Spasticity is a condition in which muscle tone becomes greatly increased. Muscle tone refers to the level of tension or resistance to movement in a muscle and is what enables people to move limbs or hold a position. For instance, to bend your arm, you must shorten or contract the biceps muscle at the front of the arm (increasing the tone) and at the same time lengthen or relax the triceps muscle at the back of the arm (reducing the tone).
When someone has spasticity in a limb, the signals from the brain are interrupted and the muscle remains in its shortened, contracted state. This causes the affected limb to feel stiff or tight and often to be difficult to move.
The instruction to contract a muscle can be triggered by sensory signals from peripheral nerves in a muscle. When the signal reaches the spinal cord, it responds with an automatic, reflex response and also by passing the message to the brain. The brain assesses the situation and sends a message back down the spinal cord telling the muscle how to respond. The whole process is normally almost instantaneous.
In MS, spasticity occurs if there is an area of scarring between the brain and the point where the nerve from the muscle joins the spinal cord. The reflex action takes place but the message to or from the brain is interrupted. Thus no message to relax the muscle is received or is delayed and the muscle remains contracted and stiff.
Depending on the area of damage in the process, spasticity can occur in any muscle in the body. This can lead to a number of problems including the walking, speech or swallowing and bladder control. The degree of spasticity can vary from mild muscle stiffness to severe, painful muscle spasms.
- More information from the MS Trust
- Article on spasticity from our Open Door newsletter
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