This chapter addresses Section L1(vi) from the 2017 CICM Primary Syllabus, which expects the exam candidates to "describe the monosynaptic stretch reflex, single twitch and tetanus". It represents a duplication of effort on the part of the author, whose younger self took it upon himself to address this subject in the "Reflex arcs" chapter probably because he felt that reflexes belonged more naturally in the neurology section. To make this look like new content, the focus of the discussion is biased in favour of the role of muscle, and specifically the specialised length and tension sensors in skeletal muscle.
- Monosynaptic reflex arcs:
- Reflex arcs consisiting of two neurons, i.e. with one synapse
- Basic components consist of:
- Proprioception sensor organ (muscle spindle and Golgi tendon organ)
- Afferent fibres, eg. sensory neuron axon
- Processor organ, eg. α motor neuron (lower motor neuron)
- Efferent fibres, eg. α motor axons
- Effector organ, eg. muscle
- Peripheral nerve (lower motor neuron) disease abolishes the reflex entirely
- Spinal cord disease abolishes the reflexes at the level of the lesion and exaggerates them below the level of the lesion
- Muscle spindles are fusiform encapsulated bundles of sensory fibres
- All muscles except facial muscles contain these
- Each spindle is bound to the outside of the perimysium of a fascicle
- Responsible for the monosynaptyic stretch reflexes
- Intrafusal fibres are proprioceptive sensor organs inside muscle spindles
- Specially modified muscle fibres, which remain contractile
- Mainly sense muscle stretch and muscle length
- Information about stretch is encoded into the rate of the afferent neuron firing
- Innervated with fast afferent fibres (Group Ia and II)
- Also receive efferent γ-motor neuron innervation, so that they may contract in cohesion with the rest of the muscle and never becoem "slack", which would prevent them from accurately assessing the length information.
- Golgi tendon organs
- Fusiform bundles of collagen fibres at the musculotendinous junction
- Innervated with fast Group Ia fibres, and purely sensory (receive no efferents)
- Mainly sense tendon stretch
- Involved in di- and tri-synaptic reflex arcs which coordinate the synergistic inhibition of antagonist motor neurons, and mediate a reflexive relaxation of the same muscle when the tendon is stretched (inverse myotatic reflex).
As always with something like this, there is no single unifying resource that could be recommended, and the trainees must piece together their understanding from several separate papers or book chapters.
It is not essential, and perhaps even harmful, to direct the reader to these resources for exam revision, as CICM First Part exams have so far never included any muscle spindle content. The reader is warned that their time spent revising this material could be better spent on becoming more familiar with neuromuscular junction blockers.
The monosynaptic muscle stretch reflex represents the most basic level of proprioceptive position control for muscle, of which the simplest version consists of only three components: a contractile structure that senses stretch, and two neurons, one sensory and the other motor, connected by the single synapse that gives this thing the right to call itself "monosynaptic". That this simple mechanism might be widespread in the animal kingdom should surprise nobody, and it appears that basically anything with a nervous system tends to have these (seriously, ctenophores). The easiest way to represent it would be something like this:
In humans, most reflexes are polysynaptic (i.e involving an interneuron between the afferent and efferent cells), and monosynaptic reflexes are mostly limited to "deep tendon reflexes", i.e. those you can bang with a reflex hammer during your hot case examination:
These are extensively modulated by descending fibres from the central motor cortex, where the modulation usually takes the form of inhibition, and is to some extent conscious (weird reflex facts include the way the descending inhibition is decreased, and in fact reversed, during sleep). However, all this sounds suspiciously like neurology, and there is plenty of reflex neurology elsewhere. To focus on muscle was the objective here. Apart from the effector role, muscle also contributes to monosynaptic reflex arcs by supplying the proprioceptive stretch sensor organs, specifically muscle spindles and Golgi tendon organs.
"Spindle" is a term which was already archaic by the time it was first used by Kühne in 1863, as by that stage the textile industry had become sufficiently industrialised that nobody spun yarn manually anymore. The bottom line here is that this elongated organ is fusiform, which gives rise to the term intrafusal to describe its contents. the Journal of Physiology has some excellent high resolution images of the earliest depictions of muscle spindles by Angelo Ruffini (1898), and as is conventional, Deranged Physiology will now challenge the reader's broadband connection by forcing them to download it inline with text:
Of course the modern reader would probably be more interested in electron microscopy and immunofluorescence. Below, an excellent image from Gerwin et al (2020) shows off a nice single muscle spindle from a mouse soleus, fluorescing with the light of a probe against VGLUT1, a a glutamate transporter in the membrane of synaptic vesicles that acts as a neuron-specific marker.
There was no scale associated with the image, but these things are said to be about 1-5mm long. They are small encapsulated bundles of specialised muscle fibres, swaddled together in some connective tissue and containing 8-20 intrafusal cells (Macefield & Knellwolf, 2018). Detailed analysis of muscle spindle physiology is probably not expected from the CICM exam candidate, but a few quotable spindle facts are probably worth knowing. Here's a list of possibly useful items extracted from the excellent paper by Kröger & Watkins (2021):
Which brings us to:
Yes, intrafusal cells remain contractile, in some awkward way. The "polar" regions on either end of the elongated cell contain just enough sarcomeres to remind you that these are actually heavily modified muscle fibres. They are much smaller and shorter than the rest of skeletal muscle fibres (rarely more than 25 μm in diameter); the length is only the same as the length of the spindle, which means most would be less than 5mm in length. They usually extend from one pole of the spindle to the other, spanning the entire length of the fusiform structure. The central region of each cell, described as "equatorial", is where all the sensory nerve endings are. Banks et al (1982) laboriously reconstructed these representations of that central region from hundreds of 1μm thick sections:
There are several distinct types of intrafusal fibres, named on the basis of what they do with their nuclei (i.e. there are "nuclear bag" and "nuclear chain" types), and beyond this there is a huge amount of subtle heterogeneity among them, with a different selection of fibre types deployed to different muscles (suggesting all kinds of functional specialisation). The mechanism of mechanotransduction here remains "largely unknown despite some tantalising clues", which is good for the CICM exam candidate, because it probably means there will be no exam questions about it; and so therefore one can make the broad generalisation that it would make no sense to go into any further detail here, as these matters are of very limited interest to the intensivist outside of their exams.
It is generally said that muscle spindles monitor muscle length, and Golgi organs monitor muscle tension (Jami, 1992). Everybody who makes this statement seems to refer to a myology textbook from the 1970s as their main resource, and it is not clear what kind of experiments led to this conclusion, but the reader is reminded (to snatch back their wandering attention) that this is again a thing that a CICM trainee needs to know only enough about to pass a potential exam question, and no more. The following is therefore written in a pragmatic format, aiming to identify nuggets of potentially examinable gold among the mess of muscle histology.
In case a picture can be worth some number of words, here's a reconstruction of some sliced calf Golgi organs from Blumer et al (2003), which shows just enough fine structure to give a casual reader some idea of what this thing should look like. For a sense of scale, the length of the entire depicted organ is about 798 μm; they are usually much smaller than muscle spindles.
Of the two proprioceptive sensors in muscle, the muscle spindles are said to contribute the most to the proprioceptive feedback required for fine position adjustments and monosynaptic reflexes. The Golgi tendon organs mostly coordinate the synergistic inhibition of antagonist motor neurons, i.e they ensure that the triceps does not also contract while the biceps is contracting. This is in fact a di- or tri-synaptic reflex arc, as it involves one or two inhibitory interneurons. They also mediate a reflexive relaxation of the same muscle when the tendon is stretched, which is sometimes mentioned as the "inverse myotatic reflex".