Following spinal cord injury , there is transient loss of all or most spinal reflex activity below the level of injury this is known as spinal shock . Possible mechanism for this is sudden loss of bombardment of spinal neurons by excitatory impulse from descending pathway .The duration of spinal shock in human last for minimum of 2 weeks 

At end of spinal shock , spinal reflex reappear and become hyperractive due to development of denervation hypersensitivity to the mediators released by the remaining spinal excitatory endings and formation of additional excitatory endings from existing neurons .

Below the level of lesion following upper motor neuron sign seen 

• Spasticity ( increased resistance to passive movement )
• Positive  Babinski sign 
• Exaggerated reflexes ( Knee jerk and Ankle  jerk )
• Clonus  
• Hyporeflexia of superficial reflexes  ( abdominal reflex , cremaster reflex )