Taste receptor cells. These are clustered in taste buds



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Physiology of Taste

Taste is ability to respond to dissolved molecules and ions.

Human detect taste with taste receptor cells. These are clustered in taste buds. Each taste bud has a pore that opens out to the surface of the tongue enabling molecules and ions taken into the mouth to reach the receptor cells inside.

Adults have 3000 to 10,000 taste buds, and children have a few more. Beyond the age of 45 years, many taste buds degenerate, causing the taste sensation to become progressively less critical in old age.


Properties of the taste system

  • A single taste bud contains 50-100 taste cells representing all 5 taste sensations.

  • Each taste cell has receptors on its apical surface. These are transmembrane proteins and ions.

  • One type of receptor may be more active than the others on the same cell. And no single taste cell contains receptors for both bitter and sweet.

  • A single sensory neuron can be connected to several taste cells in each of several different taste buds(each sensory neuron responds best to one of the 5 taste sensations).

There are five primary taste sensations: salty, sour, sweet, bitter, umami.


  1. Salty

(Like table salt) the receptor is an ion channel that allows sodium ions (Na+) to enter directly into the cell. This depolarizes it allowing calcium ions (Ca 2+) to enter and triggering an action potential in the attached sensory neuron.

  1. Sour

The proton(H+) liberated by sour substances(acids) block potassium channels thus interrupting the normal outflow of K+ that creates the resting potential of the cell. The resting potential of the cell is reduced and if this reaches threshold, an action potential is generated in the attached sensory neuron.

  1. Sweet

Sweet substances(like table sugar) bind to G-protein-coupled receptors at the cell surface.

  1. Bitter

The binding of substances with a bitter taste (like quinine) also takes place on G-protein-coupled receptors at the cell surface.



  1. Umami

Umami is the name for the taste sensation produced by amino acids such as glutamate which present in meats and cheeses.

The binding of amino acids, including glutamic acid ,takes place on G-protein-coupled receptors at the cell surface.

Taste is brought to the brainstem by 3 different cranial nerves:


  • Facial Nerve for the anterior 2/3 of the tongue.

  • Glossopharyngeal Nerve for the posterior 1/3 of the tongue.

  • Vagus Nerve for the small area on the epiglottis.




Physiology of Vision

The sense of vision is a very complex sense involving several processes that occur simultaneously to allow us to perceive the world around us. The eyes are complex sense organs . each eye has a layer of receptors which able to distinguish differences in the wavelength or color of light( present in the retina of the eye) , a lens system that focuses light on these receptors, and a system of nerves that conducts impulses from the receptors to the brain. The ability of any lens to refract properly depend on 2 factors:

1. Distance of the object from the lens.

2. Shape of the lens.

The iris adjusts the intensity of light by adjusting pupil size. Extrinsic eye muscles work so that both eyes are directed to the same point.

When light rays strike the retina, they stimulate chemical reactions in the rods and cones (photoreceptors cells) This chemical reaction generates an electrical impulse (nerve impulses) through optic nerves and then to visual areas of the cerebral cortex in the brain.



Errors of vision:

  • Astigmatism : results in an irregular curvature of cornea or lens, where parts of the visual field appear blurred.

  • Myopia (nearsightedness) : occurs when distant objects appear blurred. It happens when the eyeball is too long or the lens too thick.

  • Hyperopia (farsightedness): occurs when near objects appear blurred. It happens when the eyeball is too short or the lens too thin.

  • Color Blindness : one or more classes of cones are non-functional and males get it more than females.





Snellen eye chart


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