Muscle I PDF

NMJ and Muscle PL1400 & PL1001 Dr Therese Ruane-O’Hora [email protected] 1  LEARNING OUTCOMES 1 Recall the definition of a synapse What is an alpha motor neuron? Outline the structure of the NMJ, identifying the different components...

NMJ and Muscle PL1400 & PL1001 Dr Therese Ruane-O’Hora [email protected] 1  LEARNING OUTCOMES 1 Recall the definition of a synapse What is an alpha motor neuron? Outline the structure of the NMJ, identifying the different components Outline the process of neurotransmitter release into the synaptic cleft at the NMJ Outline the sequence of events following release of neurotransmitter into the synaptic cleft of the NMJ Recall the names of the different components of a muscle fiber Outline the sequence of events as they occur, when a skeletal muscle contracts and relaxes (Cross-bridge Formation) 2  Overview of the Neuromuscular Junction (NMJ) Example of a synapse Motor Nerve meets the muscle > synaptic cleft Release of Neurotransmitter ACh ACh binds to receptors on Muscle Sets up muscle action potential 3 Diagram for Structure of the NMJ Somatic motor neuron also a called Lower motor neuron or Alpha (α) motor neuron Neuromuscular/synaptic cleft of about 20um so no physical contact between the nerve and the muscle Lots of Mitochondria in pre synaptic nerve terminal and post synaptic terminal 4 This Diagram shows the Sequence of events at the NMJ NMJ is chemical synapse Nerve meets muscle > No direct contact between the neurone and muscle > synaptic cleft of about 20um the nerve and the muscle. If no physical contact > How does the message transfer across? ANS: release of neurotransmitter acetylcholine (ACh) into the synaptic cleft ACh is produced in the nerve terminal and stored there is synaptic vesicles After release, ACh binds to receptors on Muscle Triggers EPP in muscle cell Sets up action potential in skeletal muscle and the muscle contracts ACh is then broken down by AChE and the components are recycled 5  Types of Skeletal Muscle Fibers Type I Slow twitch (Red) Type II Fast twitch (White) muscle fibers e.g Postural muscle fibers e.g Muscles extraocular muscles Myoglobin +++ Less Myoglobin Mitochondria +++ Less Mitochondria Oxidative metabolism +++ Some Oxidative metabolism More Glycolytic metabolism SR Moderately Developed SR Highly developed Myosin ATPase isoform Myosin ATPase isoform hydrolyses ATP quickly hydrolyses ATP more slowly More Fatigue prone Fatigue resistant 6 Gross Structure of Muscle Muscle fibres > Myofibrils > Myofilaments of Actin and Myosin giving a striated appearance Organisation of Skeletal Muscle Fibres Proteins Actin and Myosin make up the myofibril Note the network of T- tubules for spread of the muscle action potential Lots of Mitochondria > ATP Sarcoplasmic Reticulum > Calcium storehouse 8  Organisation of Skeletal Muscle Fibers/Histology Recall the term Myofibrils from previous presentation > here we see that they are divided into structures called Sarcomeres Sarcomere is the contractile unit of skeletal muscle Sarcomere extends from one Z disc to the next Gives muscle a striated appearance Composed of a variety of different Proteins > Actin > Thin Filament Myosin > Thick Filament Titin Also Troponin and Tropomyosin > additional protein components of the actin filament 9 Summary of a Contraction - Relaxation (cross-bridge ) Cycle What happens within a sarcomere when the muscle contracts and relaxes? (1) In a resting muscle sarcomere actin and myosin are detached (2) Muscle action potential triggers release of Ca++ from SR and Ca++ binds to troponin C component of actin filament, allowing myosin crossbridge to interact with actin (3) and (4) Inorganic phosphate detaches itself from myosin head > allowing the Power stroke i.e. the sliding of the 2 filaments actin and myosin (5) A new ATP molecule attaches to myosin and now myosin detaches from actin (6) muscle relaxes and Ca++ pumped back into SR. ATP hydrolysed 10