What is the Reflex that causes reciprocal activation of antagonist muscle?

The reflex responsible for the reciprocal activation of antagonist muscles is known as the stretch reflex, also called the myotactic reflex. This reflex operates based on the principle of reciprocal innervation, ensuring the coordinated and smooth movement of skeletal muscles.

Reciprocal activation involves the contraction of one muscle group (agonist) while the opposing muscle group (antagonist) relaxes. When a muscle is stretched, sensory neurons called muscle spindles detect the stretch and send signals to the spinal cord.

1. Sensory Signal:

- When a muscle is stretched, specialized sensory receptors within the muscle, known as muscle spindles, are activated.

- These muscle spindles contain specialized sensory neurons that detect the length changes and send signals to the spinal cord.

2. Activation of Ia Afferent Neurons:

- The sensory information from the muscle spindles is transmitted through specialized Ia afferent neurons.

- Ia afferents are rapidly conducting neurons that convey information about muscle length and the rate of change in length to the spinal cord.

3. Synaptic Transmission:

- In the spinal cord, the Ia afferent neurons form synaptic connections with two sets of motor neurons:

a) Alpha motor neurons that supply the agonist muscle (same muscle being stretched)

b) Gamma motor neurons that innervate the muscle spindles, setting their sensitivity

4. Excitatory and Inhibitory Effects:

- Once the Ia afferent signals reach the spinal cord, the following processes occur:

a) Excitation of Agonist Motor Neurons: The Ia afferents make excitatory synaptic connections with the alpha motor neurons of the agonist muscle. This excitation leads to muscle contraction.

b) Inhibition of Antagonist Motor Neurons: Simultaneously, inhibitory interneurons receive signals from the Ia afferents and transmit inhibitory signals to the alpha motor neurons of the antagonist muscle. This leads to reciprocal muscle relaxation.

5. Coordinated Movement:

- The reciprocal activation and inhibition of agonist and antagonist muscles result in coordinated muscle contractions and relaxations.

- The stretch reflex rapidly corrects any disruptions in muscle length, maintaining muscle tone, posture, and smooth movements.

The stretch reflex is a fundamental component of proprioception, providing feedback on muscle length changes, joint positions, and maintaining overall muscle coordination and balance.