The Tribune, Chandigarh, India

Sources of pleasure and pain

The five senses are vision, hearing, taste, smell and touch. Touch is a composite sense since, through it, we can perceive temperature, vibration, pressure, pinprick and pain, says Dr Robert M. Youngson

THIS list of the sources and routes of information input to the brain is not exhaustive and the sensory receptors also include those sensitive to head position in the inner ears, the stretch receptors in the muscles and tendons, those in the joints, and the pressure sensors in the neck arteries, aorta and heart that inform the brain of the blood pressure and volume. All the information passing to the brain is coded in the form of repetitive nerve impulses, and all these nerve impulses are the same. In this, sensory information differs radically from, say, the electrical information passing from an amplifier to a loudspeaker. Individual nerve impulses follow the all or none' law. This means that they occur either fully or not at all. There is no question of amplitude modulation as in the case of audio signals in a microphone cable. The only thing that can vary, when information is conveyed along a single nerve fibre, is the repetition frequency. The maximum repetition frequency of nerve impulses is not high — only a few hundred per second (Hz). Fibres in the acoustic nerve seldom fire at a frequency of more than 200 Hz. But in spite of this seemingly severe limitations, a considerable range of intensities (amplitude) can be represented to the brain by differences in nerve-impulse frequency. Sound frequency (pitch) variations are conveyed by differences in the location of the fibres stimulated.

In general, information in the nervous system is conveyed in terms of the numbers and location of fibres firing. Electronic information systems operate with single channels that are modulated in various ways to convey data. Bio-logical information systems, on the other hand, use bundles of thousands or millions of channels, each one representing to the brain a position or a particular type (modality) of sensation. Each optic nerve contains about one million separate fibres, every one coming from a particular point on the retina and informing the brain that point has, or has not, been stimulated by a photon of light. The point-to-point correspondence persists all the way from the retina to the visual cortex. The same kind of point-to-point correspondence occurs in all the senses. Medical illustrators have often drawn grotesque human figures to represent the mapping on the brain's sensory cortex of the skin sensation. Such figures are distorted because different weighting has to be given to the higher concentration of sensory nerve endings in different parts of the body, such as the tongue and the sex organs. Simultaneous pinpricks between the shoulder-blades must be separated by several millimetres to be appreciated as separate; on the fingertip they are felt as distinct even if very close together.

In electronic systems the coded information is converted back, at the receiving end, into a familiar form— sound, pictures, text on a screen. In the brain, no such conversion occurs. The coded information itself is perceived as sound, vision, touch, taste, smell, and so on. Thus the means by which information is conveyed to the brain differs fundamentally from the thing it represents. The ultimate philosophical question is whether our perception of the 'outer world' bears any meaningful resemblance to what is really out there.

Much of the information received by the brain is not perceived in the conscious mind. This is fortunate, as we have quite enough to attend to without being constantly informed of such parameters as the levels of oxygen and carbon dioxide in our blood or the degree of tension in the arteries supplying our intestines. Even the sensory modalities of which we are often fully aware do not intrude at all times on our consciousness. Touch and pressure sensation tend to get through to us only when a significant change occurs or when they become excessive. The stimulation of the nerve endings subserving pain may or may not even give rise to awareness of pain; much depends on the circumstances.

The nature of pain

Pain differs from the other kinds of sensation in that not only is it unpleasant but it usually has a major psychological component. Pain is a localised sensation caused by stimulation strong enough to damage tissue or to threaten damage to tissue. Unless chronic (long lasting), it commonly serves as a warning of danger and prompts action tending to end it. People who, because of disease, such as leprosy, tabes dorsalis (degeneration of part of the spinal cord, caused by syphilis), or other conditions affecting the sensory nerves, are unable to experience pain, invariably suffer serious and cumulative bodily damage.

The response to pain may be reflex, involuntary and rapid, or conscious, deliberate and purposeful. Persistent pain is usually associated with distress and anxiety — and often with fear and there may be physiological changes similar to those experienced during anger and aggression. The heart beats faster, the blood pressure and the rate of respiration rise, the pupils dilate and the skin sweats. There is an increased secretion of adrenaline from the adrenal glands and increased mobilisation of glucose from the glycogen stores that are contained in the liver.

Our perception of the significance of pain is often more related to these secondary effects than to the intensity of the pain itself. If pain is separated from its mental component, as is possible by the use of drugs such as morphine, it may still be felt but may no longer be unpleasant. The distress caused by pain depends also, to a large extent, on our awareness of the cause and is modified by past experience. Even minor pain inflicted by a torturer may seem more severe than the same physical hurt resulting from an innocent cause such as an accident.

The origins of pain

The nerve endings subserving pain are called nociceptors. These do not appear to differ physically from one another, but different nociceptors seem to respond to different kinds of painful stimuli mechanical, thermal or chemical. Tissue damage results in the release of various strongly stimulating substances such as prostaglandins, and these are the principal stimulators of nociceptors, causing the nerve fibres to fire and conduct impulses to the brain. Drugs such as aspirin inhibit the enzymes that cause the release of prostaglandins from damaged cells and this is how they act as painkillers. Aspirin has no effect on pain caused by pinprick, however, in which nerve endings are directly stimulated without the intermediate stage of tissue damage.

Different nociceptors show different sensitivities. Some are stimulated by lowgrade 'warning' events of insufficient force to cause actual pain. Others respond only to strong stimuli such as pricking, cutting or burning. The stronger the stimulus, the higher the frequency of the nerve-impulse sequence sent to the brain. If a nerve fibre subserving pain is stimulated, pain will be perceived in the area of the nerve ending, irrespective of how the nerve impulse originated. Such a fibre can be stimulated at a point much nearer the brain than the remote nerve ending. In the condition of post-herpetic pain following shingles, for instance, sensory nerves are stimulated near the spinal cord by an inflammatory reaction caused by herpes zoster viruses. This causes pain that is perceived as coming from the skin.

Although the nerves carrying pain impulses terminate in the brain, and give rise to neurological activity there, the pain is usually felt in the region in which the nerve endings are situated. If the conduction of pain impulses is prevented, by, for example, injecting a local anaesthetic around the trunk of the sensory nerve or near the spinal cord, no pain will be felt, although the damaging events at the nerve ending are continuing unabated. Referred pain is pain experienced in an area other than the site of the cause because the same sensory nerve supplies both areas. Gall bladder inflammation, for instance, causes pain in the tip of the right shoulder because both the diaphragm and the skin of the shoulder are supplied by the same nerve. After amputation, phantom-limb pain, experienced exactly as if the limb were still present, can occur from irritation to the cut nerve ends in the stump. The brain can interpret the resulting nerve endings only as coming from the lost limb.

Passage of pain nerve impulses may also be blocked by the arrival of impulses caused by the stimulation of other sensory nerves. These second impulses may be stimulated by rubbing, scratching or stroking the skin, by electrical stimulation applied through the skin, or by acupuncture.

The "gate" theory

Computers operate by an elaboration of logical gates through which a stream of electrical pulses (1s or -0s) passes. These impulses may be blocked by a secondary controlling electrical signal of the same kind or may be compared with other similar pulses. Most physiologists now accept that the nervous system contains analogous arrangements of neurons operating as gates, and that pain impulses travelling up the spinal cord pass through such gates and can be blocked, or allowed to pass, by controlling signals. It seems probable that all the neuronal signals that are concerned with pain must pass through such gates.

Nerve fibres carrying pain impulses may be large or small. Both, however, affect the state of the gates. The tendency is for small fibres to open the gates and large fibres to close them. Large-fibre stimulation also sends messages to a higher level, which in addition also act to close the gate. As far as we know gates are also probably under the control of the brain.

Contact lens
Have a nice look
By Anil Kalia

THE history of contact lenses reveals a rapid and dynamic evolution of new materials and designs over the past 100 years worldwide. Indeed, research and developmental efforts continue at a rapid pace. Further improvements and additions are expected.

Today, no single lens completely satisfies all of the criteria for every contact lens patient.

Contact lenses are designed to rest on the cornea, the clear outer surface of the eye, just in front of the iris. The lens is held in place mainly by adhering to the tear film that covers the front of the eye and to a lesser extent by pressure from the eyelids.

It is a well accepted fact that contact lenses create a ‘‘barrier effect’’ that naturally decreases oxygen supply to the cornea. As the eyelid blinks, it glides over the surface of the contact lens and causes it to move slightly. This movement allows tears to provide the necessary lubrication to the cornea and helps to flush away debris.

Contact lenses are optical medical devices which are primarily used to correct various refractive errors of the eye, including myopia (near sightedness), hyperopia (far sightedness), astigmatism variation in the shape (radius of curvature) of the cornea, and presbyopia (gradual loss of the ability to focus on near objects with age). In these conditions light is not focused properly on the retina, resulting in blurred vision.

In most cases, visual acuity with contact lenses should be equivalent to or better than the best corrected spectacle acuity. In some cases (i.e., uncorrected or residual astigmatism) it is expected that the visual acuity with contact lenses, compared to the acuity through spectacles, will not be as sharp or clear. Contact lenses have an advantage over spectacles: they reduce reflections and aberrations that are strongly associated with spectacles.

In night-driving the reflections from headlights of on-coming vehicles are lesser in well-fitted contact lenses than in the spectacles. Car drivers train attendants and other people moving in and out of air-conditioned zones (like operation theatres, computerised control rooms, etc.) benefit because the contact lenses stay at body temperature and do not fog.

On the other hand the spectacles get fogged on sudden exposure to a warmer environment having some level of humidity. In professions where microscopes, telescopes, gas masks, etc., are to be used, contact lenses are more practical to use than the spectacles.

Contact lenses allow a wider field of view than spectacles. First, there are no spectacle frames to block peripheral vision (side vision). Because the contact lens is actually in contact with the eye and directly follows eye movements, the optical correcting properties are maintained in all directions of gaze. Therefore, the pupil is centered behind the optical centre of the lens at all times which allows a wider field of view.

Contact lenses are superior to spectacles for aphakia patients, since aphakic spectacles are heavy, uncomfortable and cause various visual distortions. However, intraocular lenses (IOL’s) are now the optical correction of choice for aphakia due to their superior optical and clinical aspects.

Selecting the best contact lens for a given patient requires skill and training on the part of contact lens practitioner. The contact lens practitioner must select the best contact lens for each individual patient based upon the unique visual, ocular, and lifestyle needs of that person. Not everyone is a suitable candidate for contact lens wear — especially patients with a history of repeated eye infections, allergic reactions, or low tear productions. And as with any other medical device, contact lenses are not without a degree of risk of adverse effects. The major risks of contact lens wear include eye infection, corneal abrasion, and allergic reaction. Fortunately, most of these complications are not frequent and can be avoided by careful fitting and proper follow-up care by the practitioner and compliance with instructions by the patient. The needs of the hour are convenience, comfort and health.

Mr Anil Kalia is an optometrist at Government Medical College and Hospital, Chandigarh.

Immortal cells
By Tim Radford

PATIENTS crippled by stroke could next year have stem cells injected into their brains to restore movement in paralysed limbs.

The cells are part of an "immortal" stockpile of cells taken from an aborted foetus, treated with a gene taken from a monkey cancer virus, and grown in laboratory dishes by a British biotech company called ReNeuron.

Tissue from one foetus could be used to treat 1,000 patients, the British Association for the Advancement of Science's annual festival, Creating Sparks, at Imperial College, London, was told the other day.

There are 1,40,000 stroke victims a year in the UK. Blood clots interrupt the supply of oxygen to the brain and cells die.

Two years ago, scientists learned how to manipulate stem cells — the natural machinery that allows a fertiliser egg to turn into a human composed of 100 trillion cells of more than 200 different kinds.

But stem cells are a challenge to science. They make new blood, liver, or nerve tissue, but there is nothing to distinguish them from the cells around them.

"We define them on the basis of their behaviour and what we know now is that they exist throughout our lives," said Dr John Sinden, chief scientist of ReNeuron. "Stem cells are normal cells. They keep dividing but they can only divide a limited number of times, so that we gradually lose them through life."

Stem cells also exist in the brain. Most of the body rejects transplanted tissue, but other experiments have shown that the brain will accept foreign cells.

The ReNeuron researchers injected mouse brain stem cells into rats damaged by stroke, and watched the rodents recover. The hope is that under the right conditions, foetal brain stem cells implanted into the brains of adult stroke victims will move around and begin repairing damaged parts of the brain.

The scientists used a mutant gene from a simian virus to "immortalise" the donated foetal stem cells, and kept them multiplying at 33C. The gene is switched off at 36C. The normal human temperature is 38C. So once the stem cells were implanted in a human brain, they would stop multiplying indiscriminately — that is, they could not start a tumour. But they could, in theory at least, begin to form new cells for injured brains.

"The brain is a highly specialised organ and it seems to have lost the capacity to use stem cells through evolution, probably because it is a specialisation: it needs to retain a kind of fixedness about its make-up," said Dr Sinden.

"But nevertheless it retains a capacity to respond to stem cells. So if we can actually apply stem cells, then the brain will use those stem cells, particularly the damaged brain."

The ReNeuron stem cells are capable of making the three major cell types in the brain and have already been tested in animals. "The cells get into the brain and migrate. They differentiate, they form into the neurons, the information trafficking cells of the brain, and also the glial cells, the support matrix cells of the brain."

Clinical trials on a small group of stroke patients could begin next year. The technique could be developed to help other brain damage.

"Our approach is to take stem cells from different regions of the developing brain, so that we can take a cell from the region that is most affected by stroke," he said. "That is going to be our stroke cell line.

"We can take cells from the region affected by Alzheimer's disease and that is going to be our Alzheimer's cell line. And cells from the region affected by Parkinson's are going to be our Parkinson's cell line.

"What we are trying to do here is to generate a product that could be used in hundreds, maybe thousands of patients." — The Guardian, London

In zone of light
By Dr Rajeev Gupta

(Continued from last week's Health Tribune)

PLEASE add the following pieces of advice to those given in the first part of this article.

9. Avoid excessive coffee and cola drinks:

Such drinks contain caffeine. This chemical may stimulate the nervous system in the beginning, but its regular use makes you exhausted. Caffeine is a habit-forming substance and its regular use depletes mental energy.

10. Visit your village, home-town or your own people:

Depression is commonloy experience by such people as stay away from their home-town or who migrate to other countries. A visit to their home-town — back to their roots — makes them get rid of depression. It makes them feel aware of their past and enhances their mental security.

11. Have adequate sleep:

Regular sleeplessness is bad for mental health. It makes a person irritable and fatigued. One feels constantly low. Depressed people should avoid late night social functions and TV programmes. These have a draining effect on mental activity and alertness.

12. Write down your problems:

Write clearly on a piece of paper. Write whatever comes to your mind and whatever disturbs you. It will help you to unload your problems and do a better analysis of your situation.

13. Give a positive feedback to your mind:

Depression makes you negative. The negative mind perceives everything as lost. Try to read positive books, scriptures of quotations. Assure yourself that you will come out of the depression. It is only a temporary stage and it is not going to last long. When you wear red glasses, everything appears red; if you wear green glasses, everything appears green. That does not mean that things around you have become red or green. In fact, your perception changes. Similarly, in depression, negativity colours your mind, everything looks negative. You have to assure yourself that it is not the reality. Behind the layer of negativity lies positivity.

14. Seek medical help:

Many people do not want to seek medical help. They feel that doctors have no remedy for their problems. Others feel that medicines have many side-effects and their condition may worsen after taking medicines. I have seen even doctors who hesitate to consult their colleagues for emotional problems not only for months but for years. They keep on having immense but completely avoidable suffering.

I have treated thousands of patients of various shades of depression. I know that medciines have proved beneficial to most of them. As compared to the benefits, the side-effects are negligible. They have removed sufferings of millions of people in the world and saved those innumerable persons who were going to end their lives. Never do self-medication with anti-depressant drugs and take them under strict medical supervision.

Your next-door family doctor should be in a position to help you. If he finds any difficulty, he may refer you to a physician or a psychiatrist.

15. Do not feel shy of asking for help:

Many people feel shy of seeking professional help. They think that depression is a sign of "weakness". Others might look down upon them. It is important to remember that everyone gets one or the other illness or a bad phase in life. This is the way all of us live life on this planet.

Learn to accept the fact that you are depressed and you need professional help. You can talk to those who have undergone treatment for their depression. They will be able to guide you and help you.

(Concluded)

Dr Gupta is a practising psychiatrist based in Ludhiana.