Sensation and the Sensory Organs (Gustation, Olfaction, Somatosensation, Proprioception and Kinesthesia)

Gustation (Sense of Taste)

The gustatory system comprise of three crucial parts: the mouth, tongue and also the taste buds which helps us in perceiving the sense of taste. Both the gustatory system and the olfactory system (sense of smell) follow chemoreception process. The process of Chemoreception allows the human sense organs to interpret the various taste molecules for discovering various tastes or differences in flavours. The same applies to olfactory system even.

The specialized cells or the taste buds which are situated sense various tastants or the taste molecules. These taste buds send the information about these tastants to the brain, where these molecules or tastants are processed as specific tastes. Taste can be of five different kinds like sweet, salty, sour and umami (savory).

The Tongue and the Taste Buds

The main sensory organ of the gustatory system is the tongue which has taste buds by way of which one can feel the sense of taste. The tongue has papillae or epithelial cells which have the taste buds on the surface. Papillae are of three types:

• Fungiform papillae, which are mushroom-shaped and remain on the tip of the tongue;
• Foliate papillae, are the grooves and ridges which are exist at the back of the tongue;
• Circumvallate papillae, is circular in shape and located towards the end of the tongue in a row.

The flask like shaped taste buds is formed by gustatory cells and supporting cells. The gustatory cells have a short life and regenerate on a continuous basis. Each taste bud comprise of a taste pore on the tongue surface where the sensation of taste takes place.

Tastes: Tastes can be bitter, sour, sweet, salty and umami. In general sense, taste can be either pleasant or unpleasant depending upon the materials which is being tasted. For each flavour, there will be one taste receptor. Umami, sweet and bitter tastes, use similar taste receptors called GPCR or G-Protein coupled receptor.

Olfaction (Sense of Smell)

With the help of olfactory system we can perceive the sense of smell around us by inhaling the environmental odorants and translating them to neural signals. The olfaction is physiologically quite similar to the gestation system because of the use of the chemoreceptors which play a key role in discerning the information about the substances. The senses of taste and smell of various flavours are perceived at the same time due to a process of interaction called chemoreceptive sensory interaction. Because of this foods taste different, due to a compromised olfactory system. But anatomically, the olfactory system is different from the gustatory system.

The Nose and the Nasal Cavity

The olfactory epithelium is responsible for the odour reception to occur through olfactory receptor cells. The olfactory receptor cells are located near the septum, which is a dime-sized region in the nasal cavity area known as the olfactory mucosa. On an average, humans have about 10 million olfactory cells which are located in the olfactory mucous membrane. Each has 350 different types of receptors composed of mucous membranes receptor types and each 350 receptor type characterizes only one odour type. These function with the help of celia which contains olfactory receptor proteins, which are responsible for transductions and the neural processing of the electrical signals.

In the process of transduction, the olfactory receptors detect the odour molecules and transform the chemical signals to electrical signals for transmitting to the brain as neural impulses, as a result of which one perceives the sense of smell. Odour molecules have unique and individual features, as a result of which it is possible for the olfactory system to perceive or detect from broad range of odours or smells. Humans have the ability of being able to differentiate between 10, 000 different odours.

Somatosensation: Pressure, Pain and Temperature

Somatosensory organs help the humans to perceive the sensations of pain, pressure or temperature. Somatic is associated with the sense of touch and the system which studies the somatic sensations is known as the Somatosensory system. The skin is the most important and the most complex organ of the somatosensory system. Skin gathers information from the external stimuli, translates and interprets the information for the nervous system and allows a normal functioning of the human body. The touch receptors of the skin can be categorized into three: mechanoreception related to the sense of pressure, thermoreception which is responsible for the sense of heat and nociception related to the sense of pain. The somatosensory system is further supported by the proprioceptors which comprise of the receptor cells of the joints and muscles, but sometimes they are categorized into a separate category called as the Kinesthesia.

The Somatosensory System: This system uses specialized receptor cells of the skin and the body for detecting the environmental changes. The receptors convert the physical stimuli to chemical and electrical signals and send these signals to the brain for further processing through transduction process. The cutaneous receptors or the receptors of the skin, provide information about the senses of pressure, pain and temperature. The receptors of the joints and the muscles provide information about the muscle length & tension and joint angles.

• Mechanoreception: The mechanoreceptors provide a sense of texture and pressure. These receptors are of four types and vary in terms of field size (large or small) and the speed of the adaptation (slow or fast).
• Thermoreception: The thermoreceptors can be of 2 types i.e. warm and cold receptors, which help us in deselecting the temperature changes.
• Nociception: These receptors help in the detection of pain. The pain signals can also be categorized into three different types which correspond to three different types of nerve fibers which help in the transmission of these signals. The first type is called the first pain, which can be easily located and tolerated easily. It is also called the coetaneous pricking pain. The second type is called the burning pain or the second pain which is usually difficult to locate or cannot be tolerated easily. The third type of pain often described as the deep pain arise from the joints, musculature or viscera and is usually difficult to be located or is intolerable as well as chronic.

Proprioception and the Vestibular system

Apart from the 5 senses, the Vestibular system and Proprioception are the recent advances. Proprioception is the sense of the relative position of various neighbouring parts of the body and the effort applied for the movement. Kinesthesia is the sense of being aware of the position and movement of various parts of the body using various sensory organs. Kinesthesia is the main component which helps in describing the memory, muscle and hand-eye coordination. The vestibular system, which is situated around the inner portion of the ear, provides balance and provides a sense of spatial awareness. Both the vestibular system and proprioception, contribute to providing a sense of balance but differently.