Final answer:
The withdrawal reflex from fast pain activates ventral motor neurons, which leads to the immediate contraction of muscles such as the biceps brachii, while inhibitory interneurons relax antagonist muscles like the triceps brachii.
Step-by-step explanation:
Fast pain may also elicit a withdrawal reflex, which stimulates ventral motor neurons resulting in almost immediate muscle contraction. During a withdrawal reflex, such as pulling your hand away from a hot stove, the biceps brachii contracts while the antagonist triceps brachii relaxes. This relaxation is achieved through the action of an inhibitory interneuron in the spinal cord, which inhibits the motor neuron for the triceps brachii. Similar mechanisms operate in the lower limbs where, for instance, stepping on a tack causes contraction of the tibialis anterior muscle and inhibition of the gastrocnemius and soleus muscles to prevent further injury.