Scientists have for the first time pieced together the workings of a mechanism found within nerve endings that detect muscle movements. Their discovery could clear the way for the creation of drugs to tackle a number of conditions including arthritic joint pain, high blood pressure and even tinnitus.
The findings of researchers at the Universities of Aberdeen and Durham give a new insight into a mystery about our muscles which has remained unsolved - and largely ignored - for over 30 years.
Dr Guy Bewick, Senior Lecturer at Aberdeen, and his Durham colleague, Dr Bob Banks, spent five years studying stretch sensors within muscles, known as muscle spindles, which contain sensory nerve endings.
It is already well established that motor nerve cells within muscle trigger contraction by sending "outgoing" electrical signals from the brain along nerves to their endings in the muscle. This causes the release of "chemical packets", known as vesicles, from the nerve endings in the muscle. The chemical, called acetylcholine, plays a crucial role in relaying the signal because it then binds to a special protein, called a receptor, on the muscle fibre surface, telling the fibre to contract.
Muscle spindle nerve endings detect the muscle movements and send electrical signals back to the brain to help control what the muscle is doing. It has been known since the 1960s that spindle sensory endings have vesicles of their own. However, it has not been clear what function these vesicles serve because it was thought that they were not necessary for the sensory endings to detect muscle movement.
The researchers used fluorescent dyes and electrodes to monitor what the vesicles in the sensory endings did when muscles were stretched, and how much electrical activity the stretch caused in nerves going to the brain. They discovered that these vesicles were released when muscles were stretched activating receptors on the self-same spindle endings that release them. However, the chemical they release (called glutamate), is different from that of muscle motor endings. Glutamate is actually one of the main chemical transmitters in the brain.
Importantly, the scientists also discovered a way of blocking the glutamate receptor, and this switches off the sensory response to stretch.
As these vesicles are found in nerve endings in all mechanical sense organs in the body, scientists believe it is a significant finding. Such organs include blood pressure sensors some joint pain sensors, and the ear. If you can switch off the effect of chemicals produced by these vesicles in muscle spindles, then perhaps you can switch off their effects elsewhere. This in turn could “switch off” a number of medical conditions.
Dr Guy Bewick, who is based at the University of Aberdeen’s Institute of Medical Sciences, said: "I think it’s an important step forward in understanding our own physiology, since this signalling pathway was unsuspected until now. While it’s at a very early stage, our finding could pave the way for the development of drugs to treat conditions like arthritic joint pain, high blood pressure and tinnitus.”
The findings are published in the Journal of Physiology on Saturday, January 15.