MUSCLE

1. MUSCLE CONTRACTION

2. Yesterday we finished off talking about the different kinds of muscle contractions. Today we are going to look at how a muscle produces these contractions. Think of jumping, maybe going to block a shot in basketball or spiking a volleyball. I want you to write down everything that goes into you being able to jump. So how do muscles know how and when to contract?

3. Neuromuscular System “nerve” “muscle” • complex linkage between the nervous system and the muscular system NEURON • transmits demands of the brain, via ELECTRICAL IMPULSES • 2 types of neurons • 1) Sensory –detects a stimulus; message to brain • 2) Motor –produces movement; message from brain • the movement of an impulse from the cell body to terminal branches is called an ACTION POTENTIAL • -only one direction

4. When the terminal branches of a motor neuron attach to a muscle, we get a. . . • A single action potential to the muscle will cause a twitch, but nerves transmit impulses continuously to ensure smooth movement • 2 types: • Small: stimulates few fibres fine motor movements (eye) • Large: stimulates many fibres  gross motor movements (quads) Doesn’t denote size

5. In order for a muscle force to be produced, ALL MOTOR UNITS in the group MUST BE RECRUITED All the motor units need to have a sufficient impulse reaching them ALL-or-NONE PRINCIPLE “ when a motor unit is stimulated to contract, it will do so to its fullest potential, or not at all” WHERE ARE WE? We have an electrical impulse (action potential) sent from the brain (motor neuron) and it has reached the muscle But how does the muscle receive the impulse? How is the connection made from the nervous to the muscular system?

6. Neuromuscular Junction -video • simply put, the neuromuscular junction is a space between the terminal branches of the neuron (axon) and the sarcolemma of the muscle fibre • the junction is called a “synapse” • this is the point where electrical energy transforms into chemical energy

7. The crossing of the NMJ boils down to 3 main steps: • Action potential (AP) moves along the axon till it reaches the presynaptic cleft; causes an influx of Ca 2+ ions into the cleft • The Ca 2+ ions cause a release of a neurotransmitter acetylcholine (Ach) that diffuses across the synapse • ACh is detected by receptors on the postsynaptic cleft; causes influx of Na + ions into the sarcolemma causing a new AP to propagate into the muscle fibre

9. WHERE ARE WE? We have an electrical impulse (action potential) sent from the brain (motor neuron) and it has reached the muscle. The action potential crossed a synapse at the neuromuscular junction via a neurotransmitter (Acetylcholine) and entered the muscle -we have seen a change from electrical energy (AP) to chemical energy (Ca and Ach) and back to electrical energy (new AP) -Video But how does this secondary AP cause a muscle to contract and how does the muscle itself contract?

10. Excitation-Contraction Coupling This process is one of changing chemical energy into mechanical energy The excitement of a muscle leading to its contraction • The AP is rapidly conducted along the muscle fibre till it reaches the transverse tubules (This is occurring all over the muscle fibres) -video • The AP causes a change in the tubules, which in turn forces the terminal cisternae to release Ca 2+ ions into the sarcoplasm

11. So calcium is not just important for keeping our bones strong Calcium ions are also the reason that our muscles contract. But how do calcium ions cause a contraction? Sliding Filament Theory We have now reached the smallest parts that make up a muscle: the filaments. Before we look at how the filaments work, we need to look at what they look like MYOSIN • protein • made up of a “head” and “tail” portion • contains an attachment site for actin • simpler of the two filaments

12. Structure of a myosin filament

13. ACTIN • protein • also contains 2 other proteins • 1) Troponin –binding site for calcium! • 2) Tropomyosin –cord like structure covering actin binding sites Working together, these two accessory proteins will NOT allow the myosin to interact with actin unless calcium is present

14. So, how does the interaction of actin and myosin result in a contraction? ** Muscles ALWAYS PULL, never push ** ** This PULL is a result of the OVERLAPPING of actin and myosin ** Part 1 • Released Ca2+ binds to the troponin. • Excited troponin causes a shift in the tropomyosin, which exposes actin (binding site for myosin)

15. Part 2 • Myosin will attach to the exposed actin, (forming a myosin crossbridge) flex rapidly, causing the actin to slide over the myosin • The presence of ATP (Adenosine triphosphate) or energy causes the bridge to “break” • Since calcium is still present, the actin will remain exposed and therefore the myosin will continue to attach, pull, detach, etc

16. The result. . . CONTRACTION!

17. The Sarcomere Z-line:edge of a sarcomere I-band: distance between successive myosin A-band: length of a myosin filament M-line: midpoint of myosin H-zone: distance between successive actins

18. What is happening to all those areas of the sarcomere during contraction and relaxation? H-zone decreases I-band decreases Video

19. WHERE ARE WE? That’s your job to finish In groups of 2 or 3 you are going to write out the entire process of muscle contraction from impulse to myosin crossbridges You’re not done yet! Now you are going to look over your process and eliminate everything unnecessary. Your job is to make the process as straightforward and simple as possible without losing the meaning.