The senses

The senses are our means of contact with the outside world. Although the senses can pick up a wealth of information, they are also very limited. Many animals can hear, feel and see things we cannot. Part of the reason is that we do not make the most use of our senses. If a person loses one of their senses the others suddenly seem to become stronger.

In reality, of course, they are simply being more efficiently.

The sense are of the greatest importance for our survival in modern society. A child who is born deaf or blind, for example, lags behind in many learning processes, both mental and physical. Not only are the senses evidently indispensable to the development of normal skills, but they provide our contact with other people.

The loss of sight or hearing can lead to enormous isolation. People often ask themselves: would I rather be blind or deaf? Most of them ‘choose’ deafness, because blindness has much stronger emotional associations.

Nonetheless, it has been found that people have more difficulty in adapting to deafness than to blindness; the absolute silence is almost unbearable. Moreover, background noises serve as a fixed point in the continuous, unconscious process of orienting oneself to the surroundings.

Perception

Perception is the means by which we become aware of the world around us. This awareness is achieved by gathering information via specialized nerve endings or ‘sense receptors’ situated all over the body. There are five classical senses: touch, taste, sight, smell and hearing. Through these five senses we are able to form a complex picture of our environment and interact with it. But our senses cannot react to all the messages coming from our environment, because they have a limited range of sensitivity. For instance the human eye can only see light waves from 400 to 760 nanometres in wavelength. This means we cannot detect – and therefore cannot react to – the presence of longer light waves like infra-red, or shorter ones like ultraviolet, X-rays or radioactive rays. That is why we do not notice that we have been burnt by the sun’s ultraviolet rays until the day after we return from the beach. Other species have different ranges of sensitivity: the honeybee can see light waves from 310 to 650 nanometres in length. The human ear can react to sounds with a frequency from 20 to 20,000 waves per second, whereas in the case of the bat this maximum is almost nine times as high! Apart from the classical senses there are also other senses of which we are much less aware. These are internal senses, concerned with the detection of changes in our internal environment. They monitor blood pressure, the temperature of the body, the amount of glucose in the blood, the position and movements of our body or parts of it (this is called proprioception) and balance.

Types of sense receptors

There are several types of sense receptors, each specially adapted to pick up information of a specific kind. This means, for example, that a punch in the eye makes us see stars: the photoreceptors in the eye can translate the mechanical stimulus only into vision. Photoreceptors are specialized nerve cells found in the retina, lining the inside of the eyeball. They are sensitive to light and colour and, with the aid of the brain, give us our sense of sight. Mechanoreceptors are sensitive to movement and pressure, and are found in the skin. They are responsible for our sense of touch. They are also found in hair follicles of the skin, where they sense minute movements of the hair root. One very specialized type of mechanoreceptor is the cochleal hair cell; 20,000 of these are found in the inner part of each ear, where they are sensitive to the vibrations produced by sounds. The movement of the cochleal hair cells stimulates the auditory nerve, which passes the nervous messages to the brain and allow us to hear. Chemoreceptors, sensitive to chemicals, are found in the tongue and mouth and are responsible for our sense of taste. Similar receptors in the nose give us the sense of smell.

Thermoreceptors are sensitive to temperature and are found both in the skin, where they sense the tempera- ture of things we touch, and in the blood vessels and tissues, where they sense body temperature. Nocioreceptors are sensitive to stimuli that threaten to damage the body in any way. For example, the pain receptors in the skin enable us to sense painful stimuli such as a pin prick or a burn. These receptors have a protective function and cause a reflex reaction on the muscles to remove the endangered part of the body away from the danger.

How strong does a stimulus have to be before it is painful? When does a warm sensation become a burning sensation? It is all a question of thresholds. The pain threshold is little understood. It may be altered by pain-killing drugs and may vary not only from person to person but also within the same person under the influence of strong emotion, such as anger, anxiety or fear.

Receptor thresholds

As has been noted, each sensory receptor responds to a specific stimulus. When it does so it sends a message or impulse via its nerve fibre to the brain for interpretation. The stimulus must be of a certain strength before the receptor will respond to it, but once it reaches that strength the same message will be triggered regardless of how strong the stimulus becomes. In other words it is an ‘all or nothing’ response. The level above which a message is triggered is called the stimulus threshold. The stimulus is described as either adequate (above the threshold) or inadequate . For example, a particular thermoreceptor in the skin will fire off a message whether it is in contact with a hot water bottle or a scalding kettle. However, with the latter the skin’s nocioreceptors will respond as well – and we feel pain.

Proprioception

Proprioception is the sense that conveys information about our ‘internal world’. It tells us what is happening within our own body; the position of our limbs, the movement of our joints and the activity of our gut, lungs and other internal organs. Our sense of balance is part of this system too. Mechanoreceptors – hair cells – in the organs responsible for balance in our inner ear are sensitive to the settling of small particles on them. This tells us the position and movement of our head and so enables us to balance. Much of the proprioceptive sense is at least partly subconscious. We do not have to think about the precise position of our limbs, the movement of our head or the placement of our joints in order to walk, nor do we have to think about the movement of our lungs to breathe. Yet without proprioceptors we could do none of these things.

Adaptation

When a stimulus is applied to a nerve ending which is of sufficient strength to trigger a response, a series of electrical impulses is fired and transmitted along the nerve fibre to the brain. Even if the stimulus remains constant, in most nerve endings the rate of discharge of these impulses tails off. This is called adaptation. A decrease in sensitivity to a stimulus may follow prolonged recent exposure to a similar stimulus. An everyday example of this phenomenon is the sensation provoked by a ring that has just been put on a finger. At first receptors in the skin constantly transmit impulses to the brain. After a while the frequency of these impulses gradually decreases and in the end the ring is not felt anymore. Most receptors have a strong propensity towards adaptation, such as the retina in the eye. If you stare at an object it soon becomes vague. Normally, however, one rarely perceives the process of adaptation. Because the eyes move constantly there is a different projection of an image on the retina all of the time. That is why the. Image does not fade out. Other receptors have little or no ability to adapt – the pain receptors for example. Adaptation is not the same as growing accustomed to certain stimuli. In contrast to adaptation, getting used to something is a function of the brain, of awareness. If we hear a constant high ringing tone during a conversation, after a while we do not notice it anymore, although our ears keep on receiving the sound and conveying the message. It is a matter of selection, of what we choose to hear. Another example is the so-called ‘cocktail party effect’. A person in a noisy room surrounded by other people talking can focus his or her attention on just one particular voice. The ears receive all the sounds; the selective discrimination is a function of the brain. More or less the same goes for pain stimuli. Although pain receptors do not adapt to stimuli, it is possible to regulate attention to their message. This is especially useful when one gets hurt in a dangerous environment and paying too much attention to a wound could be counter-productive. There are many reports, for example, of people who have been seriously hurt in an accident but do not notice their injuries until after rescue operations have been completed.