Muscle spindles are considered one of the most vital parts of neuromuscular system and have been found to be responsive to muscle length changes. Muscle spindles are also significant part of propriorective body qualities and aid in maintaining body balance.
Whenever the muscles move, it’s the responsibility of the brain to follow this muscle movement and also the muscle state so that the brain can determine the exact positioning of the muscle and the connected joints in motion. This information is provided to the brain’s central nervous system by the muscle spindles. The muscle spindles come in the category of sensory organs and move opposite to muscle fibers. Their main function is to respond to the changes in length of muscle and also the rate at which these changes occur. Whenever a muscle is undergoing stress, muscle spindles act to make it contract and greater the stress more is the amount of contraction. Since these are considered as highly specialized sensory type organs, it very effectively provides the movement feedback to the brain depending upon the changes in muscle length and the rate which this change occurs. This detection is performed by muscle spindles by enhancing the generation of electrical signals so that the sensory nerve endings get activated.
Each muscle spindle is composed of a tiny capsule of collagen having tapering ends. In this capsule are located highly specialized intrafusal fibers which are a type of muscle fibers. These fibers run opposite to the major muscle fibers that are embedded in skeletal muscle tissue and are known to actually cause the muscle movement. These muscle fibers are known as extrafusal fibers and their movement is caused by alpha motor neurons.
Gamma motor neurons are the nerve endings that perform the function of controlling contractile proteins located at the tapering ends of fibers. These neurons maintain the tension and sensitivity of muscle spindle irrespective of its length. The extrafusal muscle fibers on receiving an impulse from central nervous system for movement send it to muscle spindle via gamma motor neurons due to which the contractile proteins make changes in the length of intrafusal fibers and perform the action of contraction or release at the fiber’s centre.
Basically the main nerve ending are the Annulospiral nerve endings which are further divided into primary nerve endings, and secondary nerve endings.
Annulospiral Nerve Endings: These are located in the central part of intrafusal proteins and are placed as an option to contractile proteins. Since these form spiral endings around the intrafusal fiber’s centre it is called as annulospiral nerve endings.
Primary Nerve Ending: The primary annulospiral nerve ending is the fastest as well as the biggest firing present in the body and is known to detect nerve endings.
Secondary Nerve Ending: This is joined with the intrafusal fiber and performs the function of detecting the position of stimulus.
Muscle spindles take active participation in the functioning of proprioceptive activity as well as the efficiency of neuromuscular which is related to postural and balancing function of the body. If muscle spindles become weak owing to some reasons, it can lead to body imbalance causing injury.