Physiology of gustation

• Taste buds are located on the tongue, soft palate, epiglottis, pharynx, larynx, esophagus, and upper part of the trachea.
• Taste receptors and specialized epithelial cells form sensory structures called taste buds.
• Superior surface of the tongue has epithelial projections called lingual papillae.
• Filiform papillae have no taste function but aid in food manipulation.
• There are four types of lingual papillae: filiform, fungiform, foliate, and circumvallate.
• Fungiform papillae contain taste buds that respond to all five tastes (salty, sour, bitter, sweet, umami).
• Foliate papillae are found at the back of the tongue and contain taste buds with a preference for salty or savory flavors.
• Circumvallate papillae are large and contain many taste buds, particularly sensitive to saltiness and bitterness.
• Taste buds contain basal cells (continually dividing to produce daughter cells that mature in stages) and gustatory cells (extend microvilli (taste hairs) through taste pores and survive only 10 days before replacement).
• The four primary taste sensations are sweet, sour, salty, and bitter.
• Umami refers to the fifth basic taste sensation, which is associated with glutamates and other amino acids.
• Receptor potentials generated from stimulation of taste buds are transmitted via afferent fibers to the brainstem nucleus, where they synapse onto second-order neurons.
• Salt and sour receptor molecules are chemically gated ion channels whose stimulation produces depolarization of the cell. This depolarization is known as a Receptor potential. These receptor potentials are graded (varying in magnitude).
• Receptors responding to stimuli that produce sweet, bitter, and umami sensations are linked to G proteins called gustducins that use second messengers to produce their effects.
• The binding of a tastant with a receptor cell alters the cell’s ionic channels to produce depolarizing receptor potential. Receptor potential releases neurotransmitter which initiates action potentials within terminal endings of afferent nerve fibres with which receptor cell synapses. Signals are conveyed via synaptic stops in the brain stem and thalamus to the cortical gustatory area.
• The number of tastebuds begins rapidly declining at age 50.
• The facial nerve (CN VII) innervates all the taste buds on the anterior two-thirds of the tongue. From the tip to the line of the circumvallate papillae.
• The glossopharyngeal nerve (CN IX) innervates the circumvallate papillae which is the posterior one-third of the tongue.
• The vagus nerve (CN X) innervates taste buds scattered on the surface of the epiglottis.
• The sensory afferent fibres carried by these cranial nerves synapse in the solitary nucleus of the medulla oblongata.
• Taste information from the solitary nucleus travels along the lateral lemniscus to the ventral posteromedial nucleus of the thalamus, then to the primary gustatory cortex located in the insula of the cerebrum.
• Axons of the postsynaptic neurons enter medial lemniscus Tract. (Gustation nerves)
• Sensory afferent neurons in the trigeminal cranial nerve V provide information about the texture of food, taste related information e.g. peppery or burning hot.
• After synapsing in the thalamus, the information is projected to the appropriate portions of the gustatory cortex of the insular.