Skeletal muscles and visceral smooth muscles are controlled by motoneurons that descend efferently from the CNS. Thus, motoneurons are responsible for reflex motor function as well as overall voluntary motor function. Damage to the central motoneurons manifests symptomatically in so-called pyramidal tract signs.
What are motor neurons?
Motoneurons are motor neurons in the central nervous system. They belong to the efferent neurons that descend from the central nervous system. Motoneurons innervate skeletal muscle as well as smooth muscle. Contraction of muscles is the main function of motoneurons. With their axons, they control the muscles either directly or indirectly. The motoneurons of skeletal muscles are also called somatic motoneurons. They are either alpha or y neurons and are referred to as lower and upper motoneurons. The a-motoneurons innervate the extrafusal muscle fibers and enable their contraction. The skeletal muscle y-motoneurons, on the other hand, are contained in the intrafusal muscle fibers and regulate the sensitivity of the length receptors, which transmit current information about the degree of contraction to the central nervous system. The smooth muscle motor neurons are either specifically visceral or generally visceral. In a narrow sense, only the superior and inferior motoneurons of the smooth muscle are referred to as motoneurons.
Anatomy and structure
Each motor neuron receives information through the cell membrane of the dendrites and cell bodies with its receptors. In the internal organelles, this information is processed and through the axon it is transmitted chemically or electrically. For ideal conductivity, the axons are surrounded by a fatty insulating layer called myelin. The receptors on the cell membrane play an important role, especially for information processing. Transmitters in the extracellular fluid can bind to them. The receptors of motor neurons are either ionotropic or metabotropic. The ionotropic receptors, upon receipt of information, change the action potential at maximum speed and rapidly transmit the information. The metabotropic receptors conduct information through numerous intermediate steps into the nucleus. In the nucleus, the information is deposited into the DNA. Thus, motoneurons are capable of learning processes. The synapses of motoneurons form the junctions to the subsequent neuron.
Function and tasks
In the narrow definition, the most important task of motoneurons is motor control of skeletal muscle. Thus, the are responsible for all movements of this muscular apparatus and control both voluntary and involuntary movements. Above all, the lower motor neuron in the anterior horn of the spinal cord is a superior control and switching point. It primarily assumes the role of a pulse generator. The lower motoneuron is thus the execution leg of all reflexes and voluntary movements that affect the skeletal muscles. The nerve cell bodies of the lower motoneurons supply, for example, the trunk and neck muscles or the muscles of the limbs with this aim. The nerve cell bodies supplying these muscles are embedded in the gray matter of the anterior horn of the spinal cord. They extend along the entire length of the spinal cord, forming what is known as the motor nucleus. In the individual segments, the axons break out of the spinal canal with the help of the respective spinal nerve and thus reach the motor end plate of the respective muscles. The nerve cell bodies for the motor function of the striated head muscles are also subject to control by the lower motor neuron. However, they are not located in the spinal cord, but in the motor nuclei of the cranial nerves. The upper motoneuron responsible for voluntary motor activity and control of posture. The cell bodies of this motor neuron are called Betz giant cells and are located in the motor cortex of the brain. With their axons, they form the pyramidal tract and, more broadly, the extrapyramidal system. The lower motor neuron acts as a mediator in all actions of the upper motor neuron. Thus, voluntary motor activity is only indirectly controlled by the upper motoneuron and is closely related to reflex motor activity.
Diseases
Diseases of the motoneurons affect motor function and are often associated with an overriding loss of control over the musculature.In particular, muscle weakness, paralysis, and spasticity are often the result of motoneuronal damage. Although both spinal infarcts and cerebral infarcts can damage motoneurons, the best known causes of lesions to these nerve cell bodies are degenerative and autoimmune inflammatory diseases, such as multiple sclerosis. While MS is considered a central nervous system disease, the degenerative disease ALS explicitly affects the motor nervous system. In the disease, the motor neurons in the central nervous system degrade step by step. Lesions of the lower motoneuron, for example, paralyze the muscles connected to it, trigger a loss of strength or are associated with a loss of reflexes. Those of the upper motoneuron, on the other hand, are associated with spastic exaggerated muscle tone in the muscles interconnected with it. In all motoneuronal damages, so-called pyramidal tract signs appear. These are pathological reflexes, which are also called Babinski group. The reflex group corresponds to a foot limb reflex group and is still interpreted as one of the most significant indications of damage to central motoneurons. In infants, the reflexes of the Babinski group are not pathological, but are physiological. Thus, pyramidal pathway signs do not have pathological value until the infant is about one year old. Although examination for pyramidal tract signs is still a standard diagnostic test in neurology, the reliability of pathological reflexes is now viewed critically.