Inferior Ganglion: Structure, Function & Diseases

The inferior ganglion switches fibers from the glossopharyngeal and vagus nerves. It is the first ganglion encountered by the two cranial nerves outside the cranial cavity and includes both the petrosal ganglion and the nodosal ganglion. The inferior ganglion is involved in gustatory and sensory perception. Nerve damage to the gustatory pathway can cause taste disorders.

What is the inferior ganglion?

Physiology uses the term inferior ganglion or inferior (vagus) ganglion to refer to several nerve clusters. They are located on the 9th and 10th cranial nerves, the glossopharyngeal nerve and the vagus nerve. The nerves previously meet the ganglion superius – inside the cranial cavity, but outside the central nervous system – and exit the skull inside the head, where they directly meet the respective ganglion inferius. Medical science originally delineated the ganglia more sharply; even today, the ganglion of the glossopharyngeal nerve is known as the petrosal ganglion, while the inferior ganglion of the vagus nerve is also called the nodosal ganglion.

Anatomy and structure

The petrosal ganglion or ganglion inferius nervi glossopharyngei belongs to the 9th cranial nerve. It is connected to the otic ganglion by several nerve fibers; this pathway is also known as the Jacobson anastomosis. The petrosal ganglion is located in the petrosal fossa. This bony fossa lies beneath the cranial cavity between the canalis caroticus, through which the internal branch of the carotid artery passes, and the fossa jugularis, the bony fossa of the temporal bone (os temporale). The fossula petrosa owes its nickname “fossula” to its relatively small size. The petrosal ganglion belongs to the gustatory pathway; its nerves innervate the posterior third of the tongue. The ganglion nodosum or ganglion inferius nervi vagi forms a switch point for the 10th cranial nerve. The vagus nerve carries general viscerosensory signals from the viscera to the ganglion nodosum. The afferent nerve pathways also travel from there to the superior ganglion and then to the brain. In addition, the vagus nerve includes specific-viscerosensitive fibers that transmit sensations from the root of the tongue (radix linguae) and epiglottis to the inferior nervi vagi ganglion.

Function and Tasks

The inferior ganglion represents a collection of nerve cell bodies. Preganglionic neurons transmit information passing through their fibers to postganglionic neurons; accordingly, in this context, the ganglion serves as a switching point of the peripheral nervous system. The petrosal ganglion includes nerve fibers that lead to the posterior third of the tongue, where they connect the sensory cells to the nervous system. The taste cells are embedded in so-called taste buds and react specifically to chemical stimuli. Food particles serve as triggers. The taste buds at the back of the tongue transmit information about gustatory stimuli to their axons in the form of electrical signals. This is the beginning of the taste pathway, which runs to the brain via the inferior nervi glossopharyngei ganglion and the superior nervi glossopharyngei ganglion. The nerve fibers belong to the 9th cranial nerve, the glossopharyngeal nerve. The nerves innervating the posterior third of the tongue are of great importance, since this part of the tongue carries most of the taste buds. If perception in this area fails, the sense of taste as a whole is severely impaired. The wiring in the inferior ganglion is usually not 1 : 1, but in a larger ratio. In this way, the inferior ganglion reduces sensory information from the respective sensory cells. If the taste buds in the tongue perceive only a weak gustatory stimulus, this may lead to an action potential in the first nerve fiber, but it may be lost in the downstream cell. Accordingly, a corresponding stimulus is below the perception threshold and does not lead to a subjective taste impression in the brain. Early filtering protects downstream neurons from overload and ensures that unimportant stimuli do not block capacities of the nervous system. Spontaneous activity is also filtered out as a result in normal cases

Diseases

The inferior ganglion plays a role in gustatory perception through its connection to taste cells in the posterior third of the tongue.Lesions on the nerve cells involved can cause the taste pathway to transmit only incomplete, no, or faulty information to higher processing centers. As a result, tasting disorders may manifest. The type of disorder depends on which nerve cells are specifically affected and whether other types of tissue may have suffered damage. The complete loss of tasting is referred to in medicine as ageusia. In the case of total ageusia, affected persons can no longer perceive any of the tastes (sweet, sour, salty and bitter), while partial ageusia leads only to the loss of certain gustatory qualities. Individuals with hypogeusia can taste, but perceive the taste as significantly weaker. The opposite of this is represented by hypergeusia: Affected individuals suffer from high sensitivity that significantly exceeds a normal-good sense of taste. All these tasting disorders form quantitative gustatory disorders. In addition, qualitative disorders of gustatory perception exist, occurring simultaneously or independently of them: Parageusia leads to misperception of taste stimuli, so that, for example, a sugary food tastes bitter. People suffering from phantogeusia, on the other hand, perceive a stimulus even though it is not actually present. Doctors can use electrogustometry to determine whether nerves on the tongue are damaged. In this procedure, they stimulate the nerves with a very weak electrical current. The causes of taste disorders are varied and do not necessarily have to be neurological in origin. Instead, they can also be the side effect of a medication or the result of another underlying disease.