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October, 2006
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JoVE 10859

The somatosensory system relays sensory information from the skin, mucous membranes, limbs, and joints. Somatosensation is more familiarly known as the sense of touch. A typical somatosensory pathway includes three types of long neurons: primary, secondary, and tertiary. Primary neurons have cell bodies located near the spinal cord in groups of neurons called dorsal root ganglia. The sensory neurons of ganglia innervate designated areas of skin called dermatomes. In the skin, specialized structures called mechanoreceptors transduce mechanical pressure or distortion into neural signals. In hairless skin, most disturbances can be detected by one of four types of mechanoreceptors. Two of these, Merkel disks and Ruffini endings, are slow-adapting and continue to respond to stimuli that remain in prolonged contact with the skin. Merkel disks respond to light touch. Ruffini endings detect deeper static touch, skin stretch, joint deformation, and warmth. The other two major cutaneous mechanoreceptors, Meissner corpuscles and Pacinian corpuscles, are rapidly-adapting. These mechanoreceptors detect dynamic stimuli, like those required to read Braille. Meissner corpuscles are responsive to delicate touch and pressure, as well as low-frequency vibrations. Pacinian corpuscles respond best to deep, repetitive pressure and high-frequency vibrations. Information detected

 Core: Biology


JoVE 10858

Vision is the result of light being detected and transduced into neural signals by the retina of the eye. This information is then further analyzed and interpreted by the brain. First, light enters the front of the eye and is focused by the cornea and lens onto the retina—a thin sheet of neural tissue lining the back of the eye. Because of refraction through the convex lens of the eye, images are projected onto the retina upside-down and reversed. Light is absorbed by the rod and cone photoreceptor cells at the back of the retina, causing a decrease in their rate of neurotransmitter release. In addition to detecting photons of light, color information is also encoded here, since different types of cones respond maximally to different wavelengths of light. The photoreceptors then send visual information to bipolar cells near the middle of the retina, which is followed by projection to ganglion cells at the front of the retina. Horizontal and amacrine cells mediate lateral interactions between these cell types, integrating information from multiple photoreceptors. This integration aids in the initial processing of visual information, such as detecting simple features, like edges. Along with glial cells, the axons of the retinal ganglion cells make up the optic nerve, which transmits visual information to the brain. The optic nerve partially cro

 Core: Biology

The Retina

JoVE 10857

The retina is a layer of nervous tissue at the back of the eye that transduces light into neural signals. This process, called phototransduction, is carried out by rod and cone photoreceptor cells in the back of the retina.

Photoreceptors have outer segments with stacks of membranous disks that contain photopigment molecules—such as rhodopsin in rods. The photopigments absorb light, triggering a cascade of molecular events that results in the cell becoming hyperpolarized (with a more negative membrane potential) relative to when it is in the dark. This hyperpolarization decreases neurotransmitter release. Thus, unlike stimuli for most other sensory neurons, light induces a reduction in neurotransmitter release from photoreceptors. Although rods and cones both detect light, they play distinct roles in vision. Rods are highly sensitive to light, and therefore predominate in low-light conditions, such as at night. Cones are less sensitive and are used for most daytime vision. Cones are densely concentrated in the fovea—a small depression near the center of the retina that contains very few rods—and provide a high level of visual acuity in the area where the eye is focused. Cones also convey color information, because the different types—S (short), M (medium), and L (long) in humans—maximally absorb different wa

 Core: Biology

The Vestibular System

JoVE 10856

The vestibular system is a set of inner ear structures that provide a sense of balance and spatial orientation. This system is comprised of structures within the labyrinth of the inner ear, including the cochlea and two otolith organs—the utricle and saccule. The labyrinth also contains three semicircular canals—superior, posterior, and horizontal—that are oriented on different planes. All of these structures contain vestibular hair cells—the sensory receptors of the vestibular system. In the otolith organs, the hair cells sit beneath a gelatinous layer called the otolithic membrane, which contains otoconia—calcium carbonate crystals—making it relatively heavy. When the head is tilted, the otolithic membrane shifts, bending the stereocilia on the hair cells. In the semicircular canals, the cilia of the hair cells are contained within a gelatinous cupula, which is surrounded by endolymph fluid. When the head experiences movements, such as rotational acceleration and deceleration, the fluid moves, bending the cupula and the cilia within it. Similar to the auditory hair cells, displacement towards the tallest cilium causes mechanically-gated ion channels to open, depolarizing the cell and increasing neurotransmitter release. Displacement towards the shortest cilium hyperpolarizes the cell and decreases neurotr

 Core: Biology

The Cochlea

JoVE 10855

The cochlea is a coiled structure in the inner ear that contains hair cells—the sensory receptors of the auditory system. Sound waves are transmitted to the cochlea by small bones attached to the eardrum called the ossicles, which vibrate the oval window that leads to the inner ear. This causes fluid in the chambers of the cochlea to move, vibrating the basilar membrane.

The basilar membrane extends from the basal end of the cochlea near the oval window to the apical end at its tip. Although the cochlea itself narrows towards the apical end, the basilar membrane has the opposite geometry—becoming wider and more flexible towards the apical end. Primarily because of these physical characteristics, the apical end of the basilar membrane maximally vibrates when exposed to low-frequency sounds, while the narrower, stiffer basal end maximally vibrates when exposed to high frequencies. This gradient of frequency response creates tonotopy—a topographic map of pitch—in the cochlea. The hair cells are stimulated by the shearing force created by the vibration of the basilar membrane below them, relative to the stiffer tectorial membrane above them. Because of the tonotopy of the basilar membrane, hair cells are maximally stimulated by different frequencies depending on where they are in the cochlea. Those at the basal end respond be

 Core: Biology

Hair Cells

JoVE 10854

Hair cells are the sensory receptors of the auditory system—they transduce mechanical sound waves into electrical energy that the nervous system can understand. Hair cells are located in the organ of Corti within the cochlea of the inner ear, between the basilar and tectorial membranes. The actual sensory receptors are called inner hair cells. The outer hair cells serve other functions, such as sound amplification in the cochlea, and are not discussed in detail here. Hair cells are named after the hair-like stereocilia that protrude from their tops and touch the tectorial membrane. The stereocilia are arranged by height and are attached by thin filaments called tip links. The tip links are connected to stretch-activated cation channels on the tips of the stereocilia. When a sound wave vibrates the basilar membrane, it creates a shearing force between the basilar and tectorial membranes that moves the hair cell stereocilia from side to side. When the cilia are displaced towards the tallest cilium, the tip links stretch, opening the cation channels. Potassium (K+) then flows into the cell, because there is a very high concentration of K+ in the fluid outside of the stereocilia. This large voltage difference creates an electrochemical gradient that causes an influx of K+ once the channels are opened. This influx o

 Core: Biology


JoVE 10853

When we hear a sound, our nervous system is detecting sound waves—pressure waves of mechanical energy traveling through a medium. The frequency of the wave is perceived as pitch, while the amplitude is perceived as loudness.

Sound waves are collected by the external ear and amplified as they travel through the ear canal. When sounds reach the junction between the outer and middle ear, they vibrate the tympanic membrane—the eardrum. The resulting mechanical energy causes the attached ossicles—a set of small bones in the middle ear—to move. The ossicles vibrate the oval window, the outermost part of the inner ear. In the labyrinth of the inner ear, the sound wave energy is transferred to the cochlea—a coiled structure in the inner ear—causing the fluid within it to move. The cochlea contains receptors that transduce mechanical sound waves into electrical signals that can be interpreted by the brain. Sounds within the hearing range vibrate the basilar membrane in the cochlea and are detected by hair cells on the organ of Corti, the site of transduction. Along the primary auditory pathway, the signals are sent through the auditory nerve to the cochlear nuclei in the brainstem. From here, they travel to the inferior colliculus of the midbrain and up to the thalamus, and then to the primary auditory cortex. Along this pat

 Core: Biology


JoVE 10852

The sense of smell is achieved through the activities of the olfactory system. It starts when an airborne odorant enters the nasal cavity and reaches olfactory epithelium (OE). The OE is protected by a thin layer of mucus, which also serves the purpose of dissolving more complex compounds into simpler chemical odorants. The size of the OE and the density of sensory neurons varies among species; in humans, the OE is only about 9-10 cm2. The olfactory receptors are embedded in the cilia of the olfactory sensory neurons. Each neuron expresses only one type of olfactory receptor. However, each type of olfactory receptor is broadly tuned and can bind to multiple different odorants. For example, if receptor A binds to odorants 1 and 2, receptor B may bind to odorants 2 and 3, while receptor C binds to odorants 1 and 3. Thus, the detection and identification of an odor depend on the combination of olfactory receptors that recognize the odor; this is called combinatorial diversity. Olfactory sensory neurons are bipolar cells with a single long axon that sends olfactory information up to the olfactory bulb (OB). The OB is a part of the brain that is separated from the nasal cavity by the cribriform plate. Because of this convenient proximity between the nose and brain, the development of nasal drug applications is widely studied, especially in cases

 Core: Biology


JoVE 10851

Gustation is a chemical sense that, along with olfaction (smell), contributes to our perception of taste. It starts with the activation of receptors by chemical compounds (tastants) dissolved in the saliva. The saliva and filiform papillae on the tongue distribute the tastants and increase their exposure to the taste receptors.

Taste receptors are found on the surface of the tongue as well as on the soft palate, the pharynx, and the upper esophagus. On the tongue, taste receptors are contained within structures called taste buds. The taste buds are embedded within papillae, which are visible on the tongue surface. There are three types of papillae that contain taste buds and their receptors. Circumvallate papillae are the largest papillae and are located near the back of the tongue. Foliate papillae resemble folds on the side of the tongue. Fungiform papillae are found across the front three-quarters of the tongue but are less concentrated in the middle of the tongue. There are five basic tastes: salty, sour, sweet, bitter, and savory (or umami). The perception of salty taste is caused by tastants that release sodium ions upon dissolution. Sour taste, by contrast, is produced by the release of hydrogen ions from dissolved acidic tastants. Salty and sour tastants produce a neural response by depolarizing the membrane directly (salty tastants) or via ion chan

 Core: Biology

The Tongue and Taste Buds

JoVE 10850

The surface of the tongue is covered with various small bumps called papillae, which either distribute what has been ingested (filiform papillae) or contain the sensory taste (or gustatory) receptor cells (fungiform, circumvallate, and foliate papillae). Embedded within each taste-related papilla are the taste buds—clusters of 30 to 100 gustatory receptor cells.

Gustatory receptor cells extend finger-like projections called gustatory hairs (or microvilli) into a region known as the taste pore. Here, many of the cells contain receptors that detect different tastants—the molecules that can be tasted. The average number of taste buds varies significantly among individuals, with estimates ranging from 2,000-10,000 taste buds. Taste cells have a lifespan of about 10-14 days and are continually replaced. Thus, each taste bud contains taste cells at different stages of development. Aside from the filiform papillae, which do not contain taste buds, the mushroom-shaped fungiform papillae are the most numerous. Fungiform papillae are predominantly located on the anterior two-thirds of the tongue and contain between one and eight taste buds each. In contrast, the other two types of papillae—circumvallate and foliate—contain more than 100 taste buds per papilla. Circumvallate papillae, the largest type, are located at the back o

 Core: Biology
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