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2009-06-05
TOUCH is fundamental to life


Touch is so fundamental to life that most of us never think of how the many sensations we feel are produced: how, for instance, we can tell silk from sandpaper, or recognize an object simply by the way it feels on the skin.

Q Why do babies touch everything around them as well as looking at them?

A As babies we train our brains to be able to match the sight of an object with its feel. When older, these earlier experiences enable us to predict what the texture of an object or surface is without touching.

Q When someone has had a stroke and is paralyzed down one side, does this mean that he or she will have lost the sense of touch on that side?

A Not necessarily. Some people who have been paralyzed by a stroke will have retained their sense of touch on that side, provided the damage has been confined to the movement control parts of the brain. If the area of damage is sufficiently large to have involved the touch analysers in the brain or their connections, then the sense of touch will be damaged.

Q I have noticed that on very cold days my sense of touch is poor. Why is this?

A In very cold weather two things are working against the touch receptors just below the skin in the fingers. First, the cold itself will be reducing their efficiency, and second, blood will have been diverted away from the skin in order to minimize heat loss; this relatively poor blood supply will further impair the ability of these nerve endings to send concise messages to your brain.

Q Is it true that blind people have a better sense of touch than the sighted?

A A blind person will have the same equipment in his or her nervous system for touch perception. What makes blind people able to use it more effectively than those with full sight is the practice that this sense has had in the absence of sight. The brain has come to rely on touch to a much greater extent, and so the analysis of touch has become more efficient, enabling, for example, the rapid reading of Braille.

Touch is one of the first ways in which young babies explore their world, and it remains our most intimate way of rela­ting to our environment. It is through a wide range of receptors in our skin, sen­si ti ve to different types of pressure, that we are continually able to monitor our immediate surroundings and keep our brains 'in touch' with the surfaces on which we sit, the objects we grasp, and so on. However, our sensation of touch is complex and is therefore sensitive to disturbances in many parts of the ner­vous system.

least sophisticated in structure and rapidly stop firing if the hair continues to be stimulated. Receptors found in greater numbers in the hairless part ofthe skin. for example on the fingertips and lips, are formed into tiny discs. Because the nerve fibres are embedded within these disc~ they respond more slowly to pressure and continue to fire when the pressure i~ maintained. Other more structurally complicated receptors are formed by many membranes being wrapped around a nerve ending like an onion skin, ane give responses to more maintainee pressure. In addition, all the receptor~ tend to be influenced as to what in­formation they send into the nervou~ system by the temperature at which the:. are operating. This explains why ow sense oftouch tends to be impaired in cold weather.

The distribution of the different typeo of touch receptors reflects their particulc: job. The receptors around the base ofb06: hair send messages from large areas ::

The sensory receptors

Just below the surface of the skin there are many nerve. endings whose varying degrees of sensitivity allow the nervous system to be supplied with different types of touch sensations.

Wrapped around the base of the fine hairs of the skin are the free nerve end­ings which respond to any stimulation of the hair. These touch receptors are the

The touch receptors in our skin are sensitive enough to respond to the gentlest stroking of a blade of grass.

From an early age we use our sense of touch to help us to become acquainted with the shape and feel of things around us - even

a familiar object like Mum!

the skin about the pressure stimulating them. They rapidly stop their flow of information once we have been warned of the presence of objects, for example, an insect on the skin. On the hairless skin the more sophisticated receptors give continuous information, allowing objects to be felt as the brain assembles this informHtion into a coherent picture.

Parietal lobe

Cerebral Touch pathways

cortex

 

Touch receptors in the skin relay their messages to the cerebral cortex via two specific pathways in the spinal cord: one for well-localized touch sensations; the other more diffuse touch.

 

Hairless skin

Midbrain

Merkel's disc Iwell·localized touch)

Analysis in the spinal cord

Some of the fibres conveying touch in­formation pass into the spinal cord and, without stopping, go straight up to the brain-stem. These fibres deal mainly with sensations of pressure, particularly a specific point of pressure. They there­fore need to send their messages rather directly to the higher centres ofthe brain,

so that this well-localized sensation can Brainstenl be assessed without confusion from any

analysis in the spinal cord.

Other nerve fibres bringing informa­tion of more diffuse touch enter the grey matter of the spinal cord, and there meet a network of cells which perform an initial analysis of their information. This is the same area which receives messages

from the pain receptors in the skin and

elsewhere. The meeting in the spinal cord

of messages dealing with both touch and Spinal cord pain allows for the mixture of these two

sensatioris and explains such events as

the relief of painful stimuli by rubbing.

This spinal cord analysis filters the

Seeing is believing, but touching is twice the fun when you come face-to-face with a 'real-live' woolly sheep for the very firsr time (top left).

The various touch receptors in our skin aTE very discerning. If a shape is drawn on thE hand,fur instance, we can often tell what ir is without looking (top). This is obvious ly important to someone who is blind, as the ability to define shapes is fundamental to reading Braille (above).

We can all get enjoyment from the way

" things feel- from the delicious sensation 0.1 i kneading soft, pliable modelling clay (lefr ~ to the decidedly more delicate sensuousnt 5 of soft, tactile material worn next to the ski:. (far left).

sensations which are then sent upwards to the brain. The grey matter of the spinal cord here acts as an electronic gate, so that pain information can be suppressed by the advent into the cord of certain types of touch impulse, limiting the amount of trivial information that needs to be transmitted to the higher centres.

This division of the touch pathways to the brain into two streams - one of which goes fairly directly up to the brain-stem and the other which is first analysed by the cells of the spinal cord - enables the fine discriminating aspects of touch to be preserved. We can, therefore, estimate accurately the amount of pressure in a touch and its position, but if the pressure

is too great or too sharp. the pain ana­lysers become inyalwd through the con­nections in the spinal cord and tell us that the touch is painful as well.

The sensory sorting house

Whether the tauch sensations from the skin have came by the more direct route or after analysis in the spinal cord, they eventually end in the campact knot of grey matter deep in the centre of the brain, called the thal2mus.

The direct toucr. fibres \\'ill have al­ready relayed once in the brain-stem and then will haw crassed a\'er to the other side, streaming ta' the thalamus in a compact bundle. H.e 'Other fibres will have crossed O\'er to the appasite side of the spinal cord afte,' their relay in the grey matter there: SD all 'Our touch sen­sations fram 'One side 'Of the body are

~. analysed by appasite sides 'Of the brain.

~ In the thalamus ~hese pieces of in­j formation from \'2riaus different types of ~ receptor in the ski:cl are assembled and ; co-ordinated. This e::-.ables the brain's if. highest centres i:cl ~l-.e cerebral cortex to

put together a picture 'Of the sensations of touch of which \\'e became conscious.

The final analYSis

The area of the braiL \\'hich enables the complex array 'Of touch sensations entering the neryaus s\'stem to be con­sciously perceiwd is the middle section of the cerebral cortex. Like all other sensory information, touch is analysed by the cortex in a series 'Of steps. each increasing the complexity 'Of the sensory perception. From the thalamus. the raw data is projected to a narr,,\\' strip in the front of the parietal lobes.

This primary sensory area of the cortex processes the infarmatian before passing it on to the secandary and tertiary sen­sory areas. In these latter areas the full picture ofthe site. type and significance of the touch sensations \ve feel is produced and correlated along with memories of previous sensatians. as well as sensory stimuli coming via the ears and eyes. The latter co-ordination is achieved easily since the areas for vision and hearing back on to the areas for touch.

The touch sensations are also, and very importantly, co-ordinated at this point with the sensations of what position our limbs, joints and digits are in: this is of importance since it enables us to deter­mine an object's size and shape and helps us to distinguish one object from another.

Problems

Damage to the nervous system at many different levels can alter our ability to feel and notice things that touch our skin. How this affects us depends to a large

extent on the exact place in the nervaus system that the damage occurs.

Damage to the peripheral neD-es. which may happen in diabetes 'Or in alcoholism, to give twa camman examples, can affect the sense 'Of tauch. However it takes quite extensive damage for the sense of touch to be lost completely or severely diminished.

Often people with such disorder'l notice pins and needles in their hands and feet for some time before any alteration in their sense of touch. The ability of the fingers to make fine touch discrimination may be involved, and sufferp~'s may report that it feels as if they have gloves on all the time. Instead of being lost or diminished, the sense of touch can also become distorted as a result of damage to the peripheral nerves, so that a sufferer may say that smooth surfaces feel like sandpaper or warm surfaces feel hot.

Much greater distortion of the sense of touch, however, arises from disease in the spinal cord, for example in multiple sclerosis. The cross connections which arise in the spinal cord ifit is diseased or even pressed upon from the outside produce distortions oftouch which can be quite disabling and unpleasant. Apart from noticing a feeling of numbness, the hands may have lost their ability to make properly co-ordinated touch perception, for example in picking the correct coin from a pocket, or the feet may feel as if they are walking on cotton wool instead of firm ground.

Similar types of symptoms can arise from damage to the same touch pathways through the brain-stem all the way to the thalamus. Thalamic damage, which happens after strokes, for example, can produce bizarre alterations of touch so that a simple pinprick produces un­pleasant spreading electric shock-like sensations or the gentle stroking of a finger may be felt as an unpleasant burning spreading over the skin.

Damage to the parietal lobes of the cerebral cortex, common in strokes and tumours of the brain, may disrupt touch sensations in other ways. If the thalamus is still intact (it is often involved in the disease as well) then the touch will be felt, but the localization of the touch will not be accurate - it may, for example, be felt on the other side of the body. If the parietal lobe is not functioning, the cor­relation of different types of sensation will not occur.

For instance, usually, when the hand or skin is drawn upon, a person will not have trouble in distinguishing letters and numbers, but someone with parietal lobe damage will not recognize the shape, although he or she will be aware that a touch has occurred.

 

Taken from The Marshall Cavendish A  to Z GUIDE IN WEEKLY PARTS,  DOCTOR’S ANSWERS: PART 88, TOUCH, Page 2420 to 2423.

 

(Sorry. Due to the urgency of education on this site, spelling will be corrected at a later stage….All photos in the  script have been left out) 

 


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