Renshaw Cells: Characteristics And Functions Of These Interneurons
Renshaw cells are a group of inhibitory interneurons which 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.
The concept of Renshaw cells was postulated when it was discovered from antidromic signals (that move in the opposite direction to the physiological direction) a motor neuron that traveled collaterally backwards, from the ventral root to the spinal cord, and that there were interneurons firing with a high frequency and resulting in inhibition.
In several investigations it was also shown that these interneurons, Renshaw cells, 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 the motoneurons, along with hyperpolarization (increase in the absolute value of the cell’s membrane potential) of other groups of motoneurons.
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 anterior motor neuron, it generates collateral branches that project to neighboring Renshaw cells.
How Renshaw cells couple to motoneurons has been investigated with particular interest, as well as their role in negative feedback network models operating in different parts of the central nervous system.
α motoneurons give rise to large motor nerve fibers (with an average of 14 nanometers in diameter) and along their path they branch several times to then enter the muscle and innervate the large skeletal muscle fibers.
Stimulation of an α nerve fiber excites three to several hundred skeletal muscle fibers at any level, which together are called a “motor unit.”
Renshaw cells are associated with this type of motoneurons 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 (firing action potentials).
For the other, through sending inhibitory axons to synapse with the cell body of the initial 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 motoneurons and Renshaw cells is very high, since the latter can be activated, although with bursts of shorter duration, by a single motoneuron. 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 and in the posterior and intermediate horns that remain between them. These cells are much more numerous than motor neurons.
They are small in size and have a very excitable nature, since They are capable of spontaneously emitting up to 1,500 discharges per second They have multiple connections among themselves, and many of them, as occurs with Renshaw cells, establish direct synapses with motor neurons.
Renshaw cells inhibit the activity of motor neurons, limiting their stimulation frequency, which directly influences the force of muscle contraction That is, they interfere with the work of the motor neurons, reducing the force of muscle contraction.
In some ways, this mechanism can be beneficial because allows us to control movements so as not to cause unnecessary damage, perform precise movements, etc. However, in some sports greater strength, speed or explosiveness is required and the mechanism of action of Renshaw cells can make these objectives difficult.
In sports that require explosive or quick actions 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).
Furthermore, this system 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 a short distance away, since normally, at first he will do it with much more force than necessary. And this is due, in part, to the poor “action” of Renshaw cells.
This system of inhibitory interneurons develops and shapes over time, given the need of the musculoskeletal system itself to perform more or less precise actions Therefore, if we need to take precise actions, this system will be noticed and developed further; and on the contrary, if we opt for more violent or explosive movements and actions.
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, 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 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 (state of slight tension to remain upright and prepared to move), since it receives afferent information about the position of the limbs and the degree of muscle contraction.
