Alpha Motor Neuron Population
Neurons of the anterior horns are divided into several types based on their size, targets, and regulatory inputs. Alpha motor neurons play a leading role in movement execution.
Large alpha motor neurons have an irregular shape and are the largest cells in the spinal cord, serving as a reliable diagnostic feature of the anterior horns on histological sections. They are grouped into five somatomotor nuclei. These cells receive extensive afferent input: thousands of axosomatic synapses cover their cell bodies. Their signal sources include:
- descending pyramidal tracts originating from the cerebral cortex;
- associative neurons of spinal reflex arcs;
- sensory neurons (in simple monosynaptic arcs).
The primary function of large alpha motor neurons is the innervation of extrafusal (regular contractile) muscle fibers, facilitating conscious (voluntary) and unconditioned reflex movements.
Small alpha motor neurons are controlled by subcortical nuclei of the brain. Their task is to coordinate complex "unconscious" movements, such as posture maintenance and conditioned reflex actions, as well as to maintain baseline 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 function operates via a specialized mechanism known as the gamma loop:
- A gamma motor neuron sends an impulse that causes the contraction of the peripheral regions of an intrafusal fiber.
- Consequently, the central region of the fiber is stretched, leading to the excitation of its receptors.
- Afferent signals from these receptors travel via sensory neurons to associative cells in the posterior horns.
- The signal is relayed to alpha motor neurons, triggering the contraction of the working muscle (extrafusal fibers).
The outcome of the gamma loop is the fine regulation of muscle tone and high-precision voluntary movements.
Interneurons of the Anterior Horns
In addition to motor neurons, the anterior horns contain cells responsible for internal signal coordination and regulation.
Renshaw cells are specialized inhibitory interneurons (the fourth type of neuron in the anterior horns). They operate via negative feedback: they receive signals from axon collaterals of the motor neurons themselves. If motor neuron excitation becomes excessive, Renshaw cells inhibit their activity, thereby protecting the nervous system from pathological hyperexcitation.
Associative cells are located within the anterior horns (including diffusely) as well as within the interstitial nucleus of Cajal. Their primary role is to receive signals from sensory neurons of the spinal ganglia, linking sensory input to motor output. In simple three-neuron (mono- or oligosegmental) reflex arcs, these diffuse cells act as the associative elements.
Classification of Spinal Cord Neurons
All spinal cord neurons can be classified based on the destination of their axons.
- Radicular neurons: Their axons participate in the formation of anterior roots. This group includes all motor neurons and central neurons of the autonomic nervous system (sympathetic cells of the lateral intermediate nucleus and parasympathetic nuclei in the lower segments).
- Projection (tract) neurons: Their axons enter the white matter to form conduction tracts (ascending spinocerebellar and spinocortical pathways in the lateral and anterior funiculi). These include neurons of the proper, thoracic, and medial intermediate nuclei. A specific subgroup comprises diffuse tract neurons, whose axons form fasciculi proprii adjacent directly to the gray matter.
- Internal (local) neurons: Their processes never leave the boundaries of the gray matter. This group includes inhibitory Renshaw cells and interneurons of monosegmental arcs.