Skeletal Muscle (II) Flashcards
What is muscle innervation?
- One motor neuron innervates a group of muscle fibres.
- Motor neuron divides into terminals that form neuromuscular junctions on muscle fibres.
- Neuromuscular junctions are located in the middle of the fibres.
Neuromuscular transmission
- Nerve fibres (axons) conduct action potentials (APs) at a fast rate (~40-80 m/s).
- Terminals of motor nerves form neuromuscular junctions (also called motor end plates).
- The key function of the neuromuscular junctions is to transmit the activation from nerve to muscle fibres.
Neuromuscular transmission (cont.)
- Arrival of the AP at the nerve terminal triggers release of acetylcholine (Ach) into the synaptic cleft (space between nerve and muscle fibre).
- Ach is made from acetyl-CoA released from mitochondria and choline in nerve terminals.
- Released Ach binds to receptors on the muscle fibre and triggers influx of Na+ followed by generation of action potential (AP) in muscle fibres.
- Ach acts for a short time since it is degraded by Ach esterase enzyme.
- Choline is taken up by nerve terminals for re-synthesis of Ach.
Synaptic vesicles of axon terminals
The process leading to the release of Ach involves docking of the synaptic vesicles to the active zone, fusion with the membrane and release of the mediator.
Action potentials
After the depolarisation of postsynaptic membranes, an action potential is generated.
The actin potential spreads from both ends of muscle fibres at about 6m/s
Contractile properties: Human muscles
- Electrical stimulation is applied over the surface electrodes to induce action potentials.
- Torque of the isometric contraction is usually recorded.
Applications of Electrical Stimulation
Neuromuscular electrical stimulation (NMES) is used for functional and therapeutic applications in subjects with spinal cord injury or stroke.
Muscle responses to action potentials:
Muscle starts to contract at about 10 ms.
Muscle reaches peak contraction at about 38-40 ms.
What is single twitches and Summation?
Single twitches: Muscle relaxes completely between stimuli.
Summation: The stimuli are closer together so they don’t allow the muscle to relax fully.
How is muscle force regulated through frequency of action potentials?
- Contraction force in muscle fibres depends on the frequency of action potentials.
- A change in frequency of action potentials results in a proportional change of force.
- Muscle fibres are activated at 5-100 Hz frequencies in voluntary contractions.
What is the role of Calcium in tetanic contractions and fatigue?
- A continuous stimulation at 100 Hz causes a sustained increase in intracellular Ca2+ -[Ca2]I and generates tetanic contractions (smooth contractions).
- [Ca2]I matches force in repeated contractions.
- A decrease in [Ca2]I results in a lower contractile force - one of the mechanisms of muscle fatigue.
Muscle fibre types: (Muscle fibre heterogeneity)
- Skeletal muscle is not a homogeneous tissue.
- There are Type I and Type II muscle fibres.
- Type IIA and Type IIX.
- Rodents contain a large number of Type IIB muscle fibres.
How do you determine the type of a fibre?
- The isoform of expressed myosin.
- The metabolic profile. (Mitochondrial content and the capacity of glycolytic enzymes).
ATPase activity in the types of muscle fibres:
- Type I has the lowest ATPase activity.
- Type IIA has moderate ATPase activity.
- Type IIX has the highest ATPase activity.
Shortening velocity in muscle fibres:
- The fibre shortening velocity is the lowest in Type I.
- Is moderate in Type IIA.
- Is the highest in Type IIX.
