A stimulus creates a voltage change called a/an [1] receptor potential on a receptor cell, and if this is sufficient to cause sensory signals to be transmitted to the brain, a/an [2] sensation , or conscious awareness of the stimulus, may result. Stimuli can be classified by type, or [3] modality , such as taste and vision, which the brain identifies by a [4] labeled line code—that is, signals are translated as taste or visual signals depending on which nerve pathway, or "line," they arrive on. This also enables the brain to identify the location of stimulation. Stimulus [5] intensity , however, is encoded partly by the firing frequency of sensory nerve fibers. [6] Phasic receptors fire rapidly when first stimulated and soon stop or slow down their response; [7] tonic receptors continue firing much longer under continual stimulation.

Receptors are classified as [8] mechanoreceptors if they respond to physical deformation of a plasma membrane, [9] photoreceptors if they respond to light, [10] chemoreceptors if they respond to chemicals binding to the membrane, or [11] nociceptors if they respond to tissue damage. They are also called [12] exteroceptors if they respond to stimuli arising outside the body, [13] proprioceptors if they respond to changes in joint tension or limb position, and [14] interoceptors if they respond to other internal stimuli. The [15] special senses, such as vision and hearing, are located in sense organs in the head, whereas the [16] general (somesthetic) senses, such as heat, touch, and pain, have a broad distribution over the entire body.

Sensory dendrites without specialized associated connective tissue elements are called [17] unencapsulated nerve endings. They include receptors for heat and cold; [18] Merkel discs , which sense light touch; and [19] hair receptors , which sense movements of the hair. [20] Encapsulated nerve endings have specialized tissues that make the receptors more sensitive or more specific. Two of these are the [21] lamellated (pacinian) corpuscles deep in the dermis, which sense deep pressure, and the [22] tactile (Meissner) corpuscles in the more superficial layer of the dermis, which sense touch and texture. Nociceptors are very widespread, although lacking from the brain. Their first-order neurons enter the spinal cord through the [23] dorsal root and end in the [24] medulla oblongata , where they synapse with second-order neurons leading to the thalamus. Because of convergence in these neural pathways, the brain is sometimes confused about where a stimulus comes from, and interprets visceral pain as if it were arising from an area of the skin. This phenomenon is called [25] referred pain . A nociceptor stimulates the second-order neuron with a peptide neuromodulator called [26] substance P , but the brain can block this action, and thus stop some pain signals from reaching it, by sending signals back to the spinal cord through fibers that secrete another neuropeptide, called [27] enkephalins . Such mechanisms or substances that lessen the sense of pain are said to have a/an [28] analgesic effect.

The sense of taste, or [29] gustation , begins when food molecules stimulate taste buds located on bumps of the tongue called [30] papillae . Each taste cell in these structures has microvilli called [31] taste hairs to increase its receptive surface area. Taste buds at the tip of the tongue are most sensitive to [32] sweet , those at the rear of the tongue to [33] bitter , and those along the sides of the tongue to two tastes, [34] sour and salty . All taste fibers project to the [35] solitary nucleus of the medulla oblongata.

Receptor cells for smell form the [36] olfactory mucosa in the roof of the nasal cavity. Axons from these cells end at synapses in the [37] olfactory bulbs of the brain. The second-order neurons form olfactory [38] tracts leading to the medial side of the temporal lobes. Unlike other senses, signals for smell can reach the cerebral cortex without first passing through the [39] thalamus .

Hearing begins when vibrating air molecules strike the [40] tympanic membrane. Three auditory [41] ossicles transfer these vibrations to the inner ear, and this creates vibrations within a spiral, snail-shaped organ called the [42] cochlea . Two middle-ear muscles, the [43] stapedius and [44] tensor tympani , dampen intense vibrations and thus protect the inner ear from damage by extremely loud sounds. The receptor cells for hearing are called [45] hair cells ; these cells, along with supporting cells, rest on the [46] basilar membrane of the 42. There are about 20,000 [47] outer hair cells arranged in three rows, with a V-shaped array of [48] stereocilia on the apex of each cell. These 47s are embedded in a gelatinous [49] tectorial membrane above them, and these cells serve only to tune the 42 and enable it to better distinguish different frequencies of sound from each other. What we actually hear comes from a single row of about 3,500 [50] inner hair cells. All the 47 and 50 cells synapse with neurons whose axons form the [51] cochlear nerve. This joins the vestibular nerve from another part of the inner ear, and together they form cranial nerve VIII, the [52] vestibulocochlear nerve. The fibers of this nerve end in the [53] cochlear nucleus of the medulla oblongata. Second-order neurons lead to the [54] superior olivary nucleus of the medulla, which issues efferent signals concerned with cochlear tuning by the 47 cells and with the protective reflex of the middle-ear muscles, as well as issuing signals to the higher brain centers. The conscious awareness of hearing resides in the [55] temporal lobe of the cerebrum.

The inner ear is also concerned with [56] equilibrium , the sense of the orientation of the head and movements of the body. Two of the structures involved are fluid-filled chambers in the vestibule that each have a patch of receptor cells called a macula. In the [57] utricle , the macula is nearly horizontal. If the body accelerates horizontally, or if the head is tilted, the [58] otolithic membrane bends the stereocilia of this macula. In the other chamber, the [59] saccule , the macula is more nearly vertical, and the stereocilia are bent if the body accelerates vertically, as in standing up or riding an elevator. Three other tubes, the [60] semicircular ducts , sense angular accelerations of the head. Each has a dilated sac called a/an [61] ampulla containing a mound of hair cells and supporting cells called the [62] crista ampullaris . When the head rotates, the fluid in the duct, called [63] endolymph , lags behind and stimulates the receptor cells of the 62.

The eye, the tear-producing [64] lacrimal apparatus, and the [65] extrinsic muscles that move the eye are housed in the orbit. The eye itself has a three-layered wall. The cornea and sclera make up the outer layer, or [66] tunica fibrosa ; the iris and choroid are the major components of the middle layer, or [67] tunica vasculosa ; and the retina constitutes the inner layer, or [68] tunica interna . The interior of the eye consists of two major fluid-filled spaces. The fluid anterior to the lens, called [69] aqueous humor , fills the anterior chamber between the cornea and [70] iris and the posterior chamber between 70 and the lens. The fluid between the lens and retina is a gel called the [71] vitreous body . The retina is attached to the rest of the eye at just two points: an anterior ring called the [72] ora serrata just behind the lens and a posterior [73] optic disc where the optic nerve leaves the eye. The 73 is the only place where receptor cells are lacking from the retina, so it is also called the [74] blind spot . The sharpest vision, on the other hand, is at a small pit called the [75] fovea centralis , directly behind the central axis of the lens. The focusing of light on the retina involves the bending, or [76] refraction , of light rays. This is achieved mainly by the [77] cornea , since the difference between its [78] refractive index and that of the air is greater than the difference encountered anywhere else in the light path to the retina. The [79] lens bends the light rays still more to fine-tune the focusing of the image. When viewing an object close to the eye, it thickens to bend light rays more strongly. This adjustment is called [80] accommodation . The [81] pupil, or iris also constricts to screen out peripheral light rays that cannot be focused well.

Receptor cells called [82] cones are responsible for high-resolution daytime vision, whereas receptor cells called [83] rods are responsible for high-sensitivity but low-resolution vision in dim light (night vision). Day and night vision are also known as [84] photopic and [85] scotopic vision, respectively. Color vision is also attributable to the [86] cone cells, which are of three kinds that differ in their visual pigments.

When a photon of light is absorbed by a rod cell, it isomerizes a pigment called [87] rhodopsin . The [88] retinal moiety of the pigment changes from an 11-cis to an all-trans isomer and dissociates from the [89] opsin moiety. This process is called [90] bleaching , because purified pigment is seen to lose its color in the light. While the eye is in the dark, rod cells release a neurotransmitter, [91] glutamate , that inhibits the [92] bipolar cell with which the rod synapses. This prevents the generation of signals in the optic nerve. The secretion of 91 ceases when the rod absorbs light. The 92 cell then stimulates a [93] ganglion cell, which sends signals through the optic nerve to the brain. Cone cells probably work the same way but require higher light intensities. According to the [94] duplicity theory of vision, a single type of receptor cell could not produce both the high sensitivity of night vision and the high resolution of day vision.

The two optic nerves meet at an X called the [95] optic chiasm , where signals from the medial half of each retina cross over to the opposite side of the brain. This crossing over of half of the optic nerve fibers from each eye is called [96] hemidecussation . These fibers end in the [97] lateral geniculate nucleus of the thalamus, where second-order neurons begin and conduct the signal to the [98] occipital lobe of the cerebrum.