You feel tired, so you put your hand out onto a flat surface so you can lean on it. Only it isn't the sideboard, but the hob, which, coincidentally, happens to be on. You pull your hand away quickly, and curse the hob with a variety of swear words. Your hand is in pain, so you stick it under the cold tap.

You have just experienced the process of nociception, literally the sensing of noxious feeling - simply, the feeling of pain.

The Receptors and their Fibres

Pain, to state the obvious, is painful. However, there are different types of pain. Burning causes a very distinctive feeling of excessive warmth, whereas, if you were to have a dull ache, the pain is very exhausting. Predictably, there are different kinds of receptors that are designed to sense the different painful stimuli. These different receptors are also sensitive to different stimuli as well, hence as they have different tasks to perform, they must also be wired up to the nervous system in a different manner. This necessitates the use of one or more types of sensory fibre: A-delta fibres and C fibres. These will eventually run together in a cable-like structure called a sensory nerve.

Thermonociceptor

This receptor deals with the differences in temperature that cause pain. Predictably, it senses when things are really hot: generally, if an object is over 42°C¹, then it feels painful. Similarly, if an object is really cold - ice-cold, then it feels painful. The sensation of such an object is wired up by A-delta fibres.

Chemonociceptor

This is a slightly different type of receptor. It is usually found within the body, rather than being on the surface, ie, in the skin. It senses changes in its chemical environment, more specifically those of the leukotriene family of molecules: hydrogen ions and potassium ions.

So how do these chemicals cause pain? An explanation of how the leukotriene family of molecules causes pain, can be found in the entry on aspirin. Lactic acid building up in muscles after heavy exercise² also causes pain since hydrogen ions are found in this substance. Similarly, potassium ions are released when a tissue of the body is deprived of oxygen. This gives the characteristic 'burning' feeling when a person suffers pains in the chest due to angina or indeed, a heart attack. This type of receptor is wired by C fibres.

Mechanonociceptor

This is the type of receptor that recognises changes in movement, for instance that of the limbs. When you pull a muscle, or, indeed, receive a punch, then excessive force and pressure is applied that is felt as pain. This type of receptor is wired by A-delta fibres.

Certainly, it may seem strange at first to think that the variety of pain stimuli is only picked up by three receptors. However, think about your TV set. If you look closely, you can only see three colours: red, blue and green. If you stand back, then you can appreciate the whole picture in full Technicolor glory. The same thing happens with the many and multitudinous pain stimuli. A mixture of signals from the pain receptors listed above come together to be integrated in the brain, and this is what distinguishes the pin prick from the stomach ache.

However, before all this analysis can take place, the signals from the nociceptors first have to get to the brain.

The Pain Pathway - the Analyst's Route

So once a painful stimuli has been received, what then? As all roads supposedly lead to Rome³, all sensory nerves will find their way to the Central Nervous System (consisting of the brain and the spinal cord only), or CNS for short. The fibres then join together with other fibres, much like small B-roads join up to make A-roads⁴. These sensory nerves then enter the spinal cord via a place called the Dorsal⁵ Horn, called thus because it is shaped like a horn. This is where it synapses with another nerve. Synapsing is pretty much like a relay race - the painful signal is being passed, like a baton, to a larger nerve - the motorway if you like. This motorway is called the Spinothalamic Tract, for reasons which will become apparent later.

Life in the Fast Lane

Like a motorway, the Spinothalamic Tract has its own 'get in lane' rules. Sensory nerves coming from the upper limbs take a lane called the Cuneate Fasiculus, those from the lower limbs take the Gracile Fasiculus. Both are in the Anteriolateral⁶ pathway. On the way, there are a few junctions to which the signal could turn off. One of these is the junction to the cardiovascular system (the heart and associated blood vessels), and the other is to the respiratory system. This area is called the Recticular Formation. However, the pathway continues onwards and upwards through the neck region, firstly through one of two regions, depending on whether the pain is from the leg or the arm. These two regions are the Cuneate or Gracile nucleus, and an area called the Medial Lemniscus, towards the brain.

Before the sensation reaches either region, the baton must be passed on again. This occurs in a region of the midbrain called the Thalamus, hence the name of the tract (spino- as it is within the spine, and thalamic as it goes towards the thalamus). For all sensory nerves, not just those concerned with nociception, this is the major changeover point. Each different type of sensory nerve gets its own changeover point, so that the brain can easily localise the pain to the area of the body. Hence, for nerves which are carrying signals from nociceptors, there are four areas of the thalamus where the synapsing would occur, depending on whether the stimuli was detected on the head and neck or the rest of the body.

Nerves coming from the head and neck would synapse in the Ventroposteromedial⁷ nucleus. Those nerves coming from the rest of the body would synapse in the Ventroposteriolateral nucleus. Of course, as these are such cumbersome names, they are often referred to by their acronyms, VPM and VPL respectively. Other fibres from the rest of the body can also synapse in a third and subsequently fourth nucleus, the Intralaminar and Posterior nuclei.

Final Changeover

These fibres then travel upwards and onwards to the cortex of the brain, through an area called the Internal Capsule. Eventually they reach their destination, the Primary Somatosensory Cortex. Further localisation then takes place. Fibres from the VPL synapse with an area of the Primary Somatosensory Cortex called, enigmatically, S1. Those from the Posterior nucleus synapse with another area of the aforementioned cortex, S2. Confusingly enough, some fibres from S1 then go on to synapse in S2, partly because of their close proximity together in the brain. Then, once analysed here, the signals go on to the Limbic System, the part of our brain that deals with emotions (fibres from the Intralaminar and VPM go directly to this system).

One might think that the case would now be closed. However, as with many aspects of the body, it is not. The scenario described in the introduction to this entry doesn't require all this analysis, although the cursing of the cooking hob does. The reflex of the arm to pull your hand away from the heat source need not have to go via a long winded route.

The Pain Pathway - The Quick Route

This starts off in the same way to the previous method - the nerves enter the spinal cord and synapse in the dorsal horn. They synapse with one nerve going towards the brain, and another which remains in the cord. This other nerve is called an Interneurone, and this passes the baton, so to speak, on to a motor nerve in the anterior region of the spinal cord. This then travels out of the spine in the ventral aspect, and the signal travels back towards the arm. This message is travelling fast, at up to 120 metres per second. This nerve then synapses with the muscle, causing it to contract away from the heat source and thus preventing any damage occurring to your hand.

'Blocking Out' Pain

Some people have developed a way to 'block out' pain, for example in sports such as rugby, or even in tennis. How can such a thing be done? Well, the body has developed two biochemical ways in which this can happen.

The First Block

This occurs before the nerve even enters the spinal cord - in fact, just before the point at which it enters the cord. This gating occurs with the transmission of two substances, Glutamate and the mysterious Substance P. These inhibit the transmission of the pain signal (although not stopping it altogether), and thus the painful sensation is dulled.

The Second Block

This is when pain reaches the midbrain. Areas within this part of the CNS can release substances called Endorphins - literally, endogenous (produced within the body) opiates. Like the drug that they are named after, they 'kill' pain. They are also responsible for the 'high' that many people experience after heavy exercise - you don't notice the pain, but you feel great.

Why the Control of Pain?

So why do we have a mechanism by which you experience pain, yet dull it at the same time? The answer is the old 'fight or flight' response. Take a random and unlikely scenario. You're swimming in the sea, and you get bitten by a shark. It hurts a lot, yet you still manage to punch the shark on the nose, and it swims away. You swim very fast but then collapse on the beach, screaming in agony. What we can see here is that the initial pain was blocked out by the gating system and the endorphins for long enough so that you could punch the shark and swim back to safety. However, they only work for a certain length of time, and once you're on the beach, and safe, they have worn off, and the real magnitude of the pain can be appreciated. In short, it works for long enough so that you can take appropriate action, and then take evasive manoeuvres to land you in a safe place.

Of course, this is all assuming that pain has an organic cause. However, this is not always the case.

Psychogenic Pain

The common myth is that some pain with no obvious physical cause is 'all in the mind' and is hence fake. Psychogenic pain does hurt. Characteristically it occurs long after an injury has healed up, and may become much worse and spread over a larger area than that covered by the original injury.

There are three types of pain that have no obvious physical basis:

Neuralgia

This is characterised by:

• Extremely painful recurring episodes of 'stabbing' pain along the course of a nerve.

• Episodes of pain occurring suddenly with no apparent cause.

• The stimulus for the pain being innocuous, eg stroking the area gently.

An example of this is Trigeminal neuralgia, in which incredibly painful spasms occur along the Trigeminal nerve and throughout the face.

Causalgia

The is characterised by:

• Recurrent episodes of burning pain.

• The origination of pain from an area of previous injury that has long since healed, for example an old gunshot or stab wound. However, it is only found in a minority of such cases.

• Pain may increase over the years and spread to distant parts of the body.

Phantom Limb Pain

This occurs mainly with amputees or where the nerves in a limb become irreparably damaged. Pain is felt in a limb which isn't there any more, or in a limb that has no functioning nerves left.

It is further characterised by:

• A feeling that the limb is in pain and may move.

• The persistence of this pain for many years; this may be similar to the pain that required the amputation to take place.

The potential for a general decrease with time, but some may experience 'shooting' or 'burning' pains or cramps which may get worse.

Why Does this Occur?

This may come down to how the body localises pain in the Primary Somatosensory Cortex, and the neural damage that came before it. However, only a few people experience this kind of pain, and the latter explanation may not be the whole story. Indeed it is not, and may be to do with the 'conditioning' or association of pain with reward. A classic example of this is the association of pain with sexual pleasure, as is the case with sado-masochism. However, most cases are due to other associations. One example is the association of pain with sympathy and affectionate attention from peers. Thus a person may inadvertently learn to exaggerate their pain, or to even continue the pain long after the cause has gone as this has been reinforced by positive benefits.

The Psychology of Chronic Pain

However, what has been discussed previously is in fact acute pain, ie, that which you experience within a few minutes or days, and subsides as soon as it had arrived. Chronic pain is insidious; it creeps up on you slowly, and lasts for months, years, and unfortunately, maybe for the rest of your life.

So why the consideration of the psyche with chronic pain? Well, studies have shown that chronic pain which lasts for more than six months is inextricably linked with rising levels of stress and anxiety. There are, unsurprisingly, more prominent feelings of helplessness and hopelessness because of the failure of various medical treatments. As a result, a person with this kind of pain will find that pain will be the main focus of their life.

One psychologist, by the name of Sternbach, made several studies of those experiencing chronic pain in 1979, and this was his summary of what he found:

Pain patients frequently say that they could stand the pain if only they could get a good night's sleep... They feel worn down, worn out, exhausted. They find themselves getting more and more irritable with their families, they have fewer and fewer friends, and fewer and fewer interests.

Gradually, as time goes by, the boundaries of their world seem to shrink. They become more and more preoccupied with their pain, less and less interested in the world around them.

Their world begins to centre around home, the doctor's clinic and the chemist.

The 'Meaning' of Pain

The 'meaning' of pain was investigated by a psychologist called Beecher in 1956, when he interviewed soldiers and civilians who were involved in World War II. He found that 49% of soldiers claimed to be in 'moderate or severe pain', yet only 32% requested medication to relieve it. By contrast, 75% of civilians claimed to be in moderate or severe pain, and 83% requested medication to relieve it.

Beecher's conclusion was this:

The battlefield wound marked the end of the disaster of war for the soldiers, whereas for the civilians, the wound marked the beginning of a personal disaster and life disruption.

Pain Thresholds - the Myth Exploded

There are many who may declare that they have a 'high' pain threshold, and that as a result they may for example be able to eat vast amounts of chilli without significant effect. However, this is not the case. Experiments have been carried out where those who said that they have a 'high' or 'low' pain threshold were asked to put their arm into a basin of cold iced water for as long as possible. The amount of time that the arm was put into the water did not correlate with any 'high' or 'low' pain thresholds that the subjects of the experiment declared. In fact, there was no correlation with any variable to suggest that such thresholds exist.

So what about our person eating the chilli? Well, you can condition your mind to block out pain, so long as you expect it. This person probably enjoys eating lots of chilli, and knows what to expect when he chews one. Another person, who may not know the degree of spiciness, will, on eating, exclaim quite loudly that the roof of his mouth is on fire, and curse profusely. This is because he doesn't know what to expect. Our friend with the chilli fetish, if he were to bruise his toe on the side of his bed, would also cry out in pain. That is because he didn't expect it.

And Finally... How to Appear to be Pain Resistant

In fact, the header is a bit of a misnomer, mainly because this Researcher couldn't think of a better header. However, you can try to withstand pain, by using a few clever tricks.

Let's take the example of a typically traumatic situation, going to the dentist. People naturally associate the dentist with pain, because of stories told to them by dramatic peers, or indeed as a result of past experience. Thus the expectation that the result of a trip to the dentist will be a lot of pain is formed.

If you expect the pain to be really, really bad, then it will become manifest as such because your body gets all tensed up and you become more aware of the feeling of pain. So to combat this, have a realistic view of what lies beyond the dentist's door. Try to take the focus away from the expected pain and, try to formulate a pleasant image in your mind - think nice thoughts.

So now you are sitting in the dentist's chair. Quite often, when you are experiencing pain, you will bite your lower lip, and it may feel less painful. This is a way of distracting yourself from the pain, a way of not focusing on the larger pain at hand. In the dentist's surgery, you may not have a chance to bite your lower lip, as you will have your mouth wide open. So try counting the tiles on the ceiling, or doing times-tables in your head. This should take your mind off what pain you may be feeling. Although it won't take it away completely, it may however be a lot less worse than you may have previously thought.

So, to summarise the pain-resistance method:

• Have a realistic view of what is going on - true, it may be bad, but quite often, it will not be as painful as you think.

• Think nice thoughts! Imagine a pleasant beach, or lying in a field looking up at the stars - whatever the image you think of, it shouldn't be one associated with pain.

• Distract yourself from the pain you experience - bite your lip, count imaginary sheep or do arithmetic in your head; whatever takes your mind off the pain.