Explain the senses taste and smell ?

The pigmented cone cells of the retina transmit information to bipolar nerve cells, then to ganglion cells, and on to the optic nerve and the brain. Situated among the bipolar cells are amacrine cells, which form connections between neighboring bipolar cells and ganglion pairs. These cells adjust the sensitivity of the eyes according to light level.

Other cells called horizontal cells connect bipolar cells with photoreceptors, and serve in distinguishing between light and dark patterns. Impulses go from the optic nerve to the occipital lobe of the cerebrum, where the information is interpreted as vision.

Various degrees of colorblindedness occur if one or more of the pigments is missing. Colorblindness is inherited as a sex-linked trait. In females, the recessive allele on the X chromosome is offset by a dominant gene on the homologous chromosome, but the portion of the X-chromosome carrying the gene is absent on the Y-chromosome, so males exhibit the trait. Colorblind individuals usually cannot distinguish between red and green.

Taste : The sense of taste results from chemical reactions occurring in the 10 thousand or more chemoreceptors in taste buds scattered on the surface of the tongue. Most taste buds are found between projections called papillae on the surface of the tongue, but others occur on the roof of the mouth and in the throat. Humans can taste only sweet, sour, bitter, and salt.

Complex flavors are actually distinguished by our sense of smell. Stimulation of nerve endings in the taste buds result in messages carried by nerves to the cerebrum, where the sensation is interpreted.

Smell :  Chemoreceptors responsible for smell are located in the mucus-covered lining of the upper nasal cavity. The sense of smell in humans is surprisingly sensitive. Before the dendrites of specialized sensory neurons, or olfactory receptors, in the nasal epithelium are activated, substances must be dissolved in the watery mucus secreted by the mucus membrane.

Each olfactory receptor is specialized to respond to a certain chemical or group of chemicals. The nerve impulses travel through the olfactory nerve to the cerebral cortex, where the information is interpreted. The brain can interpret over 1000 different odors, some of which may be perceived through combinations of signals from close to 1000 different types of receptors.