Introduction and Anatomical Overview:
Muscle is made up of two types of fibers, intrafusal and extrafusal. Extrafusal fibers are the contractile fibers and intermixed within the extrafusal fibers are intrafusal fibers. Housed within intrafusal fibers is a specific type of mechanoreceptor. Mechanoreceptors, in general, are interspersed through the entire body – hair, skin, ligaments – and are responsible for sensing tissue pressure and distortion and give our body a sense of proprioception by detecting position of our muscles, bones, and joint. There are many types of mechanoreceptors, but one specifically – the muscle spindle – lives within the intrafusal muscle fibers. The muscle spindle transmits sensory data regarding changes in muscle length, and therefore movement, to the central nervous system (CNS) via the primary afferent (sensory) neurons. The intrafusal fibers receive neural stimulation from gamma efferent (motor) neurons. Think of the gamma motor neuron as a type of sensitivity adjuster. The efferent input adjusts the length of the spindle so that it remains at an optimal length to detect changes within the muscle.