A muscle spindle is a sensory organ that's found in the structure of muscles throughout the human body. It's referred to as a propriceptor, which means "sense of self". The muscle spindle monitors the lengthening of a muscle, and when the information from them combines in the brain, it gives the person a sense of where their body is in space. While extremely important to regular muscle function, these propriceptors can be difficult to find if a person isn't sure what to look for.
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A muscle spindle is found inside muscle tissue. It's contained within a capsule that lies parallel to the muscle fibres. Inside of the capsule are intrafusal fibres, which are a type of specialised muscle fibre. Intrafusal fibres have thick and thin filaments (called contracile proteins) at either end of a central span that does not have any contractile proteins. In between the two ends of the intrafusal fibres is a central region which is wrapped in the sensory dendrites of the muscle spindle afferent. What happens is that when the muscle lengthens, the muscle spindle is stretched, and this opens up an ion channel in the sensory dendrites. These ions cause a potential reaction in the action potentials of the muscle spindle afferent.
What all of that means can be broken down fairly simply. When a muscle lengthens, it stretches out the muscle as well as the muscle spindle. This activates the central area, the sensory dendrites, which opens an ion channel. As long as the muscle spindle remains taught it acts as an effective sensor as to how stretched the muscle is or isn't. Those signals are sent back through the body and interpreted by the brain. This is what gives a person a sense of where their body is, when all of the muscle spindles act as they should.
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