Lecture 113 of 373: Sense Organs: Hearing and Organ of Equilibrium (25 mins) | CUET (Common University Entrance Test) PG Zoology (SCQP28) | Complete Video Course 373 Lectures [222 hrs : 42 mins]

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Statocyst- is an organ of equilibrium that detects animal՚s position and acceleration with respect to gravity. Present in invertebrates like lobsters, prawns etc. It consists of hollow fluid filled cavity lined with mechanoreceptors and has a statolith (grains + calcareous secretions). Statolith is a dense material which has higher specific gravity than surrounding fluid. As the position of the animal changes the statolith rests on different regions of statolith epithelium that activates the receptor cell.

Hair cells

Hair cells are the sensory mechanoreceptor cells in the vertebrate acoustico-lateralis system, which includes the vestibular organs (for balance and detection of acceleration), the lateral line system of surface receptors in fish and amphibians (which detect water flow as well as other stimuli), and the mammalian cochlea, an auditory organ.

A vertebrate hair cell is an epithelial cell, and like all epithelial cells, it has an apical surface that faces an overlying lumen and a basal surface that faces underlying tissues.

A hair cell has at its apical end a tuft of microvilli or stereocilia (not true cilia), arranged in an increasing order. These microvilli are collectively called a hair bundle because they resemble microscopic hairs.

Hair cells do not possess axons and do not generate action potentials. Instead, they release neurotransmitter substance onto afferent neurons that conduct action potentials into the CNS.

Hair cells of some species may also have a single true cilium, called a kinocilium but mammalian auditory hair cells lack it.

Displacement of the hair bundle toward the tallest of the stereocilia depolarizes the hair cell and increases the amount of neurotransmitter it releases.

Displacement toward the shortest of the stereocilia hyperpolarizes the cell, decreases the transmitter released from the resting tonic level, and decreases cranial sensory neuron activity

Human ear (for equilibrium)

The mammalian ear consists of three parts: an external ear distal to the eardrum, an air-filled middle ear, and a liquid-filled inner ear, which consists, in part, of the cochlea. It has vestibular organs too which have sensory functions of acceleration and balance and make use of hair cells.

Each vestibular organ comprises three semi-circular canals, which detect angular acceleration of the head and body, and two otolith organs, called the sacculus and utriculus, which detect linear movement and acceleration.

The cochlea is involved in hearing. All of these chambers are filled with fluid, and many are continuous with each other.

The three semi-circular canals on one side of the head are oriented at approximately right angles to one another.

At the base of each canal, a region called the ampulla contains a cluster of hair cells in a structure called the crista ampullaris.

Movement of head leads to movement of fluid inside the canals to push against the crista ampullaris and deflects the bundles of the hair cells, opening or closing mechanoreceptive channels as in the hair cells.

The utriculus and sacculus also contain hair cells. The hair cells in these structures are contained in a region called the macula

The macula is oriented approximately horizontally for the utriculus and vertically for the sacculus. The hair cells are covered by a gelatinous mass called the otolithic membrane into which the hair bundles protrude. A dense network of crystals of calcium carbonate lies on top of the otolithic membrane. The maculae are sensitive to orientation with respect to gravity as well as head movements. When the head moves, these crystals (collectively called an otolith) tend to lag behind as an inertial mass, causing the otolithic membrane to slide against the hair bundle. This deflects the hair bundle and produces a change in hair cell membrane potential.

Human ear (for hearing)

Sound-pressure waves vibrate the eardrum (tympanic membrane), and this vibration is transmitted to the membranous oval window of the inner ear by three middle-ear ossicles: the malleus, incus, and stapes.

The middle ear contains two muscles, the tensor tympani (associated with the eardrum) and the stapedius (associated with the stapes) to damp the movements of the ossicles, protecting the auditory membranes from damage by loud sounds.

The eustachian tube connects the middle ear with the pharynx, equalizing pressure in the middle ear with environmental pressure.

The major function of the middle-ear ossicles is to transfer sound energy from air to the liquid of the inner ear

The cochlea is a coiled tube containing chambers filled with fluid. Basilar membrane separates the cochlea into an upper chamber (scala vestubuli) and a lower chamber (scala tympani). As the oval window moves, it creates fluid movement in the cochlea. This movement vibrates the basilar membrane and stimulates the auditory hair cells that sit on it.

When the basilar membrane moves, it stimulates the hair cells in a region of the cochlea called the organ of Corti.

Specific groups of the hair cells respond to specific sound frequencies, and for their respective afferent neurons to send action potentials to the CNS.