How does the knee-jerk reflex arise and how does it occur so quickly?

Reflexes occur independently of conscious awareness - i.e. you don't have to think about moving you're leg when it's hit by a tendon hammerThis is because the information isn't transmitted up to the brain, all the processing happens within the spinal cord.1) When a tendon hammer hits the knee, the sensation is detected by muscle spindles 2) Nerve impulses travel through the sensory neurone to convey this information to the spinal cord3) This sensory neurone synapses directly onto a motor neurone within the spinal cord (there are no relay neurones involved in between which is why it's an example of a monosynaptic reflex)4)The motor neurone synapses onto the leg muscle which causes it to contract which causes the leg to extend So in summery the rapid response produced in the knee jerk reflex is due to the lack of brain processing and the few synaptic connections involved.