SPECIAL SENSES

Overview of Special Senses

  • Special senses correspond to dendrites of neurons.

  • Three types of sensory receptors based on location:

    • Exteroceptors: Located near body surfaces.

    • General Somatic Afferent (GSA): Detect temperature, pressure, touch, pain.

    • Special Somatic Afferent (SSA): Involved in vision and hearing.

    • Special Visceral Afferent (SVA): Responsible for smell and taste.

    • Proprioceptors: Found in joints, tendons, and intrafusal fibers (GSA).

    • Interoceptors: Located within organs (General Visceral Afferent - GVA).

Mechanoreceptors

  • Found in the epidermis of the skin and dermis, specializing in touch sensitivity.

  • Free nerve endings penetrate nearly to the stratum corneum, especially sensitive areas like the face and cornea.

  • They respond to:

    • Touch pressure (some function also as nociceptors or thermoreceptors).

    • High and low temperatures, pain, and itching.

  • Important for mediating cutaneous sensation and pain (nociceptors), heat and cold sensations (thermoreceptors).

Types of Mechanoreceptors

  1. Merkel Disks: For tactile discrimination, fine touch, sensitive to edges and points.

    • Location: Non-hairy skin (fingers, palms, eyelids, lips, tongue, nipples, skin of feet, forearms).

    • Composed of 3-4 nerve terminals and Schwann cells.

  2. Meissner Corpuscles: Sensitivity to light touch; located in dermal papillae with long axes perpendicular to the skin surface.

  3. Pacinian Corpuscles: Respond to pressure, touch, and vibration; located in deep dermis or hypodermis.

    • Structure: Large, ovoid, 1-2 mm long; outer capsule made of concentric lamellae.

  4. Ruffini Endings: Respond to stretch and pressure in skin and joint capsules; spindle-shaped, capable of sensing temperature increases.

  5. Thermoreceptors: Two types based on temperature response:

    • Warm Receptors: Respond to temperature differences above 30 °C30 \text{ °C}.

    • Cold Receptors: Respond to temperatures below 30 °C30 \text{ °C}.

Eye Anatomy

  • Highly developed photosensory organ, located within the skull's orbits for protection and allowing a range of movements.

  • Photoreception: Conversion of light energy into action potentials.

  • Photoreceptors: Modified dendrites of rods and cones located in the retina.

  • The eye consists of three layers:

    • Fibrous Tunic: Comprises the cornea and sclera.

    • Vascular Tunic (Uvea): Includes the choroid, ciliary body, and iris.

    • Sensory Tunic (Retina): Contains the photoreceptors and neurons for signal processing.

Fibrous Tunic

  • Cornea: Transparent front part of the eye.

    • Thickness: 0.5 mm0.5 \text{ mm} at the center, 1 mm1 \text{ mm} at the periphery.

    • Avascular; nourished by the aqueous humor and peripheral capillaries via diffusion.

    • Functions:

    • Principal refractive medium (anterior curvature)

    • Five layers:

    • Corneal Epithelium: Non-keratinized stratified squamous cells, rich in sensory nerve supply.

    • Bowman’s Membrane: Provides stability and protection against infection; contains randomly arranged type I collagen fibers and an underlying basement membrane for corneal epithelium support.

    • Stroma: Main bulk of the cornea containing keratocytes (modified fibroblasts) that produce collagen and ground substance, and highly organized collagen fibers arranged in lamellae, contributing significantly to transparency.

    • Descemet’s Membrane: Provides support to the internal endothelium, functions as a filtration barrier; composed of collagen and elastic fibers; regenerates if damaged.

Sclera

  • Opaque layer covering approximately 85%85\% of the eyeball.

  • Few blood vessels; consists predominantly of dense connective tissue (Type I collagen).

  • Pierced by the optic nerve and vascular supply.

  • Functions to protect delicate inner structures and provide sites for muscle insertion.

Vascular Layer (Uvea)

  • Contains pigmented structures and vascular support.

  • Components:

    • Iris: Adjustable diaphragm controlling light entry through the pupil.

    • Contains two muscles: the sphincter pupillae (constricts pupil) and dilator pupillae (dilates pupil), both regulated by the autonomic nervous system.

    • Ciliary Body: Attaches to lens; secretes aqueous humor, enables light refraction.

    • Composed of ciliary muscle (involved in lens accommodation for focusing) and ciliary processes (produce aqueous humor).

    • Choroid: Vascular layer providing blood supply and preventing light scattering.

    • Richly vascularized, with melanocytes that produce melanin, absorbing stray light to enhance visual clarity.

Sensory Layer (Retina)

  • Inner lining that develops from the optic cup.

  • Contains photoreceptors; histologically organized into nine layers:

    1. Rods and Cones Layer: Photosensitive neurons. Rods are responsible for scotopic (dim light) vision and detect black and white; cones are responsible for photopic (bright light) vision and color perception.

    2. Outer Limiting Layer: Contains junctions connecting photoreceptors and supporting cells, particularly Müller cells.

    3. Outer Nuclear Layer: Cell bodies of photoreceptors.

    4. Outer Plexiform Layer: Axons and synapses of photoreceptors connecting to bipolar and horizontal cells.

    5. Inner Nuclear Layer: Contains nuclei of interneurons (bipolar, horizontal, amacrine, and Müller cells) that process visual information.

    6. Inner Plexiform Layer: Contains connections from interneurons to ganglion cells, further processing signals.

    7. Ganglionic Layer: Contains ganglion cell bodies that form the optic nerve. Their axons transmit processed visual information to the brain.

    8. Nerve Fiber Layer: Composed of ganglion cell axons converging towards the optic disc.

    9. Inner Limiting Membrane: Innermost layer forming the retina’s surface, composed of the basal laminae of Müller cells.

Specialized Areas of the Retina

  • Fovea Centralis: Area of high visual acuity, densely packed with cone cells, devoid of most conducting neurons.

  • Macula Lutea: Area surrounding the fovea; contains carotenoids, providing protection to the retina.

Accessory Structures of the Eye

  • Lens: Transparent, biconvex structure held by zonular fibers.

    • Changes shape (accommodates) to focus light onto the retina, a process controlled by the ciliary muscle.

  • Vitreous Body: Gel-like substance maintaining the shape of the eye.

    • Fills the space between the lens and retina; maintains intraocular pressure and helps keep the retina attached to the choroid.

  • Conjunctiva: Covers anterior sclera, consists of a stratified columnar epithelium; provides lubrication and protection.

    • Contains goblet cells that secrete mucin, contributing to the tear film.

  • Eyelids: Protect the eye, contain muscles and glands.

  • Lacrimal Apparatus: Produces and drains tears, keeping the surface of the eye moist.

    • Tears consist of water, electrolytes, antibacterial enzymes (lysozyme), and antibodies, providing lubrication, protection, and nourishment to the cornea.

Ear Anatomy

  • Comprises three sections:

    • External Ear: Captures sound waves (auricle, external auditory meatus).

    • Middle Ear: Conducts sound via ossicles to the inner ear.

    • Internal Ear: Contains sensory receptors for hearing and balance.

External Ear

  • Auricle: A funnel-shaped structure directing sound waves.

  • External Acoustic Meatus: Canal leading to the tympanic membrane; contains ceruminous glands producing earwax.

  • Tympanic Membrane: Separates external and middle ear, converts sound waves into mechanical vibrations.

Middle Ear

  • Contains the auditory ossicles (malleus, incus, stapes) which amplify sound.

    • These ossicles form a lever system that transmits vibrations from the large tympanic membrane to the small oval window, effectively increasing the force of sound by approximately 22 times22 \text{ times}.

  • Eustachian Tube: Equalizes air pressure between the middle ear and external environment, crucial for proper tympanic membrane vibration.

Internal Ear

  • Bony Labyrinth: Contains perilymph and encloses the membranous labyrinth.

  • Membranous Labyrinth: Contains endolymph and sensory structures for balance and hearing, e.g., cochlear duct, utricle, and saccule.

    • Includes the Cochlear Duct (containing the Organ of Corti for hearing), Semicircular Canals (detect angular head movements), Utricle and Saccule (detect linear acceleration and head position).

Pathway of Sound

  • Sound waves travel from the outer ear to the tympanic membrane, causing it to vibrate, which amplifies sound through ossicles to the inner ear.

  • Pressure waves are formed in the endolymph, inducing nerve signals in the cochlear division of CN VIII for sound perception.

References

  • Gartnerr, L. and Hiatt, J. (2014). Color Atlas and Text of Histology: 6th edition. Lippincott Williams & Wilkins: Philadelphia

  • Mescher, A. L. (2013). Junqueira's Basic Histology Text & Atlas 13th Edition. New York: McGraw-Hill Education.

  • Ross, M. H., & Pawlina, W. (2011). HISTOLOGY: A text and atlas with Correlated Cell and Molecular Biology 6th edition. Lippincott Williams & Wilkins: Philadelphia.

  • Young, B., Woodford, P., & O'Dowd, G. (2014). Wheater's Functional Histology: A Text and Colour Atlas (6th ed.). United States of America.