 This upper motor neuron condition results from a lack of descending input from the brain that would normally release the inhibitory neurotransmitter gamma amino butyric acid (GABA), which serves to dampen neuronal excitability in the nervous system.Spasticity is thought to be caused by an excessive increase of excitatory signals from sensory nerves without proper inhibition by GABA.Two common conditions associated with this lack of descending input are cerebral palsy and acquired brain injury.