chap 15 - special senses

Layout of the Olfactory Epithelium and Cells

The olfactory epithelium is located in the roof of the nasal cavity, lining the superior nasal concha. It measures about 5 cm² in humans.

Cells

• Olfactory receptor cells: Bipolar neurons with cilia (olfactory hairs) that detect odor molecules dissolved in mucus.

• Supporting cells: Columnar cells that provide metabolic and physical support.

• Basal cells: Undifferentiated cells that regenerate olfactory receptor neurons every 30–60 days.

• Bowman’s glands: Secrete mucus to trap odor molecules and facilitate detection.

Second Messenger System for Olfactory Transduction

• Odorant molecules bind to G-protein-coupled receptors (GPCRs) on the cilia of olfactory receptor cells.

• This activates the G-protein Golf, which in turn activates adenylate cyclase to produce cAMP (the secondary messenger)

• cAMP binds to and opens cyclic nucleotide-gated ion channels, allowing Na⁺ and Ca²⁺ influx.

• This causes depolarization, generating an action potential transmitted to the olfactory bulb.

Primary Tastes and Papillae Types

• Tastes:

  • Sweet: Sugars and some amino acids.

  • Sour: H⁺ ions (acids).

  • Salty: Na⁺ ions.

  • Bitter: Alkaloids and other complex molecules.

  • Umami: Glutamate and other amino acids.

• Papillae:

  • Fungiform: Found on the anterior 2/3 of the tongue, each has 1–5 taste buds.

  • Foliate: Located on the lateral margins of the tongue; more prominent in children.

  • Circumvallate: Form a V-shape at the back of the tongue, each contains hundreds of taste buds.

  • Filiform: Most numerous, located across the tongue’s surface, responsible for texture sensation, and do not contain taste buds.

Visible Light Range

Visible light ranges from 400 nm (violet) to 700 nm (red) in the electromagnetic spectrum.

Layers and Structures of the Eyeball

• Fibrous tunic: Outermost layer, includes the cornea (transparent, allows light in) and sclera (white, protective layer).

• Vascular tunic (choroid): Middle layer, includes the choroid (vascular, pigmented), ciliary body (controls lens shape), and iris (regulates pupil size).

• Retina: Innermost layer, contains photoreceptors (rods and cones), bipolar cells, and ganglion cells.

• Other structures: Lens, anterior and posterior chambers filled with aqueous humor, and the vitreous chamber filled with vitreous humor.

Image Formation

• Refraction: Bending of light as it passes through the cornea and lens to focus on the retina.

• Accommodation: Adjusting the lens shape for near or far objects.

• Pupil constriction: Reduces light entering the eye and increases depth of focus.

Eye Types

• Emmetropic eye: Normal vision, light focuses on the retina.

• Myopia: Nearsightedness, light focuses in front of the retina.

• Hyperopia: Farsightedness, light focuses behind the retina.

• Astigmatism: Irregular curvature of the cornea or lens causes distorted vision.

Lens Accommodation

• Distant objects: The ciliary muscles relax, lens flattens.

• Close objects: The ciliary muscles contract, lens becomes more rounded to increase refractive power.

Rods and Cones of the Retina

• Rods: Sensitive to dim light, responsible for night vision, do not detect color.

• Cones: Detect bright light and color, concentrated in the fovea.

• Disorders: Loss of rods leads to night blindness; loss of cones results in color blindness or central vision defects.

Regions of the Ear

• Outer ear: Auricle (pinna), external auditory canal, tympanic membrane (eardrum).

• Middle ear: Ossicles (malleus, incus, stapes), oval window, round window, and the Eustachian tube.

• Inner ear: Cochlea, vestibule, semicircular canals, and the auditory nerve.

Pathway of Sound

Sound waves enter the external auditory canal → Vibrate the tympanic membrane → Ossicles amplify sound → Vibrations transmitted to the oval window → Waves in cochlear fluid stimulate hair cells → Electrical signals travel via cranial nerve VIII (vestibulocochlear nerve).

Equilibrium Structures

• Semicircular canals: Detect rotational movements (dynamic equilibrium).

• Vestibule (utricle and saccule): Detect linear acceleration and head position (static equilibrium).

• Maintenance: Hair cells in these structures respond to fluid movement, sending signals to the brain to adjust posture and balance.