Chapter 13
Senses
13.1 Sensory Receptors and Sensations
Each type of sensory receptor responds to a particular kind of stimulus. Exteroceptors detect stimuli from outside the body while interoceptors detect internal stimuli. Both categories of receptors are critical to maintaining homeostasis.
Types of Sensory Receptors
There are chemoreceptors (respond to chemical substances), pain receptors (a type of chemoreceptor that responds to chemicals released by damaged tissues), photoreceptors (respond to light), mechanoreceptors (respond to mechanical forces), and thermoreceptors (respond to temperature changes).
How Sensation Occurs
When stimulation occurs, sensory receptors initiate nerve impulses that are transmitted to the spinal cord and/or brain. Sensation occurs when nerve impulses reach the cerebral cortex. Perception is an interpretation of the meaning of sensations.
13.2 Proprioceptors and Cutaneous Receptors
Proprioceptors
Proprioception is illustrated by the action of muscle spindles which are stimulated when muscle fibers stretch. A reflex action, which is illustrated by the knee reflex, causes the muscle fibers to contract. Proprioception helps maintain balance and posture.
Cutaneous Receptors
The skin contains sensory receptors for touch, pressure, pain, and temperature (warmth and cold). Three types of receptors detect touch: Meissner corpuscles, Merkel disks, and free nerve endings. Pacinian corpuscles, Ruffini endings, and Krause end bulbs detect pressure. Temperature receptors are free nerve endings.
Pain Receptors
Internal organs as well as the skin have pain receptors (nociceptors). Sometimes stimulation of internal pain receptors is felt as pain from the skin, a phenomenon known as referred pain.
13.3 Senses of Taste and Smell
Chemoreceptors in the carotid arteries and aorta are sensitive to the pH of the blood. Taste and smell are chemical senses that are sensitive to molecules in food and air.
Sense of Taste
The taste buds contain taste cells that communicate with sensory nerve fibers. The brain determines the taste according to overall pattern of incoming impulses from taste buds sensitive to sweet, sour, salty, or bitter tastes.
Sense of Smell
After molecules bind to receptor proteins on the cilia of olfactory cells, nerve impulses eventually reach the cerebral cortex, which determines the odor according to the type of olfactory cell stimulated.
13.4 Sense of Vision
Vision is dependent on the eye, the optic nerves, and the visual areas of the cerebral cortex.
Anatomy and Physiology of the Eye
The eye has three layers. The outer layer, the sclera, can be seen as the white of the eye; it also becomes the transparent bulge in the front of the eye called the cornea. The middle, pigmented layer, called the choroid, absorbs stray light rays. The rod cells and the cone cells are located in the retina, the inner layer of the eyeball. Aqueous humor fills the anterior compartment of the eye; vitreous humor fills the posterior compartment.
Function of the Lens
The cornea, the humors, and especially the lens, bring the light rays to focus on the retina. To see a close object, visual accommodation occurs as the lens rounds up
Visual Pathway to the Brain
The rods permit vision in dim light at night, and the cones permit vision in bright light needed for color vision. Breakdown of rhodopsin in rods initiates nerve impulses. There are three types of cones (blue, green, or red) that also contain pigments composed of retinal and opsin.
Function of the Retina
The retina has three layers of neurons. 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.
Blind Spot
There are no rods and cones where the optic nerve exits the retina. This is the blind spot.
From the Retina to the Visual Cortex
The visual pathway and the visual cortex take the visual field apart, but the visual association areas rebuild it so we correctly perceive the visual field.
Abnormalities of the Eye
Color Blindness
The most common abnormality is a lack of red and/or green cones.
Distance Vision
There are nearsighted (corrected by concave lens) with an elongated eyeball and farsighted (corrected by convex lens) individuals with a shortened eyeball. Astigmatism is corrected by an unevenly ground lens.
13.5 Sense of Hearing
Hearing is a specialized sense dependent on the ear and its mechanoreceptors, called hair cells.
Anatomy and Physiology of the Ear
The ear is divided into three parts: outer, middle, and inner. The outer ear consists of the pinna and the auditory canal, which direct sound waves to the middle ear. The middle ear begins with the tympanic membrane and contains the ossicles (malleus, incus, stapes). The malleus is attached to the tympanic membrane, and the stapes is attached to the oval window, which is covered by membrane. The fluid-filled inner ear contains the cochlea and the semicircular canals.
Auditory Pathway to the Brain
Hearing begins when the outer and middle portions of the ear convey and amplify the sound waves that strike the oval window. Its vibrations set up pressure waves within the cochlea, which contains the spiral organ, consisting of hair cells whose stereocilia are embedded within the tectorial membrane. When the stereocilia of the hair cells bend, nerve impulses begin in the cochlear nerve and are carried to the brain.
13.6 Sense of Equilibrium
Mechanoreceptors detect movement of the head and help achieve equilibrium.
Rotational Equilibrium
Rotational equilibrium is dependent on the stimulation of hair cells embedded in the cupula within the ampullae of the semicircular canals.
Gravitational Equilibrium
Gravitational equilibrium relies on the stimulation of hair cells on an otolothic membrane within the utricle and the saccule.
50