Chapter IX Sense System
Sensory systems process information that may lead to a sensation or to perception of an event within the body or in the outside world.
Basic Characteristics of Sensory Coding
A single afferent neuron with all its receptor endings is a sensory unit. The area of the body that, when stimulated, causes activity in a sensory unit or other neuron in the afferent pathway is called the receptive field for that neuron. The type of stimulus perceived is determined by the type of receptor activated, the specific pathways activated, and the part of the brain in which these pathways terminate. Stimulus intensity is coded by the rate of firing of individual sensory units and by the number of sensory units activated. Location of the stimulus depends on the size of the receptive field covered by a single sensory unit and on the overlap of nearby receptive fields. Information coming into the nervous system is subject to control by ascending pathways and by descending pathways.
Vision
Light is described by its wavelength or frequency. The light that falls on the retina must be focused by the cornea and lens. Lens shape is changed in response to viewing near or distant objects so that both are focused on the retina. Presbyopia interferes with accommodation. Cataract prevents the passage of light to the retina. An eyeball too long or too short relative to the focusing power of the lens causes nearsighted or farsighted vision, respectively. The photopigments of the rods and cones are made up of a protein component (opsin) and a chromophore. The rods and each of the three cone types have different opsins that make each of the four-receptor types sensitive to a different wavelength of light. When light falls upon the chromophore, the photic energy causes the chromophore to change shape, which triggers a cascade of events involving the second-messenger cyclic GMP and Ca++. When exposed to darkness, the rods and cones are depolarized and release their neurotransmitter. When exposed to light, they become hyperpolarized, and transmitter release is reduced. The rods and cones synapse on bipolar cells, which synapse on ganglion cells. Ganglion cell axons form the optic nerves, which lead into the brain. Half the optic nerve fibers cross to the opposite side of the brain in the optic chiasm. Fibers from the optic nerves terminate in the lateral geniculate nuclei of the thalamus, which send fibers to visual cortex.. Visual information is also relayed to areas of the brain dealing with biological rhythms. Coding in the visual system occurs along parallel pathways, in which different aspects of visual information, such as color, form, movement, and depth, are kept separate from each other. The colors we perceive are related to the wavelength of light. Different wavelengths excite one of the three cone photopigments most strongly. Certain ganglion cells are excited by input from one type of cone cell and inhibited by input from a different cone type. The sensation of color depends on the output of the various cone opponent-color cells and the processing of this output by brain areas involved in color vision.
Hearing
Sound energy is transmitted by movements of pressure waves. The sound wave frequency determines pitch. The sound wave amplitude determines loudness. The sound transmission sequence is as follows: Sound waves enter the ear canal and press against the tympanic membrane, causing it to vibrate. The vibrating membrane causes movement of the three small middle-ear bones, and the stapes presses against the oval-window membrane. Movements of this membrane set up pressure waves in the fluid-filled scala vestibuli, which cause movements in the cochlear duct wall, setting up pressure waves in the fluid there. These pressure waves cause vibrations in the basilar membrane, located on one side of the cochlear duct. As this membrane vibrates, the hair cells of the organ of Corti move in relation to the tectorial membrane. Movement of the hair cells' stereocilia stimulates them to release neurotransmitter that activates receptors on the peripheral ends of the afferent nerve fibers. Each part of the basilar membrane vibrates maximally in response to one particular sound frequency.
Vestibular System
A vestibular apparatus lies in the temporal bone on each side of the head and consists of three semicircular ducts, a utricle, and a saccule. The semicircular ducts detect angular acceleration during rotation of the head, which cause bending of the stereo-cilia on the hair cells. Otoliths in the gelatinous substance over the utricle and saccule hair cells move in response to changes in linear acceleration and the position of the head relative to gravity and stimulate the stereocilia on the hair cells.
Chemical Senses
The receptors for taste lie in taste buds throughout the mouth, principally on the tongue, and can respond to many different substances. Olfactory receptors, which are part of the afferent olfactory neurons, lie in the mucosa in the upper nasal cavity. Odorant molecules once dissolved in the mucus that bathes the olfactory receptors are transported to the receptors by odorant binding proteins. Olfactory pathways go to the limbic system.
