Spindle Structure

Chain fibre

Chain (or 'nuclear chain') fibres are shorter than bag fibres, being around 4 mm in diameter and 12 microns wide. There are up to 10 per spindle in man. Most of the secondary endings (Group II afferents) come from chain fibres which confer on the spindle most of its static sensitivity to length.
Fusimotor stimulation via gamma static fibres is most effective at between 50-150Hz and results in brief localised contractions which bias the discharge of both primaries and secondaries. The static and dynamic length sensitivity of primaries is disrupted, and that of secondaries enhanced.

Chain fibre primary sensory ending

Chain fibres have central annuolospiral endings which, in common with all intrafusal fibres contribute to the group Ia afferent input.

Chain fibre secondary ending

The chain fibres provide most, and in some cases all, of the secondary endings that form the Group II afferent input.

Dynamic bag 1

Dynamic bag 1 (Db1) intrafusal fibres are classified histologically as 'bag1' fibres ( Up to 100 nuclei in a central 'bag' ,low ATPase, low Glycogen, indistinct M-line) and 'dynamic' because of their mechanical properties (they respond to rapid changes in muscle length).

Dynamic bag 1 primary sensory ending

The primary (Group Ia, annulospiral) afferent of each spindle is associated with the central region of all types of intrafusal fibre, including Dynamic bag 1.

Dynamic bag 1 secondary ending

No more than 10% of all the secondary (Group II afferent) endings are found on the Db1 fibre, as spray-like endings.

Ia afferent

The Group Ia afferent relays sensory information from the central annulospiral endings of all the intrafusal fibres. Ia afferents have a powerful excitatory effect (mono- or polysynaptic) on the alpha motoneurones of the same muscle and its synergists, and inhibitory effects (via interneurone) on antagonist alpha motoneurones.

Capsule

The outer capsule of the spindle, about as long as the chain fibres, is somewhat similar to the blood brain barrier. It keeps out protein and protects the sensory endings mechanically, lubricates the moving parts and controls the local chemical environment.

II afferent

Group II afferents relay sensory information from the secondary endings, mainly associated with chain fibres.
Group II afferents excite alpha motoneurones of the same muscle via mono- and polysynaptic pathways. The monosynaptic component is about half that found for Group Ia afferents [this spindle has two secondary endings, this is quite common].

Long chain fibre

Occasionally one chain fibre is longer than its partners and receive terminations of static beta axons. The detailed innervation of chain fibres is complex.

Long chain fibre secondary sensory ending

The chain fibres provide most, and in some cases all, of the secondary endings that form the Group II afferent input.

end plate on extrafusal fibre

In about one third of spindles the Dynamic bag1 fibre is also innervated by a dynamic beta axon that additionally innervates slow fibres in extrafusal muscle.

end plate on extrafusal fibre

In a minority of spindles, static beta axons terminate on long chain and on extrafusal fibres.

Static bag 2

Static bag 2 (Sb2) intrafusal fibres are classified histologically as 'bag2' fibres ( Up to 100 nuclei in a central 'bag' , moderate to high ATPase activity, medium Glycogen) and 'static' because of their mechanical properties (they respond to slow steady changes in muscle length).

Static bag 2 primary sensory ending

The primary (Group Ia, annulospiral) afferent of each spindle is associated with the central region of all types of intrafusal fibre, including static bag 2.

Static bag 2 secondary sensory ending

No more than 20% of all the secondary (Group II afferent) endings are found on the Sb2 fibre, as spray-like endings.

Static gamma system

Static gamma motoneurones are so named because of their effect on their target intrafusal fibres (SB2 and chain fibres). When the 'static intrafusal system' is active the sensitivity to length changes of the primary endings is reduced. This may be a means of supressing the stretch reflex during voluntary movement.

Dynamic gamma system

Dynamic gamma motoneurones are so named because of their effect on their target intrafusal fibres (DB1 fibres). When the 'dynamc intrafusal system' is active the sensitivity to length changes of the primary endings is increased.