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| Anatomy and Physiology | |||||||||||||||
| PAIN This information will help you to develop your understanding of the pharmacology of the oral analgesics and local anaesthetics listed in the NPF. The function of the nervous system is to perceive stimuli, or changes in the surroundings and to co-ordinate the body's response. It is comprised of:
Part of the nervous system is voluntary, i.e. under conscious control, and part of it is automatic - it works without us thinking about it. Nerve Impulses Nerve cells or neurones are highly modified cells capable of transmitting electrical impulses.
As an impulse travels from one neurone to the next, a neurotransmitter (a chemical), bridges the synapse (the site of communication between nerve cells). The surface of a nerve fibre is positively charged. Mechanical, electrical, or chemical stimulation causes a reversal of this positive charge. This process is known as depolarisation. A wave of depolarisation sweeps along the nerve's surface. This is a nerve impulse. As a small portion of the nerve fibre becomes depolarised it quickly becomes repolarised ready to conduct another impulse. Depolarisation and repolarisation occur as a result of the distribution of sodium and potassium on either side of the nerve cell membrane. The strength of a stimulus is either sufficient to cause depolarisation or not at all. Receptors are classified into different types depending on the nature of the stimulus that excites them. The receptors that are responsible for the detection of pain are called nociceptors. Nociceptors are free nerve endings with a large receptive field. This large receptive field, sometimes, makes it difficult to identify the exact source of a painful sensation. Nociceptors and pain The characteristics and perceived location of the pain depend upon:
Nociceptors are abundant in the superficial portions of the skin, around the walls of blood vessels, and joint capsules, within the periostea (outer layer) of bones. There are various types of nociceptors. These include:
If stimuli are very powerful, all three types of receptors are stimulated. Therefore, when painful sensations are sometimes described, they are described in very similar terms. When tissue damage occurs, a sharp localised pain is felt at the time of the injury. This is then followed by longer lasting discomfort felt shortly afterwards. These two different types of sensations are sometimes referred to as fast, and slow pain. Their differences are due to the different types of receptors stimulated, the route travelled by the impulse, and its destinations within the nervous system (Rutishauser 1994). When tissue damage occurs, arachidonic acid is released into the interstitial fluid from the damaged cell membrane. Arachidonic acid is converted into prostaglandin by the enzyme cyclo-oxygenase, which is present in the interstitial fluid (Martini 1998). Prostaglandins act locally to co-ordinate cellular activity and are very powerful substances. They are effective in minute quantities and almost all tissues in the body respond to, and release, these substances (Martini 1998). The effects of prostaglandins vary depending on their nature and where they are released. When released in response to tissue damage, they stimulate nociceptors in the surrounding area. Figure 3 - A damaged cell membrane and the production of prostaglandin References Martini, F.H. (1998): in Fundamentals of anatomy and physiology (4th Ed.). New Jersey: Prentice Hall International. Rutishauser, S. (1994). Physiology and anatomy. London: Churchill Livingstone Multiple Choice Question (MCQs) |
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