Question

17) State the function of each of the following in a muscle spindle: intrafusal fibers, gamma motor neurons, and primary afferent fibers.

Answer 1

Muscle Spindle:They are stretch receptors within a muscle that first detect the changes in the length of the muscle.

Intra Fusal Fibres:Muscle fibres with in the muscle spindle Bundle are called intrafusal Fibres.

Extrafusal Fibres:Muscle fibres outsied this Muscle spindle bundle(Regular skeletal muscle fibres) are called extrafusal Fibres.

Function of Intrafusal Fibre:They act as specialized sensory organs (proprioceptors) which detect the amount of change in muscle length.

Alpha moto Neurons supplies extra fusal Fibres,Where as Gamma motor neuron supplies the Intrafusal fibres within muscle Spindle.

Function of Gamma mototr neuron:They innervate muscle spindle,activation of gamma motor neuron cause mild contraction in Intrafusal fibres,which keeps the muscle spindle taut and maintains its sensitivity to changes in the muscle length.

Primary Afferent Fibre(Type 1a Fibre) are sensory fiber of muscle Spindle ,which monitors the velocity of the stretch.

Function of Primary Afferent Fibres:They conduct impulses from muscle spindle in response to a Strech directly to spinal cord, where they are conducted suddenly through interneurons to alpha motor neurons, which stimulate muscle contraction.

For example:In case of Knee jerk Reflex,When Hit by Knee hammer---Strech in the Quadriceps Muscle----Muscle spindle Activation----The primary afferent fibres carry impulse to spinal cord---which activates Alpha motor neuron---Muscle Contraction.