Properties and Types of Sensory Receptors

A stimulus creates a voltage change called a/an [1] on a receptor cell, and if this is sufficient to cause sensory signals to be transmitted to the brain, a/an [2], or conscious awareness of the stimulus, may result. Stimuli can be classified by type, or [3], such as taste and vision, which the brain identifies by a [4] 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], however, is encoded partly by the firing frequency of sensory nerve fibers. [6] receptors fire rapidly when first stimulated and soon stop or slow down their response; [7] receptors continue firing much longer under continual stimulation.

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

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The General Senses

Sensory dendrites without specialized associated connective tissue elements are called [17] nerve endings. They include receptors for heat and cold; [18], which sense light touch; and [19], which sense movements of the hair. [20] nerve endings have specialized tissues that make the receptors more sensitive or more specific. Two of these are the [21] deep in the dermis, which sense deep pressure, and [22] in the more superficial layer of the dermis, which sense touch and texture. Nociceptors are very widespread, although lacking from the [23]. Their first-order neurons end in the [24] of the spinal cord, 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]. A nociceptor stimulates the second-order neuron with a neuropeptide called [26], 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]. Such mechanisms or substances that lessen the sense of pain are said to have a/an [28] effect.

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The Chemical Senses

The sense of taste, or [29], begins when food molecules stimulate taste buds located on bumps of the tongue called [30]. Each taste cell in these structures has microvilli called [31] to increase its receptive surface area. Taste buds at the rear of the tongue are most sensitive to [32] chemicals; those along the sides of the posterior half of the tongue are most sensitive to acids, which produce the [33] taste sensation; and those at the tip of the tongue are most sensitive to [34] (name two sensations). All taste fibers project to the gustatory nucleus in the [35] of the brain.

Receptor cells for smell form the [36] mucosa in the roof of the nasal cavity. Axons from these cells end at synapses in the [37] of the brain. The second-order neurons form olfactory [38] leading to the thalamus, hypothalamus, limbic system, and other centers. The conscious awareness of odor results when the thalamus relays signals back to the [39] gyri of the cerebrum, on the inferior surface of the frontal lobes.

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Hearing and Equilibrium

Hearing begins when vibrating air molecules strike the [40] membrane, three auditory [41] transfer these vibrations to the inner ear, and this creates vibrations within a spiral, snail-shaped organ called the [42]. Two middle-ear muscles, the [43] and [44], dampen intense vibrations and thus protect the inner ear from damage by extremely loud sounds. The receptor cells for hearing are called [45], which, along with supporting cells, rest on the [46] membrane of the 42. There are about 20,000 [47] cells arranged in three rows, with a V-shaped array of [48] on the apex of each cell. These 47s are embedded in a gelatinous [49] membrane above them, and these cells serve only to tune the 42 and enable it to better distinguish different [50] of sound from each other. What we actually hear comes from a single row of about 3,500 [51] cells. All the 47 and 51 cells synapse with neurons whose axons form the [52] nerve. This joins the vestibular nerve from another part of the inner ear, and together they form cranial nerve VIII, the [53] nerve. The fibers of this nerve end in the [54] nucleus of the medulla oblongata. Second-order neurons lead to the [55] 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 [56] lobe of the cerebrum.

The inner ear is also concerned with [57], 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 [58], the macula is nearly horizontal. If the body accelerates horizontally, or if the head is tilted, the [59] membrane bends the stereocilia of this macula. In the other chamber, the [60], 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 [61], sense angular accelerations of the head. Each has a dilated sac called a/an [62] containing a mound of hair cells and supporting cells called the [63]. When the head rotates, the fluid in the duct, called [64], lags behind and stimulates the receptor cells of the 63.

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Vision

The eye, the tear-producing [65] apparatus, and the [66] 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 [67]; the iris and choroid are the major components of the middle layer, or [68]; and the retina constitutes the inner layer, or [69]. The interior of the eye consists of two major fluid-filled spaces. The fluid anterior to the lens, called [70], fills the anterior chamber between the cornea and [71] and the posterior chamber between 71 and the lens. The fluid between the lens and retina is a gel called the [72]. The retina is attached to the rest of the eye at just two points: an anterior ring called the [73] just behind the lens, and a posterior [74] where the optic nerve leaves the eye. 74 is the only place where receptor cells are lacking from the retina, so it is also called the [75]. The sharpest vision, on the other hand, is at a small pit called the [76], directly behind the central axis of the lens. Receptor cells called [77] are responsible for high-resolution daytime vision, whereas receptor cells called [78] are responsible for high-sensitivity but low-resolution vision in dim light (night vision). Day and night vision are also known as [79] and [80] vision, respectively. Color vision is also attributable to the [81] cells, which are of three kinds that differ in their visual pigments.

The focusing of light on the retina involves the bending, or [82], of light rays. This is achieved mainly by the [83], since the difference between its [84] and that of the air is greater than the difference encountered anywhere else in the light path to the retina. The [85] 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 [86]. The [87] also constricts to screen out peripheral light rays that cannot be focused well.

When a photon of light is absorbed by a rod cell, it isomerizes a pigment called [88]. The [89] moiety of the pigment changes from an 11-cis to an all-trans isomer and dissociates from the [90] moiety. This process is called [91], because purified pigment is seen to lose its color in the light. While the eye is in the dark, rod cells release a neurotransmitter, [92], that inhibits the [93] cell with which the rod synapses. This prevents the generation of signals in the optic nerve. The secretion of 92 ceases when the rod absorbs light. The 93 cell then stimulates a [94] 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 [95] 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 [96], 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 [97]. These fibers end in the [98] body of the thalamus, where second-order neurons begin and conduct the signal to the [99] lobe of the cerebrum.

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