Nervous System

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2.23   Internal Sensory Receptors – Sense receptors located deep within the body enable a person to feel internal pain, hunger, thirst, tiredness, and nausea. Other receptors located in skeletal muscles, tendons, and the connective tissue surrounding joints are sensitive to changes in stretch and tension. They continuously relay information about body position, equilibrium, and movement to the central nervous system, and together they make up what is called the kinesthetic sense. Many internal sensations are not recognised at a conscious level. For example, some sensors monitor blood pressure; the information they send to the brain is not processed by the cerebrum, so an individual is not aware of this sensation.

2.24   Sensation and the Skin – Touch is the sensation produced by pressure on the surface of the body and is sometimes called the tactile sense. Pressure receptors are located all over the skin. On sensitive parts of the body, such as the tongue, lips and finger pads, their concentration is very high. Touch messages are transmitted by bundles of sensory nerve fibres that come through the subcutaneous layer to the dermis, or inner layer of skin. The bundle of fibres, some of which reach into the epidermis, or outer layer of the skin. These nerve endings in the epidermis are sensitive to pressure. Other sensory nerve fibres end in specialised receptors in the dermis, which sense temperature or pain.

 Smell and Taste

2.25   Your sense of taste and your sense of smell are both stimulated by chemicals. Their main purposes are to help you detect harmful substances and to stimulate your appetite and digestion. Even though taste and smell messages travel separately to different regions of the brain, they create a combined feeling of either pleasure or displeasure when you are eating. For example, think of how the aroma of a delicious meal can make your mouth water.

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.