Muscle Fibres and the stretch reflex

aka BSCC Physiology 012

Basic Science in Clinical Context Examination: 2 minutes long in 2 parts.

  • Exam candidate answering a question (under exam conditions)
  • Professor providing a more detailed explanation (with transcript)
Please explain the components of a muscle spindle and the sequence of events involved in producing a stretch reflex
Examinee response: Drawing and explanation in real-time video/audio
Examiner Explanation

Muscle Fibres and the stretch reflex

  • When a muscle is stretched it contracts.
  • The stimulus that initiates the reflex is the stretch of the muscle, and the response is contraction of the muscle being stretched.
  • The sensory organ is a fusiform shaped structure called the muscle spindle. It is located within the fleshy parts of the muscle.
  • The impulses originating form the muscle spindle are transmitted to the spinal cord by fast sensory fibres.
  • Impulses are then sent directly to the motor units in the spinal cord which innervate the same muscles and a contraction occurs.
  • (Muscle spindles are sensory receptors inside the muscle)
  • EFMF (extra fusal muscle fibre) are fibres which have an origin and an insertion/attachment to a tendon. They are the regular contractile units of muscle
  • In the connective tissue capsule within the EFMFs is the muscle spindle. The muscle spindle contains IFMFs (intrafusal muscle fibres) . The IFMFs are parallel to the EFMFs, have no origin or insertion, but are attached to the intramuscular connective tissue inside the muscle.

There are three types of IFMF

  • Nuclei bag (NB) fibres types (look like a bag in the centre of the fibre)
    1. Dynamic (NB)
    2. Static fibres (NB)
  • Nuclei chain fibres
  • The IFMFs are sensory only. Spiral annular neuron endings rap around the IFMFs and travel to the spinal cord. When the IFMFs are stretched they send impulses to the spinal cord.
  • Following a stretch or stimulus of the muscle- the IFMFs will also stretch because they are connected to the intramuscular connective tissue.
  • This generates an impulse which is conveyed via the afferent fibres: Ia and II to the spinal cord.
  • This results in a firing of the Alpha efferent motor neuron (the nucleus is located in the spinal cord and its axon projects outwards to directly or indirectly control muscle) from the spinal cord and causes the muscle bulk (MB) to contract
  • When the MB contracts and shortens, the IFMFs become slack and will thus not send any more signals to the spinal cord. The muscle will now stop contracting.
  • To prevent and protect the muscle from heavy load changes, a motor neuron called the gamma efferent motor neuron innervates the muscle spindle and causes it to contract at the same time as EFMFs are contracting.
  • Now the EFMF and IFMF contract strengthening the muscles, protecting form heavy load changes
BSCC Basic Science in Clinical Context 700 2

Basic Science

in Clinical Context

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