Renshaw cells: characteristics and functions of these interneurons.

A type of interneuron that influences movement control and Muscle activation.

Renshaw cells are a group of inhibitory interneurons that are part of our motor functions in the medulla. that are part of our motor functions in the spinal cord.

These cells (named after the first person to describe them, Birdsey Renshaw) were the first type of spinal interneurons to be functionally, morphologically and pharmacologically identified. In this article we will see their characteristics.

What are Renshaw cells?

The concept of Renshaw cells was postulated when it was discovered from antidromic (moving in the opposite direction to physiological) signals that a motor neuron traveled collaterally backward from the ventral root to the spinal cord, and that there were interneurons firing at a high frequency and resulting in inhibition.

In several investigations it was further shown that these interneurons, the Renshaw cells, were stimulated by acetylcholine from motor neurons.The neurotransmitter responsible for generating action potentials in the muscle fibers to generate contraction movements.

Other evidence was found that antidromic stimulation of nerve fibers also generated action potentials in motor neuron bodies, along with hyperpolarization (increase in the absolute value of the cell membrane potential) of other groups of motor neurons.

Mechanisms of action

Renshaw cells, located in the anterior horn of the spinal cord, transmit inhibitory signals to the surrounding motor neurons.. As soon as the axon leaves the anterior motor neuron body, it generates collateral branches that project to neighboring Renshaw cells.

How Renshaw cells couple to motor neurons has been investigated with particular interest, as well as their role in models of negative feedback networks operating in different parts of the central nervous system.

α-motor neurons

The α-motor neurons give rise to large motor nerve fibers (averaging 14 nanometers in diameter) and along their pathway branch several times to then enter the muscle and innervate large skeletal muscle fibers.

Stimulation of an α-nerve fiber excites three to several hundred skeletal muscle fibers at any level, which together are referred to as a "motor unit."

Renshaw cells are associated with this type of motor neuron in two ways. On the one hand, by receiving an excitatory signal from the axon of the motor neuron as soon as it leaves the motor root.The cells thus "know" whether the motor neuron is more or less activated (by firing action potentials).

On the other hand, through the sending of inhibitory axonsto synapse with the cell body of the motor neuron at the beginning, or with another α-motor neuron of the same motor group, or with both.

The efficiency of synaptic transmission between α-motor neuron axons and Renshaw cells is very high, since the latter can be activated, albeit with bursts of shorter duration, by a single motor neuron. Discharges are generated by long-lasting excitatory postsynaptic potentials.

Interneurons

Interneurons are present in all regions of the medullary gray matter, both in the anterior horn and in the posterior and intermediate horn between them. These cells are much more numerous than motor neurons.

They are small in size and have a very excitable nature, as they are capable of spontaneously emitting they are capable of spontaneously emitting up to 1,500 discharges per second.. They have multiple connections among themselves, and many of them, as is the case with Renshaw cells, establish direct synapses with motor neurons.

The Renshaw circuit

Renshaw cells inhibit the activity of motor neurons, limiting their stimulation frequency, which directly influences the force of contraction. directly influences the force of contraction of the muscles.. That is to say, they interfere with the work of the motor neurons, decreasing the force of muscle contraction.

In a way, this mechanism can be beneficial because it allows us to control our movements so as not to it allows us to control our movements so as not to cause unnecessary damage, to make precise movements, etc.to perform precise movements, etc. However, in some sports, greater strength, speed or explosiveness is required and the mechanism of action of the Renshaw cells can hinder these objectives.

In sports in which explosive or fast actions are requiredThe Renshaw cell system is inhibited by the central nervous system, so that a greater force of muscular contraction can be achieved (which does not mean that the Renshaw cells automatically stop working).

This system, moreover, does not always act equally. It seems that at early ages it is not very developed; and we see this, for example, when a child tries to throw the ball to another boy who is at a short distance, since normally, at the beginning he will do it with much more force than necessary. And this is due, in part, to the weak "action" of the Renshaw cells.

This system of inhibitory interneurons develops and shapes itself over time, in response to the need of the musculoskeletal system itself to perform more or less precise actions.. Therefore, if we need to perform precise actions, this system will be noticed and will develop more; and on the contrary, if we opt for more violent or explosive movements and actions.

Brain and motor functions

Beyond the Renshaw cells and at another level of complexity, the behavior of our muscles is controlled by the brain, mainly by its outer region, the cerebral cortex..

The primary motor area (located in the center of our heads) is responsible for controlling ordinary movements, such as walking or running; and the secondary motor area is responsible for regulating fine and more complicated movements, such as those necessary to produce speech or play the guitar.

Another important area in the control, programming and guidance of our movements is the premotor area, a region of the motor cortex responsible for regulating the fine and more complicated movements, such as those necessary to produce speech or play the guitar.a region of the motor cortex that stores motor programs learned through our experiences.

Next to this region we also find the supplementary motor area, responsible for the initiation, programming, planning and coordination of complex movements.

Finally, it is worth mentioning the cerebellum, an area of the brain responsible, together with the basal ganglia, for initiating our movements and maintaining muscle tone (a state of slight tension to remain upright and ready to move), since it receives afferent information about the position of the limbs and the degree of muscle contraction.

Bibliographical references:

• Renshaw, B. (1946). Central effects of centripetal impulses in axons of spinal ventral roots. Journal of Neurophysiology, 9, pp. 191 - 204.