21. The regulation of muscle work and muscle nerve connection: the motor unit

Question 1

Muscle-nerve connection

Answer 1

• The transmission of neural action potential to the muscle takes place in the area of the myoneural junction.
• The essence of this connection is the acetyl choline release induced by the action potential at the nerve terminals. This chemical sign binds to the nicotinic receptors of the muscle membrane and results in the opening of ligand activated cationic channels.
• The opening of these cationic channels produces a local current (end plate potential, EPP) which is gradually decreasing conducted to the fast, voltage gated sodium channel situated next to the myoneural junction. There, a sudden conformation change is induced and then action potential is formed.
• Muscle fibers are rich in neural branchings. Nerve fiber/muscle ratio may vary considerably according to the task of the muscle.
  
  • Muscles executing fine movements (e.g. ocular muscles) have a ratio of 1:1
  • In other skeletal muscles the ratio may even reach 1:100

Question 2

Neural signal

Answer 2

• A neural signal (AP) is transformed to muscular electrical signal (AP) by the mediation of a chemical signal (acetylcholine):
  
  • Presynaptic area:
    
    • Action potential from axon
    • Neural action potential induces calcium entry to the synaptic ending
    • Acetylcholine vesicle release
  • Synaptic cleft:
    
    • Filled with acetylcholine
  • Postsynaptic area:
    
    • Acetylcholine binds to its specific receptors on the myolemma
    • Ligand gated ion channel is opening: a local end plate potential (EPP) is developing
    • EPP activates neighboring voltage gated sodium channels, which result in a formation and propagation of action potentials.

Question 3

Muscle-type nicotinic acethylcholine receptor molecule

Answer 3

Muscle-type nicotinic acethylcholine receptor molecule is composed of two alpha, two beta, and one delta subunits. This structure makes possible the competitive blocking effect of curare and bungarotoxin (poisons).

(nicotinic receptors in CNS don’t have delta subunit – therefore this type of nicotinic acethylcholine receptor is more resistant to curare)

Question 4

Motor unit

Answer 4

• Motor unit is composed of a nerve and a muscle supplied by this nerve.
• Large motor unit:
  
  • Nerve: Large diameter, very fast conduction, difficult stimulation
  • Muscle: Large fiber number, large diameter, large force, anaerobic metabolism, speed is fast, easily fatigues, fiber length is very long.
• Small motor unit:
  
  • Nerve: Small diameter, fast conduction, easy stimulation
  • Muscle: Few fiber number, intermediate diameter, small force, oxidative metabolism, speed is slow, does not fatigue, fibre length is short.