motor unit is the type of muscle where the muscle fibers are under the control of a single motor axon.
motor unit is the type of muscle where the muscle fibers are under the control of a single motor axon.
what type of muscle are the muscle fibers under the control of a single motor axon?
motor unit
distinct skeletal muscles are controlled by groups of individual motor units. such motor units are made up of a single motor neuron and the muscle fibers that it innervates. motor neurons are located in the ventral horn of the spinal cord and the brainstem. these neurons innervate skeletal muscle fibers through the propagation of action potentials down their axons (through ventral roots and cranial nerves), and they stimulate skeletal muscle fibers at neuromuscular junctions where they synapse with the motor end plates of muscle fibers. in humans, these axons can be as long as one meter. motor neurons themselves fall into three main classes: alpha-motor neurons control extrafusal muscle fibers, meaning that they innervate skeletal muscles leading to movement; gamma-motor neurons innervate intrafusal muscle fibers, controlling the sensitivity of muscle spindles to stretch; beta-motor neurons are capable of synapsing on either type of muscle fiber. alpha-motor neurons can further be divided into three separate subclasses, distinguished according to the contractile properties of the motor units that they form: fast-twitch fatigable (ff), fast-twitch fatigue-resistant (fr), and slow-twitch fatigue-resistant (s). the composition of a motor pool may consist of multiple classes and subclasses of motor neurons.
motor pool (neuroscience)
a remarkable aspect of synaptogenesis is the fact that motoneurons are able to distinguish between fast and slow-twitch muscle fibers; fast-twitch muscle fibers are innervated by "fast" motoneurons, and slow-twitch muscle fibers are innervated by "slow" motoneurons. there are two hypothesized paths by which the axons of motoneurons achieve this specificity, one in which the axons actively recognize the muscles that they innervate and make selective decisions based on inputs, and another that calls for more indeterminate innervation of muscle fibers. in the selective paths, the axons recognize the fiber type, either by factors or signals released specifically by the fast or slow-twitch muscle fibers. in addition, selectivity can be traced to the lateral position that the axons are predeterminately arranged in order to link them to the muscle fiber that they will eventually innervate. the hypothesized non-selective pathways indicate that the axons are guided to their destinations by the matrix through which they travel. essentially, a path is laid out for the axon and the axon itself is not involved in the decision-making process. finally, the axons may non-specifically innervate muscle fibers and cause the muscles to acquire the characteristics of the axon that innervates them. in this path, a "fast" motoneuron can convert any muscle fiber into a fast-twitch muscle fiber. there is evidence for both selective and non-selective paths in synapse formation specificity, leading to the conclusion that the process is a combination of several factors.
synaptogenesis