Skeletal Muscle Skeletal muscles, also called skeletal muscles, striated muscles, or striped muscles, function to help move body parts, protect deeper organs, give the body shape, and produce body heat. Skeletal muscles work by contracting which is regulated by the system Nerves that connect to muscle fibers are called motor nerve–muscle connections ( neuromuscular junction). Within a skeletal muscle there are: - Muscle fibers:plays a major role in the contraction process which is innervated by Aα motor nerve fibers - Muscle spindles:functions as a stretch reflex receptor and smoothes contractions, innervated by motor nerve fibers A- and sensory nerves IA & II - Connective tissue / fascia / tendons; vascular system; nervous system ; fat cells Skeletal muscles work through a process of contraction and relaxation: Contractions occur when there is a shift (sliding) between actin and myosin filaments due to the process of attaching the myosin head to actin Relaxation occurs when the myosin head separates from the actin filament so that the myosin and actin separate again. Contraction and relaxation mechanism: Calcium comes out of the storage area in the cisterns and binds to troponin-C in the myosin filament chain causing the active area of actin to open so that the myosin head attaches. This bond will cause a shift in the actin and myosin filaments so that the distance between the two actin bases (sarcomeres) shortens, this event is called contraction. The calcium pump works to put calcium back into the cisterns so that the myosin heads are released from the actin filaments and the distance between the two actin bases returns to normal, this event is called relaxation. Stages of impulse propagation from the Aα motor neuron until skeletal muscle contraction occurs: 1. Release of impulse from the alpha motor neuron 2. Saltatory conduction via motor nerve Aα 3. Impulse reaches the end of the axon at the neuromuscular junction 4. Impulse causes openingvoltage-gated Ca channels 5. Ca influx into the cytosol supporting the axon stimulates docking and subsequent exocytosis of vesicles and the release of neurotransmitters (acetyl choline) into the synaptic cleft 6. Acetyl choline will bind to nicotonic type cholinergic receptors in the synaptic sarcolemma 7. Stimulation of cholinergic receptors causes the opening of Na channels resulting in an influx of Na ions, resulting in EPSP (Excitatory Post Synaptic Potential). If the EPSP is a threshold value, an action potential (impulse) will occur in the sarcolemma (muscle fiber membrane). 8. The impulse that occurs will be propagated in all directions (propagation) along the sarcolemma and also enter the muscle cells through the channel membrane of the sarcotubular system (sarcotubular reticulum (SR) + transverse tubules) 9. Impulses transmitted through the sarcotubular system will reach the triad which causes the cisterns (end of the SR) to be stimulated resulting in the release of Ca ions. Cisterna is a Ca ion depot 10. Ca ions bind to Troponin-C, which results inactive site actinopens, then the myosin head will stick and will pull the actin filament closer to the myosin filament, this is called the processslidingbetween actin and myosin. WhenslidingThe sarcomere will shorten, called contraction. The more active sarcomeres, the stronger the contractions 11. Ca ions will re-enter the SR via active transport, so that Ca ions will be released from Troponin C, as well as the release of the myosin head fromactive site, then troponin will cover itactive site actinagain, this is called relaxation. Skeletal muscle contraction disorders: - Skeletal muscle spasms (muscle cramps) : Muscles contract maximally without relaxation. Due to the lack of ATP for Ca transport to the sarcoplasmic reticulum, the accumulation of calcium in the cytosol blocks the relaxation process. - Atrophy: Old muscles do not contract, Due to damage to the motor nervous system: Aα, motoneuron - Paralysis Muscle innervation disorders resulting in paralysis Due to damage to the motor nervous system: Aα, Tractus Pyramidalis, motor center in precentral gyrus - Myasthenia Gravis Muscle contractions become weaker over time Due to impaired impulse transmission at the neuromuscular junction, due to reduced acetyl choline or cholinergic receptors at the neuromuscular junction - Rigor Mortis Muscle stiffness in corpses occurs because ATP is depleted so there is no relaxation.