special senses

Chapter 8: Special Senses

Overview of Special Senses

  • Special senses include:

    • Smell

    • Taste

    • Sight

    • Hearing

    • Equilibrium

  • Special sense receptors are large, complex sensory organs or localized clusters of receptors.

Part I: The Eye and Vision

  • 70% of all sensory receptors are found in the eyes.

  • Each eye has over 1 million nerve fibers that carry information to the brain.

Anatomy of the Eye

  • Accessory Structures of the Eye:

    • Extrinsic eye muscles

    • Eyelids

    • Conjunctiva

    • Lacrimal apparatus

  • Surface Anatomy:

    • Key features: Palpebral fissure, lateral commission, iris, eyelid, brow, eyelashes, pupil, lacrimal caruncle, sclera.

External and Accessory Structures

  • Eyelids:

    • Meet at medial and lateral commissure (canthus).

    • Eyelashes have tarsal glands that produce lubricating secretion.

    • Ciliary glands are located between eyelashes.

  • Conjunctiva:

    • Membrane lining the eyelids and eyeball, connects with transparent cornea, secretes mucus to keep the eye lubricated.

  • Lacrimal Apparatus:

    • Lacrimal gland: Produces tears, located on the lateral end of each eye.

    • Tears drain across the eye into lacrimal canaliculi, then the lacrimal sac, and into the nasolacrimal duct, emptying into the nasal cavity.

  • Composition of Tears:

    • Contain dilute salt solution, mucus, antibodies, lysozyme (bacterial enzyme).

    • Functions: Cleanse, protect, moisten, and lubricate the eye.

Extrinsic Eye Muscles

  • Six muscles attach to the eye's outer surface and produce gross movements.

Internal Structures: The Eyeball

  • Structure of the eyeball consists of three layers (tunics):

    • Fibrous Layer: Outside layer (includes sclera and cornea).

    • Vascular Layer: Middle layer (choroid).

    • Sensory Layer: Inner for light perception (retina).

  • Humors fill the interior of the eyeball:

    • Lens divides the eye into anterior and posterior segments.

  • Fibrous Layer:

    • Sclera: White connective tissue layer, visible as "white of the eye".

    • Cornea: Transparent, central anterior portion, allows light passage, self-repairing, can be transplanted without rejection.

  • Vascular Layer:

    • Choroid: Blood-rich layer that prevents light scattering, modified into:

      • Ciliary body: Controls shape of lens.

      • Iris: Regulates light entering the eye, gives eye color.

      • Pupil: Rounded opening in the iris.

  • Sensory Layer:

    • Retina:

      • Outer pigmented layer absorbs light, prevents scattering.

      • Inner neural layer contains photoreceptors: rods (dim light) and cones (color vision).

      • Electrical signals are transmitted through bipolar neurons and ganglion cells to the optic nerve.

      • Optic disc (blind spot): No photoreceptor cells.

Physiology of Vision

  • Light must be focused on the retina for clear vision:

    • Light is refracted by the cornea, aqueous humor, lens, and vitreous humor.

    • Accommodation: Lens changes shape to focus on close objects.

  • Real images on the retina are reversed and smaller than the actual object.

  • Visual Pathways:

    1. Light signals from the retina travel via the optic nerve.

    2. Cross at optic chiasma.

    3. Split into optic tracts, synapse in the thalamus.

    4. Proceed via optic radiation to the occipital lobe for interpretation.

  • Binocular Vision: Each eye sees a slightly different view, creating depth perception.

A Closer Look at Eye Conditions

  • Emmetropia: Normal vision where images focus correctly.

  • Myopia (Nearsightedness): Distant objects appear blurry; light focused in front of the retina due to elongated eyeball.

  • Hyperopia (Farsightedness): Near objects unclear; light focused behind the retina, often due to short eyeball or "lazy lens".

  • Astigmatism: Blurred vision from unequal curvatures of the cornea or lens.

Eye Reflexes

  • Convergence: Eyes move medially when focusing on close objects.

  • Photopupillary Reflex: Bright light causes pupils to constrict.

  • Accommodation Pupillary Reflex: Pupils constrict when viewing close objects.

Part II: The Ear - Hearing and Balance

  • The ear accommodates two senses: hearing and equilibrium.

  • Anatomy of the Ear:

    • Three areas: External, Middle, Internal ear.

  • External Ear:

    • Consists of auricle (pinna) and external acoustic meatus (auditory canal).

    • Involved in collecting sound waves.

  • Middle Ear:

    • Air-filled cavity with three ossicles (hammer, anvil, stirrup) transmitting vibrations from the tympanic membrane to oval window.

  • Internal Ear:

    • Contains organs for hearing and balance (cochlea, vestibule, semicircular canals), filled with perilymph and endolymph.

Equilibrium

  • Vestibular apparatus in the inner ear has two parts:

    • Static equilibrium (position of head)

    • Dynamic equilibrium (angular movements of the head).

  • Maculae: Static equilibrium receptors that send information to the brain via the vestibular nerve.

  • Crista Ampullaris: Part of dynamic equilibrium detecting rotational movements, sending impulses to the cerebellum.

Hearing

  • Spiral Organ of Corti: Contains hair cells that detect sound vibrations.

  • Pathway of Sound:

    • Vibrations move through ossicles from eardrum to oval window, amplified and stimulate hair cells, leading to nerve impulses traveling to the auditory cortex.

  • Pitch Discrimination:

    • High-pitched sounds disturb short, stiff fibers; low-pitched sounds disturb long, floppy fibers of the basilar membrane.

Hearing and Equilibrium Deficits

  • Deafness: Hearing loss classified as conduction deafness (external/middle structures) or sensorineural deafness (nervous system).

  • Ménière’s Syndrome: Affects the inner ear, causing progressive deafness and vertigo.

Part III: Chemical Senses - Smell and Taste

  • Chemoreceptors: Sensory receptors activated by chemicals.

  • Smell can differentiate a wide range of chemicals; taste has five types of receptors.

  • Olfactory Receptors: Located in the nasal cavity; impulses transmitted to the olfactory nerve.

  • Taste Buds: Most located on the tongue and other areas, contain gustatory cells that respond to flavors.

  • Five Basic Taste Sensations:

    • Sweet, sour, bitter, salty, umami (savory).

Developmental Aspects of Special Senses

  • Early formation of sense organs can be affected by maternal infections in early pregnancy.

  • Vision develops significantly from infancy to age 9; infants are born farsighted.

  • Age-related issues include presbyopia and sensorineural deafness (presbycusis).

  • Taste and smell sensitivity declines with age.