Anaesthesia itself means ‘without sensation’, and drugs that cause anaesthesia are commonly prescribed by medical staff to relieve pain. As you rightly point out, there are two main classes of drug that can bring about this state; these are known as local and general anaesthetics.
A local anaesthetic causes a small area of the body to feel numb whilst the person remains fully conscious. An example of this may be a numbing injection given by a dentist before they try to yank a tooth out. However, with a general anaesthetic, the patient will be totally unconscious and unaware of what is going on. This type of anaesthesia is usually reserved for major operations.
These pain-relieving drugs bring about these effects in different ways, and for many of them we’re not 100% sure how they work.
Local anaesthetics work by reducing the ability of nerve fibres in a small area to transmit electrical signals toward the brain. There aren’t just pain-sensing nerves in the skin, however. There are various different types of nerves – temperature, touch and pressure and joint position nerves. The thinnest of these nerves are pain fibres, and these are most susceptible to the local anaesthetic. Other nerve types (such as touch and pressure nerves) aren’t affected – meaning that you can still feel pressure and tugging throughout your tooth extraction. (If only they could stop that!)
General anaesthetics are very mysterious. They are either injected or inhaled and quickly pass from the blood into and throughout the brain. They temporarily stop various parts of the brain working normally, but in particular they act on an area called the reticular activating system (at the base of the brain) – a region that makes us awake and alert.
Each general anaesthetic, however, seems to work in a different way, making it even more difficult for researchers to figure out how they work. There are lots of complex theories that you can read about, but be advised – trying to get your head around them all could send you asleep.
Hope this helps!
Answer by Emily Hughes and Dr Stu
Image: Cyril Vallée, on Flickr