Anterior horns of the spinal cord

Population of alpha motor neurons

Neurons of the anterior horns are divided into several types depending on their size, targets, and sources of regulation. Alpha motor neurons play a leading role in the execution of movements.

Large alpha motor neurons have an irregular shape and are the largest cells of the spinal cord, serving as a reliable diagnostic sign of the anterior horns on histological preparations. They are grouped into five somatomotor nuclei. These cells receive massive afferent input: thousands of axosomatic synapses are located on their cell bodies. The sources of signals for them are:

• descending pyramidal tracts running from the cerebral cortex;
• association neurons of spinal reflex arcs;
• sensory neurons (in the simplest monosynaptic arcs).

The main function of large alpha motor neurons is the innervation of extrafusal (regular contractile) muscle fibers, which ensures conscious (voluntary) and unconditioned reflex movements.

Small alpha motor neurons are under the control of subcortical nuclei of the brain. Their task is to provide complex "unconscious" movements, such as posture maintenance and the performance of conditioned reflex actions, as well as to maintain general muscle tone.

Gamma motor neurons and the gamma loop

Gamma motor neurons are regulated by structures of the reticular formation. Unlike alpha motor neurons, they innervate intrafusal muscle fibers located inside neuromuscular spindles (proprioceptors).

Their work is carried out through a special mechanism known as the gamma loop:

1. The gamma motor neuron sends an impulse that causes contraction of the peripheral parts of the intrafusal fiber.
2. Due to this, the central part of the fiber is stretched, leading to excitation of the receptors.
3. The afferent signal from the receptors travels via sensory neurons to association cells of the posterior horns.
4. The signal is transmitted to alpha motor neurons, causing contraction of the working muscle (extrafusal fibers).

The result of the gamma loop function is the fine regulation of muscle tone and ensuring high precision of voluntary movements.

Interneurons of the anterior horns

In addition to motor neurons, the anterior horns contain cells that provide internal coordination and signal regulation.

Renshaw cells are specialized inhibitory interneurons (the fourth type of neurons in the anterior horns). They operate on a negative feedback principle: they receive signals from the axon collaterals of the motor neurons themselves. If motor neuron excitation becomes excessive, the Renshaw cell inhibits its activity, thereby protecting the nervous system from pathological hyperexcitation.

Association cells are located both within the anterior horns themselves (including diffusely) and as part of the interstitial nucleus of Cajal. Their main role is to receive signals from sensory neurons of the spinal ganglia. They link sensory input with motor output. In simple three-neuron (mono- or oligosegmental) reflex arcs, such diffuse cells act as association elements.

Classification of spinal cord neurons

All neurons of the spinal cord can be classified according to where their axon projects.

• Root neurons: their axons participate in the formation of the anterior roots. This group includes all motor neurons, as well as central neurons of the autonomic nervous system (sympathetic cells of the lateral intermediate nucleus and parasympathetic nuclei in the lower segments).
• Tract neurons: their axons enter the white matter and form conduction tracts (ascending spinocerebellar and spinocortical tracts in the lateral and anterior funiculi). These include neurons of the proper, thoracic, and medial intermediate nuclei. A special subgroup is formed by diffuse tract neurons, whose axons form the fasciculi proprii adjacent directly to the gray matter.
• Internal neurons: their processes never leave the boundaries of the gray matter. These include inhibitory Renshaw cells and association interneurons of monosegmental arcs.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Structure and functions of neuromuscular spindles
• Sympathetic and parasympathetic nuclei of the spinal cord
• Organization of ascending tracts of the lateral and anterior funiculi
• Mechanism of the closure of simple three-neuron reflex arcs

Other topics in "Nervous system: nerves, ganglia, spinal cord"

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