Renshaw cell
| Renshaw Cell | |
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| Details | |
| Neurotransmitter | Glycine |
| Identifiers | |
| NeuroLex ID | Renshaw Cell |
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Anatomical terminology
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Renshaw cells are inhibitory interneurons found in the gray matter of the spinal cord, and are associated in two ways with an alpha motor neuron.
In this way, Renshaw cell inhibition represents a negative feedback mechanism. A Renshaw cell may be supplied by more than one alpha motor neuron collateral and it may synapse on multiple motor neurons.
The concept of the Renshaw cells was postulated by Birdsey Renshaw (1911–1948), when it was discovered that with antidromic signals from a motor neuron running collaterally back via the ventral root into the spinal cord, there were interneurons firing with a high frequency, resulting in inhibition. Later work by Eccles et al., provided evidence that these interneurons, which they called “Renshaw Cells,” are stimulated by acetylcholine from motor neurons. Previous work by Renshaw and Lloyd had shown that this antidromic inhibition resembled direct inhibition from spinal nerves but resulted in relatively longer inhibition of 40-50 ms (compared to 15 ms). The antidromic stimulation of the nerve fiber also resulted in action potentials in the cell bodies of the motor neurons along with hyperpolarization of other groups of motor neurons. In the event where the initial stimulation of the motor neuron originated in a spinal tract the Renshaw cell spike occurred during the declining phase of the initial motor neuron soma spike giving an indication of the source and sequence of stimulation of the Renshaw cell.
Although during embryonic development the Renshaw cells lack synapses from the dorsal root, prenatal and postnatal stages show the development of dorsal root originating synapses, which are functional and stimulate action potentials. But these decrease during development while acetylcholine motor axons begin to synapse and proliferate with Renshaw cells, ultimately being primarily stimulated by the motor neurons.
The Renshaw cells are ultimately excited by multiple antidromic motor neuron axons, where the majority of axons originate from synergist motor neurons, and in turn the Renshaw cell synapses with multiple neurons, eliciting IPSP in alpha motor, 1a inhibitory interneurons and gamma motor neurons. The antidromic collateral circuit back to the triggering motor neuron is known as “recurrent inhibition”. This homonymous inhibition is not universal. Whereas most initial experiments have been done on cats, it has been found that in man that proximal muscles of the hand and foot do not have homonymous inhibition. Heteronymous inhibition has been found to be dominant in the leg compared to the arm, where antagonist muscles work simultaneously. (It must also be noted that Renshaw cells are activated by gamma motor neurons, but to a lesser extent). The Renshaw cells not only synapse with homonymous and heteronymous nerves, but also with the Ia interneurones, which are stimulated by the Ia afferents from the same muscle group activated by the motor neurons, which have an inhibitory effect on the antagonist muscle group. This “recurrent facilitation” causes reduced inhibition of the reciprocal inhibition of the Ia interneuron of the antagonist group (Baret et al.; 2003), which may in turn also be inhibited by signals from the corticospinal tract. It has been shown that:
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