**4.1 The cochlea**

The bony cochlea lies in front of the vestibule and has an external appearance rather like the shell of a snail. The shell has approximately two and one half turns and its height is about 5mm while the greatest distance across the base is about 9 mm.

Fig. 1. Cross section of the cochlea (SOURCE: www.nap.edu/openbook. page=35 Modified from Davis and Associates (1953)

Hearing Loss in Minor Head Injury 143

the organ of corti by several types of specialized, highly differentiated cells. These are the pillar cells, Dieters cells and Hensen's cells. In the fetus and the newborn there are about

It is the most commonly used method of measuring hearing acuity. It is a subjective test. The frequencies usually tested are at octave steps i.e., 125, 250, 500, 1000, 2000, 4000, 8000 Hz. A pure tone audiometer is an electronic instrument capable of producing pure tone sound of different frequencies at variable intensities. It helps in qualitative and quantitative diagnosis

It is the measurement of acoustic emissions in the external auditory meatus as a function of air pressure within the external auditory meatus. It provides a rapid atraumatic and objective technique for evaluating the integrity of (a) Middle ear transmission system, (b) Estimating middle ear pressure, (c) Estimating volume of ear canal or middle ear, (d)

Type A Tympanogram indicate normal middle ear pressure as indicated by tympanogram peak at 0 daPa. Normally middle ear pressure typically falls between +50 and –100 daPa.

Volume measurements more than 2 ml in children and 2.5ml in adults are usually indicative

Mild cochlear hearing loss has little effect on acoustic reflex thresholds for tonal stimuli. for

As early as 1948, Gold (Gold.T,1948) discovered that the outer hair cells of the cochlea could produce energy by an active mechanical process. However it was not until 1978 that Kemp (Kemp 1978) by a series of basic and clinical experiments demonstrated that the cochlea was capable of producing low intensity recordable sounds called oto-acoustic emissions (Fig-3). Oto-acoustic emissions (OAEs) can be defined as the audio frequency energy which originates in and is released from the cochlea, transmitted through the ossicular chain and tympanic membrane and measured in the external auditory meatus. They can occur either spontaneous or in response to acoustic stimulation. OAEs are believed to reflect the active biomechanical movement of the basilar membrane of the cochlea (Fig-4). This retrograde traveling wave is thought to be responsible for the sensitivity, frequency selectivity and wide dynamic range of the normal auditory system. Oto-acoustic emissions (OAEs) are believed to be the by product of pre-neural mechanisms of the cochlear amplifier and in particular, to be linked to the normal functioning of the outer hair cells. Oto-acoustic emissions are vulnerable to a variety of agents such as acoustic trauma (Hamernik RP, 1996) hypoxia, (Rebillard.G Lavigne& Rebillard.M) and oto-toxic medications (Ress .B D et al,

of tympanic membrane perforation or patent pressure equalization tube.

patients with hearing losses that exceed 70 dB reflexes are typically absent.

1999) that cause hearing loss by damaging outer hair cells.

3500 inner hair cells and 13000 outer hair cells.

**5. Audiometric tests 5.1 Pure tone audiometry** 

of hearing loss.

**5.2 Tympanometry** 

Evaluating Eustachian tube function.

**5.3 Oto-acoustic emissions** 

The basilar membrane which separates the scala media from the scala tympani consists of connective tissue fibers embedded in an acellular matrix. The organ of corti, tectorial membrane along with the basilar membrane makes up the cochlear partition. (Fig-1).

Fig. 2. Organ of corti (Nolte (1993) The Human Brain 3rd Ed. Fig. 9-34B, p. 213. Cross-section through the Organ of Corti)

The organ of corti (fig-2) is a ridge like structure containing the auditory sensory cells and a complex arrangement of supporting cells. The sensory cells are arranged in two distinct groups as inner and outer hair cells. There is a single row of inner hair cells, although occasionally extra hair cells may be apparent, and also three, four or five irregular rows of outer hair cells, with frequent gaps where individual hair cells are absent. Each hair cell consists of a body, which lies with in the organ of corti, and a thickened upper surface called the cuticular plate, from which projects a cluster of stereocilia or hairs. The stereocilia contains a core of actin molecules packed in a para crystalline array and covered with a cell membrane. The stereocilia are connected to each other along the sides by fine filaments called the side links. The tip of each stereocilium is connected to the sides of the next tallest stereocilium by a longer filament known as a tip link. The body of the inner hair cells is flask shaped, with a small apex and large cell body. The long axis of cell is inclined towards the tunnel of corti, and nerve fibers and nerve endings are located around the lower half of the body. The stereocilia projecting from the thickened cuticular plate are arranged in two or three rows parallel to the axis of the cochlear duct. The body of the outer hair cell is cylindrical with the nucleus lying close to the lower pole, where afferent and efferent nerve endings are attached. There are several rows of stereocilia but the configuration varies from a W shape at the base, through a V shape in the middle coil, to almost a linear array at the apex. The number of stereocilia also decreases in the passage from base to apex, where as the length increases, although not in a linear fashion. The hair cells are supported with in

The basilar membrane which separates the scala media from the scala tympani consists of connective tissue fibers embedded in an acellular matrix. The organ of corti, tectorial

Fig. 2. Organ of corti (Nolte (1993) The Human Brain 3rd Ed. Fig. 9-34B, p. 213. Cross-section

The organ of corti (fig-2) is a ridge like structure containing the auditory sensory cells and a complex arrangement of supporting cells. The sensory cells are arranged in two distinct groups as inner and outer hair cells. There is a single row of inner hair cells, although occasionally extra hair cells may be apparent, and also three, four or five irregular rows of outer hair cells, with frequent gaps where individual hair cells are absent. Each hair cell consists of a body, which lies with in the organ of corti, and a thickened upper surface called the cuticular plate, from which projects a cluster of stereocilia or hairs. The stereocilia contains a core of actin molecules packed in a para crystalline array and covered with a cell membrane. The stereocilia are connected to each other along the sides by fine filaments called the side links. The tip of each stereocilium is connected to the sides of the next tallest stereocilium by a longer filament known as a tip link. The body of the inner hair cells is flask shaped, with a small apex and large cell body. The long axis of cell is inclined towards the tunnel of corti, and nerve fibers and nerve endings are located around the lower half of the body. The stereocilia projecting from the thickened cuticular plate are arranged in two or three rows parallel to the axis of the cochlear duct. The body of the outer hair cell is cylindrical with the nucleus lying close to the lower pole, where afferent and efferent nerve endings are attached. There are several rows of stereocilia but the configuration varies from a W shape at the base, through a V shape in the middle coil, to almost a linear array at the apex. The number of stereocilia also decreases in the passage from base to apex, where as the length increases, although not in a linear fashion. The hair cells are supported with in

through the Organ of Corti)

membrane along with the basilar membrane makes up the cochlear partition. (Fig-1).

the organ of corti by several types of specialized, highly differentiated cells. These are the pillar cells, Dieters cells and Hensen's cells. In the fetus and the newborn there are about 3500 inner hair cells and 13000 outer hair cells.
