A reflex is a fast, automatic response that does not wait for the brain to decide. The simplest reflex arcs pass through the spinal cord in a single loop.
Parts of the arc
A receptor senses the stimulus, a sensory neuron carries it to the spinal cord, and a motor neuron carries the response out to the muscle. Some arcs add interneurons in between.
Stretch reflex
Tapping a tendon stretches the muscle spindle, which fires the sensory neuron; it connects directly to the motor neuron and the muscle contracts. The knee jerk tests the L3 and L4 levels.
Withdrawal reflex
Stepping on something sharp pulls the foot away through several interneurons, while the other leg straightens to hold the body up. This happens before the pain is felt.
Brain control
Upper motor neurons from the brain normally damp reflexes down. Damage to the brain or spinal cord removes this brake and reflexes become brisk, while damage to the nerve or root makes them weak or absent.
| Reflex | Nerve root | Muscle |
|---|---|---|
| Biceps | C5, C6 | Biceps |
| Triceps | C7 | Triceps |
| Knee | L3, L4 | Quadriceps |
| Ankle | S1 | Calf (gastrocnemius) |
At the bedside
Grade reflexes from 0 (absent) to 4 (with clonus) and always compare both sides. An upgoing big toe (positive Babinski sign) in an adult points to an upper motor neuron lesion.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).