What are the parts of a muscle cell Muscles cells contain many nuclei and mitochondria. Cytoplasm contains myofibrils which lie parallel to one another Thick fibers are myosin Thin fibers are actin Arrangement of actin and myosin makes the light and dark bands or striations on skeletal muscle
Parts of a muscle cell (continued) Light bands are called I bands - Made up of actin fibers hooked to Z lines. A bands - dark - myosin overlapping with actin. From 1 Z line to the next is a sarcomere. Network of sarcoplasmic reticulum surround the myofibrils and are parallel. Transverse tubules are perpendicular and go out into the extracellular fluid.
What is the structure of a motor unit Neuromuscular junction - each skeletal muscle fiber is connected to a motor neuron. The muscles contracts when stimulated. Where the nerve and muscle fiber meet is called a neuromuscular junction.
Motor End Plate Muscle fiber membrane is specialized to form a motor end plate - nuclei and mitochondria are abundant and cell membrane is highly folded. (Why?) Neuron forms presynaptic knob which releases neurotransmitters into the synaptic cleft. The neurotransmitter is picked up by the muscle which if stimulated to threshold will contract. Acetylcholine is the neurotransmitter that stimulates skeletal muscle contraction.
Motor unit One motor fiber may connect to many muscle fibers to make a motor unit. The fewer muscle fibers in the motor unit, the finer the movement.
How do muscle fibers contract? Myosin is a twisted protein with cross-bridges which are structures that project outward. In the presence of Calcium, the crossbridges of the myosin fiber react with actin to form links. Actin has a binding site that the crossbridge of myosin attachs to, the head of the myosin crossbridgeattachs to actin, the myosin crossbridge then bends slightly pulling actin with it, and then reattaches to the next actin site and the process continues - shortening the muscle fiber.
How is the muscle fiber stimulated? Sarcoplasmic reticulum contains more Ca than the cytoplasm, due to active transport. The muscle impulse or stimulus causes the membrane (of the sarcoplasmic reticulum) to become more permeable to Ca, so Ca leaves the sarcoplasmic reticulum. When enough Ca enters the cytoplasm, the myosin and actin form linkages and the muscle contracts.
How is the muscle stimulus stopped? When the neuron stops producing acetylcholine, 2 things must occur before the muscle can relax. 1. Acetylcholinesterase must decomposes acetylcholine (to stop muscle stimluation). 2. Muscle stimulus ceases, and the calcium pump moves Ca back into the sarcoplasmic reticulum, which decreases the amount of Ca in the cytoplasm. 3. The linkage between actin and myosin break, and the muscle relaxes.
Energy sources for contraction Aerobic respiration - uses oxygen supplied by lungs and circulatory system to carry out activities. Activities from low to moderate exercise would use aerobic respiration.
Anaerobic respiration Anaerobic respiration - when doing vigorous activity, such as weight lifting, sprinting,.... can’t get enough oxygen through normal respiration, so have to tap into stored oxygen. Break glucose down into lactic acid, which is carried to the liver. The person creates an Oxygen debt that must be repaid later, this repayment may take several hours after the exercise and usually will be the reason for muscle soreness.
Muscle Fatigue A muscle exercised strenously for a prolonged period of time may lose the ability to contract, which is called fatigue. (Think of running a race, such as the 400m - you run at a fast pace for 1 lap around the track - after you stop, your legs feel like jelly) This usually results from the accumulation of lactic acid in the muscle. The lactic acid lowers the pH of the muscle, so that muscles can’t respond to nerve stimulation.
Muscle Cramp A cramp is when the muscle contracts, but doesn’t relax completely. The actin/myosin links haven’t been broken.
How can athletes prevent lactic acid from building up? Why is it that after several practices, you stop being sore? The more you train, the more capillaries form to bring oxygenated blood to the muscle cells. There is also more mitochondria to undergo cellular respiration - so don’t have to kick over into anaerobic respiration as quickly.
Heat Loss 25% of the energy released during cellular respiration is used in metabolic processes. The other 75% is heat which can be used in homeostasis and temperature regulation of the body.
Odds and Ends Muscles also have a threshold stimulus that must be met (just like nerves did) All or none response - if the muscle is stimulated to threshold, the muscle contracts. Remember that a muscle is made up of several motor units which respond to different thresholds of stimulation. Only units with low thresholds might respond.
Muscle summation Recruitment is when you increase the number of motor units. Twitches are a single stimulation - think of your eye - really not a useful movement. Muscle summation is where movements are made smooth by stimulating and relaxing motor units at different times.
Muscle Tone Muscles tone is when your body undergoes a certain amount of sustained contraction. A small number of motor units are always being stimulated.