Motoneuronos

Motoneuronos, or motor neurons, are nerve cells of the somatic nervous system that transmit signals from the central nervous system to skeletal muscles, initiating contraction. They originate in the ventral horn of the spinal cord and in motor nuclei of the brainstem, and their axons project to specific muscle fibers. Motoneuronos are broadly classified into alpha (α) motor neurons, which innervate extrafusal muscle fibers responsible for force generation, and gamma (γ) motor neurons, which innervate intrafusal fibers within muscle spindles and help regulate muscle tone and length sensing.

Functionally, motoneuronos act as the final common pathway of motor control. They convert synaptic input from upper motor neurons, interneurons, and sensory feedback into action potentials that trigger muscle contraction via acetylcholine release at the neuromuscular junction. The recruitment of motor units follows the size principle: small units activate first and larger units are recruited as needed for greater force.

Motoneuronos operate within a broader motor circuit that integrates voluntary movement, reflexes, posture, and proprioception. They are subject to modulation by higher centers in the brain and by sensory input, allowing coordinated and adaptable movement.

Clinical relevance is marked by the effects of motoneuron loss or dysfunction. Upper motor neuron lesions can produce spasticity and hyperreflexia, while lower motor neuron damage causes weakness, fasciculations, atrophy, and hyporeflexia. Diseases affecting motoneurons include amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), poliomyelitis, and various motor neuropathies. Understanding motoneuronos is essential for research into motor control, neurodegenerative disease, and neuromuscular therapeutics.