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.
What are Renshaw cells?
The concept of Renshaw cells was postulated when a motor neuron was discovered from antidromic signals (moving in the opposite physiological direction) that traveled collaterally backwards from the ventral root to the spinal cord, and qu ‘there were interneurons firing at a high frequency and resulting in inhibition.
Several investigations have also shown that these interneurons, 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 line of evidence was that antidromic stimulation of nerve fibers also generated action potentials in motor neuron bodies, as well as hyperpolarization (increase in the absolute value of cell membrane potential) of others. motor neuron groups.
Mechanisms of action
Renshaw cells, located in the anterior spines of the spinal cord, they transmit inhibitory signals to surrounding motor neurons. By simply leaving the axon out of the anterior motor neuron body, they generate collateral branches that project into neighboring Renshaw cells.
Of particular interest was investigated how Renshaw cells attach to motor neurons, as well as their role in models of negative feedback networks operating in different parts of the central nervous system.
Α motor neurons
Α motor neurons give rise to large motor nerve fibers (With an average of 14 nanometers in diameter) and along its way they branch out several times and 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 one level, which together are called the “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 motor neuron axon, How much comes out of the motor root; in this way the cells “know” if the motor neuron is more or less activated (triggering of the action potentials)
On the other hand, by sending inhibitory axonsmake synapses with the main motor neuron cell body, 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, as the latter can be activated, albeit with shorter bursts, by a single motor neuron. The discharges are generated by long-lasting excitatory postsynaptic potentials.
Interneurons are present in all regions of medullary gray matter, both in the anterior and posterior rods and intermediate between them. These cells are much more numerous than the motor neurons.
They are small in size and have a very excitable nature because they are able to spontaneously emit up to 1500 downloads per second. They have multiple connections to 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 stimulation frequency, which influences the force of muscle contraction. In other words, they interfere with the work of motor neurons by reducing the force of muscle contraction.
In a way, this mechanism can be beneficial because it allows us to control the 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 may hinder these goals.
In sports where explosive or rapid action is required, The Renshaw cell system is inhibited by the central nervous system, so that a greater force for muscle contraction can be obtained (which does not mean that Renshaw cells automatically stop functioning).
Moreover, this system does not always act in the same way. 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 at another boy who is a short distance away, as he normally will, at first, 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 is developed and modeled over time, faced with the need for the musculoskeletal system itself to perform more or less precise actions. Therefore, if we are to take precise action, 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 through its external region, the cerebral cortex.
The primary motor zone (located in the center of our heads), is responsible for controlling ordinary movements, such as walking or running; and the secondary motor zone, responsible for regulating even more complicated movements, such as those necessary for producing speech or playing the guitar.
Another important area in the control, programming and guidance of our movements is the premotor zone., A region of the motor cortex that stores motor programs learned through our experiences.
Next to this region we also find the additional motor zone, responsible for starting, scheduling, planning and coordinating complex movements.
Finally, we must signal to the cerebellum, responsible brain area, in collaboration with the basal ganglia, to initiate our movements and maintain muscle tone (state of slight tension to stay upright and ready to move), because it receives related information on the position of the limbs and the degree of muscle contraction.
- Renshaw, B. (1946). Central effects of centripetal impulses on the axons of the ventral roots of the spine. Journal of Neurophysiology, 9, pages 191-204.