A reflex arc is the fast nerve pathway that produces an automatic response to a stimulus. It helps protect the body from harm before conscious thought can slow the reaction down. Touching a hot surface and quickly pulling your hand away is a classic withdrawal reflex.
The pathway matters because it shows how the nervous system can act rapidly while still sending information to the brain afterward.
In a basic reflex arc, a receptor detects a stimulus and sends an impulse through a sensory neuron to the spinal cord. An interneuron in the spinal cord may relay the signal to a motor neuron, which carries the impulse to an effector such as a muscle. The brain is not needed to start many spinal reflexes, although it receives signals that let you feel pain and understand what happened.
Examples include the knee-jerk reflex, which helps maintain posture, and the withdrawal reflex, which helps prevent tissue damage.
Key Facts
- Reflex arc sequence: stimulus → receptor → sensory neuron → spinal cord → motor neuron → effector → response.
- Sensory neurons carry impulses from receptors toward the central nervous system.
- Motor neurons carry impulses from the central nervous system to muscles or glands.
- Interneurons connect sensory neurons to motor neurons inside the spinal cord or brain.
- Reflexes are fast because many are processed in the spinal cord before the brain forms a conscious response.
- Reaction pathway speed depends on distance and nerve conduction velocity: time = distance ÷ speed.
Vocabulary
- Reflex arc
- A reflex arc is the nerve pathway that produces a quick, automatic response to a stimulus.
- Stimulus
- A stimulus is any change in the environment that can be detected by a receptor.
- Sensory neuron
- A sensory neuron carries nerve impulses from a receptor to the spinal cord or brain.
- Interneuron
- An interneuron is a neuron inside the central nervous system that links sensory and motor neurons.
- Motor neuron
- A motor neuron carries nerve impulses from the central nervous system to an effector such as a muscle.
Common Mistakes to Avoid
- Saying the brain controls every reflex is wrong because many reflexes are initiated in the spinal cord before the brain is fully aware of the stimulus.
- Reversing sensory and motor neurons is wrong because sensory neurons carry signals into the central nervous system, while motor neurons carry signals out to effectors.
- Forgetting the receptor is wrong because a reflex must begin with a structure that detects the stimulus, such as pain receptors in the skin or stretch receptors in a tendon.
- Thinking reflexes are learned decisions is wrong because reflexes are automatic responses that happen without conscious planning, although some can be modified by the nervous system.
Practice Questions
- 1 A nerve impulse travels 1.5 m during a withdrawal reflex at a speed of 50 m/s. How long does the nerve signal take to travel this distance?
- 2 In a knee-jerk reflex, a signal travels 0.8 m from receptor to spinal cord and 0.8 m back to the muscle. If the impulse speed is 40 m/s, what is the total travel time?
- 3 Explain why pulling your hand away from a hot surface can happen before you consciously feel pain, and include the roles of the spinal cord and brain in your answer.