Neural Control and Coordination

Elementary structure and functions of eye and ear

  • The organ of sight in human is located in the orbits / eye sockets
  • Three concentric layers of tissues in the eye are (i) – Fibrous coat, (ii) Vascular coat and (iii) Neural coating.
  • Sclera, cornea and conjunctiva forms the outermost fibrous layers, named so, as they are not is contact with blood vessel and hence cornea transplant became very successful as graft rejection is not involved.
  • Sclera is known as the white of the eye and it’s the largest outermost covering layer of eye.
  • Beneath the sclera, there is a pigmented layer called choroid that nourishes retina as it is rich in blood vessels
  • Ciliary body emerges out from choroid layer and it holds the lens at a fixed position, by attaching lens through a suspensory ligament.
  • Iris is known as colour of the eye that consists of radial and longitudinal muscles to contract and expand the pupil.
  • Sympathetic nervous system causes radial muscles to contract for pupillary dilation.
  • Para-sympathetic nervous system promote longitudinal muscles to contract for pupillary contraction.
  • Innermost nervous layer in composed of retinal layer, which has 7-8 layers out of which, the most important layer is the layer of rods and cones, named after their shape.
  • Photopic vision (daylight) is the function of cones and scotopic vision (dark)(or) twilight vision is the function of rods.
  • Opposite to the lens in the retinal layer there is a shallow depression called fovea centralis found over macula lutea , is a location where large numbers of coned cells than rods are found. This spot is to be referred as yellow spot.
  • The point of departure of optic nerve through the retina does not have any sensory cells and is termed as optic disc (or) blind spot.
  • Resolution of the eye is greatest at the fovea centralis.
  • Rods as well as cones are made up of 2 parts – (i) opsin (Rhodopsin/scotopsin in rods; photopsin in cones) and (ii) Retinene, (Retinal – aldehyde parts of vitamin A).
  • External light induces dissociation of retinal from opsin, which result in change is the opsins structure that produces a signal that generates action potentials in ganglion cells.
  • These Action potentials are transmitted by the optic nerves to the visual cortex area of the brain located in the occipital lobe.
  • Two important functions of ear are (i) – Hearing [Cochlea] and (ii) Body balance [vestibular apparatus].
  • The three sections of ear are (i) outer – Pinna; (ii) middle ear and (iii) Inner ear.
  • External ear is consiodered to be vestigial that contains a soft portion called lobule.
  • It collects the sound from environment and targets them into external auditory canal. It contains ceruminous glands to produce wax (Cerumen) that protects from foreign particles like dust, fungus, etc.
  • Middle ear is an air filled cavity that possess ear ossicles and a tympanic membrane covering the ear drug / tympanum.
  • The three ear ossicles present in each ear are (i) Malleus, (ii) Incus and (iii) stapes [which are in Hammer, Anvil and stirrup shape].
  • The smallest bone in human body is stapes and surrounding it there is a smallest muscle of body called stapedius
  • Ear ossicles amplify (or) increases the intensity of sound by 200 times, whereas it does not change the frequency of sound waves.
  • The internal ear [with 2 openings namely fenestra ovalis (oval window) and Fenestra rotunda (round window)] possess 2 parts called as cochlea and vestibular apparatus.
  • Inner ear is delicate and irregular in nature called as labyrinths, which are of 2 types (i) bony labyrinth, (ii) membranous labyrinth
  • Membranous labyrinth is filled with endolymph and the space separating membranous and bony labyrinth [Perilymphatic space] is filled with perilymph.
  • Cochlea contains 3 fluid filled chambers (i) – upper scala vestibuli, (ii) middle scala media and (iii) lower scala tympani.
  • Also cochlea posses three membranes (i) upper reissner’s membrane, (ii) middle basilar membrane and (iii) Tectorial membrane emerges from the basilar membrane.
  • On the basilar membrane, organ of corti is present that contains numerous type of neural cells such as pillar cells, hair cells etc.
  • The tips of the hair are embedded in a thin elastic membrane called tectorial membrane.
  • The sound vibrations induces ripple in the basilar membrane which tend to bend the hair cells of organ of corti that presses against tectorial membrane, creating an action potential in the bipolar neurons that sends the signal to auditory cortex located in the temporal lobe of brain.
  • Vestibular apparatus consists of 3 semi – circular canals, whose base is called ampulla that contain sensory patch of cells, called crista.
  • Utricle and saccule are the 2 structures of vestibule that posses other types of sensory cells called macula that posses odith membrane (CaCO3) for its support. Both crista and macula help in maintain posture and body balance

Watch this video for the topic from 1:30 to 44:31

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