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Chapter 18: The Senses. Types of Sensory Receptors. Chemoreceptors respond to chemical substances, such as changes in pH, or the senses of taste and smell. Pain receptors are chemoreceptors that respond to chemicals from damaged tissues. Mechanoreceptors respond to mechanical forces.
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Types of Sensory Receptors • Chemoreceptors respond to chemical substances, such as changes in pH, or the senses of taste and smell. • Pain receptors are chemoreceptors that respond to chemicals from damaged tissues. • Mechanoreceptors respond to mechanical forces. • The senses of hearing and balance both involve mechanoreceptors.
Proprioceptors (mechanoreceptors) in tendons around joints make us aware of position; pressoreceptors in arteries detect blood pressure changes, and stretch receptors in lungs detect degree of inflation. Thermoreceptors respond to temperature changes; there are both warm receptors and cold receptors. Photoreceptors respond to light energy. Special photoreceptors called rods result in black-and-white vision, while cones detect color.
How Sensation Occurs • Sensation occurs when nerve impulses reach the cerebral cortex. • Perception is an interpretation of the meaning of sensations. • The sensation that results depends on the part of the brain receiving the impulses. • Receptors may integrate signals before sending nerve impulses. • Sensory adaptation occurs when a stimulus continues but the receptor decreases its response.
These proprioceptors allow the muscles to maintain the proper length and tension, or muscle tone. The knee-jerk reflex involves muscle spindles. Signals to the CNS from muscle spindles help maintain balance and posture. Golgi tendon organs are proprioceptors with the opposite effect.
Cutaneous Receptors • The dermis of the skin contains sensory receptors for touch, pressure, pain, and temperature (warmth and cold). • Three types of cutaneous receptors are sensitive to fine touch: • Meissner corpuscles are concentrated in finger tips, lips, tongue, nipples, and genital areas; • Merkel discs are found where the epidermis meets the dermis; and
3) free nerve endings (root hair plexus) around hair follicles all detect touch. Three different types of pressure receptors are Pacinian corpuscles, Ruffini endings, and Krause end bulbs. Temperature receptors are simply free nerve endings in the epidermis; some are responsive to cold and others are responsive to warmth, although there are no structural differences between them.
Pain Receptors Nociceptors are pain receptors on internal organs and may be sensitive to temperature, pressure, or chemicals. Referred pain occurs when stimulation of internal pain receptors is felt as pain from the skin. Referred pain most likely happens because of shared nerve pathways between the skin and internal organs.
Sense of Taste • The taste buds located in papillae on the tongue contain taste cells that communicate with sensory nerve fibers. • Microvilli on taste cells contain receptor proteins that match chemicals in food. • The brain determines the taste according to a “weighted average” of incoming impulses from taste buds sensitive to either sweet, sour, salty, or bitter tastes.
Sense of Smell • Olfactory cells (modified neurons) are located in epithelium in the roof of the nasal cavity. • After molecules bind to receptor proteins on the varied cilia of olfactory cells, nerve impulses lead to olfactory areas of the cerebral cortex. • The perceived odor is determined by the combination of olfactory cells stimulated. • The effects of smell and taste combine.
Anatomy of the Eye • The eye has three layers. • The sclera is the outer layer seen as the white of the eye and includes the transparent bulge in the front of the eye called the cornea. • The choroid is the middle, darkly pigmented layer that absorbs stray light rays; it also becomes the iris that regulates the size of the pupil.
Behind the iris, the choroid thickens and forms the ciliary body. The ciliary body contains the ciliary muscle, which controls the shape of the lens for near and far vision. The lens divides the eye into two compartments: the anterior compartment (containing aqueous humor) and the posterior compartment (containing vitreous humor).
Rod cells and cone cells are located in the retina that forms the inner layer. The retina lines the back half of the eye and has cone cells densely packed in one area called the fovea centralis. Sensory fibers from the retina form the optic nerve leading to the brain.
Focusing • The cornea and the lens focus light rays on the retina. • To see a close object, the ciliary muscles change the lens shape to provide visual accommodation. • After age 40, the lens is less able to accommodate and near vision is less acute. • Cataracts occur when the lens becomes opaque; sun exposure might be a factor in developing cataracts.
Photoreceptors • Both rod cells and cone cells have an outer segment with membranous disks containing embedded pigments. • Rods contain a deep purple pigment called rhodopsin that is composed of retinal (made from vitamin A) and the protein opsin. • Rods are numerous and provide peripheral vision, perception of motion, and vision in dim light at night.
When a rod absorbs light, rhodosin splits into opsin and retinal, leading to a cascade of reactions and the closing of rod membrane ion channels. Inhibitory neurotransmitters are no longer released from the rod. Breakdown of rhodopsin in rods thus initiates nerve impulses. Cones have three different pigments (red, green and blue) made from retinal and opsin; opsin varies between the three.
Integration of Visual Signals in the Retina • The retina has three layers of neurons: rods and cones are near the retina, bipolar cells are in the middle, and the innermost layer contains ganglion cells that carry impulses to the optic nerve. • The rod and cones synapse with the bipolar cells, which in turn synapse with ganglion cells that initiate nerve impulses.
As signals pass from one layer to the next, integration occurs because each layer contains fewer cells than the previous layer. However each cone connects directly to one ganglion cell, while a hundred rods may synapse with only one ganglion cell. It is likely that much processing occurs in the retina before impulses are sent to the brain. There are no rods and cones where the optic nerve exits the retina; this is the blind spot.
Integration of Visual Signals in the Brain • The visual pathway begins with the retina and passes through the thalamus before reaching the cerebral cortex. • The visual pathway and the visual cortex split the visual field apart, but the visual association areas rebuild it so we correctly perceive the entire visual field.
Abnormalities of the Eye • Color Blindness • The most common abnormality is a lack of red and/or green cones. • Distance Vision • Nearsighted individuals (elongated eyeball) cannot see distant objects; this is corrected by a concave lens. • Farsighted individuals (shortened eyeball) see distant objects well but not up close; this is corrected by a convex lens. • Astigmatism occurs with an uneven cornea or lens.
Anatomy of the Ear • The ear is divided into three parts. • The outer ear consists of the pinna and the auditory canal, which direct sound waves to the middle ear. • The middle ear begins at the tympanic membrane (eardrum) and contains the ossicles: the malleus, incus, and stapes that amplify sound waves.
The malleus is attached to the tympanic membrane, and the stapes is attached to the oval window, which is covered by membrane. The inner ear contains semicircular canals and vestibule involved in equilibrium, and the cochlea for hearing.
Process of Hearing • Sound waves enter the auditory canal and vibrate the tympanic membrane. • If the vibrations are strong enough, the outer and middle portions (ossicles) of the ear convey and amplify the sound waves about 20 times and vibrate against the oval window. • These vibrations set up pressure waves within the fluid of the cochlea.
The cochlea contains the spiral organ consisting of hair cells on the basilar membrane whose stereocilia are embedded within the tectorial membrane. Vibrations within the cochlea cause the sterocilia to vibrate against the tectorial membrane, thus generating nerve impulses.
Different regions are sensitive to different frequencies or pitch. When the stereocilia of the hair cells bend, nerve impulses are generated in the cochlear nerve and are carried to the brain.
Gravitational Equilibrium Stimulation of hair cells within the utricle and the saccule, two sacs located in the vestibule, by the slippage of calcium carbonate granules or otoliths, provide impulses that tell the brain the direction of movement of the head. The movement of the otoliths provides a sense of gravitational equilibrium.
Proprioceptors in muscles and joints help the body maintain balance and posture. Cutaneous receptors in the skin respond to touch, pressure, pain, and temperature (both warmth and cold). In the mouth, the microvilli of taste cells have membrane protein receptors that respond to certain molecules.
Olfactory cells within the olfactory epithelium respond to molecules and result in a sense of smell. Photoreceptors for sight contain visual pigments, which respond to light rays. Some integration occurs in the retina of the eye before nerve impulses are sent to the brain.
Sensory receptors for hearing are hair cells in the cochlea of the inner ear that respond to pressure waves. Sensory receptors for balance are hair cells in the vestibule and semicircular canals of the inner ear that respond to the tilt of the head and to the movement of the body, respectively.
