Motor Neurons

Motor neurons, also known as motoneurons, are a type of neuron that directly or indirectly controls the effector organs of the body, primarily muscles and glands^[1]. They are an essential component of the nervous system, transmitting signals from the brain and spinal cord (central nervous system) to initiate and regulate muscle contraction^[2].

1. Spinal Motor Neurons are located in the gray matter of the spinal cord. They are further divided into two subtypes, alpha and gamma motor neurons. Alpha motor neurons innervate extrafusal muscle fibers, which are responsible for force generation during muscle contraction, whereas gamma motor neurons innervate intrafusal muscle fibers, playing a key role in maintaining muscle tone^[4].

2. Cranial Motor Neurons are located in the brainstem. They control the muscles involved in eye movement, facial expressions, chewing, swallowing, and speech^[5].

The concept of the neuron, including motor neurons, was first proposed by Heinrich Wilhelm Gottfried von Waldeyer-Hartz in 1891. This was made possible by the preceding work of several scientists, notably Santiago Ramón y Cajal, who used Golgi's staining method to describe the detailed structure of the neuron^[6].

Motor neurons have the crucial function of receiving and integrating signals from the central nervous system and translating these signals into muscle movement. This involves transmitting impulses along their axons to muscle fibers, which respond by contracting or relaxing^[7].

The interaction between motor neurons and muscles is not a one-to-one relationship. Instead, a single motor neuron and the muscle fibres it innervates constitute a "motor unit"^[8]. The size of the motor unit varies depending on the precision of muscle control needed. For example, a motor unit in the hand, where fine control is needed, might include just a handful of muscle fibres, whereas a motor unit in the leg, where coarse control is sufficient, might include hundreds or even thousands of muscle fibres^[9].

The axons of motor neurons can be very long, reaching up to a meter or more in humans. This is because they often need to carry signals from the brain or spinal cord all the way to muscles in the extremities^[10]. For example, the motor neurons controlling toe muscles have their cell bodies in the lower part of the spinal cord but must send their axons down the entire length of the leg and foot^[10]. Because these axons can be quite long, they are also thick to facilitate fast transmission of electrical signals. In fact, they are among the thickest axons in the human body, with diameters up to about 20 micrometers^[11].