Why does spasticity happen?

Author: SCIRE Community Team | Reviewer: Holly Timms , Felicia Wong | Published: 21 November 2017 | Updated: ~

Spasticity is related to several changes to the body that happen after SCI. The main reason for spasticity after SCI is a reduced ability of the brain to ‘calm down’ overactive reflexes. Over time, the muscles and tendons may also change, becoming more tense and stiff, which also contributes to the symptoms of spasticity.

The stretch reflex

The stretch reflex is an automatic movement response that happens when a muscle is stretched quickly, causing the muscle to tense. It is commonly tested as the ‘tendon tap’ below the kneecap.

Image of a man touching the flame of a candle. A red line connects to a muscle in the arm and up to the spinal cord. From there, a blue line travels from the spinal cord and back down the arm.

Pain signals from touching something hot travels to the spinal cord and back to the muscles without going to the brain first.4

When a muscle is quickly stretched, it activates special stretch sensors called muscle spindles. They send a signal through sensory neurons to the spinal cord. In the spinal cord, the message is passed along to motor neurons, which send a movement command back to the muscle, causing it to contract. This reflex happens in the spinal cord without travelling to the brain first.

Like muscle stretch, pain can also trigger spinal cord reflexes that use the same nerve pathway as the stretch reflex. For example, stepping on something sharp or touching a hot burner activates spinal cord reflexes.

The brain normally dampens spinal cord reflexes

Although the stretch reflex happens in the spinal cord, the brain influences how sensitive the reflex is. The brain normally sends signals down the spinal cord, which dampens the sensitivity of reflexes.

This is called descending inhibition. ‘Descending’ means ‘coming down from the brain’, and ‘inhibition’ means ‘reducing the activity of’ the stretch reflex. Descending inhibition is important because it tells the stretch reflex to ‘calm down’ so it doesn’t get in the way of normal movements.

Spinal cord injury prevents the brain from dampening spinal cord reflexes

When the spinal cord is injured, descending inhibition from the brain is cut off. Without its calming effects,
the stretch reflex becomes overactive. This can lead to a constant level of muscle tension (called muscle tone) and excessive reflexes which cause the muscles to tighten uncontrollably or unexpectedly and the other symptoms of spasticity.

Watch SCIRE’s YouTube video explaining why spasticity happens after SCI.5