16. SENSE ORGANS

SENSE ORGANS

Sense organs have specialised nerve cells which receive stimuli and convert them into appropriate nerve impulses. These nerve impulses are carried to the CNS by afferent or sensory nerve fibres. The sense organs – eyes, ears, tongue, skin, and nose – contain receptors that relay information through sensory neurons to appropriate places within the nervous system.

EYE (SENSE OF VISION)

Eyes are almost spherical in shape (diameter ~2.5 cm). They are situated in the orbital cavity, supplied by the optic nerve (II cranial nerve). Adipose tissue fills the space between the eye and orbital cavity.

Accessory Structures of Eyes

• Eyebrows: Resist sweat, dust, and foreign particles from entering eyes
• Eyelids: Upper and lower folds of skin protecting eyes; upper eyelid raised by levator palpebrae muscle
• Lachrymal Apparatus: Almond-shaped glands on lateral end of upper eyelids; produce tears with antibacterial and lubricating properties
• Extrinsic Muscles of Eye: Superior rectus (up), inferior rectus (down), medial rectus (inward), lateral rectus (outward), inferior oblique (clockwise), superior oblique (anticlockwise)

Layers of Eyeball

1. Outer Fibrous Layer (Sclera/Cornea): Sclera is white, smooth; cornea is transparent, replaces sclera over anterior 1/6th of eyeball
2. Middle Vascular Layer (Uveal Tract): Highly vascularised; consists of choroid, ciliary body, and iris; iris has central opening called pupil
3. Inner Nervous Tissue Layer (Retina): Contains rods (dim light, scotopic vision, rhodopsin) and cones (bright light, photopic vision, iodopsin)

Spots on Retina

• Blind Spot (Optic Disc): No rods or cones; no image formation
• Yellow Spot (Macula Lutea/Fovea Centralis): Small area (~6mm) opposite optical axis; sharpest vision

Physiology of Vision

• Light enters through cornea → iris controls pupil size → lens focuses light → vitreous humour → retina → optic nerve → optic chiasm → occipital lobe (visual cortex)
• Binocular Vision: Both eyes transmit slightly different images; brain compiles them into a single image

Disorders of the Eye

• Conjunctivitis (Pink Eye/Madras Eye): Inflammation of conjunctiva
• Keratitis: Inflammation of cornea
• Glaucoma: Increased intraocular pressure due to obstructed aqueous humour circulation; can lead to blindness
• Myopia (Near-sightedness): Image converges before retina; corrected with concave lenses

EAR (SENSE OF HEARING AND EQUILIBRIUM)

Parts of Ear

1. Outer Ear (Pinna + External Auditory Meatus): Auricle collects sound waves; meatus contains ceruminous glands secreting cerumen (earwax)
2. Middle Ear (Tympanic Cavity): Contains ear ossicles (malleus, incus, stapes) and muscles (tensor tympani, stapedius)
3. Inner Ear (Labyrinth): Bony labyrinth (vestibule, cochlea, 3 semicircular canals) and membranous labyrinth (cochlear duct, semicircular ducts, utricle, saccule)

Physiology of Hearing

• Sound waves → pinna → external auditory meatus → tympanic membrane vibrates → malleus → incus → stapes → oval window → perilymph in scala vestibuli → basilar membrane → organ of Corti → nerve impulses → auditory nerve → brain
• Organ of Corti: Contains ~16,000-24,000 auditory cells on basilar membrane

NOSE (SENSE OF SMELL)

Olfactory Epithelium Cells

• Olfactory Receptors: Bipolar neurons; first-order neurons of olfactory pathway
• Supporting Cells: Columnar epithelial cells; provide physical support, nourishment, and detoxification
• Basal Cells: Stem cells; produce new olfactory receptors (life expectancy ~1 month)

Functions of Nose

1. Smell
2. Respiration
3. Air-conditioning (moistens dry/cold air)
4. Detoxification (mucus traps dust, bacteria, pollen, viruses; cilia beat 1200 movements/minute)

TONGUE (SENSE OF TASTE)

Taste Bud Structure

• Gustato-Receptors (Taste Cells): 5-15 bipolar neurosensory cells; have microvilli; nerve fibres join to form facial (VII) or glossopharyngeal (IX) nerves
• Supporting Cells: ~40 spindle-shaped non-sensory cells

Types of Papillae

• Filiform (Thread-like): Most numerous; no taste buds; provide friction
• Fungiform (Mushroom-like): Small; contain ~5 taste buds
• Circumvallate (Ringed-circle): Large; contain ~100 taste buds; form V-shaped structure
• Foliate: 4-5 vertical folds on each side; leaf-like ridges

Functions of Tongue

1. Sense of taste (recognises delightful food, rejects unpalatable food)
2. Assists in digestion (moves food, converts into bolus)
3. Promotes speech (changes position, alters air passage shape)

SKIN (SENSE OF TOUCH)

Skin is the largest organ (surface area ~1.8 m², 16% of total body weight). Derivatives include nails, hair, sebaceous glands, and sweat glands.

Layers of Skin

1. Epidermis: Outermost stratified squamous epithelium (no blood vessels). Layers: Stratum Basale (deepest, melanin), Stratum Spinosum, Stratum Granulosum, Stratum Corneum (10-30 layers of corneocytes)
2. Dermis: Below epidermis; papillary layer (thin) and reticular layer (thick); highly vascularised
3. Subcutaneous Tissue (Hypodermis): Loose connective tissue and adipose tissue (~3 cm on abdomen)

Functions of Skin

1. Sensation (temperature, touch, pressure, pain)
2. Protection (physical barrier against microbes, dehydration, UV radiation)
3. Thermoregulation (sweating cools body; reduced sweating conserves heat)
4. Immunity
5. Excretion (sweat excretes toxins, ions)
6. Blood reservoir (8-10% of total blood volume in resting adult)
7. Drug delivery route (transdermal patches)
8. Endocrine function (Vitamin D biosynthesis)