Special Senses: Hearing And Balance
The young healthy human can detect sound wave frequencies of between 40 Hz and 20 kHz, but the upper frequency limit declines with age. When sound waves reach the ear, they pass down the external auditory meatus (the external ear) to the tympanic membrane that vibrates at a frequency and strength determined by the magnitude and pitch of the sound. The vibration of the membrane causes three ear ossicles (malleus, incus and stapes) in the middle ear (an air-filled cavity) to move, which, in turn, displaces fluid within the cochlea (the inner ear) as the foot of the stapes moves the oval window at the base of the cochlea. This mechanical link prevents the incoming sound energy from being reflected back, and the ossicles improve the efficiency with which the sound energy is transferred from the air to the fluid. Small muscles are attached to the ossicles and contract reflexly in response to loud sounds, thereby dampening the vibration and attenuating the transmission of the sound (Fig. 58a).
The inner ear includes the cochlea and also the vestibular organs responsible for balance (see later). The receptors involved in both hearing and balance are specialized mechanoreceptors called hair cells. Projecting from the apical surface of the hair cell is a bundle of over 100 small hair-like structures called stereocilia and a larger stere- ocilium called the kinocilium. Deflection of the stereocilia towards the kinocilium leads to a potential change in the cell (depolarization), the release of a transmitter substance from the base of the hair cell, and activation of the nerve fibres that convey impulses to the higher centres of the brain.
The cochlea comprises a coiled tube of about 3 cm in length (Fig. 58b), with three tubular canals running parallel to one another (scala vestibuli, scala media and scala tympani).
The scala vestibuli and the scala tympani contain perilymph (which is similar to extracellular fluid in composition), and the scala media contains endolymph (similar in composition to intracellular fluid). The scala vestibuli and scala tympani are joined at the tip of the coil (the helicotrema); at the base of the scala vestibuli is the oval window and at the base of the scala tympani is the round window, separating the fluid of the inner ear from the air in the middle ear.
The scala media lies between the two perilymph-filled canals; the boundary between it and the scala vestibuli is called Reissner’s mem- brane, and the boundary between it and the scala tympani is called the basilar membrane. On top of the basilar membrane sits the organ of Corti in which the hair cells are situated. There are around 15 000 hair cells distributed in rows along the basilar membrane. There are two types of hair cell: the inner hair cells which form a single row and the more numerous outer hair cells arranged in three rows. The hair cells are ideally placed to detect small amounts of movement of the basilar membrane. Because of the changing width of the basilar membrane, high-frequency sounds maximally displace the membrane at the base of the cochlea and low-frequency sounds maximally dis- place the membrane at the apical end of the cochlea.
The auditory signals are relayed through a complex series of nuclei in the brain stem and the thalamus, eventually reaching the primary auditory cortex in the temporal lobe of the cerebral cortex.
The system associated with balance is called the vestibular system and is not only involved with balance, but also postural reflexes and eye movements.
As mentioned earlier, the receptors involved in the vestibular system are hair cells. These hair cells are found in the inner ear in close proximity to the cochlea in two otolith organs called the utricle and saccule, and in a structure called the ampulla found in the three semicircular canals. The otolith organs are primarily involved in the detection of linear motion and static head position, and the semicircular canals in the detection of rotational movements of the head.
The four otolith organs (two on each side) each contain a structure called the macula which comprises a number of hair cells (Fig. 58c). With the head erect, the macula in each utricle is orientated horizontally and that in each saccule is orientated vertically. The base of each macula contains hair cells whose stereocilia project into a gelatinous mass called the otolith membrane. When the head is tilted, the force of gravity displaces the otolith membrane, thereby bending the stereocilia. The nerve fibres innervating the hair cells are spontaneously active: displacement in one direction increases firing and displacement in the opposite direction decreases firing of the neurones. The utricle sends signals representing forwards and backwards movements and the saccule conveys information about vertical movements.
The semicircular canals each contain an organ called the ampulla (Fig. 58d). They respond to rotational movement of the head, and the plane of each canal is perpendicular to the other two, so that, between all six (three on each side), they provide information relating to the rotational acceleration of the head during movement around any axis. Each canal contains endolymph and the ampulla comprises hair cells in which the stereocilia project into a gelatinous mass, with the same specific gravity as the endolymph, called the cupula. During acceleration in the plane of a particular canal, the endolymph tends to remain stationary because of inertia. The movement displaces the stereocilia and stimulation of the associated nerve fibres occurs. Again, movement in one direction increases firing of the nerves and movement in the opposite direction causes a decrease in firing. Vestibular afferent fibres from the auditory (VIII) nerve have their cell bodies in the vestibular ganglion and terminate in one of four vestibular nuclei in the medulla. These nuclei also receive inputs from neck muscle receptors and the visual system. They then project to a number of areas of the central nervous system, including the spinal cord, thalamus, cerebellum and oculomotor nuclei, where they are in-volved in posture, gait and eye movements. They also project to the primary somatosensory cortex and to the posterior parietal cortex.