Showing papers by "Patrick D. Wall published in 1984"

Muscle but not cutaneous C-afferent input produces prolonged increases in the excitability of the flexion reflex in the rat.

Abstract: Stimulation of cutaneous afferent fibres in the sural nerve and muscle afferent fibres in the gastrocnemius-soleus nerve at a strength that excites C axons produces a delayed and long-lasting burst of activity in posterior biceps femoris/semitendinosus flexor motoneurones. Following a 20 s stimulation at 1 Hz to the sural nerve the flexor motoneurones continue to fire for 20 s while a similar stimulus to gastrocnemius-soleus nerve results in an after-discharge lasting three times longer. Using stimuli to the sural and gastrocnemius-soleus nerves as conditioning stimuli (20 s, 1 Hz) changes in the excitability of the flexor reflex were measured by recording the discharge evoked by a test sural nerve stimulus or by a standard pinch to the ipsilateral and contralateral toes. Prior to any conditioning stimulus the flexor reflex remained stable for prolonged periods. Conditioning stimuli at strengths that activated large myelinated afferent fibres only, or large and small myelinated afferent fibres, failed to produce more than a very transient alteration in the reflex excitability. Conditioning stimuli at C-fibre strength to the sural nerve produced a marked increase in the excitability of the reflex for 10 min. C-fibre strength gastrocnemius-soleus nerve conditioning stimuli resulted in a similar increase in excitability followed by a second phase of facilitation peaking at 20-30 min and lasting for up to 90 min. The afferent barrage initiated by cutting the sural and gastrocnemius-soleus nerves resulted in similar patterns of reflex excitability increases with the muscle nerve resulting in a more prolonged effect than the cutaneous nerve. The results show that a brief C-afferent fibre input into the spinal cord can produce a prolonged increase in the excitability of the flexion reflex and that muscle C-afferent fibres evoke longer-lasting changes than cutaneous C fibres. The differences in the time course of the post-conditioning effects may be related to the well-described differences in the sensory consequences of injury to skin versus deep tissue.

Receptive fields of rat lamina 1 projection cells move to incorporate a nearby region of injury

Abstract: The effects of punctate burns to the skin on the response properties of a group of lamina 1 projection cells in the dorsal horn of the spinal cord has been studied in decerebrate rats. The burns were made 5-15 mm outside the boundaries of the receptive fields (RF) of the cells and they produced a slow increase in the excitability of the cells as seen by an enlargement of RF, towards the lesion, a decreased mechanical threshold and an increased responsiveness to transcutaneous electrical stimulation applied with needle electrodes both inside and outside the RF. These changes were first seen 10-15 min after burning and were not reversed during the time course of the investigation. Local anaesthetic injected at the site of injury produced only a partial and slow reversal of the excitability increases. Since responses to transcutaneous electrical stimulation were increased, injury-induced changes were unlikely to be due entirely to alterations in the properties of primary afferent fibres, but this was directly tested by attempting to mimic the afferent barrage produced by the burn by electrical stimulation of the central cut ends of either the sural nerve or a dorsal rootlet. In both cases a single 10 min period of stimulation (0.5 Hz supramaximal intensity) was found to induce a slow and progressive increase in RF size, confirming the existence of a central component. The changes seen with peripheral burns were probably triggered at least in part by a discharge in primary afferent C fibres since units studied in capsaicin treated animals (which have an impairment of C fibre function) did not show equivalent changes. These results suggest that mechanisms of sensory processing, especially nociception, are subject to reorganization in the face of altered input. The slow time course of the changes suggests that the underlying mechanism may not be the rapid post-synaptic potentials of traditional neurophysiology.

Acupuncture and transcutaneous electrical nerve stimulation.

Abstract: The study of folk medicine by anthropologists and medical historians has revealed an astonishing array of ingenious methods to relieve pain (Brockbank, 1954). Every culture, it appears, has learned to fight pain with pain: in general, brief, moderate pain tends to abolish severe, prolonged pain. One of the oldest methods is cupping, in which a glass cup is heated up (by coals or flaming alcohol) and then inverted over the painful area and held against it. As the air in the cup cools and contracts, it creates a partial vacuum so that the skin is sucked up into the cup. The procedure produces bruising of the skin with concomitant pain and tenderness. Cupping was practised in ancient Greece and Rome as early as the 4th century B.C., and was also practised in ancient India and China. Over the centuries, the method spread to virtually all parts of the world, and cups of various sizes, shapes and materials have evolved. Cupping has been used-and is still widely practised-for a large variety of ailments, including headaches, backaches, and arthritic pains. Cauterization is yet another ancient method. Generally, the end of an iron rod was heated until it was red-hot, and was then placed on the painful area, such as the foot in the case of gout, or on the buttock, back or leg in patients with low back pain. Often, however, the cautery was applied to specifically prescribed sites distant from the painful area. The procedure, of course, produced pain and subsequent blistering of the area that was touched by the cautery, but reportedly also led to the relief of chronic pain (Elliott, 1962). The same effect was achieved by two other procedures: rubbing blistering fluids into the skin, or applying a cone ofmoxa (made from the leaves of the mugwort plant) to a site on the body, setting the tip of the cone aflame, and allowing it to burn slowly until it approached or reached the skin. Again, the procedure produced pain and, while used for all kinds of diseases, was often prescribed specifically for painful conditions (Brockbank, 1954; Elliott, 1962). There are countless other methods that resemble the ones just described, and which are usually labelled as 'counter-irritation'. It is evident that the one factor common to all ofthem is that they produce pain to abolish pain. The pain was usually brief and moderate but its effect was to relieve or abolish a much more severe, chronic pain. These methods, of course, did not always work, but they obviously worked well enough to have survived as procedures of folk medicine throughout the world for thousands of years. Do these procedures work better than a placebo? There are no experimental studies, but the evidence from studies of acupuncture suggests that they do.

The effects of capsaicin applied to peripheral nerves on responses of a group of lamina I cells in adult rats.

Abstract: In adult rats, the sciatic and saphenous nerves on one side were treated topically with capsaicin. The capsaicin treatment had the effect of increasing the latency for withdrawal of the foot from hot water; 11–22 days later, the animals were decerebrated, and cells in the superficial dorsal horn of the lumbar cord with axons projecting in the contralateral dorsolateral funiculus (DLF) were examined electrophysiologically on the treated and untreated sides of the cord. HRP was applied to cut axons of the DLF at C4, in other rats, and retrograde labelling of cells in the lumbar cord indicated that most or all of the recordings in the capsaicin-treated animals were likely to originate from lamina 1. The dorsal horn cells, with receptive fields on the foot, showed decreased responses to electrically evoked afferent impulses in C fibres and grossly altered receptive fields. After capsaicin treatment, the proportion of cells responding to C afferents fell from 83% to 14%. The proportion responding only to C afferents and not to A afferents fell from 9% to 0%. The receptive fields (RFs) of these cells showed two gross abnormalities; 32% of the cells on the treated side had no apparent RF or an ill-defined, intermittent RF, whereas such cells were rare on the untreated side or in intact animals. By contrast 49% of the cells had grossly expanded RFs with an average area of 430 mm2 against the normal average size of 130 mm2. The antidromic conduction velocity of the axons of the cells was measured, and while there was no significant change in the number of cells with axons conducting above 3 m/second, an increased number of cells with lower velocities were recorded. It is suggested that the capsaicin treatment induced dendritic changes in cells with small axons allowing antidromic spikes to invade these cells.

The painful consequences of peripheral injury.

Abstract: Peripheral damage is immediately assessed by the central nervous system by way of a gate control system so that the sensory outcome depends not only on the fact of the injury and the injury signals but also on other convergent impulses from the periphery and on descending controls from brain to spinal cord. However peripheral injury, particularly when nerves are affected, sets off a chain of slow reactions which start in the area of damage but spread centrally. There are the local inflammatory reactions which change the sensitivity of nerves or of sprouts growing from cut nerves. There are changes which move over the entire length of the damaged axon changing the dorsal root ganglia and the terminals of afferents within the spinal cord. The arrival of injury produced impulses in the spinal cord triggers changes with a latency of many minutes which persist for hours even if no further impulses arrive. These increases of excitability and expansion of receptive fields which are triggered by C fibres may be the basis of the secondary hyperalgesias and reflex changes associated with injury.

The Hyperpathic Syndrome: A Challenge to Specificity Theory

Abstract: Yngve Zotterman influenced our way of thinking in many ways. One was the tactic he shared with his master, Lord Adrian, which was to start in the periphery and to attribute as many sensory phenomena as possible to particular types of afferent fibres. This eminently sensible initial tactic has been carried to an extreme by a number of authors, into the claim that the sole origin of pain is activity in a particular type of fibre, the nociceptor. Since they have assigned the monopoly of extracting this quality of sensory experience to a specific fibre type in the periphery, the only role they can allow to central synaptic processes is to control the amplitude of sensation without affecting its quality or location. The central nervous system is seen to consist of a series of labelled line pain specific pathways whose gain may change but not their information content. The adherents of the specificity theory are wrong for many reasons, one of which is the subject of this paper, the hyperpathic syndrome.