Pain is something we have all experienced, but it is a complicated process that not many of us understand. When we are working on education programs for veterinarians we focus heavily on providing proper pain relief, but with out an understanding of what pain is it is difficult to know how to treat it properly.
Pain begins with the peripheral nerves, which are the nerves that connect the brain and spinal spinal cord with the other parts of our body such as muscles, skin and organs. Peripheral nerves are comprised of multiple nerve fibres, which differ from each other in how they respond to stimulus and how fast they transmit to the brain.
There are three main afferent (going to spinal cord) nerve fibres that contribute to the painful response. The first is the A Beta nerves which have a low threshold for stimulus and there for transmit all sensory signals to the brain. These are also the fastest nerve fibres, and are responsible for telling our brain when, for example, someone brushes a feather over our skin. The second is the A delta nerve fibres which are slightly slower but have a higher threshold, this means they only respond to noxious stimuli (well most of the time) and are known as a type of nociceptor. The third set of nerves fibres are the C fibres, these are significantly slower but are also involved in nociception, so have a higher threshold as well.
This is why when something painful occurs, like we touch a hot plate or hit our finger with a hammer, we sometimes feel it has happened first (A beta fibres), then feel a sharp pain (A delta fibres) and later have dull throbbing pain (C fibres).
It also helps explain why rubbing a painful spot can help reduce pain levels - when rubbing our skin we are stimulating the A beta fibres which transmit to the same area of the spinal cord as the nociceptor nerve fibres. The signals from the A beta fibres can help dull the pain sensation by activating inhibitory mechanisms and because they transmit that non-noxious stimuli is occurring they can help overwhelm the pain signal transmission.
Once the fibres have detected noxious stimulus the signal is transmitted to the spinal cord.Neurotransmitters are released which stimulate the ascending nerves in the spinal cord which transmit the signal to the brain.
The brain processes these signals and we feel pain. Pain itself is the unpleasant emotional experience the brain produces, where as nociception is the detection of a noxious stimuli and the subsequent transmission. This means nociception can occur with out the feeling of pain, for example under anaesthetic our body does not respond to the pain but nociception is still occurring. Once the anaesthetic wears off our brain is bombarded with the pain signals, this is why it is important to provide (or recieve) peri-operative and post-operative pain relief.
The high threshold of A delta and C fibres means that strong stimulus is needed before the signal is sent to the brain, these nerves are important because they act as a warning system for the body. Not only do they tell the brain damage is occurring but they also activate a withdrawal response with in the spinal cord. When a noxious stimulus occurs a signal is transmitted through the spinal cord to efferent (leaving spinal cord) nerves which control muscle movement. So for example when we touch a hot plate and pull our fingers away quickly this is the withdrawal response, which is regulated entirely by the spinal cord and does not need to go the brain. This is another protective mechanism to ensure the noxious stimulus is removed quickly.
Many of us have also experienced hypersensitisation. This is when something painful occurs but then a non-painful stimulus in that area still illicits a painful response. This is because the nerve fibres are already sensitise, lowering their threshold. So instead of only reacting to strong stimulus they begin to react to low stimulus, such as touch, as well. This is why it is important to provide pain relief once pain has occurred to prevent wind up and overreaction to non-painful stimulus. Pain becomes more difficult to control the longer it goes on.
Pain is a very complicated subject, and one scientists are still studying in greater detail. But by understanding the basics of the pain pathway we can better understand why pain occurs and there for how to help treat it. Nociceptors are found in mammals, birds, reptiles and even invertebrates like worms so we can assume that all animals are capable of feeling the discomfort associated with pain and should be treated accordingly.
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