Nervous System and Spinal Cord


Reflexes are fast, unpredictable, involuntary reactions to an external stimulus.

Reflexes are involuntary actions in response to an external stimulus that are performed by the body in a fast and unpredictable way. In a reflex arc, a stimulus is detected by a sensory neuron, which sends information to the spinal cord for processing. Then a motor neuron sends a signal to an effector organ, such as a muscle. Because these signals are processed via the spinal cord and do not need to be transmitted to the brain, reflexes occur relatively quickly. Most reflex arcs involve only three neurons: a sensory neuron, an interneuron, and a motor neuron.

There are several types of reflexes. Examples of reflexes include stretch, Golgi tendon, flexor withdrawal, and crossed extension reflexes. They are categorized by unique responses to the stimulus. The stretch reflex controls muscle length to keep them at a constant length. The Golgi tendon reflex is an opposing reflex that causes muscles to relax after they contract. The flexor withdrawal reflex is a spinal reflex that allows people to quickly remove (or withdraw) their body from painful stimuli. The crossed extension reflex lets the body respond on one side as a reaction to a stimulus on the other side of the body. One example includes the human body's response to pain. When a person steps on something sharp with his or her left foot, a quick shift in body weight to the right foot happens. This shift helps ease the pain on the left side while simultaneously maintaining balance.

Reflex Arc

A reflex arc occurs when a stimulus is received and a sensory neuron is activated. The stimulus is processed by the spinal cord and information is sent to the interneuron. Then a motor neuron is activated, and the effector muscle responds.