Muscle Spindle Basics.
Muscle spindles are found within the belly of muscles and run in parallel with the main muscle fibres.The spindle senses muscle length and changes in length. It has sensory nerve terminals whose discharge rate increases as the sensory ending is stretched. This nerve terminal is known as the ANNULOSPIRAL ending, so named because it is composed of a set of rings in a spiral configuration. These terminals (shown in blue) are wrapped around specialised muscle fibres that belongs to the muscle spindle (INTRAFUSAL FIBRES) and are quite separate from the fibres that make up the bulk of the muscle (EXTRAFUSAL FIBRES).
There are two main types of intrafusal fibre .. NUCLEAR BAG and NUCLEAR CHAIN . For now we will stay with the Nuclear bag fibre .. so called because there is a bunch of about 100 nuclei in the central (or equatorial) region underlying the sensory nerve ending.
Another important feature.... a motor supply to the intrafusal muscle (shown above in red). In this region on either side of the central area the intrafusal fibres are able to contract if their motor supply is active. The motor supply comes via efferent fibres that usually fall into the gamma classification of diameters. They are often (and better) referred to as FUSIMOTOR fibres.
Two things can, in principle, cause the annulospiral ending to be stretched and so increase its discharge.
1. A stretch of the muscle as a whole will stretch the spindles within it , and thus the sensory endings.
2. Fusimotor activity will cause contraction of the intrafusal fibres below the fusimotor nerve terminals either side of the central region. This will result in stretch of the central sensory region .
Nuclear chain fibre also have annulospiral sensory endings in the central region (the nuclei are in line). It is a shared branch of the axon that supplied the central area of the nuclear chain fibre. This sensory nerve is of group Ia, the fastest found in the body.
Further out we see that there are other sensory endings, more closely associated with the chain fibres. These fall into the slower group II division of sensory nerves and are referred to as SECONDARY endings in contrast to the centrally located PRIMARY endings.
The two types of intrafusal fibre, (bag and chain) have different mechanical properties, and respond differently to their largely separate fusimotor fibres. They also differ in respect to their sensory endings. Consequently, the information relayed to the CNS by the spindle via group Ia and group II sensory endings is different.
In simple terms, the Ia afferents respond partly to muscle length, but respond more powerfully to changes in length (BLUE). The group II afferents are much better at registering length alone (RED).
Ia afferents can powerfully excite the ALPHA MOTONEURONES of the muscle containing the spindle. This is the basis of the classical STRETCH REFLEX in which extension of the muscle (and thus its spindles) cause a reflex contraction.
The spindle can therefore register muscle length and velocity. Furthermore the sensitivity to both length and velocity can be altered by the CNS via activity in the fusimotor system, the static gamma system controlling length sensitivity and the dynamic gamma system controlling velocity sensitivity.
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