The Special Senses

The Chemical Senses: Taste and Smell

  • Taste (Gustation)

    • Taste receptors are classified as chemoreceptors responding to chemicals in food dissolved in saliva.

    • Occur in taste buds, located mainly on the surface of the tongue within two types of papillae:

      • Fungiform papillae

      • Vallate (Circumvallate) papillae

    • Taste buds consist of:

    • 50-100 epithelial cells

    • Main cell types:

      • Gustatory epithelial cells

      • Basal epithelial cells

      • Contain long microvilli extending through a taste pore to the surface of the epithelium.

      • Cells in taste buds are replaced every 7-10 days.

  • Taste Sensation

    • Five basic tastes: Sweet, Sour, Salty, Bitter, and Umami (elicited by glutamate).

    • The ‘taste map’ is considered a myth; all taste modalities can be elicited from all areas containing taste buds.

  • Gustatory Pathway

    • Taste information reaches the cerebral cortex mainly through:

    • Facial nerve (VII)

    • Glossopharyngeal nerve (IX)

    • Some via the Vagus nerve (X)

    • The sensory neurons synapse in the medulla (solitary nucleus) and transmit impulses to the thalamus, ultimately reaching the gustatory area in the cerebral cortex (insula).

Smell (Olfaction)

  • Olfactory receptors are located in the olfactory epithelium (pseudostratified columnar cells) consisting of:

    • Olfactory sensory neurons

    • Supporting epithelial cells

    • Olfactory stem cells

  • Cell bodies of sensory neurons are in olfactory epithelium with apical dendrites that project to the epithelial surface, ending in a knob with olfactory cilia that act as receptors for smell. Mucus plays a role in dissolving odor molecules.

  • Olfactory Pathway

    • Axons of sensory neurons form bundles called filaments of the olfactory nerve (I), penetrate the cribriform plate of the ethmoid bone, and synapse with mitral cells in the olfactory bulbs.

    • Information is sent to the limbic system and primary olfactory cortex in the piriform lobe.

Disorders of the Chemical Senses

  • Anosmia: Absence of the sense of smell, caused by injury, colds, allergies, or zinc deficiency.

  • Uncinate fits: Distortions of smell or olfactory hallucinations often due to irritation of olfactory pathways.

Eye and Vision

  • Vision is the dominant sense in humans, with 70% of all sensory receptors located in the eyes. 40% of the cerebral cortex is involved with visual processing.

  • Accessory Structures of the Eye:

    • Conjunctiva: Transparent mucous membrane subdivided into palpebral and bulbar conjunctiva.

    • Lacrimal apparatus: Keeps the eye moist, includes the lacrimal gland (produces fluid) and lacrimal sac (empties into nasal cavity).

  • Anatomy of the Eyeball:

    • Composed of three tunics and internal cavities filled with specific fluids (humors).

    • Cornea bends light rays to form images on the retina.

  • The Vascular Layer:

    • Ciliary body encircles the lens and is involved in focusing light.

    • Contains ciliary muscles and ciliary processes.

  • The Inner Layer (Retina):

    • Composed of two layers:

    • Pigmented layer: Single layer of melanocytes.

    • Neural layer: Contains photoreceptor cells, bipolar cells, and ganglion cells, where photoreceptors signal the rest of the pathway leading to the optic nerve.

  • Photoreceptors:

    • Rod cells: Sensitive to light; allow vision in dim light.

    • Cone cells: Operate best in bright light; enable high-acuity color vision.

  • Visual Pathways:

    • Information travels from the retina through the optic nerve, synapsing at the lateral geniculate nucleus of the thalamus, and then to the primary visual cortex.

The Ear: Hearing and Equilibrium

  • The ear is divided into three main regions:

    • Outer Ear: Includes the auricle (pinna) and external acoustic meatus leading to the tympanic membrane.

    • Middle Ear: Contains ossicles (smallest bones: Malleus, Incus, Stapes). Tiny muscles (tensor tympani, stapedius) buffer loud sounds.

    • Internal Ear: Contains the bony labyrinth (semicircular canals, vestibule, cochlea) and membranous labyrinth (filled with endolymph).

  • Semicircular Ducts sense rotational movements; contain receptor cells that respond to acceleration.

  • Equilibrium and Auditory Pathways: Transmit information about head position and sound, influencing the balance and auditory processing in the brain.