Endocrine System

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2.26   Smell – Olfactory (smell) receptors are located in a dime-sized area in the roof of the nasal passages. They are able to sense minute amounts of chemicals in the air. These receptors are actually specialised neurons whose dendrites are modified into hair-like projections called cilia. Their axons transmit nerve impulses through an opening in the skull directly to the olfactory bulb in the forebrain. When a person has a stopped-up nose, little air passes over the olfactory receptor cells, resulting in a diminished sense of smell. Compared to some other mammals, humans have a poor sense of smell. For example, humans have anywhere from 5 million to 20 million olfactory receptors, while dogs have 40 million or more.

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.