BIOLOGY Volume 2 - a guide to general biology - 2004

17. COORDINATION AND REGULATION IN ANIMALS

17.5. Structure and function of receptors

17.5.4. The mammalian ear

The mammalian ear is a sensory organ containing mechanoreceptors sensitive to gravity, movement in space, and sound. The movement and position of the HEAD relative to gravity are detected by the vestibular apparatus, which consists of the semicircular canals¹, the saccule, and the utricle. The remaining structures of the ear are designed to receive, amplify, and transduce sound waves into electrical impulses, which, upon reaching the auditory areas of the Brain, produce auditory sensations.

Structure and function of the ear

The ear consists of three regions, each specialized to perform its own function (Fig. 17.41). The outer ear is formed by the pinna with supporting elastic Cartilage inside. The pinna, acting like a funnel, collects and directs sound waves into the external auditory meatus. Sound waves cause vibrations of the tympanic membrane, which separates the outer ear from the Middle ear. In the middle ear, there are three auditory ossicles—the malleus, incus, and stapes—which transmit vibrations from the tympanic membrane (60 mm²) to the membrane of the oval window (fenestra vestibuli) with an area of 3.2 mm². The geometry of this system and The ratio of these areas result in the sound wave pressure being amplified 22-fold. The middle ear is filled with air and connects to the Pharynx via the Eustachian (auditory) tube. This protects the tympanic membrane from damage during changes in atmospheric pressure. The oval window leads into the Inner ear, or Vestibulocochlear Organ, which is a complex system of canals and cavities forming a bony labyrinth (within the Temporal bone) filled with a fluid called perilymph; inside the bony labyrinth lies the membranous labyrinth, which mirrors its shape and whose sacs and ducts contain endolymph. Receptor Cells and their associated accessory structures are also located here. Auditory receptors are situated in the cochlea, while the receptors for balance and movement—the vestibular apparatus—are located in the vestibule (from the Latin vestibulum) within the saccule, utricle, and ampullae of the semicircular ducts. The middle ear is separated from the inner ear by two membranes—the oval window (fenestra vestibuli), against which the stapes rests, and the round window (fenestra cochleae). Perilymph lies between them.

Class="center">

Fig. 17.41. Schematic diagram of the main structures of the mammalian ear forming the Organs of Hearing and Balance (not to scale).

The Nature of sound

Sound is created by the vibration of particles in a medium and propagates as waves—alternating regions of high and low pressure—in liquids, gases, and solids. The number of waves occurring per unit of time is called the frequency of vibration, which in the case of sound is proportional to its pitch (high, low, etc.). The human ear is sensitive to frequencies in the range of 40–20,000 Hz (cycles per second). In dogs, this range reaches 40,000 Hz, and in bats, 100,000 Hz. The frequency of human speech ranges from 500 to 3,000 Hz, and sensitivity to high pitches declines with age.

Actual sounds usually consist of many different tones. For example, the same note played on a violin and a trumpet, say middle C, corresponds to the same fundamental frequency of 256 Hz, yet the sounds are different. This is due to the presence of additional tones (overtones, or harmonics) produced by the instrument, which determine the quality of the sound, or its timbre. The same principle applies to the human voice and determines its individual sound.

The intensity (loudness) of sound depends on the amplitude of the sound waves emitted by their source and is a measure of the energy they contain.

The cochlea and hearing

The cochlea is a spiral canal 35 mm long, divided along its length by membranes into three compartments (Fig. 17.42).

Fig. 17.42. Diagram of a cross-section of the cochlea showing The Organ of Corti (spiral organ).

The scala tympani and scala vestibuli are filled with perilymph and communicate with each other at the blind end of the cochlea through a small opening. Between them lies the cochlear duct (scala media), triangular in cross-section, which contains endolymph. It is separated from the scala tympani by the basilar membrane, which Supports Hair receptor cells whose hairs (cilia) brush against the tectorial membrane. This system, consisting of the basilar membrane, sensory (hair) cells, and the tectorial membrane, is called the organ of Corti (spiral organ). This is where the Transduction of sound waves into electrical impulses takes place.

Sound waves transmitted from the tympanic membrane to the oval window set the perilymph (inside the scala vestibuli) in motion, which in turn, via Reissner's (vestibular) membrane, moves the endolymph in the cochlear duct. Its vibration is transmitted through the basilar membrane back to the perilymph (scala tympani) and is finally dissipated in the air of the middle ear (tympanic cavity) As a result of the resulting vibrations of the round window.

The precise mechanism by which pressure waves are converted into nerve impulses is unknown, but it is believed to be based on the relative movement of the tectorial membrane and the basilar membrane of the organ of Corti. As a result of the vibrations of the basilar membrane caused by pressure waves, the two membranes slide relative to each other, causing the sensory hairs to shear against the tectorial membrane. The resulting deformation of the sensory hairs leads to the depolarization of the sensory cells and the generation of receptor potentials, which initiate action potentials in the axons of the auditory nerve.

Discrimination of pitch and intensity

The ability to distinguish pitch depends on the fact that sounds of a given frequency cause vibrations of the basilar membrane and excitation of sensory cells in a highly specific region of the organ of Corti. The excited cells send signals to the corresponding areas of the auditory cortex of the brain, where the auditory sensation is produced. As one moves away from the Base of the cochlea toward its apex, the basilar membrane becomes wider and more flexible, and its sensitivity changes such that only low-frequency sounds reach the apex. Therefore, high-frequency sounds stimulate receptors only at the base of the cochlea, while low-frequency sounds stimulate them only at its apex. A pure tone, which is a sound of a single frequency, will stimulate only one small area of the basilar membrane; however, most sounds consist of different frequencies and stimulate many areas simultaneously. The auditory cortex integrates the signals coming from different PARTS OF THE basilar membrane, resulting in the perception of a single blended sound.

The discrimination of intensity, or loudness, of a sound depends on the fact that each region of the basilar membrane contains a set of sensory cells with different thresholds of sensitivity to vibration. For example, a quiet sound of a given pitch stimulates only a few Hair cells, whereas a louder sound of the same frequency will also excite other cells with a higher threshold for vibration. Ultimately, at the level of nerve fibers, this leads to spatial summation of excitation.

Balance

Maintaining balance at rest and during movement is due to the continuous flow of sensory signals to the brain regarding THE POSITION OF various body parts. Impulses from proprioceptors in the Muscles and joints report the position and state of the limbs, but vital information about the position and movement of the head comes from the vestibular apparatus of the inner ear, which consists of the utricle, saccule, and semicircular canals.

As in the cochlea, the receptors here are ciliated hair cells located on dense structures bathed in endolymph. Head movement causes deflection of the hairs, generating a generator potential in the hair cells.

In the saccule and utricle, the hair cells are clustered in localized areas called maculae. The hairs of these cells in each sac are embedded in a shared gelatinous mass containing calcium carbonate granules—the so-called otoconia or otoliths (Fig. 17.43). Deflected by gravity during head movement, these granules bend the hairs, allowing the sensory cells to inform the brain of the head's position relative to the vertical.

Fig. 17.43. Scanning electron micrograph of the inner ear, showing the macula—a sensory area of one part of the vestibular apparatus (Organ of Balance), namely the saccule. Supporting cells (general Background) can be seen with receptor hair cells (long projections) interspersed among them. When the head tilts sideways, calcium carbonate crystals called otoconia (the polyhedral structure between the projections in the lower part) shift with the endolymph under METABOLISM/18.html">The Influence of gravity and stimulate the hair cells. Photograph taken using a Scanning Electron microscope.

The utricle responds to Head movements in the vertical plane, and the otoconia produce maximum stimulation when they pull the receptor hairs downward, for example, when the body is upside down (Fig. 17.44).

Fig. 17.44. Effect of head position on The activity of receptor cells in the utricle and saccule.

The saccule responds to lateral tilts of the head. When the head is upright, the hair Cells of the saccule are oriented horizontally. When the head tilts to the left, a different response is generated in the left and right saccules. The left saccule experiences increased stimulation as the otoconia pull the hairs downward, while stimulation on the right side decreases. This leads to A change in the impulses sent to the Cerebellum, allowing the position of the head to be perceived.

The three semicircular canals are oriented in three mutually perpendicular planes. They detect the direction and rate of change in head position. At the base of each canal is an enlargement—the ampulla, containing a conical gelatinous mass called the cupula. The cupula covers the hairs of the receptor cells and extends almost to the opposite wall of the ampulla (Fig. 17.45). During rapid head rotations, the endolymph lags behind the cupulae due to inertia, displacing them in the direction opposite to the movement. This causes the cilia to bend, to which the receptors respond with a generator potential that triggers action potentials in sensory Neurons. Hair cells of both the maculae and cupulae respond to linear acceleration.

Fig. 17.45. Diagram of The structure of the semicircular duct ampulla (cross section).

¹ Strictly speaking, the canals are bony cavities containing the membranous semicircular ducts (similar to the sacs), which actually contain the receptor cells. — Translator's Note.