QS Study

The muscle spindle is a specialized receptor within muscle fiber which provides sensory information that is used by the central nervous system in the control of muscle activity. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, by activating motor neurons via the stretch reflex to resist muscle stretch. Spindles play an important role in motor control and are used to help regulate muscle length during movement.

Structure

One muscle contains many muscle spindle, each muscle spindle composed of 2-14 slender muscle fibers termed intrafusal muscle fiber. The muscle spindle is encapsulated and ends are tapering. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, by activating motor neurons via the stretch reflex to resist muscle stretch.

There are two types of intrafusal muscle fiber.

Nuclear bag fiber:

  • 2 per spindle,
  • Longer and thicker,
  • Recognized by the presence of numerous nuclei in the equatorial region.

Nuclear chain fiber:

  • Per spindle tour or more.
  • Shorter and Thinner, Nuclei form a single longitudinal row or chain in the center of each fiber at the equatorial region.
  • Ends of both fibers striated and central end non-striated and noncontractile.

Fig: Neuromuscular spindle showing two types of intrafusal fibers; the nuclear chain fibers

Innervations (Sensory)

(A) Anulospiral or primary endings:

  • 1 for one nuclear bag taxi,
  • 1 for a second nuclear bag and nuclear chain fiber.

(B) A flower spray or secondary ending: ‘Only for chain fiber.

Innervations (Motor)

(A) Plate ending or dynamic γ – efferent: For nuclear hag fiber.

(B) Tail ending or static γ – efferent: For nuclear chain fiber.

The extrafasal fibers are supplied by α-motor neuron.

Stimulation of muscle spindle

When muscle activity occurs, either actively or passively, the intrafusal fillers are stretched and there is an increase in the rate of passage of nerve impulses to the spinal cord or brain in the afferent neurons.

Important:

(1) Maintains muscle tone,

(2) Maintains posture,

(3) Carries kinesthetic sensation,

(4) Act as a negative feedback device in maintaining muscle length,

(5) Responsible for stretch reflex during muscular contraction,

(6) Mediates protective reflex when muscle vigorously extended.