Special senses are those whose receptors occur in relatively large, complex sensory organs of the head.
1. Sense of Smell a. Olfactory receptors
(1) Olfactory receptors are chemoreceptors that chemicals dissolved in liquid stimulate.
(2) Olfactory receptors function together with taste receptors and aid in food selection.
(1) The olfactory organs consist of receptors and supporting cells in the nasal cavity.
(2) Olfactory receptors are neurons with cilia that sense lipid-soluble chemicals.
c. Olfactory nerve pathways.
(1) Nerve impulses travel from the olfactory receptors through the olfactory nerves, olfactory bulbs, and olfactory tracts.
(2) They go to interpreting centers in the limbic system of the brain.
d. Olfactory stimulation
(1) Olfactory impulses may result when various gaseous molecules combine with specific sites on the cilia of the receptor cells.
(2) Olfactory receptors adapt rapidly.
(3) Olfactory receptors are often damaged by environmental factors but are not replaced.
2. Sense of Taste a. Taste receptors
(1) Taste buds consist of receptor cells and supporting cells.
(2) Taste cells have taste hairs that are sensitive to particular chemicals dissolved in water.
(3) Taste hair surfaces have receptor sites to which chemicals combine and trigger impulses to the brain.
b. Taste sensations
(1) The four primary taste sensations are sweet, sour, salty, and bitter.
(2) Various taste sensations result from the stimulation of one or more sets of taste receptors.
(3) Each of the four primary kinds of taste cells is particularly sensitive to a certain group of chemicals.
c. Taste nerve pathways
(1) Sensory impulses from taste receptors travel on fibers of the facial, glossopharyngeal, and vagus nerves.
(2) These impulses are carried to the medulla and ascend to the thalamus and then to the gustatory cortex in the parietal lobes.
3. Sense of Hearing a. The external ear collects sound waves created by vibrating objects.
b. Middle ear
(1) Auditory ossicles of the middle ear conduct sound waves from the tympanic membrane to the oval window of the inner ear. They also increase the force of these waves.
(2) Skeletal muscles attached to the auditory ossicles provide the tympanic reflex, which protects the inner ear from the effects of loud sounds.
c. Auditory tubes connect the middle ears to the throat and help maintain equal air pressure on both sides of the tympanic membranes.
d. Inner ear
(1) The inner ear consists of a complex system of connected tubes and chambers—the osseous and membranous labyrinths. It includes the cochlea, which, in turn, houses the organ of Corti.
(2) The organ of Corti contains the hearing receptors that vibrations in the fluids of the inner ear stimulate.
(3) Different frequencies of vibrations stimulate different sets of receptor cells; the human ear can detect sound frequencies from about 20 to 20,000 vibrations per second.
e. Auditory nerve pathways
(1) The nerve fibers from hearing receptors travel in the cochlear branch of the vestibulocochlear nerves.
(2) Auditory impulses travel into the medulla oblongata, midbrain, and thalamus, and are interpreted in the temporal lobes of the cerebrum.
4. Sense of Equilibrium a. Static equilibrium maintains the stability of the head and body when they are motionless. The organs of static equilibrium are located in the vestibule.
b. Dynamic equilibrium balances the head and body when they are moved or rotated suddenly. The organs of this sense are located in the ampullae of the semicircular canals.
c. Other structures that help maintain equilibrium include the eyes and the proprioceptors associated with certain joints.
5. Sense of Sight a. Visual accessory organs include the eyelids and lacrimal apparatus that protect the eye and the extrinsic muscles that move the eye.
b. Structure of the eye
(1) The wall of the eye has an outer, a middle, and an inner tunic that function as follows:
(a) The outer layer (sclera) is protective, and its transparent anterior portion (cornea) refracts light entering the eye.
(b) The middle layer (choroid coat) is vascular and contains pigments that help keep the inside of the eye dark.
(c) The inner layer (retina) contains the visual receptor cells.
(2) The lens is a transparent, elastic structure. The ciliary muscles control its shape.
(3) The iris is a muscular diaphragm that controls the amount of light entering the eye; the pupil is an opening in the iris.
(4) Spaces within the eye are filled with fluids (aqueous and vitreous humors) that help maintain its shape.
c. Light refraction
(1) Light waves are primarily refracted by the cornea and lens to focus an image on the retina.
(2) The lens must thicken to focus on close objects.
d. Visual receptors
(1) The visual receptors are rods and cones.
(2) Rods are responsible for colorless vision in relatively dim light, and cones provide color vision.
e. Visual pigments
(1) A light-sensitive pigment in rods (rhodopsin) decomposes in the presence of light and triggers a complex series of reactions that initiate nerve impulses on the optic nerve.
(2) Three sets of cones provide color vision. Each set contains a different light-sensitive pigment, and each set is sensitive to a different wavelength of light; the color perceived depends on which set or sets of cones are stimulated.
f. Stereoscopic vision
(1) Stereoscopic vision provides perception of distance and depth.
(2) Stereoscopic vision occurs because of the formation of two slightly different retinal images that the brain superimposes and interprets as one image in three dimensions.
(3) A one-eyed person uses relative sizes and positions of familiar objects to judge distance and depth.
g. Visual nerve pathways
(1) Nerve fibers from the retina form the optic nerves.
(2) Some fibers cross over in the optic chiasma.
(3) Most of the fibers enter the thalamus and synapse with others that continue to the visual cortex of the occipital lobes.
(4) Other impulses pass into the brain stem and function in various visual reflexes.
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