Sense Organs – Eye and Ear

Sense Organs – Eye and Ear

Introduction

·       Sense organs are specialized structures that allow the body to perceive and interpret stimuli from the external and internal environment.

·       Humans have five primary sense organs – eye (vision), ear (hearing and balance), nose (smell), tongue (taste), and skin (touch).

·       Among them, the eye and ear are highly complex sensory organs

·       The eye is the organ of sight, detecting light and converting it into electrical signals interpreted by the brain as vision.

·       The ear serves dual functions: hearing (audition) and equilibrium (balance) by detecting sound waves and changes in body position.

Eye – Anatomy and Physiology of Sight

Anatomy of the Eye

The human eye is a spherical, hollow organ about 2.5 cm in diameter, located in the orbit of the skull, protected by eyelids, eyelashes, conjunctiva, and tears.

Main Parts of the Eye

1. Protective Structures
• Eyelids – protect against injury, regulate light entry.
• Eyelashes & Eyebrows – protect from dust, sweat, and excessive light.
• Conjunctiva – thin transparent membrane lining eyelids and anterior sclera.
• Lacrimal apparatus – produces tears (contain lysozyme, an antibacterial enzyme).
2. Layers of the Eyeball
   The eyeball wall has three coats:
• Fibrous layer (outermost)
• Sclera – white opaque covering, maintains shape, provides attachment to eye muscles.
• Cornea – transparent, avascular dome at the front; refracts most of the light entering the eye.
• Vascular layer (middle, uvea)
• Choroid – pigmented, highly vascular, nourishes retina, absorbs excess light.
• Ciliary body – contains ciliary muscles (control lens shape for accommodation) and ciliary processes (secrete aqueous humor).
• Iris – colored part of the eye with central pupil (regulates light entry).
• Nervous layer (innermost)
• Retina – light-sensitive layer containing photoreceptors:
• Rods – for dim light and peripheral vision.
• Cones – for color vision and visual acuity.
• Contains macula lutea (yellow spot) with fovea centralis (highest visual acuity).
• Retina is connected to the brain via the optic nerve.
3. Other Structures
• Lens – biconvex, transparent structure behind iris; focuses light on retina by changing curvature.
• Humors:
• Aqueous humor – clear fluid between cornea and lens; nourishes cornea and lens, maintains intraocular pressure.
• Vitreous humor – gel-like substance filling space behind lens; maintains shape of eye and presses retina against choroid.

Physiology of Sight (Mechanism of Vision)

1. Refraction of Light
• Light rays enter through cornea → aqueous humor → pupil → lens → vitreous humor → retina.
• Cornea does most refraction, lens fine-tunes focus.
2. Accommodation
• Adjustment of lens curvature by ciliary muscles to focus near or distant objects.
• Near vision – lens becomes more convex.
• Distant vision – lens flattens.
3. Pupil Regulation
• Iris regulates pupil size (constriction in bright light, dilation in dim light).
4. Phototransduction
• Light stimulates rods and cones → converted into electrical impulses.
• Rods: contain rhodopsin (visual purple) for dim light.
• Cones: contain iodopsin for color vision (red, green, blue).
5. Visual Pathway
• Photoreceptors → bipolar cells → ganglion cells → optic nerve → optic chiasma → optic tract → lateral geniculate body of thalamus → optic radiations → visual cortex (occipital lobe).
• Brain interprets signals as vision.

Ear – Anatomy and Physiology of Hearing

Anatomy of the Ear

The ear has two main functions: hearing and balance. It is divided into three parts:

1. External Ear
• Auricle (Pinna) – collects sound waves.
• External auditory canal – directs sound to tympanic membrane; contains ceruminous glands (earwax).
• Tympanic membrane (eardrum) – vibrates with incoming sound waves.
2. Middle Ear
• Air-filled cavity connected to pharynx via Eustachian tube (equalizes pressure).
• Contains ossicles (ear bones):
• Malleus (hammer), Incus (anvil), Stapes (stirrup).
• Transmit and amplify vibrations from tympanic membrane to oval window.
3. Inner Ear (Labyrinth)
• Bony labyrinth (filled with perilymph) and membranous labyrinth (filled with endolymph).
• Divided into:
• Cochlea (hearing) – spiral-shaped, contains organ of Corti with hair cells (mechanoreceptors) that convert vibrations into nerve impulses.
• Vestibule (balance) – contains utricle and saccule (detect linear acceleration, gravity).
• Semicircular canals (balance) – three canals at right angles, detect rotational movements.

Physiology of Hearing (Mechanism of Audition)

1. Sound Conduction
• Sound waves enter auricle → external auditory canal → vibrate tympanic membrane.
• Ossicles (malleus → incus → stapes) amplify vibrations and transmit them to oval window.
2. Wave Transmission
• Vibrations enter cochlea via oval window → create pressure waves in perilymph and endolymph.
• These waves displace the basilar membrane where the organ of Corti lies.
3. Sound Transduction
• Movement of basilar membrane bends hair cells in organ of Corti.
• Bending generates nerve impulses in cochlear nerve fibers.
4. Auditory Pathway
• Hair cells → cochlear nerve → vestibulocochlear nerve (cranial nerve VIII) → cochlear nuclei in brainstem → thalamus → auditory cortex (temporal lobe).
• Brain interprets frequency, intensity, and quality of sound.

Physiology of Balance (Vestibular Function)

• Static equilibrium – maintained by utricle & saccule (detect head position and linear acceleration).
• Dynamic equilibrium – maintained by semicircular canals (detect rotational movements).
• Impulses from vestibular apparatus travel via vestibular nerve to cerebellum and brainstem for postural control.

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