Overview
This lecture explains the withdrawal reflex, a rapid and automatic response to painful stimuli, and describes its neural pathways and role in protecting the body.
Withdrawal Reflex: Mechanism and Function
- The withdrawal reflex removes a limb from a painful stimulus without conscious thought.
- Reflex intensity increases as the stimulus becomes more painful.
- Nociceptors (pain receptors) detect the noxious stimulus and initiate the reflex.
- Sensory neurons relay signals from nociceptors to the spinal cord.
Neural Pathways Involved
- The withdrawal reflex is a polysynaptic reflex, involving interneurons to connect sensory and motor neurons.
- One pathway activates a motor neuron, causing contraction of flexor muscles to withdraw the limb.
- A second pathway inhibits the motor neuron for the opposing extensor muscle, preventing interference with withdrawal.
- A third, crossed pathway activates the extensor and inhibits the flexor in the opposite limb for balance and support.
Crossed-Extension Reflex
- The crossed-extension reflex supports the body by activating extensor muscles in the limb opposite the one experiencing pain.
- This reflex ensures posture is maintained if, for example, one leg is withdrawn from a painful stimulus.
Key Terms & Definitions
- Withdrawal Reflex — automatic removal of a limb from a painful stimulus.
- Polysynaptic Reflex — reflex involving multiple synapses via interneurons.
- Nociceptor — sensory receptor specialized for detecting pain.
- Flexor Muscle — muscle that contracts to bend a limb.
- Extensor Muscle — muscle that extends or straightens a limb.
- Crossed-Extension Reflex — reflex causing the opposite limb to extend for postural support during withdrawal.
Action Items / Next Steps
- Review the sequence of events in the withdrawal and crossed-extension reflexes.
- Be able to label and describe the neural pathways involved in a diagram.