Special Senses and Eye Anatomy Overview

Special Senses Overview

• Special senses include smell, taste, sight, hearing, and equilibrium
• Special sense receptors are large, complex sensory organs or localized clusters of receptors

The Eye and Vision

• 70% of all sensory receptors are in the eyes
• Each eye has over 1 million nerve fibers carrying information to the brain

Anatomy of the Eye: Accessory Structures

• Extrinsic eye muscles: Six muscles, attached to the outer surface of the eye, controlling gross eye movements
• Eyelids: Meet at the medial and lateral commissure (canthus), protecting the eye
• Eyelashes: Prevent foreign particles from entering the eye
• Tarsal glands: Produce oily secretions lubricating the eye
• Ciliary glands: Located between eyelashes
• Conjunctiva: Membrane lining the eyelids and eyeball, connecting with the cornea and secreting mucus to keep the eye moist
• Lacrimal apparatus: Lacrimal gland (produces tears) and associated ducts. Tears drain across the eye, the lacrimal canaliculi, the lacrimal sac, and into the nasolacrimal duct, emptying into the nasal cavity. Tears contain dilute salt solution, mucus, antibodies, and lysozyme (bacteria-destroying enzyme).

Internal Structures: The Eyeball

• Fibrous layer: Sclera (white connective tissue layer) and Cornea (transparent, central anterior part allowing light passage, easily repairs itself)
• Vascular layer: Choroid (blood-rich nutritive layer), Ciliary body (smooth muscle structures), and Iris (pigmented layer controlling light entering the pupil). The pupil is a rounded opening in the iris.
• Sensory layer: Retina (outer pigmented layer absorbing light and preventing scattering, and inner neural layer with receptor cells, Rods (gray scale vision in dim light), and Cones (color vision, concentrated in the fovea centralis)). The optic disc (blind spot) is where the optic nerve exits the eyeball.
• Humors: Aqueous humor (clear, watery fluid anterior to the lens, maintains intraocular pressure,) and Vitreous humor (gel-like substance posterior to the lens, prevents collapsing).
• Lens: Flexible, biconvex crystal-like structure, held in place by a suspensory ligament attached to the ciliary body
• Ophthalmoscope: Instrument illuminating the interior of the eyeball, detecting various eye conditions.

Physiology of Vision

• Pathway of light through the eye: Light is bent (refracted) by the cornea, aqueous humor, lens, and vitreous humor. The eye is set for distant vision (over 20ft) and must change shape (accommodation) to focus on close objects (less than 20ft).
• Real images formed on retina: Reversed left-to-right and upside down, smaller than the object.
• Visual fields and pathways: Optic nerve (bundle of axons carrying impulses from retina), optic chiasma (location where optic nerves cross), optic tracts (contain fibers from lateral side of same eye and medial side of opposite eye, synapses with neurons in thalamus), optic radiation (axons from thalamus, synapse with cortical cells in occipital lobe), optic cortex (visual interpretation occurs).
• Visual fields and binocular vision: Each eye sees a slightly different view, but field of view overlaps, resulting in binocular vision (depth perception).
• Eye reflexes: Convergence (reflexive movement of eyes medially to focus on near objects), Photopupillary reflex (bright light causing pupils to constrict), Accommodation pupillary reflex (viewing close objects causing pupils to constrict).
• Eye conditions: Emmetropia (normal vision), Myopia (nearsightedness, eyeball is too long, distant objects are blurry), Hyperopia (farsightedness, eyeball is too short, near objects are blurry), Astigmatism (light focuses as lines not points due to unequal curvature of cornea/lens, blurry images)

The Ear: Hearing and Balance

• Ear houses hearing and equilibrium (balance)
• Receptors are mechanoreceptors
• Different organs house receptors for each sense

Anatomy of the Ear

• Three areas: External (outer) ear, Middle ear, Internal (inner) ear
• External ear: Auricle (pinna), External acoustic meatus (auditory canal), Tympanic membrane (eardrum).
• Middle ear: Middle ear cavity (air-filled, mucosa-lined), Pharyngotympanic tube (auditory tube, equalizes pressure), Auditory ossicles (malleus, incus, stapes, transmit vibrations from eardrum to the inner ear).
• Internal ear: Bony labyrinth (osseous labyrinth, cochlea, vestibule, semicircular canals, filled with perilymph), Membranous labyrinth (suspended in perilymph, contains endolymph)

Equilibrium

• Vestibular apparatus: Equilibrium receptors in the inner ear, consisting of static and dynamic equilibrium components.
• Static equilibrium: Maculae (receptors in vestibule) report on head position, help maintain head erectness, send info via vestibular nerve to cerebellum.
• Dynamic equilibrium: Crista ampullaris (receptors in ampulla of each semicircular canal). Cupula (gelatinous cap) drags against endolymph during head movements, stimulating hair cells, impulses travel to vestibular nerve and cerebellum.

Hearing

• Spiral organ of Corti (located in cochlear duct): Hair cells on the basilar membrane, gel-like tectorial membrane bending cells when basilar membrane vibrates, impulses carried to auditory cortex on temporal lobe via cochlear nerve.
• Pathway of vibrations from sound waves: Vibrations move through ossicles to oval window, amplified, causing pressure waves in the basilar membrane and bending hair cells, generating action potentials. High-pitched sounds disturb short, stiff basilar membrane hairs; low-pitched sound disturb long, floppy basilar membrane hairs.
• Hearing and equilibrium deficits: Deafness (any degree of hearing loss), Conduction deafness (transmission of vibrations impaired), Sensorineural deafness (damage to ear/nervous system), Ménière's syndrome (inner ear problem causing deafness and vertigo).

Chemical Senses: Smell and Taste

• Receptors for smell and taste are chemoreceptors, responding to chemicals in solution
• Smell can differentiate wider range of chemicals than taste, both complementing each other in response to many same stimuli.
• Olfactory receptors: Located in the roof of the nasal cavity, detect chemicals dissolved in mucus by olfactory hairs (cilia of neurons), impulses are transmitted to olfactory cortex via olfactory nerve (cranial nerve I). Smell is interpreted in olfactory cortex.
• Taste buds: Receptor organs in taste papillae, on tongue, soft palate, and superior part of pharynx, cheeks. Gustatory cells (taste receptors) possess gustatory hairs. Impulses carried to gustatory complex by several cranial nerves (facial, glossopharyngeal, vagus). Taste buds are replaced by basal cells.
• Basic taste sensations: Sweet, sour, bitter, salty, umami.

Developmental Aspects of Special Senses

• Special sense organs form early in embryonic development
• Maternal infections during first 5-6 weeks of pregnancy can cause visual abnormalities and sensorineural deafness.
• Vision requires learning; infants have poor visual acuity, farsightedness, lack color vision, lack depth perception. Eyes continue growing and maturing until age 8-9. Age-related eye issues (Presbyopia, decreased lens elasticity, difficulty with close-vision, lens discoloration, Constricted pupils).
• Hearing is present at birth, but responses are reflexive. Critical listening and language imitation develop during toddler stage. Age-related issues (Presbycusis, otosclerosis). Taste and smell are acute at birth, but sensitivity decreases after age 40.

Explore the fascinating world of special senses, focusing on the eye and its anatomy. Learn about the complex structures involved in vision, including accessory eye components and sensory receptors. This quiz will enhance your understanding of how we perceive the world around us through sight.