red line represents the capsule of the muscle spindle
2 important functions:
insulates the inside of the spindle - separates it from the ionic concentrations outside - inside of muscle spindle have free nerve endings (axons), which are sensitive to ionic concentrations
contains viscous fluid inside the spindle - the viscous fluid protects the intrafusal fibres from mechanical perturbation from the outside as we only want our sensory nerve endings in the spindle to respond to mechanical perturbation that arises within the spindle, not mechanical perturbations from outside
