The skin is our largest sense organ, and is certainly not as simple as many people assume. The skin can detect differences in temperature, light touch, pressure, position, and dangerous or noxious stimuli that threaten the body and which are perceived as pain. We can locate such sensations to the patch of skin where they occur – in some cases, in the fingertips, for example, to within a millimetre of the actual stimulus. Sensations are the conscious registration and interpretation of stimuli from the external or internal environment and are an essential part of normal life. A sensation is the sum effect of a chain of events starting peripherally at the site of the stimulus and ending in the higher centres of the brain. It takes only a fraction of a second for a stimulus impinging on th skin to be perceived, identified and localized by the brain despite the large number of steps involved. The stimulus is detected by a receptor, which collects, samples and converts various forms of energy into electrical impulses (the mechanical equivalent of this is a ‘transducer’); these impulses are carried from the receptor by the sensory nerves to the brain; we become conscious of the impulses, and feel the sensations, when the impulses finally reach the cortex of the brain.
Millions of receptors situated both inside and on the surface of our bodies are receptive to all types of information. Those in the skin are known as ex-teroceptors and are stimulated by the immediate external environment.
There are several types of receptor in the skin. They are buried at different depths and each is specifically designed to respond to one (occasionally more than one) type of stimulus: chiefly touch, pressure, pain, temperature and various types of vibration. Some receptors are actual nerve, or neurons, which are modified in shape and function to detect a specific stimulus. Others are devices originating from skin tissue that have intimate connections with the neurons of the nervous sytem. These are known as encapsulated endings.
Examples of encapsulated endings are Krause’s end-bulbs, Meissner’s corpuscles, and Pacinian corpuscles, which were identified and described by the anatomists and microscopists whose names they bear. Krause’s end-bulbs detect touch and cold and are microscopic; Meissner’s corpuscles are also microscopic and are stimulated by touch and pressure; Pacinian corpuscles are up to 2mm long and respond to touch, pressure and high-frequency vibrations. Free nerve-endings that terminate in the epidermis detect pain, temperature and also touch. Nerve endings wrapped around the roots of hairs, in the hair follicles, detect touch as a result of movement of the hair shaft. In addition some nerve endings are expanded into specialized structures such as Merkel’s discs, which detect touch and pressure, and Ruffini’s corpuscles, which detect touch and warmth. These various receptors occur over most parts of the body although some areas, such as fingertips, lips and genitals, have higher densities of certain receptors making them extra-sensitive. Modern research shows that receptors may not be as rigidly compartmentalized as in the above description: it is more likely that the brain acts on the pattern of messages it receives from the skin, rather than on messages from one or two individual types of receptor.
Coding of the information
A stimulus detected by any one of the receptors activates a specific part of the brain. All nerves from a certain type of receptor join up and travel along the same route within the spinal cord to the brain – this is known as a ‘sensory pathway’. If the pathway for a particular sensation, such as pain, is stimulated then the brain identifies the stimulus as coming from the periphery of that pathway – regardless of where the actual stimulus is sited. For example, if the area of the brain that controls the left hand is stimulated directly by electrodes, the resulting sensation is felt in the left hand and not in the head. This explains why people who have had limbs amputated feel ‘phantom sensations’ coming from the site of the amputated limb. These sensations originate in the cut nerve fibres of the amputation stump, but the brain still interprets them as coming from the skin of the limb, and therefore ‘thinks’ that the limb still exists.
In the brain, specific areas of the outer layer (cortex) correspond with specific areas of the body and are responsible for interpreting all sensations originating in that part alone. Thus we are able to ‘map out’ the parts of the body on the cortex of the brain.
The ‘sensory pathway’ mentioned above is much more than a simple bundle of nerves running up to the brain from the spinal cord. One example should starve to demonstrate the complexity. Sensory nerve fibres carrying pain and temperature information enter the spinal cord at what is called the posterior nerve root. As they enter, they branch into two pathways. One branch connects to motor nervethat pass back out of the spinal cord and run to the muscles near to the origin of the impulse. This aspect of the pathway produces reflex activity: when a dangerous stimulus is detected, impulses travel to the spinal cord and then straight back to the limb, which activates the muscles to withdraw it from the stimulus. The other branch of the pain and temperature fibres connects to neurons within the spinal cord, called second-order neurons. Their fibres immediately cross over to the opposite half of the spinal cord before running upwards along a distinct tract (the sensory pathway) to a receiving area in the brain, called the thalamus. Here another set, the third-order neurons, relay and distribute the impulses to the cortex. The crossing-over of nerve fibres at the point they enter the spinal cord means that impulses travel to the thalamus on the opposite side to which they were received. For example, someone who suffers spinal cord damage on the left side may only lose pain and temperature (and other) sensations in the right side of the body, usually below the level of damage.
Dermatomes and referred pain
During embryonic development, many parts of the body originate from the same tissue but subsequently move or ‘migrate’ to their adult positions – often taking their nerve supply with them. For instance, some of theof the arm have the same origin as those of the heart. Pain felt in the heart, as in a heart attack, may confuse the brain and may be perceived as coming from the related in the arm. This phenomenon is known as ‘referred pain’. Another example is the diaphragm, which develops embryo-logically in the region of the neck and shoulder and subsequently moves to its adult location in the abdomen; hence anything irritating the diaphragm may be felt in the neck and shoulder.
The skin may be marked off into transverse segments, called dermatomes, with the nerves from each dermatome entering the spinal cord at a certain level. This reflects our humble evolutionary origins from invertebrate creatures such as the worm, in which segmentation is rather more obvious.