THE OTHER SENSES

Smell

The sense of smell is activated by substances carried by airborne molecules into the nasal cavities, where the substances activate highly specialized receptors for smell, located in the olfactory epithelium. From there messages are carried directly to the olfactory bulb in the brain, where they are sent to the brain’s temporal lobe, resulting in our awareness of smells. Pheromones are sensed by receptors in the vomeronasal organ (VNO), which sends messages to a specialized olfactory bulb.


Figure 3-20

Taste

The receptor cells for the sense of taste are housed in the taste buds on the tongue, which, in turn, are found in the papillae, the small bumps on the surface of the tongue. Each taste bud contains a cluster of taste receptors, or taste cells, that cause their adjacent neurons to fire when they become activated by the chemical substances in food, sending a nerve impulse to the brain.

We experience only four primary taste qualities: sweet, sour, salty, and bitter. All other tastes derive from combinations of these four. Flavor is a complex blend of taste and smell.


Figure 3-21

Kinesthetic and Vestibular Senses

The kinesthetic senses relay specific information about muscle movement, changes in posture, and strain on muscles and joints. They rely on feedback from two sets of specialized nerve endings: stretch receptors, which are attached to muscle fibers, and Golgi tendon organs, which are attached to the tendons.

The vestibular senses control equilibrium and create an awareness of body position. The receptors for these senses are located in the vestibular organs in the inner ear. The sensation of body rotation stems from the three semicircular canals of the inner ear. The sensation of gravitation and movement forward and backward, as well as up and down, arises in the two vestibular sacs that lie between the semicircular canals and the cochlea.

Sensations of Motion

The vestibular organs are also responsible for motion sickness, which triggers strong reactions in some people. Motion sickness may be caused by discrepancies between visual information and vestibular sensation.

The Skin Senses

The skin is the largest sense organ, with numerous nerve receptors distributed in varying concentrations throughout its surface. The nerve fibers from these receptors travel to the brain.

Skin receptors give rise to what are known as the cutaneous sensations of pressure, temperature, and pain. Research has not established a simple connection between the various types of receptors and these separate sensations. Because the brain uses complex information about the patterns of activity on many different receptors to detect and discriminate among skin sensations, a direct connection between receptors and sensations has so far eluded researchers.

Pain

People have varying degrees of sensitivity to pain. The most commonly accepted explanation of pain is the gate control theory, which holds that a "neurological gate" in the spinal cord controls the transmission of pain impulses to the brain. Studies of pain relief suggest the existence of the placebo effect, which occurs when a pain sufferer feels relief from pain when given a chemically neutral pill but told that it is an effective pain reliever.