By John N Wood, Simon Beggs and Liam J Drew
Tissue damage is detected by sensory neurons that relay messages to the central nervous system. A torrent of information is emerging about the molecular control of these cells, and is opening exciting new opportunities for pharmaceutical development.
Potentially injury-causing stimuli (noxious stimuli) are detected by specialized nerves that are found in the skin, muscle and viscera. These damage-sensing neurons, nociceptors, respond to tissue damage and can cause a sensation of pain when they are activated.
Nociceptors were first described by Charles Sherrington, a highly influential physiologist who worked in London in the early twentieth century. In humans, these nerve cells may be up to a metre in length, and convey sensory information, via electrical impulses, to the spinal cord from all areas of the body.
Nociceptors respond to multiple types of stimuli, such as high levels of pressure, high or low temperatures and chemicals, including acids. However, these types of stimuli will only generate electrical activity in such neurons when they are of an intensity sufficient to cause, or potentially cause, injury to the animal.
To convert such stimuli into electrical activity, the ends of these cells (the sensory terminals) contain a variety of specialized proteins, termed receptors, that are activated by mechanical, thermal or chemical insults. These receptors change their conformation when activated to form pores in the membrane of the cells. These pores, or ion channels, allow positively charged ions, such as sodium and calcium, to rush into the cell and in doing so they will lower the voltage across the membrane. In this way the intensity of the stimulus is encoded; the greater the stimulus, the larger the change in voltage. If the membrane voltage is sufficiently lowered (to the electrical threshold), other protein channels that are voltage sensitive are activated and these generate electrical impulses. Such impulses then travel along the length of the neuron. In terms of sensory coding, the larger the change in voltage at the terminal, the greater the number of impulses generated.
In the example in Figure 1, mechanical stimuli (top panel) at different intensities have been applied to a sensory neuron. At low intensities, the stimulus has little impact, but at the higher level of mechanical stimulation the electrical threshold is surpassed and an electrical impulse, or action potential, is induced (see bottom panel).
Specific receptors for noxious stimuli
Some chemical and thermal stimuli activate receptors via molecules released from damaged cells. One example is the universal energy storage molecule ATP, which is normally found only inside the cell. The fact that it is present outside is evidence of leakage from damaged cells, and triggers an electrical impulse in the nociceptor. ATP binds to a specific receptor on the outside of the cell, thereby opening pores in the membrane and exciting the neurons electrically. A number of chemicals can act synergistically in this process, as shown in Figure 2.
Tissue-damaging heat seems to act in a related way; in this case the high temperature appears to directly open the ion channel by changing the conformation of a specific receptor. Interestingly, this receptor is also activated by extracts of chilli peppers – explaining the hot sensation associated with spices such as chilli.
Damaged tissue also tends to become more acidic, and high acidity also activates the heat/pepper receptor. This effect underlies the sensation of ‘burning’ pain, and also explains why acid ‘burns’ the skin. Cold stimuli, by contrast, activate different (but related) receptors.
Perhaps the least well understood aspect of pain induction is the mechanisms by which pressure activates sensory neurons.
A multitude of sensory neurons
Sensory neurons conducting messages to the spinal cord come in a number of different types, each with a distinct role. Molecularly, their functions are reflected in the specific complement of receptor proteins that they express. For example, most nociceptors express the heat/pepper receptor, whereas sensory neurons conveying information about innocuous touch will express receptors that respond to low levels of pressure.
Functionally, one way in which sensory neurons can be distinguished is on the basis of the speed at which they transmit information. Some sensory neurons are responsible for the sensation of light touch, and others transmit information about muscle position (known as proprioreception), allowing us to sense how our limbs and body are positioned in three-dimensional space. The nerves of these neurons transmit messages rapidly, and are known as fast or ‘Aα/β-fibres‘ (Figure 3).
Damage-sensing neurons are of two types; most have C-fibresthat transmit information slowly and underlie slow, dull pain. Another class have Aδ-fibres, these have an intermediate conduction velocity and mediate the initial, sharp pain associated with tissue damage.
The different types of sensory neurons can be identified using antibodies against specific proteins present on their cell surfaces, allowing living cells to be extracted and studied outside the body. The cells can be kept alive for weeks in tissue culture, and their responses to various chemical and physical insults can be studied in isolation. In Figure 4, the green and blue cells represent two classes of nociceptors; the vast majority of these cells will respond to hot pepper extracts, which excite only this class of cells.
A further distinguishing feature of the sensory neurons is their termination point within the spinal cord (Figure 5). Most neurons involved with pain signals (coloured green and red) terminate in regions known as laminae I and II at the top, or dorsal, end of the spinal cord. The cells that they contact are organized to transmit information to structures in the brain that are involved with the responses to pain and its perception.
Dissecting pain pathways
It is only within the past few decades that the neuronal pathways of pain have begun to be analysed in detail. It is becoming clear that there is great complexity in the receptive and electrical properties of the different types of sensory neuron, their anatomical arrangement within the spinal cord, and the nature of their synaptic connections with spinal cord neurons – not least because the nature of the connection is altered by the very act of transmission. [see also Pain hypersensitivity]
Rapid progress is however being made, thanks in particular to new molecular technologies and imaging techniques, and the use of reliable and informative animal models. These approaches are enabling us to distinguish seemingly identical types of neuron, to identify the molecular components that give them their specific properties, and to determine their specific role in pain detection.
Of particular interest are the distinct types of receptors and ion channels that detect noxious stimuli, conduct electrical impulses, and are involved in transmission across synapses. These show great diversity of form, and a major challenge is to understand how this diversity is related to differences in function (and hence to different aspects of pain).
Also crucial are the intracellular signalling pathways that are activated in nociceptors, for example during inflammation. These pathways have short-term and long-term effects, which change the responsiveness or excitability of the nociceptor. [see also Pain hypersensitivity] Again, this will be manifest as changes in pain responses.
Molecular studies can begin to dissect out the individual roles of proteins involved in these two aspects of nociceptor behaviour. The specific role of these molecules can then be explored in animal models, for example ‘knockout’ mice in which particular genes have been deleted. To assess the role of a particular receptor, for example, we can knock out the corresponding gene in the mouse and see what impact this has on pain responses. There is also extensive use of in vitro systems; when we discover that a specific receptor is functional in nociceptors we can either study its function in cultured neurons or we can express the gene for that receptor in other cell types to study its function in isolation. Moreover, such systems are a useful way to study the way that different gene products interact with one another.
As well as providing fundamental insights into the molecular mechanisms of pain, these approaches are identifying new targets for pharmaceutical intervention – and heralding a new era of targeted analgesics.