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

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 look at their characteristics.

What are Renshaw’s cells?

Renshaw’s concept of cells was postulated when it was discovered from the anti-drom signals (moving in the opposite direction to the physiological one) a motor neuron traveling collaterally backwards from the ventral root to the spinal cord, and that there were interneurons firing with a high frequency and resulting in an inhibition.

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

Another evidence was to find 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 motor neuron groups.

Mechanisms of action

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

How Renshaw cells attach 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.

Motoneurons α

Motor neurons α give rise to large motor nerve fibres (averaging 14 nanometres in diameter) and along their path branch out several times to then enter the muscle and innervate the large skeletal muscle fibres.

Stimulation of a nerve fibre α excites three to several hundred skeletal muscle fibres at any level, which together are called a “motor unit”.

Renshaw’s cells are associated with this type of motor neuron in two ways. On the one hand, by receiving an excitatory signal from the motor neuron axon , as soon as it leaves the motor root; in this way the cells “know” if the motor neuron is more or less activated (triggering action potentials)

On the other hand, by sending inhibitory axons to perform synapses with the cell body of the motor neuron at the beginning, or with another motor neuron α of the same motor group, or with both.

The efficiency of synaptic transmission between the axons of the motor neurons α and the Renshaw cells is very high, as the latter can be activated, although with shorter bursts, by a single motor neuron. The discharges are generated by long-lasting excitatory post-synaptic potentials.

Interneurons

Interneurons are present in all regions of the medullary gray substance, both in the anterior and posterior and intermediate antlers 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 up to 1500 shocks per second . They have multiple connections with each other, and many of them, like Renshaw cells, establish direct synapses with motor neurons.

The Renshaw circuit

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

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

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

This system, moreover, does not always act in the same way. 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 a short distance away, since normally, at the beginning he will do it with much more strength than necessary. And this is partly due to the poor “action” of Renshaw’s cells.

This system of inhibitory interneurons is developed and shaped 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 further; and conversely, 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 needed 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 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, the cerebellum, the area of the brain responsible, along with the basal ganglia, for initiating our movements and maintaining muscle tone (a state of slight tension in order to remain upright and ready to move) should be noted, as it receives afferent information about the position of the extremities and the degree of muscle contraction.

Bibliographic references:

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