Sensory System

Sensory systems are important to us; they let us perceive the environment. The senses can be broadly divided in to the sense of touch, smell, taste, vision and hearing. The seemingly simple perception is in fact not as simple as it sounds there is a lot of chemistry and physics that takes place. When we touch a hot surface, smell that delicious food, see beautiful colors, or hearing that beautiful music, our nervous system is working at an incredible rate, to make sense of these signals and let us enjoy them or run away from them.

Sense of Taste: Sense of taste is related to the sense of smell but there are only five kinds of tastes that we can detect. They are sweet, sour, bitter, salty and umami. Detection of taste is done via special epithelial cells in the taste buds of the tongue. In case of salty taste the sodium ions leads to opening of ion channels and the signal is sent as an electrical impulse to the brain. In case of sweet, bitter an umami 7TM receptors are involved, when a “tastent “ binds to these receptors they produce cAMP, which in turn open channel proteins which allow ions to pass through and the neuron send the impulse to the brain which detects it.

Sight: Sense of sight or vision is the ability to see, this is done through the photoreceptor cells, which are either Rods or cones. Rod cells contain a pigment rhodopsin, which in turn consists of opsin protein and retinal. When retinal is exposed to light it changes its conformation. This then lead to activation of a G Protein called Transducin. Transducin then breaks cGMP to GMP, which leads to closing of ion channels, sending an electrical signal to the brain. Cone cells on the other hand contain pigment specific receptors; there are receptors of red, green, and blue light.

Hearing: In our inner ear, we have a fluid filled sac, which is coiled like a snail. This sack is called the cochlea. The cochlea has small hair cells. These hair cells have many stereocillia of different lengths. When the sound waves hit these stereocillia, it changes the membrane potential of the hair cell due to the mechanical stress. The whole process is mechanical and the movement of stereocillia, which creates the membrane potential, is transported to the brain via nerves and is perceived as sound.

Touch: This includes sensing many different factors like pressure temperature and pain. Sense of touch is not very well understood. It is thought that the sense of touch is processed by pain processing centers in the brain and the spinal cord. The receptors, which sense this, are called nocireceptors. A study of spicy food and how it is perceived provided insight into the understanding of the sense of touch. It was determined that capsaicin receptors when activated open up cation channels leading to membrane potential and a nerve impulse in generated.

References

Jermey M Berg, John LTymokzo, Lupert Stryer. (2012). Sensory systems Biochemistry (7th ed,). New York : W.H. Freeman and Company.