a. Modality refers to the type of stimulus or sensation it produces (vision, taste, etc.).

b. Location is also indicated by which nerve fibers are firing. Sensory projection is the ability of the brain to identify the site of stimulation.

c. Intensity can be encoded by: firing frequencies of nerve fibers, recruitment of more fibers, and stimulation of fibers that vary in their thresholds.

d. Duration is encoded in the way nerve fibers change their firing frequencies over time. Phasic receptors tend to generate a burst of action potentials then quickly adapt and stop transmitting impulses. Tonic receptors adapt slowly and continue to transmit impulses.

a. Chemoreceptors respond to chemicals.

b. Thermoreceptors respond to temperature changes.

c. Nociceptors are pain receptors and sense tissue damage.

d. Mechanoreceptors respond to a physical change in their shape.

e. Photoreceptors respond to light.

a. General senses (also called somatic, somatosensory, or somesthetic) have receptors that are widely distributed throughout the body. These detect touch, pressure, heat, cold, pain, and chemistry.

b. The special senses are limited to the head, including vision, hearing, taste, smell, and equilibrium.

a. Interoceptors detect stimuli from internal organs.

b. Proprioceptors sense position and movement of the body or its parts.

c. Exteroceptors detect external changes.

a. Free nerve endings are abundant in epithelial and connective tissues. They include thermoreceptors and nociceptors.

b. Merkel discs are flattened nerve endings at the base of the epidermis that sense light touch and pressure.

c. Hair receptors (peritrichial endings) consist of dendrites wrapped around hair follicles, and are stimulated when the hair is touched.

a. Tactile (Meissner) corpuscles occur in the dermal papillae of the skin, especially in the hairless areas (fingertips, nipples, lips, genitals). They consist of 2-3 nerve fibers within a mass of connective tissue, and are phasic receptors.

b. Krause end bulbs are similar to tactile corpuscles but occur in mucous membranes.

c. Lamellated (pacinian) corpuscles occur both in certain areas of the viscera and deep within the dermis of the hands, feet, breasts, and genitals. They consist of a core of nerve fibers wrapped in Schwann cells. They are phasic for deep pressure and vibration.

d. Ruffini corpuscles are located in the dermis, subcutaneous tissue, and joint capsules. They respond tonically to heavy pressure and movement. Each has an elongated, flattened capsule containing a few nerve fibers.

a. Fast pain, the sharp, stabbing feeling perceived at the time of injury is carried on myelinated type A fibers.

b. Slow pain is carried on unmyelinated type C fibers, for a sensation of diffuse, dull ache.

a. Somatic pain arises from the skin, muscles, and joints, and can be superficial or deep.

b. Visceral pain is less localized due to fewer nociceptors in the viscera, and is commonly caused by stretch, chemicals, or ischemia.

a. Pain signals traveling on first order neurons travel to an interneuron hooked up to the spinothalamic tract, then to the thalamus, which relays the signal to the cerebral cortex.

b. Pain signals also travel up the spinoreticular tract to the reticular formation where the state of arousal may be affected.

a. Sometimes it is difficult to locate the source of pain, particularly for visceral pain.

b. Referred pain occurs when pain fibers from deep tissues merge with those of the skin, and follow the same pathway to the thalamus.

c. Knowledge of origins of referred pain can be a useful diagnostic tool.

a. Damaged tissues release a number of chemicals that stimulate nociceptors, with bradykinin as one of the most potent stimulators.

b. The most studied neuropeptide released by first-order nerve fibers in the dorsal horn is substance P.

a. The CNS has analgesic mechanisms that help alleviate the pain of childbirth, for example. Oligopeptides with analgesic qualities are the enkephalins and endorphins (collectively, the endogenous opiates).

b. The endogenous opiates act as neuromodulators to block the transmission of pain and produce feelings of pleasure.

c. The reticular formation may also moderate sensitivity to pain by means of endorphins.

d. Some interneurons of the dorsal horn inhibit second-order neurons of the pain pathway, especially after receiving input from touch fibers.

a. Taste results from the action of chemicals on the taste buds located on the tongue.

b. Lingual papillae are of four types: filiform papillae that have no taste buds and are important to animal grooming; foliate papillae that occur along the back lateral edge of the tongue and house few taste buds; fungiform papillae, located in the apex with about five taste buds each; and vallate papillae, surrounded by deep trenches, located at the back of the tongue. These also house taste buds.

c. Taste buds house taste cells, supporting cells, and basal cells. The taste cells support microvilli called taste hairs that have surface receptors for chemicals (flavors) in food. Taste cells divide by mitosis and live for 7-10 days. They synapse with neurons.

a. To be tasted, molecules have to be dissolved in water. Four primary taste sensations exist: salty, sweet, sour, and bitter.

b. The flavor of food also involves smell, texture, and appearance.

a. Cranial nerves VII, IX, and X send taste sensations to the gustatory nucleus of the medulla oblongata. Signals are then relayed either to other brainstem nuclei involved with autonomic reflexes (gagging and the like) or to the thalamus. The thalamus signals the gustatory cortex.

a. Olfactory mucosa contains 10-20 million olfactory neurons, each of which bears 10-20 cilia called olfactory hairs. These have binding sites for odor molecules, and lie in a thin layer of mucus.

b. Olfactory cells are the only sensory neurons that lie in direct contact with the outside environment, and are replaced about every 60 days.

a. To be detected, chemicals must be volatile and water soluble.

b. When an odor molecule binds with a specific receptor, a second messenger is produced, opening ion channels in the membrane. Sodium ions enter the cell and depolarize it, creating a receptor potential.

c. Olfactory receptors are quick to adapt.

d. Some odors, such as the ammonia smell of smelling salts, stimulate nociceptors that trigger the trigeminal nerve.

a. Olfactory axons pass through the cribriform plate to the olfactory bulbs to the olfactory tracts. These tracts follow a complex pathway that involves the medial temporal lobes. Olfactory signals reach the cortex before the thalamus if the person is unaware of them; conscious awareness of smell travels through the thalamus to the neocortex in the frontal lobes.

a. A vibrating object sets up vibrations in nearby air molecules which travel to the eardrum and cause it to vibrate.

a. Pitch is determined by the frequency of vibration. Human ears can hear frequencies from 20-20,000 Hz, but are most sensitive to frequencies ranging from 1,500 to 4,000 Hz.

a. Loudness is the perception of amplitude of frequency. Loudness is expressed in decibels (dB), with 120-140 dB causing pain in most people. Prolonged exposure to sounds greater than 90 dB can cause permanent hearing loss.

a. The external ear consists of three parts. The outer auricle funnels vibrations toward the auditory canal and eardrum. The auditory canal leads to the eardrum, and is lined with protective ceruminous glands and hairs. The eardrum (tympanic membrane) separates the outer ear from the middle ear, and is highly sensitive to pain.

a. The middle ear lies in an air-filled tympanic cavity in the temporal bone. Air enters from the throat via the auditory (eustachian) tube. Infections often spread to the middle ear through the auditory tube.

b. Three small bones, the auditory ossicles, span the distance between the tympanic membrane and the inner ear. First is the malleus, then the incus, and last, the stapes, which is suspended by a ligament into the oval window of the inner ear.

c. Two muscles of the middle ear, the stapedius and tensor tympani, have protective functions.

a. The inner ear is housed within a bony labyrinth. Passageways within the bone are lined with membranous labyrinth. Between bone and membrane is a fluid called perilymph; endolymph fills the chamber within the membranous labyrinth.

b. The labyrinths begin at a chamber called the vestibule which houses the organs of equilibrium. Anterior to the vestibule is the cochlea, which is the organ of hearing.

c. The cochlea has three fluid-filled chambers: the scala vestibuli is superior and begins near the oval window; the inferior scala tympani that terminates at the round window; and the middle cochlear duct, which is bounded by the vestibular membrane on the top and basilar membrane on the bottom. The cochlear duct contains endolymph; the other two chambers contain perilymph.

d. The basilar membrane supports the organ of Corti containing hair cells, each with stereocilia. The tips of the stereocilia shear against an overlying tectorial membrane.

e. Within the organ of Corti, inner hair cells (IHCs) send actual hearing impulses. Outer hair cells (OHCs) adjust the response of the cochlea to different frequencies.

a. The function of the auditory ossicles is to concentrate the energy from the eardrum to a smaller oval window, overcoming the impedance of the endolymph (impedence matching).

b. The ossicles and eardrum are protected by the tympanic reflex in response to loud noises, but it is not effective for sudden loud noises.

c. The middle-ear muscles tighten up before you speak to protect your ears from the volume of your own voice.

a. Auditory ossicles vibrate against the oval window, which sets up vibrations within the fluid-filled inner ear. The endolymph of the cochlear duct vibrates, causing the hair cell stereocilia to move against the tectorial membrane.

b. Bending the stereocilia causes depolarization of the hair cell; bending in the opposite direction closes the potassium ion channel while the cell hyperpolarizes. The hair cell releases neurotransmitter during depolarization, generating an action potential to the cochlear nerve.

a. Loud sounds produce vigorous vibrations of the organ of Corti, exciting a greater number of cells over a larger area. The brain interprets a higher frequency of action potentials as a loud sound.

b. A sound causes a standing wave in the basilar membrane. Low-frequency sounds would cause a peak amplitude at the distal end of the organ of Corti; higher frequency sounds are detected closer to the proximal end.

a. The cochlea can "tune in" to receive some frequencies better than others. Outer hair cells are supplied with both sensory and motor fibers. The motor fibers can cause an OHC to shorten.

a. Each of these chambers within the vestibule has a patch of hair cells and supporting cells called a macula. There are two such patches per ear, one in the saccule and one in the utricle. These help in the perception of the orientation of the head when the body is stationary.

b. Each hair cell within the maculae has 40-70 stereocilia and one motile true cilium, called a kinocilium. The tips of these extensions are embedded in the gelatinous otolithic membrane, which is weighted with otoliths.

c. When the position of the head is moved, the otoliths shift, causing the hair cells to bend, and generate a nerve signal.

a. Rotational acceleration is detected by three endolymph-filled semicircular ducts, each of which houses an ampulla. Within the ampulla is a mound of hair cells with supporting cells called the crista ampullaris. Hair cells are embedded in a gelatinous cupula that responds to motion, bending the stereocilia and stimulating hair cells.

a. Hair cells for the sense of equilibrium synapse with the vestibular nerve, which joins the vestibulocochlear nerve. From there, impulses travel to the vestibular nucleus of the pons. From there, projection fibers lead to the spinal cord and to brainstem nuclei that control eye movements.

a. Eyelids close to protect the eye and remove debris. They are bordered with tarsal glands that secrete oil to coat the eye and reduce tear evaporation.

b. The conjunctiva is the pink inner lining of the eyelid. It is highly sensitive to pain, and keeps the eyeball moist. It is very vascular.

c. The lacrimal apparatus consists of a lacrimal gland, ducts, and short lacrimal canals that lead to a lacrimal sac. Tears wash foreign debris from the eye, moisten it, and contain lysozyme to keep the eye surface free from bacteria.

d. Six extrinsic eye muscles are responsible for movements of the eye. These are the superior, inferior, medial and lateral rectus muscles, and a superior and inferior oblique pair of muscles.

a. The eyeball is made of three layers, or tunics.

b. The outermost tunica fibrosa consists of the sclera and transparent cornea.

c. The tunica vasculosa is the middle, vascular layer that has a layer to keep the inner eye dark (choroid) and supplies eye tissue with oxygen and nutrients. A ciliary body forms a muscular ring around the lens and secretes aqueous humor. The iris controls the diameter of the pupil, and contains chromatophores with varying quantities of pigment.

d. The tunica interna is the retina.

a. The optical apparatus of the eye admits and refracts light rays, then focuses them on the retina.

b. The lens is connected to the ciliary body by the suspensory ligament. These structures adjust the shape of the lens and focus light rays on the retina.

c. The anterior and posterior chambers at the front of the eye are filled with aqueous humor. It is produced by the ciliary body and drains out the scleral venous sinus.

d. Behind the lens, the cavity of the eyeball is filled with vitreous humor.

a. The retina is really an outgrowth of the telencephalon. The most posterior layer of the retina is pigmented epithelium. The neural apparatus consists of three principal cell layers.

b. At the back of the retina lie the photoreceptor cells called rods and cones. Rod discs contain the pigment rhodopsin. Rods are responsible for colorless vision in dim light. Cones function in bright light and are responsible for both sharp and color vision.

c. Bipolar cells are those that rods and cones synapse with after receiving an image.

d. The third layer is made up of ganglion cells that receive input from bipolar neurons.

e. Horizontal and amacrine cells form horizontal connections between rod, cone, and bipolar cells but do not form a true layer of cells.

a. Directly posterior to the lens lies the macula lutea on the retina - a small patch containing the fovea centralis that produces the most detailed image in vision. The nearby optic disc occurs where the optic nerve pierces the retina; no vision occurs here, and thus it is called the blind spot.

a. The iris contains two sets of muscles. The pupillary constrictor narrows the pupil to admit less light to the eye. The pupillary dilator widens the pupil to admit more light. Pupillary contriction in response to light is called the photopupillary reflex.

a. The bending of light rays is called refraction. As light enters the eye, it is first refracted by the cornea. The aqueous humor does not greatly refract light.

a. To focus on something up close, the lens must change shape to bend light rays to focus on the retina. Adjustments to close range vision are called near response, involving three events that focus the image on the retina.

b. Convergence of the eyes orients the visual axis of each eye toward the object in order to focus on the fovea centralis of each eye.

c. Constriction of the pupil adjusts the amount of light and reduces spherical aberration.

d. The lens accommodates by means of the ciliary muscle. The closest an object can be and still come into focus is called the near point of vision. This distance increases with age as the lens loses elasticity.

a. The visual pigment of rods, rhodopsin or visual purple, is composed of the protein opsin and a vitamin A derivative called retinal.

b. In cones, the pigment is photopsin or iodopsin. Three kinds allow peak absorption of light of different wavelengths, enabling color vision.

a. In the dark, retinal exists as cis-retinal. When it absorbs light, it is converted to trans-retinal. This isomer then separates from the opsin. The pigments in cones behave in a similar manner.

a. In the dark, rods are very active, producing a steady ion flow called the dark current. The production of a nervous impulse stems from shutting off this current.

b. The dark current is maintained by cyclic guanosine monophosphate (cGMP) that holds sodium gates open. Potassium ions diffuse outward continually. At the same time, the cell releases glutamic acid from its synaptic end.

c. When a rod absorbs light, the dark current drops or ceases. The rhodopsin molecule becomes enzymatically active and triggers a cascade of reactions that break down molecules of cGMP. The effect is amplifying, which explains why rods are sensitive to low light.

d. As cGMP is degraded, the sodium channels close, the dark current declines, and the glutamic acid secretion is halted.

e. When bipolar cells detect fluctuations in light intensity, they communicate this to ganglion cells. Ganglion cells produce all-or-none action potentials, while all other retinal neurons produce graded local potentials.

a. When trans-retinal dissociates from rhodopsin, it is transported back to the pigmented epithelium, converted back to cis-retinal, carried back to the rod and reunited with opsin.

a. Light adaptation occurs when we go from the dark into bright light. At first, pain is experienced from overstimulated retinas. The pupils quickly constrict. Rods bleach quickly in bright light; the cones take over.

b. Dark adaptation occurs when you are in bright light and suddenly go into a very dark room. The rate rhodopsin regenerates begins to exceed the rate of bleaching. Dilation of pupils also helps to get more light into the eyes.

a. The duplicity theory suggests that a single type of receptor cell cannot produce both high sensitivity and high resolution. Therefore, two types of visual receptors, namely rods and cones, are needed.

CHAPTER ESSAY: Anesthesia - From Ether Frolics to Modern Surgery (p. 595)