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

MECHANISM OF ACTION OF GAMMA MOTOR NEURONS

• Excitation of these neurons ends up in contraction of intrafusal muscles.
• This in turn stretches the nuclear bag fibers and nuclear chain fibers.
• Sensory receptor of the muscle spindle in limited to these areas and can detect the tension upon these receptors.
• Stretch initiates depolarization in the sensory receptor membrane.
• Intrafusal fibers have no control on the tension developed in the extrafusal fibers.
• The nuclear bag fibers are tonic and sarcolemma of these are not excitable.
• No appreciable action potential is generated, hence no muscle twitch occur.
• On contrary nuclear chain fibers respond to neural excitation and are phasic in nature. It is excitable when stimulated and generates action potential in response to gamma trail motor neurons.
• This end up in the twitch type of contraction.

Nature of Sensory fibers 

• Afferent fibers from nuclear bag spiral around the intrafusal muscle and form Annulospiral or Nuclear bag endings of the muscle spindle. These fibers are large and myelinated, responsible for initiation of myotatic reflex.
• Sensory fibers associated with nuclear chain intrafusal fibers terminate in plaque like ending namely the Floewer Spray ending. These fibers are small and are myelinated and are responsible for flexor reflex. These system functions to maintain posture of an animal and muscle tone of the skeletal muscles.
• The effect exerted by Gamma plate motor neurons is referred to as Gamma loop mechanism.