptor cells and is in contact with the hair of the receptor cells.
t. Different frequencies of vibration move different parts of the basilar membrane.
u. A particular sound frequency causes the hairs of a specific group of receptors cells to bend against the tectorial membrane.
6. Auditory Nerve Pathway
a. The cochlear branch of the vestibulocochlear nerve carries hearing impulses to the medulla oblongata.
b. The medulla oblongata conveys the hearing impulses through the midbrain to the thalamus.
c. From the thalamus, hearing impulses go to the temporal lobes where they are interpreted.
E. Sense of Equilibrium
1. Introduction
a. The organs of static equilibrium sense the position of the head, maintaining stability and posture when the head and body are still.
b. The organs of dynamic equilibrium sense movements when the head and body suddenly rotate.
2. Static Equilibrium
a. The organs of static equilibrium are located within the vestibule.
b. The two expanded chambers within the vestibule are the utricle and saccule.
c. The macula is a patch of hair cells and supporting cells of the utricle and saccule.
d. When the head is upright, the hairs of the macula in the utricle project vertically and those of the saccule project horizontally.
e. The otolithic membrane is a gelatinous membrane.
f. Gravity stimulates hair cells to respond.
g. When hair cells bend, they signal their associated nerve fibers.
h. The nerve impulse generated by the bending of the hair cells travels to the brain by means of the vestibular branch of the vestibulocochlear nerve.
i. The brain responds to equilibrium information by sending motor impulses to skeletal muscles.
j. The maculae also participate in the sense of dynamic equilibrium.
3. Dynamic Equilibrium
a. The three bony semicircular canals lie at right angles to each other and occupy three different planes in space.
b. The ampulla is a swelling of the membranous labyrinth of a semicircular canal that communicates with the utricle of the vestibule.
c. The crista ampullaris is a sensory organ composed of hair cells and supporting cells.
d. The cupula is gelatinous mass that covers hair cells of the crista ampullaris.
e. The semicircular canals move with the head or torso but the fluid inside the membranous canals tend to remain stationary and this bends the cupula.
f. The bending of hairs stimulates the hair cells to signal their associated nerve fibers and, as a result, impulses travel to the brain.
g. Parts of the cerebellum are particularly important in interpreting impulses from the semicircular canals.
F. Sense of Sight
1. Introduction
a. Visual accessory organs assist the visual receptors.
b. Examples of visual accessory organs are eyelids, lacrimal apparatus, and extrinsic eye muscles.
2. Visual Accessory Organs
a. Each eyelid is composed of skin, muscle, connective tissue, and conjunctiva.
b. The orbiculais oculi muscle functions to close the eyelids.
c. The levator palpebrae muscle functions to raise the upper eyelids.
d. Tarsal glands are modified sebaceous glands in eyelids.
e. Conjunctiva is a mucous membrane that lines the inner surfaces of the eyelids and covers a portion of the surface of the eyeball and functions to keep the surface of the eyeball moist.
f. The lacrimal apparatus consists of the lacrimal gland and a series of ducts and functions to secrete tears and to drain them into the nasal cavity.
g. A lacrimal sac is a structure, which collects tears from superior and inferior canaliculi.
h. A lacrimal duct is a duct, which collects tears from the lacrimal sac and empties tears into the nasal cavity.
i. Tears contain water and en enzyme called lysozyme.
j. The extrinsic muscles of the eye function to move the eyeball.
k. The six extrinsic muscles of the eye are superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique, and inferior oblique.
l. The superior rectus muscle moves the eye upward and medially.
m. The inferior rectus muscle moves the eye downward and medially.
n. The medial rectus muscle moves the eye medially.
o. The lateral rectus muscle moves the eye laterally.
p. The superior oblique muscle moves the eye downward and laterally.
q. The inferior oblique muscle moves the eye upward and laterally.
3. Structure of the Eye
a. The three layers of the eyeball are outer fibrous, middle vascular, and inner nervous.
b. The spaces within the eye are filled with fluids that support its wall and internal structures and help maintain its shape.
c. The two parts of the outer tunic are the cornea and sclera.
d. The cornea is transparent and functions to allow light to enter the eye.
e. The sclera is white and opaque and functions to protect internal structures of the eyeball.
f. In the back of the eye, the optic nerve pierces the sclera.
g. The middle tunic of the eye includes choroid coat, ciliary body, and iris.
h. The choroid coat is the posterior five-sixth of the middle layer
and its functions include to supply nutrients to surrounding tissues and to absorb excess light.
i. The ciliary body is the thickest part of the middle layer and forms a ring around the front of the eye and its functions include holding and moving the lens.
j. Ciliary processes are radiating folds within the ciliary body.
k. Ciliary muscles are muscles within the ciliary body.
l. Suspensory ligaments extend from the ciliary processes and hold the lens in position.
m. The lens is made up of specialized epithelial cells and lens fibers.
n. When ciliary muscles relax, suspensory ligaments become tight and the lens becomes thinner.
o. When ciliary muscles contract, suspensory ligaments become
loose and the lens becomes thicker.
p. The iris is a thin diaphragm and functions to control the amount of light that enters the eye.
q. The anterior cavity of the eye is the portion of the eye in front of the lens.
r. The anterior chamber of the eye is the portion of the eye in front of the iris.
s. The posterior chamber of the eye is the portion of the eye between the iris and lens.
t. Aqueous humor is located in the anterior cavity of the eye and functions to provide nutrients to surrounding tissues.
u. The pupil is a hole in an iris.
v. The size of the pupil changes in response to light intensity.
w. The inner tunic of the eye consists of the retina which contains the visual receptor cells.
x. The retina has distinct layers including pigmented epithelium, neurons, nerve fibers, and limit in membranes.
y. The five major groups of retinal neurons are receptor cells, bipolar neuron, ganglion cells, horizontal cells and amacrine cells.
z. Receptor cells, bipolar cells and ganglion cells provide a direct pathway for impulses triggered in the receptors to the optic nerve and brain.
aa. The horizontal cells and amacrine cells function to modify the impulses transmitted on the fibers of the direct pathway.
bb. The macula lutea is yellowish spot in the central region of the retina.
cc. The fovea centralis is a depression in the center of the macula lutea.
dd. The optic disc is the area where nerve fibers from the retina exit the eye.
ee. The posterior cavity of the eye is the portion of the eye behind the lens.
ff. Vitreous humor is located in the posterior cavity and functions to support the internal structures of the eye and helps maintain its shape.
gg. Light waves entering the eye must pass through the cornea, aqueous humor, lens, vitreous humor, and several layer of the retina before they reach the photoreceptors.
4. Light Refraction
a. Light refraction is the bending of light and occurs when light moves at an oblique angle from one medium to another medium of a different density.
b. A convex surface causes light waves to converge.
c. A concave surface causes light waves to diverge.
d. Light is refracted by cornea and lens as it enters the eye.
e. If the shape of the eye is normal, light wave are focused sharply onto the retina.
f. The image focused on the retina is upside down and reversed from left to right.
g. Divergent light waves focus behind the retina unless something increases the refracting power of the eye.
h. Accommodation accomplishes the increase in refracting power by thickening of the lens.
5. Visual Receptors
a. Two kinds of photoreceptor cells are rods and cones.
b. Rods and cones are found in a deep layer of the retina.
c. Rods and cones are stimulated when light reaches them.
d. Rods are more sensitive to light than cones.
e. Rods provide vision in dim light.
f. Rods produce colorless vision, whereas cones detect colors.
g. Cones provide sharp images, whereas rods produce broad outlines of objects.
h. The area of sharpest vision is the fovea centralis.
i. The concentration of cones decreases in areas farther away from the macula lutea.
6. Visual Pigments
a. Rhodopsin is the light sensitive pigment of rods.
b. Rhodopsin is located in membranous discs of rods.
c. In the presence of light, rhodospin breaks down into molecules of opsin and retinal.
d. A series of reactions cause nerve impulses to travel away from the retina, through the optic nerve, and into the visual cortex where visions are interpreted.
e. In bright light cones detect colors.
f. Dark adapted eyes are those whose rods have increased amounts of available rhodospin.
g. Light adapted eyes are very sensitive to light and have decreased amounts of rhodopsin available.
h. Three pigments found on cones are erythrolabe, chlorolabe, and cyanolabe.
i. Erythrolabe is most sensitive to red light waves.
j. Chlorolabe is most sensitive to green light waves.
k. Cyanolabe is most sensitive to blue light waves.
7. Stereoscopic Vision
a. Stereoscopic vision perceives distance, depth, height, and width of objects.
b. Stereoscopic vision depends on vision with two eyes.
c. A person with one eye is less able to judge distance and depth accurately.
8. Visual Nerve Pathways
a. Axons of ganglionic cell leave eye through the optic nerve.
b. The optic chiasm is where the optic nerves cross.
c. Impulses leave the optic chiasm through optic tracts and most are carried to the thalamus.
d. From the thalamus, visual impulses travel to the visual cortex.
e. Visual impulses that do not go to the thalamus go to the midbrain and are important in visual reflexes.
V. Life-Span Changes
A. By age fifty the senses of smell and taste begin to diminish.
B. By age sixty, a fourth of the population experiences hearing loss.
C. Age-related hearing loss may be due to decades of cumulative damage to the sensitive hair cells of the organ of Corti.
D. Presbycusis is hearing loss due to degeneration of auditory pathways.
E. Tinnitus is an abnormal ringing in the ear.
F. Vision may decline with age because of dry eyes, too few tears, crystal-like deposits in the vitreous humors, or cataracts in lens.
G. “Floaters” are due to crystal-like deposits in the vitreous humors.
H. Presbyopia is the inability to read small print up close
I. Glaucoma is increased pressure in the eye due to accumulation of aqueous humor.
J. Cataracts are eye disorders in which the lenses become clouded and somewhat opaque.
K. Retinal detachment is a condition in which the retina becomes detached from the posterior surface of the middle layer of the eye.
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