Nervous System
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2.27 Taste – The tongue is covered with small, rough bumps called papillae. Each papilla is covered with taste buds, clusters of taste sensors. Taste receptors sense chemicals – classified as sweet, sour, salty, or bitter – in foods and beverages. When chewed-up food mixed with saliva washes across the papillae, the chemicals in the food stimulate the taste receptors. The receptors transmit nerve impulses to the brain through the gustatory nerve, and the brain interprets the taste. Babies have about 10,000 taste buds, but this number decreases as a person gets older. There are other reasons why the ability to taste can diminish. For example, there is good evidence that long-term cigarette smoking damages taste receptors, adding to the loss that normally comes with age.
The Vision
2.28 Human beings have excellent eyesight. In fact, only birds can see better than humans can, although squids and octopuses can see just as well. Your vision is a response to light falling on the retina of the eye. Light rays enter your eye through the pupil and are focused onto the retina, which is actually an extension of your brain. Nerve fibres of the retina join to form the optic nerve, which transmits nerve impulses to the occipital lobe of your brain. Your brain then interprets these impulses as an image, and you “see” it.
2.29 Eye – The outer surface of the eye is covered by the cornea, a transparent membrane that protects the eye but allows light to enter. Behind the cornea is the iris, a thin, coloured, circular membrane. In the centre of the iris is an opening called the pupil. Muscles in the iris control the amount of light that is let in by altering the diameter of the pupil. The pupil dilates, or enlarges, to let more light in and contracts to let in less light. Behind the pupil is the lens, a thick, curved structure that focuses the light rays so that they will fall on the retina, the thin lining on the back of the eye. The image on the retina is inverted.
30. Receptor Cells – There are approximately 1 billion receptor cells in the retina. Rods are cells that are extremely sensitive to light; however, they cannot detect colour, and they produce poorly defined images. Cones are cells that detect colour and produce sharp images. At the centre of the retina is a concentration of about 3 million cone cells. People usually move their eyes so that the light rays from an object they want to see fall on this area. There are no cones or rods at the point where the optic nerve enters the retina. Light that falls on this area does not stimulate any sight receptors, resulting in a “blind spot” in the field of vision.
