Skeletal Muscle Neuromuscular junction with ACh and nicotinic receptors. Start o

Question

Skeletal Muscle Neuromuscular junction with ACh and nicotinic receptors. Start of action potential, why skeletal muscle is all or none. Excitation-contraction coupling between T-tubles and sarcoplasmic reticulum to release Ca+2 for allosteric regulation of contraction cycle, troponin, tropomyosin actin and myosin cross-bridge cycling and where and how ATP is used. Define motor unit. Know about Spatial summation (recruitment) and temporal summation (tetany/ fatiguel isometric vs. isotonic contractions. Understand how the length- tension relationship works for passive lengthening and shortening of the muscle. What is the metabolism of skeletal muscle cells, how does this effect speed of movement, size of cells and recruitment order? What about fatigue?

Explanation / Answer

All or none potential means that the action potential will be generated only for a particular threshold only and would be of the same amplitude no matter how strong is the threshold. This simply means either it will be generated or it wont be and if at all it is generating it will be of a particular amplitude no matter how high is the threshold.

When the action potential from a motor neuron reaches the end plate it release acetylcholine which then binds to the acetylcholine receptors on the muscle fibres. This open up channels that bring in sodium depolarizing the muscel fibre. If the membrane reaches the threshold that it requires then an action potential is generated which travels down the membrane of the myofibrils called sarcolemma. It travels though the T-tubule system and reaches the sarcaplasmic reticulum that stores calcium and upon being excited by the action potential releases the calcium ions into the sarcoplasma. As the calcium ion's concentration increases, they bind to the troponin. Troponin associates with tropomyosin that blocks the interaction of myosin heads on actin filaments. As calcium ion binds troponin it changes shape, remodulates tropomyosin so as to allow myosin heads to interact with actin filaments forming a cross bridge. ATP binds to the nucleotide site of the myosin head which is hydrolyzed to cause power stroke which causes contraction of muscle fibre. During this the ADP is bound to themyosin head, as ADP releases and ATP binds , the myosin head detaches again and comes back to the original position.

Motor unit is the assembly of motor neurons innervating the skeletal muscle fibres.

Spatial summation is the summation of signals from various sources which add up at the axon hillock. They can be excitatory or inhibitory. Temporal summation mean repetition of signals from the same source.

Fatigue is a state when there is no more ATP left in the muscles to keep contracting.

The length of muscles remain samein isometric contractions, there is range of motion required, they are not dynamic whereas in isotonic contraction the tension remains the same while the length changes.

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