Gustatory System

The gustatory system utilizes taste receptors in the oral cavity to differentiate various stimuli through several neurological pathways. The gustatory system is also intricately linked to many of the physiological responses that control gastric secretions, electrolyte balance and the avoidance of toxins.

The gustatory system is comprised of anatomical features of the oral cavity, taste receptors, facial nerves and regions in the brain. The anatomical features of the oral cavity include the tongue, salivary glands and structures involved with swallowing.

Taste receptors are found throughout the oral cavity and most notably on the tongue. Taste receptors are harbored within small, visible protuberances or bumps on top of the tongue called papillae or taste buds. More specifically, papillae found on the anterior portion of the tongue are called fungiform papillae for their broad, flat structures resembling a fungus, and those found on the posterior portion of the tongue are called circumvallate papillae, which are large, dome-shaped structures forming two rows of eight to twelve papillae across the tongue.

The facial (VII), glossopharyngeal (IX) and vagal (X) cranial nerves innervate the taste receptors of the oral cavity. Additionally, each cranial nerve innervates a specific region of the oral cavity. The facial (VII) nerve and its branches are responsible for innervating the anterior and lateral surfaces of the tongue as well as the roof of the oral cavity. The glossopharanygeal (IX) nerve and its branches innervate taste receptors on the anterior, lateral and posterior regions of the tongue. Finally, the vagus (X) nerve is responsible for innervating taste receptors on or in the regions of the larynx and epiglottis. When taste receptors are stimulated, the message is sent from the oral cavity via cranial nerves to an anatomical structure in the brain called the nucleus of the solitary tract (NST) located within the medulla and a portion of the pons. The NST acts as a relay center in the brain, transmitting sensory and motor information to and from the brain in response to gustatory sensations.

The neurophysiologic role of the gustatory system is expansive. Some of the functions of the gustatory system include, distinct taste sensations, maintaining homeostatic and energy levels, recognizing varying degrees of temperature, and palpable stimuli. These perceived stimuli are related to a complex milieu that also encompasses many visceral (autonomic nervous system) and motor reflexes.

There are four taste sensations, which include sweet, salty, sour and bitter. These four taste sensations enable the body, physiologically, to control intake and monitor nutrient levels. Monitoring salt and sugar concentrations is necessary for the body to maintain proper electrolyte and energy levels respectively. Additionally, bitter and sour taste sensations provide the body with a defense mechanism, as these sensations are often associated with poisons and toxins.

Taste sensations are perceived in various locations on the tongue, but some taste receptors exhibit more specificity than others. Taste sensations are detected using ion-channels, which are pore forming proteins that allow for the passage of ions, and G-protein coupled receptors—a system that utilizes ligand receptors on the cell's membrane to activate mechanisms within the cell.

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