The Reflex Arc
When you accidentally touch a burning stove you will instantly pull your hand away before you realize that it’s hot. You don’t have to think about it – it feels like it happens even before you feel the pain. This is because the process is a reflex and doesn’t involve the brain.
Receptors and Effectors
The diagram shows someone stepping on a drawing pin. Receptors in the skin of the foot will register the fact that this has happened and will send a signal along a sensory neuron to the spinal cord. Inside the spinal cord an interneuron will transfer the nerve signal to another nerve cell. This cell is a motor neuron and it carries the signal to the muscles in the leg. The leg muscles will contract and, hopefully, the foot will be moved away from the source of pain.
Each of the nerve cells is separated by synapses.
The Patellar Reflex (or Knee-Jerk)
You may have seen doctors on the television (or maybe it’s happened to you) where they test someone’s reflexes by tapping just under the kneecap (patella) with a small rubber hammer. If this spot is hit just right then it will cause some special sensory cells (called a muscle spindle) to send signals off to the spinal cord. The signal passes through the interneuron and then via a motor neuron to the quadriceps muscle. This causes the leg to kick forward.
No matter how much you concentrate you cannot stop this from happening as it’s a reflex action and doesn’t involve the brain.