Structural elements and receptors
The functioning of the segmental apparatus is based on specific receptors and spinal cord neurons integrated into reflex arcs.
Muscle receptor apparatus includes:
- Muscle spindles. These are intrafusal fibers that are excited when the muscle is stretched.
- Golgi tendon organ (GTO). Located in tendons, it responds to tension (contraction) of muscle fibers.
Neuronal composition of the segment:
- α-motor neurons — large cells whose axons go to extrafusal fibers (the main working mass of the muscle). Their excitation directly causes contraction.
- γ-motor neurons — innervate intrafusal fibers of the spindles, regulating their sensitivity to stretch.
- Interneurons (including inhibitory ones) and γ-afferent neurons, providing complex coordination within the segment.
Mechanisms of action: from rest to autogenic inhibition
The activity of the spinal segment can be divided into several functional stages:
- Stretch and response. During passive muscle stretch, γ-afferent pathways are activated. The signal reaches α-motor neurons, whose excitation leads to muscle contraction.
- Protective inhibition. Muscle contraction excites GTO receptors. The signal from them is transmitted to inhibitory interneurons, which suppress the activity of α-motor neurons. This mechanism, called autogenic inhibition, protects muscle tissue and tendons from excessive strain.
- Gamma loop. Descending signals from the extrapyramidal system stimulate γ-motor neurons, causing intrafusal fiber contraction. This changes the sensitivity of the muscle spindle and triggers a reflex loop leading to the activation of α-motor neurons and contraction of the main muscle.
Descending control and movement preparation
The segmental apparatus is the "lower tier" of voluntary motor control, which is tightly regulated by the brain.
Influence of descending pathways:
- Pyramidal tract is directed primarily at activating α-motor neurons.
- Extrapyramidal tract mainly controls γ-motor neurons.
- Rubrospinal tract stimulates flexor motor neurons (and inhibits extensors), whereas vestibulospinal tract acts oppositely, activating extensors.
Phases of movement preparation: Motor neurons prepare for voluntary movement in stages. First, a "pre-tuning" process occurs (the excitability of the agonist increases before the signal arrives). At 60 ms before the onset of muscle activity, the "tuning" process is engaged under the control of the extrapyramidal system. Finally, at 30 ms before the start, the pyramidal system initiates the "trigger process" — a sharp increase in the excitability of the agonist motor neurons.
Reciprocal regulation and stepping movements
Complex motor acts, such as walking, are based on the reciprocity mechanism — coupled inhibition and excitation of antagonistic muscles.
For example, during load on the right leg, its knee joint begins to flex. This causes a stretch of the muscle spindles of the right leg extensors, triggering the myotatic reflex — the leg straightens, bearing body weight. Simultaneously, on the opposite (left) leg, a reciprocal process occurs: flexor motor neurons are activated and extensor motor neurons are inhibited, leading to flexion of the left knee. When body weight is shifted to the left leg, the phases swap.
Briefly: answers to topical questions
How is the topographic distribution of motor neurons organized in the spinal cord?
Motor neurons of limb muscles form columns. Their topographic arrangement:
- Medially: motor neurons of proximal muscles.
- Laterally: motor neurons of distal muscles.
Somatotopy is also noted for the anterior horns of the grey matter in the lumbar segment: the medial part contains neurons innervating trunk muscles, while the lateral part contains neurons innervating leg muscles.
What types of intrafusal fibers are part of the muscle spindle?
The muscle spindle contains two types of intrafusal muscle fibers:
- Nuclear bag fibers — their middle part is expanded by a "bag" containing about 50 nuclei. A spindle typically contains 2 such fibers. They function as stretch receptors (dynamic component).
- Nuclear chain fibers — shorter and thinner, lying adjacent to the nuclear bag fibers. There are 4–5 of them in a spindle. They function as tension receptors (static component).
Class and exam questions
What is the purpose of autogenic inhibition?
It is necessary to protect the muscle from excessive tension. During strong contraction, Golgi tendon organs are activated, which inhibit $\alpha$-motor neurons via interneurons.
Which system controls γ-motor neurons?
The activity of γ-motor neurons is predominantly controlled by the extrapyramidal system of the brain, which regulates muscle tone via the gamma loop.
What else to study on this topic
- Influence of force and speed on motor cortex neuron activity
- Flexor reflex
- Crossed extensor reflex
- Postural reactions exemplified by a decerebrate animal
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