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Neuromuscular transmission

Neuromuscular transmission. Synaptic transmission ***. Synapse is the junction between two neurones where electrical activity of one neurone is transmitted to the other. Motor Unit. Motor Unit : is the motor neuron and all the muscle fibers it supplies

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Neuromuscular transmission

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  1. Neuromuscular transmission

  2. Synaptic transmission *** • Synapse is the junction between two neurones where electrical activity of one neurone is transmitted to the other

  3. Motor Unit • Motor Unit : • is the motor neuron • and all the muscle • fibers it supplies • all of these fibers will have the same type (either fast twitch or slow twitch). When a motor unit is activated

  4. The Neuromuscular junction consists of Axon Terminal : contains around 300,000 vesicles which contain the neurotransmitter acetylcholine (Ach). Synaptic Cleft : 20 – 30 nm ( nanometers ) space between the axon terminal & the muscle cell membrane. It contains the enzyme cholinesterase which can destroy Ach .

  5. The Neuromuscular junction consists of • Ach is synthesized locally in the cytoplasm of the nerve terminal , from active acetate (acetylcoenzyme A) and choline. • Then it is rapidly absorbed into the synaptic vesicles and stored there. • The synaptic vesicles themselves are made by the Golgi Apparatus in the nerve soma ( cell-body). • Then they are carried by Axoplasmic Transport to the nerve terminal , which contains around 300,000 vesicles .

  6. Acetylcholine (1) • Ach is synthesized locally in the cytoplasm of the nerve terminal, from active acetate (acetylcoenzyme A) and choline. • Then it is rapidly absorbed into the synaptic vesicles and stored there. • The synaptic vesicles themselves are made by the Golgi Apparatus in the nerve soma ( cell-body). • Then they are carried by Axoplasmic Transport to the nerve terminal , which contains around 300,000 vesicles . • Each vesicle is then filled with around 10,000 Ach molecules .

  7. Steps involved: • AP at the synaptic knob -----» Ca channels open (increase Ca permeability) -----» • release of neurotransmitter (NT) from synaptic knob to synaptic cleft -----» • NT combines with specific receptors on the other membrane -----» postsynaptic potential -----» AP will result

  8. Neuromuscular transmission

  9. Neuromuscular transmission *** • Transmission of impulse from nerve to muscle (neuromuscular junction) • Steps: AP at nerve knob -----» increase Ca permeability (Ca inter synaptic knob) -----» release of Acetylcholine (Ach) -----» Ach combine with receptors on motor end plate -----» Na permeability increase -----» end plate potential develop -----» AP spread on the membrane -----» muscle contraction

  10. Molecular basis of muscle contraction *** • Anatomical consideration: • Muscle fibre • Sarcomere • Myosin (thick filament): • Cross-bridge • Actin (thin filament) • Regulatory protein: (Troponin,Tropomyosin) • Actin

  11. Events of muscle contraction: *** • Acetylcholine released by motor nerve »»»»» EPP »»»»» depolarization of CM (muscle AP) »»»»» • Spread of AP into sarcoplasmic reticulum »»»»»release of Ca into the cytoplasm • »»»»» Ca combines with troponin »»»»» troponin pull tropomyosin sideway »»»»» exposing the active site on actin »»»»» myosin heads with ATP on them, attached to actin active site • »»»»» Resulting in formation of high energy actin-myosin complex »»»»» activation of ATP ase (on myosin heads) »»»»» energy released, which is used for sliding of actin & myosin

  12. Events of muscle contraction: • When a new ATP occupies the vacant site on the myosin head, this triggers detachment of myosin from actin • The free myosin swings back to its original position, & attached to another actin, & the cycle repeat its self

  13. Events of muscle contraction: • When ca is pumped back into sarcoplasmic reticulum • »»»»» ca detached from troponin »»»»» tropomyosin return to its original position • »»»»» covering active sit on actin »»»»» prevent formation of cross bridge »»»»» relaxation

  14. Muscle contraction **** • 1- simple muscle twitch: The mechanical response (contraction) to single AP (single stimulus) • 2- Summation of contraction: Spatial summation: the response of single motor unites are added together to produce a strong muscle contraction Temporal summation: when frequency of stimulation increased (on the same motor unite), the degree of summation increased, producing stronger contraction

  15. Types of muscle contraction: • 1- Isometric contraction: • No change in muscle length, but increase in muscle tension (e.g. standing) • 2- Isotonic contraction: • Constant tension, with change in muscle length (e.g. lifting a loud)

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