Muscle Spindles & Neuromuscular Junction Flashcards
What is a motor unit?
An alpha motor neurone and all the extrafusal skeletal muscle fibres it innervates.
Where in the spinal cord are alpha motor neurones that control distal muscles?
Lateral
Where in the spinal cord are alpha motor neurones that control proximal muscles?
Medial
Which neurotransmitter is found in neuromuscular junctions?
Acetylcholine
What is the motor end plate?
The region of muscle fibre that lies directly under the terminal portion of the axon.
What is a neuromuscular junction?
The junction of an axon terminal with the motor end plate.
Outline the mechanism of neuromuscular transmission.
- Action potential opens the voltage gated Ca2+ channel, allowing Ca2+ ions to diffuse into the axon terminal
- Ca2+ binds to proteins that enable vesicles containing ACh to fuse with the plasma membrane and release ACh into the cleft
- ACh diffuses across the cleft and binds to cholinergic nicotinic receptors, opening ion channels
- More Na+ moves in than K+ out, producing an end-plate potential
- Action potential is propagated into the T tubules and spreads throughout muscle
What do stretch receptors in the muscle spindle monitor?
Muscle length & rate of change of muscle length
What are the muscle fibres within the spindle called?
Intrafusal fibres
What are intrafusal fibres innervated by?
Gamma motor neurones, a type of lower motor neurone
Name the 2 type of stretch receptor in the spindle.
Nuclear chain fibre
Nuclear bag fibre
Where are fast type 1a afferent sensory nerves associated with on the spindle?
Middle third
Where are slow conducting type 2 afferent sensory nerves associated with on the spindle?
Inferior and superior thirds
What do golgi tendon organs monitor?
How much tension the motor units are exerting.
What are golgi tendon organs?
Endings of afferent fibres that wrap around collagen bundles in the tendons
How is the golgi tendon organ activated?
When the muscle is stretched, tension is exerted on the tendon. The tension straightens the collagen bundles and distorts the golgi tendon receptor endings, activating them.
Give an example of a stretch reflex.
Knee jerk reflex.
Describe the knee jerk reflex.
- Patellar tendon is tapped, pushing the tendon and stretching the thigh muscles
- Stretch receptors are activated, stimulating a burst of action potentials
- Motor units are stimulated, causing the muscles to contract resulting in extension of the lower leg = knee jerk
- This reflex is monosynaptic as interneurons are not involved
Describe the withdrawal reflex.
Painful stimulation activates the flexor muscles and inhibits the extensor muscles of the ipsilateral (same side) leg. The leg moves away from the harmful stimulus.
Describe the inverse stretch reflex.
1b fibres of the golgi tendon inhibit alpha motor neurones and so inhibit muscle contraction. This regulates muscle tension and protects the muscle from overload.
What is clasp knife reflex caused by?
An upper motor neurone lesion
What is the clasp knife reflex?
When one bends a limb, initially there is resistance. After a certain point, resistance falls dramatically. 1b fibres inhibit the alpha motor neurones.
What is muscle tone?
The degree of contraction of a muscle
What do muscles with high and low tone feel like?
High tone: firm + rigid
Low tone: Soft + flaccid
What is hypertonia?
Abnormally high muscle tone. Increased level of alpha motor neurone activity keeps the muscle contracted. Disorder of descending pathway/upper motor neurones.
What is hypotonia?
Abnormally low muscle tone. Weakness and absent reflex. Disorder of alpha motor neurones i.e. lower motor neurone disorder
