Muscle stretch reflex | Organ Systems | MCAT | Khan Academy

Summary

This video explains the muscle stretch reflex, a fundamental, involuntary response to stimuli. The reflex consists of an afferent part, which brings sensory information into the central nervous system, and an efferent part that triggers a muscle response. Using the knee-jerk reaction as an example, the video illustrates how a sudden stretch in a muscle triggers contraction via muscle spindles and lower motor neurons. It also discusses how reflexes operate without involving the higher brain, offering an efficient protective mechanism, and touches on how imbalance between opposing muscles enhances the response.

Takeaways

• 😀 Reflexes are automatic responses to stimuli that do not require conscious thought or involvement of the brain's higher centers.
• 😀 The muscle stretch reflex has two parts: afferent (information traveling to the central nervous system) and efferent (information traveling from the CNS to the muscle).
• 😀 A muscle stretch reflex can occur when a muscle is stretched rapidly, triggering a protective contraction to avoid injury.
• 😀 The knee-jerk reflex is a well-known example where tapping below the kneecap causes the leg to involuntarily kick out.
• 😀 The afferent part of the reflex involves sensory neurons that detect muscle stretch and send signals to the spinal cord or brainstem.
• 😀 The efferent part of the reflex involves lower motor neurons that communicate back to the muscle, causing it to contract in response to the stretch.
• 😀 Reflexes like the muscle stretch reflex can happen on the same side of the body, with afferent and efferent signals traveling on the same side.
• 😀 The stretch reflex can be enhanced by reciprocal inhibition, where the muscle opposite the one contracting (antagonistic muscle) relaxes.
• 😀 Muscle spindles are specialized receptors in skeletal muscles that detect changes in muscle length, particularly rapid stretches.
• 😀 A diminished muscle stretch reflex can occur if there's damage to the afferent or efferent pathways, either in the sensory or motor neurons.
• 😀 Reflexes are fast, automatic responses that bypass higher brain centers and occur at the spinal cord or brainstem level, ensuring a quick protective reaction.

Q & A

What is a reflex, and what are its two parts?

A reflex is an automatic response to a stimulus that does not require conscious involvement. It has two parts: the afferent part, which carries information about the stimulus into the central nervous system, and the efferent part, which carries response information from the central nervous system to the body.

What is the role of the afferent part in a reflex?

The afferent part of a reflex is responsible for detecting the stimulus through specialized receptors, such as muscle spindles, and transmitting the information to the central nervous system for processing.

How does the efferent part contribute to a reflex?

The efferent part of the reflex carries the response information from the central nervous system to the muscles or glands in the periphery, causing the appropriate action, such as muscle contraction.

What is the muscle stretch reflex, and how does it work?

The muscle stretch reflex occurs when a skeletal muscle is stretched rapidly, triggering a response to contract the muscle in order to prevent injury. The reflex pathway involves the muscle spindle detecting the stretch and sending information via afferent neurons to the central nervous system, which then sends a signal through efferent neurons to the muscle to contract.

Why does the leg kick when a doctor hits below the knee with a rubber hammer?

The rubber hammer strikes the tendon below the kneecap, causing the muscle in the front of the thigh to stretch. This activates muscle spindles, sending a signal to the central nervous system, which sends a response back to the muscle to contract, resulting in the involuntary leg kick.

What is the function of muscle spindles in the muscle stretch reflex?

Muscle spindles are specialized receptors in skeletal muscles that detect stretch. When the muscle is stretched, the muscle spindle senses the change and sends signals through afferent neurons to the central nervous system to initiate the muscle stretch reflex.

What happens when there is a problem with the lower motor neurons or somatosensory neurons?

If there is a problem with either the lower motor neurons or the somatosensory neurons, it can lead to diminished or lost muscle stretch reflexes because the communication between the muscle and the central nervous system is disrupted.

What makes reflexes different from higher-level nervous system functions?

Reflexes are automatic responses that occur at lower levels of the nervous system, such as the spinal cord or brainstem, without the need for involvement of higher functions like cognition, emotion, or consciousness in the cerebrum.

How does the inhibition of muscles on the opposite side of the body enhance the muscle stretch reflex?

When the muscle on one side of the body contracts, the muscle on the opposite side relaxes due to inhibitory signals. This balance prevents the muscles from working against each other and enhances the reflex response, such as a more pronounced leg kick during the knee-jerk reflex.

What is the role of inhibitory neurons in the muscle stretch reflex?

Inhibitory neurons are involved in the muscle stretch reflex by preventing the contraction of muscles on the opposite side of the body. This inhibition helps to enhance the reflex response by preventing the opposing muscle from resisting the contraction of the stretched muscle.