Hearing

Sound causes vibration of the tympanic membrane (otherwise known as the eardrum). These vibrations produce movements of the middle-ear ossicles, which press against a membrane called the fenestra ovalis in the cochlea. These vibrations produce pressure waves within the fluid of the cochlea, which cause movements of a membrane called the basilar membrane. Sensory hair cells are located on the basilar membrane and the movements of the basilar membrane bend the hair cell processes. This stimulates action potentials that are transmitted to the brain in sensory fibers and interpreted and sound.

The human ear is capable of hearing sounds that range from 20 Hertz to 20,000 Hertz (Hertz are equal to cycles per second). This distinguishes the frequencies of sounds that humans can hear. The intensity or loudness that humans can detect ranges from 0.1-0.5 decibels to sounds that are so loud that they produce pain.

Sound waves are funneled by the pinna into the external auditory meatus. These two structures composed the outer ear. The external auditory meatus channels the sound to the tympanic membrane. The sound waves will produce tiny movements in the tympanic membrane. Between the internal side of the eardrum and the cochlea is the middle ear. The middle ear consists of three middle-ear ossicles, the malleus, incus and stapes. The vibrations of the tympanic membrane are transduced to the cochlea by the ossicles. The stapes is attached to the fenestra ovalis, which is the window to the cochlea. The movements of the ossicles are transmitted to the cochlea by virtue of the direct connection between the stapes and the cochlea.

A few small muscles provide protection for the ossicles. The stapedius is attached to the neck of the stapes, and it contracts if sounds become too intense. This prevents damage to the cochlea. If a sound reaches high amplitude very quickly, as in the case of a gunshot, then the stapedius may not respond quickly enough to prevent damage.

The cochlea constitutes the inner ear. This structure is about the size of a pea and is shaped like the shell of a snail. Vibrations of the stapes displace perilymph fluid within a part of the osseous labyrinth called the scala vestibuli, which is the upper of the three chambers within the cochlea. The lower of the three chambers is also a part of the osseous labyrinth and is known as the scala tympani. The middle chamber is a part of the membranous labyrinth called the scala media. The scala media is part of the membranous labyrinth, and therefore, contains endolymph rather than perilymph. The perilymph of the scala vestibuli and the scala tympani is continuous, since the apex of the cochlea contains a small space called the helicotrema. Vibrations of the fenestra ovalis cause pressure waves within the scala vestibuli, which pass to the scala tympani. Movements of perilymph with the scala tympani travel to the base of the cochlea where they cause displacement of a membrane called the fenestra rotunda into the middle-ear cavity.

The movements of the perilymph of the scala vestibuli to the scala tympani produce displacement of the vestibular membrane and the basilar membrane. Each sound frequency displaces a different portion of the basilar membrane. The sounds of higher frequency cause maximum vibrations of the basilar membrane closer to the stapes, whereas sounds of lower frequency cause movements of the basilar membrane further away from the stapes.

The sensory hair cells are located in the basilar membrane, with their stereocilia projecting into the endolymph of the scala media. The stereocilia are embedded in a membrane called the tectorial membrane, which overhangs the hair cells within the scala media. Collectively, the basilar membrane, hair cells with sensory fibers, and tectorial membrane form a functional unit called the organ of Corti. The organ of Corti is unique to mammals. Displacement of the perilymph creates a shearing force between the basilar membrane and the tectorial membrane. This causes the stereocilia to move and bend. Such movement causes ion channels in the membrane to open and this depolarizes the hair cells. The depolarized hair cells release a neurotransmitter (probably glutamate), which stimulates an associated sensory neuron. The greater the intensity of sound, the greater the amount of neurotransmitter released by the hair cell and the greater the generator potential produced in the sensory neuron.

The standing wave generated in the basilar membrane will peak at different places in the cochlea. These points of highest amplitude will be the ones that achieve the greatest stimulation. Different sounds will cause peaks at specific places in the cochlea, and this is the main mechanism of pitch discrimination.

Sensory neurons in the vestibulocochlear nerve synapse with neurons in the medulla oblongata that project to the inferior colliculus in the midbrain. Neurons in this area project to the thalamus, which sends axons to the auditory lobe of the temporal lobe. By means of this pathway, neurons in different regions of the basilar membrane stimulate neurons in corresponding areas of the auditory cortex. Each area of this cortex represents a different part of the basilar membrane and a different pitch.

There are two major categories of deafness, conduction deafness and sensorineural (or perceptive) deafness. Conductive deafness involves defects in the transmission of sound waves through the middle ear to the oval window. Senorineural deafness is due to deficiencies in the cochlea, auditory cortex or in between that prevent hearing. Conductive deafness can be caused by middle-ear damage by otitis media, or otosclerosis. Exposure to excessively loud noise, or other incidents can cause sensorineural deafness. The hair cells of mammals cannot regenerate, even though the hair cells of reptiles and birds can. Why this is the case is an active field of research.

Sensorineural deafness can affect only a few frequencies, and as one gets older, it is common to lose the higher frequencies (18,000-20,000 Hertz). In fact, it is common to lose a large range of hearing, and this condition is called presbycusis, or age-related hearing loss. Hearing aids can help people with this problem.

The eyes and vision

The eyes transduce electromagnetic energy into nerve impulses. In humans, only a limited part of this spectrum can excite the photoreceptors. Wavelengths between 400-700 nanometers are the wavelengths of light that can be perceived by the human eye. The yellow tint of lens filters out UV light, but people who have had their lens removed are able to see UV light. Infrared light lacks sufficient energy to excite our receptors.

The structure of the eyeball is macroscopically simple. The outermost layer is a tough coat of connective tissue called the sclera, which can be seen externally as the white of the eye. The tissue of the sclera is continuous with the transparent cornea. Light passes through the cornea to enter the anterior chamber. Light then passes through an opening called the pupil, which is surrounded by a pigmented muscle known as the iris. After passing though the pupil, light enters the lens. The iris is capable of increasing or decreasing its diameter, like the diaphragm of a camera to admit more or less light. Constriction of the pupils is produced by contraction of circular muscles within the iris. Dilation of the pupil is produced by contraction of the radial muscles. Constriction of the pupils is the result of parasympathetic stimulation, whereas dilation is due to sympathetic stimulation.

The posterior part of the iris contains a pigmented epithelium that gives the iris its color. The color of the eye is determined by the amount of pigment. Blue eyes have the least amount of pigment, brown eyes have even more and black eyes have the most. In albino individuals, the complete lack of pigment causes the eyes to be pink, since the absence of pigment allows blood vessels to be visible.

The lens of the eye is suspended from a muscular process called the ciliary body, which connects the sclera and encircles the lens. Zonular fibers suspend the lens from the ciliary body, forming a suspensory ligament that supports the lens. The space between the cornea and iris is the anterior chamber and the space between the iris and the lens is the posterior chamber. The anterior and posterior chambers are filled with a fluid called aqueous humor. This fluid is secreted by the ciliary body into the posterior chamber, where it provides nourishment to the avascular lens and cornea. The aqueous humor is drained from the anterior chamber into the canal of Schlemm, which returns it to the venous blood. Inadequate drainage of aqueous humor can lead to excessive accumulation of fluid, which causes increased intraocular pressure. This condition is called glaucoma, and can produce serious damage to the retina and our vision.

Behind the lens lies a compartment called the vitreous chamber, that is filled with a thick viscous substance called vitreous humor. Light that passes through the vitreous humor enters the neural layer, which contains photoreceptors at the back of the eye. This neural layer is called the retina. A darkly pigmented choroid layer underneath absorbs light that passes through the retina. While passing through the retina, some of the light activates photoreceptors, which in turn activate other receptors. Neurons in the retina contribute fibers that are gathered together at a region called the optic disc, where they exit the retina as the optic nerve. This region lacks photoreceptors and the known as the blind spot. The optic disc is the site of entry and exit of blood vessels.

When light passes from a medium of one density into another medium of a different density, the light is bent or refracted. The degree of refraction depends upon the comparative densities of the two media, which is indicated by the refractive indices of the media. The refractive index of air is set at 1.00 as a standard and all other media are set relative to air. The refractive index of the cornea is 1.38 and the refractive index of the lens is 1.40. However, the light has to pass through the aqueous humor, which has a refractive index of 1.33. Since the greatest difference in refractive index occurs at the air-cornea interface, the light is refracted most at the cornea. The degree of refraction also depends on the curvature of the lens. The curvature of the cornea remains constant, but that of the lens can vary. Contraction of the ciliary muscle narrows the aperture of the ciliary body and reduces the tension on the zonular fibers that suspend the lens. Relaxation of the ciliary muscle places tension on the zonular fibers of the suspensor ligament that pulls the pens taut. When an object is viewed far away, the ciliary muscles relax, and the lens is at is relaxed, least convex form. As an object is moved closer to the eye, the muscles of the ciliary body contract and relax the suspensor ligament, which makes the lens more spherical. This process, by which the lens changes shapes to allow visual examination of objects at different distances, is called accommodation.

The retina

The retina consists of a single-cell-thick pigmented epithelium. This epithelium is full of two types of photoreceptor neurons, the cone cell and the rod cells. The neural layers of the retina are actually forward extensions of the brain. In this sense, the optic nerve is a tract. The myelin sheath of the optic nerve is derived from oligodendrocytes instead of Schwann cells.

The neural layers of the retina face towards the incoming light. The light passes several neural layers before striking the photoreceptors. The photoreceptors synapse with other neurons and conduct nerve impulses outward in the retina. The outer layers of the retina that contribute axons to the optic nerve are called ganglion cells. These neurons receive synaptic input from bipolar cells, which lie underneath. The bipolar cells receive input from the rods and cones underneath them. Neurons called horizontal cells synapse with several photoreceptors. Neurons called amacrine cells synapse with several ganglion cells.

The activation of the photoreceptors is caused by a chemical change that occurs in response to a photon of light. Rods contain a pigment called rhodopsin. Rhodopsin appears purple (a combination of red and blue) because it transmits light in the red and blue regions of the spectrum and absorbs light energy in the green region. The wavelength of light that is absorbed best is at about 500 nm, and this point is called the absorption maximum. Green objects are seen more easily at night, when rods are used for vision, than red objects. Red light is not absorbed by rhodopsin, and only absorbed light can generate the chemical reaction that results in vision. In response to absorbed light, rhodopsin dissociates into its two components; the pigment retinal and a protein called opsin. This reaction is known as the bleaching reaction.

Retinal is capable to existing is one of two conformations, 11-cis-retinal and 11-trans-retinal.

Dark adaptation occurs when a light-adapted person enters a dark room. Rod and cone cells synthesize more pigments, and this allows the person to see better in conditions of low light intensity. Full adaptation occurs after about 20 minutes, but slight dark adaptation can be noticed after five minutes. Because retinal is a derivative of vitamin A, deficiencies in vitamin A can lead to severe night blindness. Other changes in the rods also cause dark adaptation, but these changes are poorly understood.

Within the retina, only the ganglion and amacrine cells are capable of generating an all-or-nothing action potential. The photoreceptors, bipolar cells and horizontal cells produce graded potentials or hyperpolarizations that are analogous to ESPSs or ISPSs. Nerve transmission to the ganglion cells is unusual in that the photoreceptors release an inhibitory neurotransmitter that prevents the bipolar cells from exciting the ganglion cells. Once light strikes the photoreceptor cells, the inhibitory neurotransmitter is no longer released, and the bipolar cells are free to stimulate the ganglion cells.

The membranes of both cone and rod cells contain a huge number of sodium channels that remain open in the dark. The large influx of sodium ions in the absence of light stimulation results is what is known as the dark current. The dark current keeps the photoreceptor depolarized and induces the release of an inhibitory neurotransmitter, which prevents the bipolar cell from exciting the ganglion cell. Once light strikes the photoreceptor, the isomerization of retinal occurs, and the dissociation of the retinal from opsin causes opsin to activate a special G-protein called transducin. The a subunit of transducin activates the enzyme phosphodiesterase, which degrades cyclic guanosine monophosphate (cGMP). Cyclic GMP is needed to keep the sodium channels open, and the decrease in intracellular cGMP causes the sodium channels to close. This hyperpolarizes the membrane of the photoreceptor, which ceases the release of the inhibitory neurotransmitter. Without the inhibitory neurotransmitter, the bipolar cell forms a graded potential that excites the ganglion cell.

Cones are less sensitive to light than rods, but they provide color vision and greater visual acuity. During the day the rods are bleached out by daylight, and the cones provide color vision and high acuity. Our perception of colors is due to stimulation of three types of cones. Each type of cone contains retinal, but the opsin with which retinal is associated in cone cells is special, and is called photopsin. There are three types of photopsins and these three different photopsins are able to absorb light at three different frequencies. Furthermore, cone cells carry one but not the other two photopsins. Therefore, cone cells are specialized to respond to one range of light frequencies, and therefore, one range of colors. These cones are designated blue, red, and green cones. The blue cones contain a photopsin whose absorption maximum is about 430 nm. The absorption maximum of the green cones is approximately 540 nm, and that of the red cones is approximately 585 nm. Our perception of any given color is due to an activation of one or a combination of these three different types of cones. This theory of color vision is called the trichromatic theory of color vision. This explains why red lights in photographic darkrooms allow sight without affecting the dark adaptation of the eyes, since red light allows the red cones to detect the vision without bleaching the rods.

The connections of rod and cone cells to the bipolar and ganglion cells differ, and these differences are the reasons that rods are able to maximize sensitivity while sacrificing acuity, while cone cells sacrifice intensity for acuity. Within the retina, several rod cells synapse with one bipolar cell, and more than one bipolar cell synapses with one ganglion cell. This way, the inputs of several rod cells are given to just one ganglion cell, and this is the reason that rod cells are able to derive a visual signal from very little light. Cone cells only synapse with one bipolar cell, which synapses with one ganglion cell. This way, the ganglion cell transmits the input of each individual cone cell, and this is the reason for the great visual acuity of the cone cells. However, because cone cells are only giving the input of one photoreceptor cell to the ganglion cell, the signal is weaker and this means that cone cells need much more light to generate a signal.

During the daylight, people will usually look at something with the center of their gaze. This center of their gaze is an attempt to focus the image upon a very cone-rich portion of the retina called the fovea centralis. Because of the high ration of cones to rods at this portion of the retina, daylight vision is the best when the image is focussed on this part of the eye.

The activation of the ganglion cells brings the image to the brain by means of the optic nerve. The optic nerve crosses at the optic chiasm, which is just anterior to the hypothalamus. Even though the optic nerve decussates at the optic chiasm, not all fibers cross over. Therefore each side of the cerebral cortex receives information from both eyes. The axons from the ganglion cells in the left temporal half of the eye pass to the left lateral geniculate nucleus of the thalamus. Axons from the ganglion cells from the nasal half of the right retina decussate and also synapse with the left lateral geniculate nucleus. The right lateral geniculate nucleus receives input from both eyes relating to the left half of the visual field. Neurons in both lateral geniculate bodies of the thalamus project to the striate cortex of the occipital lobe in the cerebral cortex. The neurons here project to the more anterior areas of the occipital lobe, which are known as the visual association areas. A small minority of the fibers from the retina goes to the superior colliculus, where they activate motor pathways that lead to eye and body movement. This is known as the tectal system.
 

Muscles of the Eye
Muscle	Innervation	Action
Superior rectus	Oculomotor nerve	Rotates eye upward and toward midline
Inferior rectus	Oculomotor nerve	Rotates eye downward and toward midline
Medial rectus	Oculomotor nerve	Rotates eye toward midline
Lateral rectus	Abducens nerve	Rotates eye away from midline
Superior oblique	Trochlear nerve	Rotates eye downward and away from midline
Inferior oblique	Oculomotor nerve	Rotates eye upward and away from midline
Ciliary muscles (smooth)	Oculomotor, parasympathetic fibers	Cause suspensory ligaments to relax
Iris, circular muscles (smooth)	Oculomotor, parasympathetic fibers	Cause pupil to constrict
Iris, radial muscles (smooth)	Sympathetic fibers	Cause pupil to dilate