Special Senses: Taste and Smell

• Special senses include taste, smell, sight, hearing, and equilibrium.
• These senses use specialized receptor cells mostly located in the head.
• Cranial nerves process sensory information, which is then interpreted in specific brain areas.

Taste (Gustation)

• Taste receptors are in taste buds, mainly on the tongue's papillae.
• Gustatory epithelial cells detect chemicals and Basal epithelial cells regenerate taste cells every 7–10 days.
• The five basic tastes are sweet, salty, sour, bitter, and umami.
• The facial nerve (VII) transmits taste from the anterior 2/3 of tongue.
• The glossopharyngeal nerve (IX) transmits taste from the posterior 1/3 of tongue.
• The vagus nerve (X) transmits taste from the epiglottis and pharynx.
• Impulses travel through the solitary nucleus in the medulla, then to the thalamus, and finally to the gustatory cortex in the insula.

Smell (Olfaction)

• Receptors are in the olfactory epithelium on the nasal cavity’s roof.
• Olfactory sensory neurons (bipolar) have cilia (olfactory hairs) that bind odor molecules.
• Supporting cells and olfactory stem cells assist and regenerate neurons.
• Odorants dissolve in mucus and stimulate receptors, which send signals to the brain.
• Axons pass through the cribriform plate (ethmoid bone) into the olfactory bulb, synapsing with mitral cells in glomeruli.
• Signals proceed to the limbic system (for emotions) and olfactory cortex (for interpretation).

Sight (Vision)

• The eyeball has three layers: fibrous, vascular, and inner.
• The fibrous layer includes the sclera (white) and cornea (transparent, refracts light).
• The vascular layer includes the choroid (pigmented), ciliary body (controls lens), and iris (controls pupil).
• The inner layer (retina) contains photoreceptors: rods (dim light, no color) and cones (bright light, color vision).
• The lens focuses light onto the retina.
• The vitreous humor (posterior) maintains eye shape, and the aqueous humor (anterior) nourishes the lens and cornea.
• Light is refracted by the cornea and lens to focus on the fovea centralis (high cone density) in the macula lutea for sharp vision.
• Signals from photoreceptors pass through bipolar cells to ganglion cells; their axons form the optic nerve.

Hearing (Audition)

• The external ear includes the auricle and external auditory canal.
• The middle ear includes the tympanic membrane and auditory ossicles (malleus, incus, stapes).
• The inner ear includes the cochlea (hearing), which is filled with endolymph.
• Sound waves vibrate the eardrum and ossicles amplify the vibrations.
• The cochlea converts vibrations into neural impulses via hair cells.
• Signals travel via the cochlear nerve (part of cranial nerve VIII) to the auditory cortex.

Equilibrium (Balance)

• Located in the vestibular apparatus of the inner ear: semicircular canals, utricle, and saccule.
• Maculae and cristae sense head position and movement.
• It detects changes in head position (static equilibrium) and rotation (dynamic equilibrium).
• Signals are sent via the vestibular branch of the vestibulocochlear nerve (VIII) to the cerebellum and brainstem.