Neuromuscular Junction and Muscle Contraction Lecture (6) PDF

Lecture(6) Neuromuscular junction and muscle contraction Prof. Dr. Nanees El-Malkey Assistant professor, Sinai University sinaiuniversity.net  Item no.1: define NMJ Item no.2: describe NMJ structure Item no.3: functional structure of skel...

Lecture(6) Neuromuscular junction and muscle contraction Prof. Dr. Nanees El-Malkey Assistant professor, Sinai University sinaiuniversity.net  Item no.1: define NMJ Item no.2: describe NMJ structure Item no.3: functional structure of skeletal muscles Item no.4:Excitation contraction coupling INDEX @Sinaiunieg [email protected] www.su.edu.eg  Nervous system @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg Spinal cord segments @Sinaiunieg [email protected] www.su.edu.eg  Structure of the motor end plate (MEP): Neuromuscular junction the area between a nerve fiber and muscle fiber. @Sinaiunieg [email protected] www.su.edu.eg  1. Motor neuron 3.Muscle fiber 2.  Neuromuscular transmission (The end plate potential, EPP) 1 3 2 5 4  Motor unit: consisted of one anterior horn cell (AHC) + its axon + muscle fibers (3-300 fiber) ‫خليه عصبيه واحده و مجموعه الياف‬ ‫عضليه‬ The motor pool: consisted of all AHCs + nerve + skeletal muscle supplied by it. ‫كل الخاليا العصبيه وكل الياف‬ ‫@العضله‬Sinaiunieg [email protected] www.su.edu.eg  Functional structure of skeletal muscles Muscle proteins: Contractile proteins regulatory proteins @Sinaiunieg [email protected] www.su.edu.eg  Contractile proteins Myosin Actin 6 polypeptide chains It is small globular protein. that form: 1. 2 heavy chains : long tail and arm. attached to each other to form 2. 4 light chains: globular heads: filamentous structure arranged as -One : actin-binding sites - The other : contain sites of ATP long double helix hydrolysis.. @Sinaiunieg [email protected] www.su.edu.eg  Myosin molecule The 2 heavy chains: form tail and arm The 4 light chains: forming 2 globular heads. @Sinaiunieg [email protected] www.su.edu.eg  ❑ The arrangement of these contractile proteins in the filament shows transverse striations. ❑ alternative dark (A) bands and light (I) bands Sarcomere is the contractile unit of the muscle. @Sinaiunieg [email protected] www.su.edu.eg  B. Regulatory protein: Tropomyosin and Troponin @Sinaiunieg [email protected] www.su.edu.eg  Tropomyosin Troponin - Long filaments -Small globules -between actin filaments -located on tropomyosin at intervals -Cover actin binding sites. 3 subunits: -Keep actin structure Troponin T:Binds to tropomyosin. Troponin C: Binds with calcium Troponin I: Inhibit binding of action to myosis.  - Sarcotubular system: It is a system of tubules present in the sarcoplasm of the muscle fiber and made of: 1)T- tubules (transverse system): rapid spread of the action potential 2) The sarcoplasmic reticulum: a source of Ca++ ions. @Sinaiunieg [email protected] www.su.edu.eg  Mechanism of muscle contraction (Excitation - contraction coupling) 1. action potential propagates and transmitted to all fibers via the T tubules. 2. Release of Ca++ ions from the sarcoplasmic reticulum. 3. Ca++ binds to troponin-C leading to: initiation of contraction by…….. @Sinaiunieg [email protected] www.su.edu.eg  Ca++ initiates contraction by binding to troponin-C leading to: a. Weakness of the binding of troponin I to actin. b. Movement of tropomyosin laterally → uncovering the binding sites of actin for the myosin heads. 1. The interaction between actin and myosin heads leads to sliding of actin filaments between myosin filaments → muscle contraction. ‫اكتين بيتحرك الميوسين ثابت‬ @Sinaiunieg [email protected] www.su.edu.eg The energy required for this mechanism is provided by breakdown of ATP to ADP by ATPase activity of myosin heads in the presence of Ca++ ions. @Sinaiunieg [email protected] www.su.edu.eg @Sinaiunieg [email protected] www.su.edu.eg  Mechanism of muscle relaxation 1. The Ca++ is actively re-up taken back to the SR by active Ca++ pump to be stored in the cisterns (Ca++ pump need breakdown of ATP). 2. ↓↓ intracellular Ca++ ions → the troponin-tropomyosin complex return to its original position→ separation of myosin head from actin sites → muscle relaxation. So, ATP hydrolysis is required for contraction and relaxation and without ATP relaxation will not occur. @Sinaiunieg [email protected] www.su.edu.eg  Wrap up 4. S.C consists of 31 segments. 5. Each segment has: posterior horn, and anterior horn. 6. Posterior root is sensory 7. Ventral root is motor. 8. Some segments have lateral horns. MCQ 1. Light bands are formed of…. A. Actin B. Myosin C. Troponin D. tropomyosin ATP required for muscle contraction is found At ……… A. actin heads B. Troponin head C. myosin head D. tropomyosin True or false Relaxation process is a passive process The interaction between actin and myosin leads to sliding of myosin filaments between actin filaments T- tubules is the source of calcium during excitation contraction coupling THANK YOU Dr. Nanees El-Malkey Assistant professor, Sinai University