At Newcastle University's School of Biomedical, Nutritional, and Sport Sciences, students are asked to complete an answer cover sheet that requires them to provide detailed explanations that showcase their knowledge of the subject matter. In this particular question, students are required to discuss the types of inhibition that Renshaw cells and 1a inhibitory interneurons provide, the neurotransmitters they use, and how they integrate into the spinal cord's synaptic circuitry to generate alternating activity in flexor and extensor motoneurons.

Both Renshaw cells and 1a inhibitory interneurons are present in the spinal cord, and they inhibit motor neurons using GABA and/or glycine as inhibitory neurotransmitters. Renshaw cells are activated by motoneuron axon collaterals, which create inhibitory synaptic connections to various motor neurons, including the same motor neurons that excite them and 1a inhibitory interneurons. This negative feedback system is known as recurrent inhibition and is likely to stabilize the firing rate of the motor neurons. Renshaw cells receive significant synaptic input from descending pathways and distribute inhibition to task-related groups of motor neurons and 1a interneurons, thereby contributing to establishing the pattern of transmission in divergent group 1a pathways according to the motor task.

On the other hand, 1a inhibitory interneurons send inhibitory signals to the antagonist motoneurons once the muscle spindles in the agonist muscle are activated, providing reciprocal inhibition. The interneuron receives complex excitatory and inhibitory inputs, including direct input from the motor cortex, which allows the use of reflex circuits to simplify the motor cortical program.

Renshaw cells and 1a inhibitory interneurons complement each other, working together to provide fluid movement. For instance, when the agonist (flexor) muscle spindle is stretched, it sends an electrical signal via 1a sensory fibers to activate the agonist motoneuron to contract the agonist muscle and 1a inhibitory interneuron to relax the antagonist (extensor) muscle, i.e., reciprocal inhibition. Once activated, the flexor motoneuron activates the Renshaw cell, which inhibits the neuron, i.e., recurrent inhibition. Furthermore, the Renshaw cell inhibits the 1a inhibitory interneurons, which stops extensor inhibition and allows for extension. The same process is repeated for the antagonist muscle using extensor-specific V1 interneurons, leading to the constant alternation between the extensor and flexor motoneurons that provide fluid movement.