Chapter 17: Special Senses

Chapter 17: Special Senses

Olfaction

  • Olfactory Epithelium

    • Occupies the superior part of the nasal cavity.

    • Covers the inferior surface of the cribriform plate and extends along the superior nasal concha.

    • Composed of:

    • Olfactory receptor cells

    • Supporting cells

    • Basal cells

  • Olfaction

    • Defined as the sense of smell.

    • Classified as a CHEMICAL sense, similar to gustation (taste).

Olfactory Receptor Cells

  • Characteristics:

    • Serve as the 1st order neurons of the olfactory pathway.

    • Each olfactory receptor cell:

    • Is a bipolar neuron.

    • Has an exposed dendrite and an axon that projects through the cribriform plate, terminating in the olfactory bulb.

    • Olfactory cilia extend from the dendrite; these are sites of olfactory transduction.

    • Contain olfactory receptors which detect inhaled odorants.

Supporting and Basal Cells

  • Supporting Cells

    • Columnar epithelial cells located in the mucous membrane lining the nose.

    • Functions:

    • Provide physical support and nourishment to olfactory receptor cells.

    • Offer electrical insulation for olfactory receptor cells.

    • Help detoxify chemicals contacting the olfactory epithelium.

  • Basal Cells

    • Stem cells located between the bases of supporting cells.

    • Continuously undergo cell division to produce new olfactory receptor cells approximately every 30 days.

Olfactory Transduction

  • Process involves:

    • Binding of an odorant molecule to an olfactory receptor protein.

    • Activation of a G protein and adenylate cyclase.

    • Production of cyclic AMP (cAMP).

    • cAMP opens cation channels allowing Ca^2+ and Na^+ ions to rush into the cell.

    • Results in depolarization, generating an action potential that propagates along the axon of the olfactory receptor cell.

Olfactory Pathway

  • Olfactory Nerves (CN I)

    • Terminate in the olfactory bulbs, which are paired masses of gray matter in the brain.

    • In the olfactory bulbs, axon terminals of olfactory receptor cells form synapses with dendrites and cell bodies of olfactory bulb neurons.

    • Axons of olfactory bulb neurons extend posteriorly to form the olfactory tract.

    • Some axons project to the primary olfactory area of the cortex located in the temporal lobe, while others project to the limbic system and hypothalamus.

    • Key Point: Olfaction is unique as the only senses that reach the cortex without synapsing in the thalamus.

Olfactory Cortex

  • Main regions include:

    1. Anterior olfactory nucleus

    2. Olfactory tubercle

    3. Piriform cortex

    4. Entorhinal cortex

Gustation

  • Papillae Types:

    • Vallate Papillae (Circumvallate):

    • Each contains 100-300 taste buds.

    • Approximately 12 of these present on the tongue.

    • Fungiform Papillae:

    • Mushroom-shaped, scattered across the tongue surface.

    • Each contains about 5 taste buds.

    • Foliate Papillae:

    • Located in small trenches on the lateral margins of the tongue.

    • Most taste buds in this region degenerate in childhood.

    • Filiform Papillae:

    • Distributed over the entire tongue surface.

    • Do NOT contain taste buds but have tactile receptors, increasing friction.

  • Gustation (Taste)

    • Defined as the sense of taste, also classified as a CHEMICAL sense.

Gustatory Receptor Cells

  • Characteristics:

    • Lifespan of approximately 10 days.

    • At the base, they synapse with the dendrites of 1st order neurons.

    • Dendrites of each 1st order neuron contact multiple gustatory receptor cells across several taste buds.

  • Supporting Cells

    • Contain microvilli.

    • Surround and support gustatory receptor cells.

  • Basal Cells

    • Stem cells located at the periphery of the taste bud, near the connective tissue layer.

    • Produce supporting cells that develop into gustatory receptor cells.

Gustatory Transduction

  • Process involves:

    • Tastant must be dissolved in saliva.

    • Contact with the plasma membrane of gustatory microvilli (sites of transduction).

    • Generates a receptor potential which stimulates exocytosis of synaptic vesicles.

    • Neurotransmitters released trigger nerve impulses in the 1st order sensory neuron.

  • Note: The taste map of the tongue is now considered obsolete.

Gustatory Pathway

  • ▶Tastant → Gustatory Receptor Cell → Gustatory Nucleus (medulla) → 1st order neuron (CN VII, IX, or X) → Thalamus → Primary Gustatory Cortex

Vision

  • Electromagnetic Spectrum:

    • Wavelengths determine what humans can see, with visible light ranging approximately from 400 nm (violet) to 700 nm (red).

Eye Anatomy and Functions

  • Cornea

    • Transparent coating covering the colored iris that helps in focusing light onto the retina.

  • Sclera

    • The white part of the eye, dense connective tissue covering most of the eyeball, except the cornea.

  • Choroid

    • Highly vascularized structure providing nutrients to the posterior surface of the retina.

    • Contains melanocytes that produce melanin, preventing reflection and scattering of light.

  • Iris

    • Colored portion of the eyeball comprising melanocytes that regulate the amount of light entering the eyeball.

    • Appears black due to underlying heavily pigmented structures.

  • Pupil

    • The part of the eye that regulates light intake.

Visual Pathway

  • Structure of the retina includes different types of cells, including:

    • Photoreceptors: Rods and cones.

    • Bipolar cells and Ganglion cells.

  • The Photopigment consists of:

    • Opsin: a glycoprotein.

    • Retinal: a derivative of Vitamin A.

Phototransduction

  • Mechanism of action for rods:

    • Isomerization:

    • In darkness, cis-retinal fits snugly in opsin, but when it absorbs light, it converts to trans-retinal, generating a receptor potential.

    • Bleaching:

    • Trans-retinal separates from opsin, resulting in colorless components.

    • Regeneration:

    • Retinal isomerase converts trans-retinal back to cis-retinal for reuse.

Visual Pathway Sequence

  • Path of visual signals from retina to brain includes:

    • Optic nerve → Optic chiasm → Optic tract → Lateral geniculate nucleus of thalamus → Optic radiation → Primary visual cortex (Area 17) in occipital lobe.

Hearing

  • Anatomy of the ear includes:

    • Auricle, Auditory Canal, Tympanic Membrane, Middle and Inner Ear components.

Hearing Mechanism

  1. Auricle detects sound waves and funnels them to the auditory canal.

  2. Sound strikes the tympanic membrane, causing it to vibrate.

  3. Vibration transferred through ossicles (malleus, incus, stapes).

  4. Stapes vibrates the oval window, generating waves in the perilymph.

  5. Waves travel through the cochlea, stimulating hair cells to generate nerve impulses in 1st order neurons.

Equilibrium

  • Otolithic organs (saccule and utricle)

    • Contain otoliths, dense calcium carbonate crystals that help detect changes in head position in relation to gravity.

  • Endolymph: A high-potassium fluid that helps determine balance during dynamic movements.

Final Note

  • Reference to relevant educational videos including topics surrounding olfaction, gustation, vision, hearing, and balance for further learning and understanding of these systems.