Renshaw cells are inhibitory
interneurons
Interneurons (also called internuncial neurons, relay neurons, association neurons, connector neurons, intermediate neurons or local circuit neurons) are neurons that connect two brain regions, i.e. not direct motor neurons or sensory neurons. I ...
found in the
gray matter
Grey matter is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil (dendrites and unmyelinated axons), glial cells (astrocytes and oligodendrocytes), synapses, and capillaries. Grey matter is distingui ...
of the
spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
, and are associated in two ways with an
alpha motor neuron
Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their co ...
.
* 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, the Renshaw cell action 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 neuron
A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectl ...
s.
Function
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
interneuron
Interneurons (also called internuncial neurons, relay neurons, association neurons, connector neurons, intermediate neurons or local circuit neurons) are neurons that connect two brain regions, i.e. not direct motor neurons or sensory neurons. I ...
s and
gamma motor neurons
A gamma motor neuron (γ motor neuron), also called gamma motoneuron, or fusimotor neuron, is a type of lower motor neuron that takes part in the process of muscle contraction, and represents about 30% of ( Aγ) fibers going to the muscle. Like ...
. 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. (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 (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid (carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinogeni ...
, but
GABA may also hyperpolarize the Renshaw cell - for a prolonged time relative to glycine. It has also been shown 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
Negative feedback (or balancing feedback) occurs when some function of the output of a system, process, or mechanism is fed back in a manner that tends to reduce the fluctuations in the output, whether caused by changes in the input or by other ...
, 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, also known as lockjaw, is a bacterial infection caused by ''Clostridium tetani'', and is characterized by muscle spasms. In the most common type, the spasms begin in the jaw and then progress to the rest of the body. Each spasm usually ...
.
Renshaw cells utilize the neurotransmitter
glycine
Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid (carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinogeni ...
as an inhibitory substance that synapses on the alpha motor neurons.
Clinical significance
Renshaw cells are also the target of the toxin of ''
Clostridium tetani
''Clostridium tetani'' is a common soil bacterium and the causative agent of tetanus. Vegetative cells of ''Clostridium tetani'' are usually rod-shaped and up to 2.5 μm long, but they become enlarged and tennis racket- or drumstick-shaped wh ...
'', a Gram positive, spore-forming anaerobic bacterium that lives in the soil, and causes
tetanus
Tetanus, also known as lockjaw, is a bacterial infection caused by ''Clostridium tetani'', and is characterized by muscle spasms. In the most common type, the spasms begin in the jaw and then progress to the rest of the body. Each spasm usually ...
. When wounds are contaminated with ''C. tetani'', the toxin travels to the spinal cord where it inhibits the release of
glycine
Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid (carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinogeni ...
, an inhibitory neurotransmitter, from Renshaw cells. As a result, alpha motor neurons become hyperactive, and muscles constantly contract.
Strychnine
Strychnine (, , US chiefly ) is a highly toxic, colorless, bitter, crystalline alkaloid used as a pesticide, particularly for killing small vertebrates such as birds and rodents. Strychnine, when inhaled, swallowed, or absorbed through the eye ...
poison also specifically acts on these cell's ability to control alpha motor neuron firing by binding to the
glycine
Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid (carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinogeni ...
receptors on the alpha motor neuron and thus muscles continually contract and may prove fatal if the diaphragm is involved.
History
The concept of the Renshaw cells was postulated by
Birdsey Renshaw
Birdsey Renshaw (October 10, 1911, Middletown, Connecticut – November 23, 1948, Portland, Oregon) was an American electrophysiologist and neuroscientist. He is known for his 1941 discovery of the eponymous Renshaw cells and the Renshaw inhibiti ...
(1911–1948), when it was discovered that with
antidromic An antidromic impulse in an axon refers to conduction opposite of the normal (orthodromic) direction. That is, it refers to conduction along the axon away from the axon terminal(s) and towards the soma. For most neurons, their dendrites, soma, or a ...
signals from a
motor neuron
A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectl ...
running collaterally back via the
ventral root
In anatomy and neurology, the ventral root of spinal nerve, anterior root, or motor root is the efferent motor root of a spinal nerve.
At its distal end, the ventral root joins with the dorsal root
The dorsal root of spinal nerve (or posterior ...
into the
spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the sp ...
, 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
Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Part ...
from motor neurons (
nicotinic receptor). Previous work by Renshaw and Lloyd
[ Lloyd, D. P. C., After-currents, after-potentials, excitability, and ventral root electrotonus in spinal motoneurons, J.gen. Physiol..,1951,35 ,289] 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
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, c ...
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.
References
External links
Diagram at pediatricneuro.comNIF Search - Renshaw Cellvia the
Neuroscience Information Framework
The 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 and provides many aut ...
{{DEFAULTSORT:Renshaw Cell
Central nervous system neurons