Muscle Spindles & SNARE Proteins: Proprioception and Vesicle Fusion
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Muscle Spindles: Sensory Encoding of Length and Velocity
Muscle spindles are collections of 6–8 specialized muscle fibers located within the muscle mass itself (Figure 1.7). These fibers do not contribute significantly to the force generated by the muscle. Rather, they are specialized receptors that signal (a) the length and (b) the rate of change of length (velocity) of the muscle. Because of the fusiform shape of the muscle spindle, these fibers are referred to as intrafusal fibers.
Each muscle contains many muscle spindles; muscles that are necessary for fine movements contain more spindles than muscles that are used for posture or coarse movements.
Afferen t Types and Their Responses
Group Ia afferents provide information about both length and velocity. Group II afferents signal information about muscle length only. Because of their patterns of innervation onto the three types of intrafusal fibers, Group Ia and Group II afferents respond differently to different types of muscle movements.
- Initially, both Group Ia and Group II fibers fire at a certain rate, encoding the current length of the muscle.
- During a stretch, the two types differ in their responses:
- The Group Ia afferent fires at a very high rate during the stretch, encoding the velocity of the change in muscle length; at the end of the stretch, its firing decreases because the muscle is no longer changing length. However, its firing rate remains higher than before the stretch, as it now encodes the new length of the muscle.
- The Group II afferent increases its firing rate steadily as the muscle is stretched. Its firing rate does not depend on the rate of change of the muscle; rather, it depends only on the immediate length of the muscle.
SNARE Proteins and Vesicle Fusion Mechanism
SNARE (an acronym derived from 'SNAP (Soluble NSF Attachment Protein) REceptor') proteins are a large protein superfamily that mediate vesicle fusion, that is, exocytosis.
Docked vesicle. The vesicle contains the SNARE protein synaptobrevin and the calcium sensor synaptotagmin. Two additional SNARE proteins, syntaxin and SNAP-25, are anchored to the nerve terminal plasma membrane at the active zone. Syntaxin is held in a folded, inactive configuration by Munc18-1.
When the vesicle is in the prime position, syntaxin has entered its open configuration and formed a ternary SNARE complex with synaptobrevin and the two arms of SNAP-25. The complex is stabilized by the presence of complexin. Ca2+ binds to synaptotagmin, which in turn binds to the SNARE complex, displacing complexin and inducing pore formation.
Key molecular players
- Synaptobrevin (vesicular SNARE)
- Syntaxin (plasma membrane SNARE)
- SNAP-25 (plasma membrane SNARE)
- Synaptotagmin (calcium sensor)
- Complexin (stabilizes SNARE complex)
- Munc18-1 (regulates syntaxin conformation)
The sequence of events can be summarized as: docking of the vesicle, priming with formation of the ternary SNARE complex, stabilization by complexin, Ca2+-triggered binding of synaptotagmin to the complex, displacement of complexin, and pore formation leading to exocytosis.