VPB 121: Veterinary Physiology - II (2+1)

MUSCLE SPINDLE ORGAN

• These are located within the belly of the skeletal muscles.
• They are in the shape of a spindle and are made up of specialized striated muscle fibres which are known as Intrafusal fibers. These intrafusal fibers are placed within the connective tissue capsule attached to the perimysium of the skeletal muscle.
• The longitudinal axis of them are oriented parallel to that of skeletal muscle fibre which contains them.
• Structure of muscle spindles
  • There are two types of intrafusal fibers. One of them is known as Nuclear bag where their centre portion is interrupted and dilated which houses many nuclei. Nuclear bag region tapers towards the pole. Another intrafusal fibre is known as the Nuclear chain. They have uniform diameter throughout, but near the middle small, but appreciable increased diameter due to the increased number of cell nuclei.
• Nerve Supply
  • It is innervated by the Gamma motor neurons or Intrafusal fibers. The cell bodies of the Gamma motor neurons are located in the ventral horn of the grey matter of the corresponding spinal segment. The skeletal muscles are innervated by α-motor neurons. These are referred to Extrafusal fibers.

 

• The Gamma motor neurons are of two types,
  • Gamma plate neurons.
  • Gamma trail motor neurons.
• The Gamma plate neurons innervate nuclear bag intrafusal fibers and are tonic neurons generating sustained action potential.
• Gamma trail motor neurons supply nuclear chain intrafusal fibers. These are phasic neurons and produce action potential of short duration with high frequency.