Renshaw cell
Encyclopedia
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.
signals from a motor neuron
running collaterally back via the ventral horn 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 interneurones, which they called “Renshaw Cells,” are stimulated by acetylcholine
from motor neurons. Previous work by Renshaw and Lloyd had show that this antidromic inhibition resembled direct inhibition from spinal nerves but resulted in relatively longer inhibition of 40-50msec (compared to 15msec). The stimulation of the antidromic nerve fiber also resulted in action potentials of the central part of the motoneurones that would generate the original impulse, along with hyperpolarization
of other groups of motor neurons. In the event where the initial stimulation of the motoneuron originated in the spinal track, these impulses occurred after the initial stimulation, and where the Renshaw cell spike occurred during the declining phase of the initial motoneuron soma spike had thus giving an indication of the source and sequence of stimulation of the Renshaw cell.
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 interneuron
s 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. Where as 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:
The Renshaw cells may also be inhibited by both proprioceptive dorsal root afferents], antidromic ventral axons as well as “descending” inhibition. The hyperpolarization of Renshaw cells by afferent and descending neurons have been shown to be caused by the release of glycine
, but GABA
may also hyperpolarize the Renshaw cell- for a prolonged time relative to glycine. It has also be show that glycine is the inhibitory transmitter released by the Renshaw cells.
In essence the Renshaw cells regulate the firing of the alpha motor neuron leaving the ventral horn. Conceptually they remove “noise” by dampening the firing frequency of over-excited neurons with a negative feedback loop, which prevents weakly excited alpha motor neurons from firing. Descending spinal cord nerves in turn regulate the Renshaw cells.
The rate of discharge of the Renshaw cell is broadly proportional to the rate of discharge of the associated motor neuron(s), and the rate of discharge of the motor neuron(s) is broadly inversely proportional to the rate of discharge of the Renshaw cell(s). Renshaw cells thus act as "limiters," or "governors," on the alpha motor neuron system, thus helping to prevent muscular damage from tetanus
.
Renshaw cells utilize the neurotransmitter glycine
as an inhibitory substance that synapses on the alpha motor neurons.
Strychnine
specifically acts on these cell's ability to control alpha motor neuron firing by binding to the glycine receptors on the motor neuron. This antagonistic poison will predispose someone to tetanic contractions, and can prove fatal if the diaphragm becomes involved.
, a spore-forming anaerobic bacterium that lives in the soil. When wounds are contaminated with C. tetani, the toxin travels to the spinal cord where it inhibits the release of glycine
from Renshaw cells. As a result, alpha motor neurons become hyperactive, and muscles constantly contract.
Gray Matter
"Gray Matter" is a short story by Stephen King, first published in the October 1973 issue of Cavalier magazine, and later collected in King's 1978 collection Night Shift. It is set in the same area as King's novel Dreamcatcher.-Setting:...
of the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
, and are associated in two ways with an alpha motor neuron
Alpha motor neuron
Alpha motor neurons are large lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction...
.
- They receive an excitatory collateral from the alpha neuron's axon as they emerge from the motor root, and are thus "kept informed" of how vigorously that neuron is firing.
- They send an inhibitory axon to synapse with the cell body of the initial alpha neuron and/or an alpha motor neuron of the same motor pool.
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.
Discovery
The concept of the Renshaw cells was postulated by B. Renshaw, when it was discovered that with antidromicAntidromic
An antidromic impulse in an axon refers to conduction opposite of the normal direction. That is, it refers to conduction along the axon away from the axon terminal and towards the soma...
signals from a motor neuron
Motor neuron
In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system that project their axons outside the CNS and directly or indirectly control muscles...
running collaterally back via the ventral horn into the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
, there were interneurons firing with a high frequency, resulting in inhibition. Later work by Eccles et al., provided evidence that these interneurones, which they called “Renshaw Cells,” are stimulated by acetylcholine
Acetylcholine
The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...
from motor neurons. Previous work by Renshaw and Lloyd had show that this antidromic inhibition resembled direct inhibition from spinal nerves but resulted in relatively longer inhibition of 40-50msec (compared to 15msec). The stimulation of the antidromic nerve fiber also resulted in action potentials of the central part of the motoneurones that would generate the original impulse, along with hyperpolarization
Hyperpolarization
Hyperpolarization has several meanings:* Hyperpolarization occurs when the strength of the electric field across the width of a cell membrane increases...
of other groups of motor neurons. In the event where the initial stimulation of the motoneuron originated in the spinal track, these impulses occurred after the initial stimulation, and where the Renshaw cell spike occurred during the declining phase of the initial motoneuron soma spike had thus giving an indication of the source and sequence of stimulation of the Renshaw cell.
Physiology
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
Synergist
A Synergist is a kind of muscle that performs, or helps perform, the same set of joint motion as the agonists. Synergists muscles act on movable joints. Synergists are sometimes called as "neutralizers" because they help cancel out, or neutralize, extra motion from the agonists to make sure that...
motor neurons, and in turn the Renshaw cell synapses with multiple neurons, eliciting IPSP in alpha motor, 1a inhibitory interneuron
Interneuron
An interneuron is a multipolar neuron which connects afferent neurons and efferent neurons in neural pathways...
s 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. Where as 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:
- Recurrent inhibition is depressed during strong voluntary contractions (presumably due to inhibition of the Reshaw cell by descending input).
- Renshaw cells are more inhibited at the same level during a dynamic contraction compared with sustained contraction.
- Renshaw cells are facilitated during weak voluntary contractions.
- Renshaw cells are facilitated during co-activation of antagonists.
The Renshaw cells may also be inhibited by both proprioceptive dorsal root afferents], antidromic ventral axons as well as “descending” inhibition. The hyperpolarization of Renshaw cells by afferent and descending neurons have been shown to be caused by the release of glycine
Glycine
Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...
, but GABA
Gabâ
Gabâ or gabaa, for the people in many parts of the Philippines), is the concept of a non-human and non-divine, imminent retribution. A sort of negative karma, it is generally seen as an evil effect on a person because of their wrongdoings or transgressions...
may also hyperpolarize the Renshaw cell- for a prolonged time relative to glycine. It has also be show that glycine is the inhibitory transmitter released by the Renshaw cells.
In essence the Renshaw cells regulate the firing of the alpha motor neuron leaving the ventral horn. Conceptually they remove “noise” by dampening the firing frequency of over-excited neurons with a negative feedback loop, which prevents weakly excited alpha motor neurons from firing. Descending spinal cord nerves in turn regulate the Renshaw cells.
The rate of discharge of the Renshaw cell is broadly proportional to the rate of discharge of the associated motor neuron(s), and the rate of discharge of the motor neuron(s) is broadly inversely proportional to the rate of discharge of the Renshaw cell(s). Renshaw cells thus act as "limiters," or "governors," on the alpha motor neuron system, thus helping to prevent muscular damage from tetanus
Tetanus
Tetanus is a medical condition characterized by a prolonged contraction of skeletal muscle fibers. The primary symptoms are caused by tetanospasmin, a neurotoxin produced by the Gram-positive, rod-shaped, obligate anaerobic bacterium Clostridium tetani...
.
Renshaw cells utilize the neurotransmitter glycine
Glycine
Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...
as an inhibitory substance that synapses on the alpha motor neurons.
Strychnine
Strychnine
Strychnine is a highly toxic , colorless crystalline alkaloid used as a pesticide, particularly for killing small vertebrates such as birds and rodents. Strychnine causes muscular convulsions and eventually death through asphyxia or sheer exhaustion...
specifically acts on these cell's ability to control alpha motor neuron firing by binding to the glycine receptors on the motor neuron. This antagonistic poison will predispose someone to tetanic contractions, and can prove fatal if the diaphragm becomes involved.
Target of neurotoxins
Renshaw cells are also the target of the toxin of Clostridium tetaniClostridium tetani
Clostridium tetani is a rod-shaped, anaerobic bacterium of the genus Clostridium. Like other Clostridium species, it is Gram-positive, and its appearance on a gram stain resembles tennis rackets or drumsticks. C. tetani is found as spores in soil or in the gastrointestinal tract of animals. C...
, a spore-forming anaerobic bacterium that lives in the soil. When wounds are contaminated with C. tetani, the toxin travels to the spinal cord where it inhibits the release of glycine
Glycine
Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...
from Renshaw cells. As a result, alpha motor neurons become hyperactive, and muscles constantly contract.
External links
- Diagram at pediatricneuro.com
- NIF Search - Renshaw Cell via the Neuroscience Information FrameworkNeuroscience Information FrameworkThe Neuroscience Information Framework is a repository of global neuroscience web resources, including experimental, clinical, and translational neuroscience databases, knowledge bases, atlases, and genetic/genomic resources.-Description:...