Modification of pain within the spinal cord - the basis of counterstimulation and endorphin like substances
In its simplest role the spinal cord acts as a relay station for impulses travelling between peripheral nerves and the brain. We now know however, that incoming information is subject to modification within the spinal cord in such a way that it may be exaggerated or underestimated by the time it reaches the brain. This is referred to as spinal cord modulation.
Impulses carrying pain information arrive at the spinal cord via the nerve. Within the spinal cord they are transferred to a number of intermediary nerves or neurones for forward carriage to the brain. Before transmission to the brain can take place special pain neurones or ‘T’ (for target) cells must be stimulated. T cells which are in a state of excitation, discharge pain impulses more readily than those which are in a state of inhibition. The excitatory state of individual T cells is influenced by chemical substances released from adjacent nerve endings. These chemical substances are referred to as neurotransmitters. The nerve endings which release them belong to nerves from a variety of sources including the brain (descending inhibition). Two approaches to childbirth pain act by increasing the amount of inhibitory neurotransmitters within the spinal cord.The first is counterstimulation (massage, acupuncture, heat, TENS). The second involves generating or mimicking naturally occurring morphine-like substances (endorphins) in spinal cord.
Certain kinds of pain can be inhibited by intense sensory stimulation; such as rubbing the skin, hot or cold compresses, electricity and injecting water into the skin. It is by a similar mechanism that an itch can be relieved by scratching. Labour pain can be eased by using techniques such as back rubbing, hot packs, warm showers and transcutaneous electrical nerve stimulation (TENS). Information like touch tends to be carried by fast transmitting nerves which terminate in the spinal cord in close proximity to slow transmitting nerves carrying pain. Stimulating the activity of these fast transmitting nerves causes them to release neurotransmitters which inhibit the transfer of pain impulses arriving via the slow transmitting nerves. This phenomenon was first proposed by Melzack and Wall in 1965 and is known as the ‘Gate-Control’ theory of pain. It is the mechanism explaining counterstimulation.
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