2-Minute Neuroscience: Knee-jerk Reflex

Summary

In this video, the knee-jerk reflex, also known as the patellar reflex, is explained as a simple, automatic response that helps maintain balance and posture. It occurs when the patellar tendon is tapped, causing the lower leg to kick out. The reflex involves a direct communication between sensory and motor neurons within the spinal cord, bypassing the brain. Stretch receptors in the quadriceps muscle, known as muscle spindles, trigger this reflex. The reflex arc is considered monosynaptic but actually involves complex neuron interactions, including inhibition of opposing muscles like the hamstring to allow for unopposed movement.

Takeaways

• 😀 The knee-jerk reflex, also known as the patellar reflex, is a simple reflex arc that helps maintain posture and balance.
• 😀 It is often tested by doctors by tapping the patellar tendon with a small hammer, causing the lower leg to automatically kick outward.
• 😀 A lack of response or excessive response during the knee-jerk reflex test could indicate a neurological disorder or damage.
• 😀 The knee-jerk reflex occurs at the spinal cord level, without involving the brain; the brain receives information about the movement afterward.
• 😀 When the patellar tendon is stretched, muscle spindles in the quadriceps detect the stretch and send signals to sensory neurons.
• 😀 These sensory neurons transmit the information to the spinal cord, where they synapse with motor neurons controlling the quadriceps muscle.
• 😀 The motor neurons trigger the contraction of the quadriceps muscle, causing the leg to move.
• 😀 The knee-jerk reflex is considered a monosynaptic reflex because it involves a direct connection between sensory and motor neurons without intermediate neurons.
• 😀 In reality, the reflex involves several hundred sensory neurons and about 50 motor neurons, rather than just one of each.
• 😀 Sensory neurons from muscle spindles also activate interneurons that inhibit motor neurons supplying opposing muscles like the hamstrings.
• 😀 The inhibition of the hamstring motor neurons ensures that the action of the quadriceps muscle is not opposed, enabling the knee-jerk reflex to occur smoothly.

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