Equine Gustatory System - Horse Anatomy

Introduction

Gustation is the sense of taste, and is a system involving chemoreceptors. The gustatory system can usually detect four different types of taste: bitter, sweet, sour and salt. These tastes are detected by taste buds that are contained within papillae, which are mainly found on the dorsal surface of the tongue. These tastes are relayed from the taste buds, via the olfactory nerve (CN I), to the brain.

The tongue is the main structure involved in taste. The tongue is covered by a lingual mucosa, which is tough, and most of its surface is covered with papillae. The papillae are a local modifictaion of the lingual mucosa. There are also a few taste buds present on the epiglottis and the pharynx. They are grouped according to their function: mechanical papillae are cornified and protect the deeper structures of the tongue, and gustatory papillae, which are covered in taste buds.

Group Name	Types of Papillae	Function
Mechanical Papillae	1. Filiform papillae 2. Conical papillae	1. The smallest and most numerous papillae 2. Larger, but less numerous
Gustatory Papillae	1. Fungiform papillae 2. Vallate papillae 3. Foliate papillae	Taste buds are contained within the epithelium of the gustatory papillae, the taste buds being sensitive to taste

See the link for further information on types of papillae: The Tongue and Taste Buds

There are salivary glands in the regions of the taste buds. These salivary glands remove small particles of food from the papillae, to make the papillae free for new food entering the mouth.

Taste Buds

Taste Bud - Copyright David Bainbridge

Taste buds are made up of a group of eptihelial cells, and are contained within papillae. They contain three major cell types:

1. Supporting (sustentacular) cells - these cells mainly form the outer layer of the taste bud

2. Gustatory cells - these cells are chemoreceptors, and are located in the centre of the taste bud

3. Basal cells

The soft palate, pharynx and nasal cavity contribute to taste sensation, mainly due to olfactory information. Other factors that contribute to taste are consistency and temperature of food. Taste buds can detect four different types of taste: salt, sweet, bitter and sour. There are no structural differences among the taste buds that detect these different types of taste. A taste receptor is a chemoreceptor that allows taste. There are two types of taste receptor:

1. Salt and sour (acid): ion channels

2. Bitter and sweet: G-protein coupled receptors (GPCRs) and ion channels

Each taste receptor allows a different sort of sensory transduction. After the taste receptors have detected the presence of a certain compound, they start an action potential which reaches the brain. These action potentials are conveyed to the brain via three of the cranial nerves:

• Facial nerve (CN VII): carries action potentials from the rostral two-thirds of the tongue
• Glossopharyngeal nerve (CN XII): carries action potentials from the caudal third of the tongue
• Vagus nerve (CN X): carries some of the action potentials from the back of the oral cavity

Sensory neurones synapse in the solitary nucleus of the medulla.

Vasculature

The main blood supply to the tongue is via the lingual artery, which is a branch of the external carotid artery. A secondary blood supply to the tongue is provided via the tonsillar branch of the facial artery and the ascending pharyngeal artery.

Innervation

The rostral 2/3 of tongue is innervated by the lingual branch of the trigeminal nerve (CN V), which is sensory supplying temperature, touch and pain. The chorda tympani of the facial nerve (CN VII) supplies the taste. The caudal 1/3 of tongue is innervated by the glossopharyngeal (CN IX), providing motor function for taste.

Central Gustatory Pathways

Central Gustatory Pathways - Copyright David Bainbridge

Receptor cells have a single receptor type, yet afferent nerve fibres carry information from several different cell types. This means that the brain has to re-discriminate between the tastes by cross-comparison between inputs from many fibres.

Salt

There is an ion channel in the wall of the taste bud cells, which allows sodium ions (Na+) to enter the cell. This causes depolarisation of the cell, which causes the opening of voltage regulated calcium ion (Ca2+) gates, causing calcium enters to flood into the cell, which then causes the release of a neurotransmitter.

Sweet

Sweet tastes are conveyed via G-protein coupled receptors (GPCRs). Sweet compounds such as saccharides activate the GPCR, which causes the release of a substance called gustducin, which itself then activates a molecule called adenylate cyclase, which is present inside the cell. Adenylate cyclase causes an increase in the concentration of the molecule cAMP, which itself will cause the closure of potassium ion (K+) channels. This will lead to depolarisation, and then the release of a neurotransmitter.

Bitter

Bitter tastes are conveyed via G-protein coupled receptors (GPCRs). Bitter compounds activate the GPCR, which causes the release of a substance called gustducin. Gustducin is made up of three subunits, which, when activated by the GPCR, break apart and activate a local enzyme, phosphodiesterase. Phosphodiesterase then converts a precursor within the cell into a secondary messenger, which itself causes the closure of potassium ion (K+) channels. The secondary messenger can also stimulate the endoplasmic reticulum to release calcium ions (Ca2+), which help to cause depolaristion. Depolarisation leads to accummulation of potassium ions within the cell, then depolarisation, which leads to release of a neurotransmitter.

Sour

Sour taste indicates the presence of acidic compounds. Three different recptors are present for the detection of sour taste:

1. An ion channel that allows hydrogen (H+) ions to flow into the cell.

2. A potassium ion (K+) channel, which allows potassium ions to escape from the cell. These channels are blocked by hydrogen ions, so the potassium ions are trapped inside the cell.

3. A protein which opens to sodium (Na+) ions when a hydrogen (H+) ion attaches to it. This allows sodium ions to flow down its concentration gradient into the cell. This influx allows opening of the voltage regulated calcium ion (Ca2+) gates

These receptors work together, leading to depolarisation of the cell, which then leads to the release of a neurotransmitter.

Histology

Filiform Papillae - Copyright John Bredl

Circumvallate Papillae - Copyright John Bredl

Foliate Papillae - Copyright John Bredl

The tongue is lined by stratified squamous epithelium.

• Filiform Papillae: no glands, no taste buds, no lymphatic tissue.
• Circumvallate Papillae: contain glands, taste buds and lymphatic tissue. The glands open into the moat around the papillae. The taste buds are present on the sides of the papillae. The lymphatic tissue is found deeper into the papillae.
• Foliate Papillae: form a series of parallel folds. They contain glands, taste buds and some lymphatic tissue. The glands are found deep inside and between the papillae. The taste buds are found on the sides of the papillae.