Renshaw cells: characteristics and functions of these interneurons

Renshaw cells are a group of inhibitory interneurons that are part of our motor functions of 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 its characteristics.

• Related article: "Types of neurons: characteristics and functions"

What are Renshaw cells?

The concept of Renshaw cells was postulated when it was discovered from antidromic signals (which move in the opposite direction to the physiological one) a motor neuron that traveled collaterally backwards, 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 also shown that these interneurons, the Renshaw cells, They were stimulated by acetylcholine from motor neurons., the neurotransmitter responsible for generating action potentials in muscle fibers to generate contraction movements.

Another piece of evidence was finding that antidromic stimulation of nerve fibers also generated action potentials in the bodies of motor neurons, together with hyperpolarization (increase in the absolute value of the cell's membrane potential) of other groups of motor neurons.

Action mechanisms

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

Particularly interesting has been investigated how Renshaw cells couple to motoneurons, as well as their role in models of negative feedback networks operating in different parts of the nervous system central.

α motor neurons

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

Stimulation of one α nerve fiber excites from three to several hundred skeletal muscle fibers at any level, collectively 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; in this way the cells “know” if the motor neuron is more or less activated (firing action potentials)

For the other, via inhibitory axon deliveryto synapse with the cell body of the beginning motor neuron, or with another α motor neuron of the same motor group, or with both.

The efficiency of synaptic transmission between the axons of α motor neurons and Renshaw cells is very high, since the latter can be activated, although with bursts of shorter duration, for a single motor neuron. The 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 horns, as well as in the posterior and intermediate horns that lie between them. These cells are much more numerous than motoneurons.

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

Renshaw's Circuit

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

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

In sports that require explosive or fast actions, the Renshaw cell system is inhibited by the central nervous system, so that a greater force of muscle contraction (which does not mean that Renshaw cells automatically stop function).

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

This system of inhibitory interneurons develops and shapes over time, given the need of the musculoskeletal system itself to carry out more or less precise actions. Therefore, if we need to perform precise actions, this system will be noticed and further developed; and vice versa, if we opt for more violent or explosive movements and actions.

Brain and motor functions

Beyond 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.

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

Another of the important areas in the control, programming and guidance of our movements is the premotor area., a region of the motor cortex that stores motor programs learned through our experiences.

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

Finally, it is worth noting the cerebellum, the area of ​​the brain responsible, together with the basal ganglia, to start our movements and to maintain muscle tone (state of slight tension to stay upright and ready to move), since it receives afferent information about the position of the extremities and the degree of contraction muscular.

Bibliographic references:

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