The Anatomy of the Tongue and Gustation Process
Over the past days, I have been carefully explaining the anatomy of the sense organs. I started with the Ear, in the topic The Anatomy, and Physiology of the Ear, to the eye, starting with the Anatomy and Physiology of the Eye, then I went further to explain how light ray is converted to electrical signals from the Retina to the Optic nerve, in the post Explaining Phototransduction in the Eye. Taking it further today, We will be discussing the anatomy of the Tongue and Gustation.
The tongue cannot be overlooked as one important sense organ of the body, seeing that the tongue responsible for taste. But we should also know that the tongue has a lot to do with our speech accuracy, pronunciation and speech functioning, and Food Manipulation Functions
The Tongue Anatomy
The tongue, held by muscles called the Hyoglossus Muscle to the hyoid bone, and divided into two part by a V-shaped structure known as the Sulcus terminalis. The two parts are the anterior and the posterior part of the tongue. The anterior part of the tongue is two-third of the tongue, while the posterior part of the tongue is one-third of the tongue. From the styloid process close to the ear, is the muscle called the styloglossus muscle that runs to the tongue, helping the tongue to retract back when it is stretched. Still on the muscle, there is the palatoglossus muscle that connects the tongue to the soft palate. The palatoglossus muscle pulls the base of the tongue to the palate when swallowing bolus. From the Mandible [chin] to the tongue is another muscle called the Genioglossus muscle. The Genioglossus muscle is responsible for the protruding/sticking the base of the tongue anteriorly (the process of sticking out) of the tongue.
The Gustation Process
The tongue is made up of taste buds (something we are familiar with) known as Fungiform Papillae, which are found virtually everywhere but majorly in the dorsal part of the anterior two-third of the tongue. Just on the V shaped Sulcus terminalis is another set of taste buds called Vallate papillae/Circumvallate Papillae. In children, there is another special taste bud, called Foliate papillae which has to do with helping to taste milk. The Filliform Papillae is another papillae which doesn't have taste buds in them, but are responsible for friction. The Filliform Papillae is the most abundant papillae in the tongue, basically found in the dorsum part of the anterior two-thirds of the tongue. The taste buds goes down to the epiglottis and pharynx.
The Taste bud of the tongue is made up of several chemosensory cells known as taste cells, connected to synapses and linked to the dendritic processes. The Synapse help the taste cells to get taste. Tastes can be categorized into sweet, salty, sour, bitter, and umami (associated with beef tastes).. The Sweet, Umami, and Bitter, taste triggers the G Protein-Coupled Receptors which is bound to Transducin which then activates phospholipase C which is an effector enzyme which generates the messengers, Diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (InsP3). Diacylglycerol activates protein Kinase C which initiates phos-phorylation cascade of protein channels. Inositol 1,4,5-trisphosphate (InsP3) on the other hand activates the Smooth Endoplasmic Reticulum channels to produce calcium which will signal neurotransmitters including noradrenaline and serotonin to stimulate the nerve terminal synapses. Like I said above, the synapse then links to the dendritic process, and then to the cranial nerves (I will be discussing cranial nerves in the future)..
The cranial nerves responsible for transmiting signals from the tongue to the brain are the Cranial Nerve VII, Cranial Nerve IX, and Cranial Nerve X. For the sake of ease, the Cranial nerve VII is the Facial nerve, The Cranial Nerve IX is the Glossopharyngeal nerve, and Cranial Nerve X is the vagus nerve. The Cranial nerve receive taste signals from the anterior two-third of the tongue, where the taste signals are taken through the Chorda Tympani to the Nucleus of tractus Solitarius in the Pons at the brain steam and in the Medula of the brain. The Cranial nerve IX carries sensory taste signals from the Vallate papillae/Circumvallate Papillae, and taste buds (Fungiform Papillae) from the posterior part of the tongue. This sensory taste signals are carried through the Petrosal ganglia/inferior ganglia and sent to the Nucleus of tractus Solitarius in the Medulla.. The Cranial nerve X which is the vagus nerve pick sensory signals/fibers from the taste buds in the epiglottis, soft pallets, and the pharynx. The Nerve runs through the Nodose ganglion and through the jugular foramen then to the Nucleus of tractus Solitarius in the Medulla.
From the Nucleus of tractus Solitarius in the Medulla, the information is sent to the pontine taste area, after which the pons will send the signals to the ventral posteromedial nucleus in the thalamus, and send it to the primary gustatory cortex in the insula where the conscious perception of taste is felt. It is important to know that both Olfactory and Taste pathways are intertwined..
It is important to know that the tongue is made up of several muscle fibers running through it which allows it to form into shapes, when doing different processes such as chewing, speech, swallowing and other tongue activity. It is important that the tongue muscles are innervated by the Cranial Nerve 12. I can see that in my posts, there have been a lot of mentions of Cranial nerves, and I promise that once I am done with the sense organs, I will pick up the Cranial Nerves.