[Neurology] Muscle tone and reflexes

Every muscle in the body has stretch receptors which are stimulated by tension. Signals of high tension are sent up a sensory nerve to the anterior horn of the spinal cord. Here the sensory nerve synapses with a motor nerve which descends back to the muscle and stimulates contraction. The sensory nerve and motor nerve form a reflex arc. This means that an incoming sensation of muscle tension can produce an effector response of muscle contraction as quickly as possible.

The brain sends inhibitory signals down nerves which synapse with the reflex arc. This allows the brain to adjust the sensitivity of this reflex arc, and this is what regulates muscle tension.

Lower motor neuron lesions

When the lower motor neurons are damaged, there is no signal to the muscle fibers. This means that the pathways which produce muscle contraction are inactive and the sarcomeres remain at their maximal length. The muscles remain flaccid and powerless. With time, the muscles will atrophy as there are no signals promoting muscle growth. There will be almost no muscle tension (hypotonia) and no deep tendon reflexes (arreflexia).

These lesions result in flaccid paralysis.

 

Upper motor neuron lesions

Above the anterior horn of the spinal cord, the neurons are known as upper motor neurons. These convey the signal from the brain to the lower motor neurons. When they are damaged, no signal reaches the reflex arc in the spine. This means that no signal for voluntary movement can pass from the brain to the muscle. However the reflex arc is still intact so these muscles are able to maintain high tension. Without the inhibitory signals from the brain, the reflex arc becomes unrestrained and the muscle tone increases (hypertonia). The deep tendon reflexes in the affected limbs are heightened (hyperreflexia).

These lesions result in spastic paralysis.