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Lec 8 physiology

الكلية كلية التمريض     القسم قسم العلوم الطبية الاساسية     المرحلة 1
أستاذ المادة اسراء حرجان محسن خشان       05/05/2019 20:24:29
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lec 8 physiology dr.israa h.mohsen
muscular system
muscles are a unique form of tissue that transform energy into motion.
the body contains three types of muscle: cardiac muscle, smooth muscle, and skeletal muscle.
functions of the muscular system
the muscular system plays four important roles in the body:
• produces movement
• stabilizes joints
• generates heat.
structure of muscle fibers
because of their long, thread-like appearance, muscle cells are called muscle fibers. unlike other cells, muscle fibers have multiple nuclei pressed against the side of the plasma membrane. furthermore, even though muscle fibers are extremely thin, they contain a complex interior—just like other human cells.
? the plasma membrane surrounding each fiber is called a sarcolemma,while the cytoplasm of the cell is called sarcoplasm.
? long protein bundles called myofibrils fill the sarcoplasm. myofibrils store glycogen (which is used for energy) as well as oxygen.
? sarcoplasmic reticulum (sr)—the smooth endoplasmic reticulum of a muscle fiber—surrounds each myofibril. this is where calcium ions are stored.
? myofibrils consist of even finer fibers, called myofilaments. there are two types of myofilaments: thick and thin. thick myofilaments are made of a protein called myosin, while thin myofilaments consist of a protein called actin. the arrangement of actin and myosin gives skeletal muscle its striated appearance.
? a system of tubules, called transverse (t) tubules, extend across the sarcoplasm. formed from inward projections of the sarcolemma, the t tubules allow electrical impulses to travel deep into the cell.
thick filaments
each thick myofilament consists of hundreds of myosin molecules stacked together, with the myosin heads facing outward.
thin filaments
consisting of two chains of the contractile protein actin, thin myofilaments look like a string of beads. entwined with the actin are two other proteins: tropomyosin and troponin.
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? in a relaxed muscle, the myosinand actin lie side by side, partially overlapping. the myosin and actin are completely detached from one another.
? contraction occurs when the myosin heads latch onto the actin myofilaments. this forms what is known as a cross bridge between the actin and myosin. the myosin heads latch onto and release the actin repeatedly, creating a series of “power strokes” that propel the actin myofilaments forward, toward the center of the sarcomere.
muscle contraction and relaxation
to contract, a skeletal muscle must be stimulated by a nerve, specifically a motor neuron. the cell bodies of motor neurons reside in the brainstem and spinal cord. extensions from the cell bodies, called axons, carry impulses to skeletal muscles. each axon branches numerous times, with each branch stimulating a different muscle fiber.
the connection between a motor neuron and a muscle fiber is called a neuromuscular junction. between the end of the motor nerve and the muscle fiber is a narrow space called the synaptic cleft.
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how muscle fibers contract
1. when an impulse reaches the end of a motor neuron, it causes small vesicles to fuse with the cell membrane and release a neurotransmitter (a chemical messenger) called acetylcholine (ach) into the synaptic cleft.
2. the ach quickly diffuses across the synaptic cleft, where it stimulates receptors in the sarcolemma (the membrane surrounding the muscle fiber).
3. in turn, this sends an electrical impulse over the sarcolemma and inward along the t tubules. the impulse in the t tubules causes the sacs in the sarcoplasmic reticulum to release calcium.
4. the calcium binds with the troponin on the actin filament to expose attachment points. in response, the myosin heads of the thick filaments grab onto the thin filaments, and muscle contraction occurs.
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how muscle fibers relax
when nerve impulses stop arriving at the neuromuscular junction, ach is no longer released. the enzyme acetylcholinesterase breaks down any remaining ach while calcium ions are pumped back into the sarcoplasmic reticulum.
with the calcium removed, troponin and tropomyosin again prevent the myosin heads from grasping the thin filament, and the muscle fiber relaxes.
contraction of an entire muscle
now that we’ve discussed how individual muscle fibers contract, we need to apply that knowledge to the muscle as a whole. to begin, recall that one motor neuron stimulates a group of muscle fibers. the neuron and all the fibers it stimulates are called a motor unit. a single motor unit can consist of a few fibers…or a few hundred.
these fibers are scattered throughout the muscle rather than bunched together, allowing the contraction to be spread over a wide area.
to contract, muscle fibers must receive an electrical stimulus. the stimulus needs to be of a certain strength, or voltage. if the stimulus is too weak, the muscle fiber won’t respond. the minimum voltage needed to cause a muscle fiber to contract is called the threshold. when a fiber receives a stimulus at or above threshold, it responds after a brief lag by quickly contracting and then relaxing. this single, brief contraction is called a twitch.
obviously, one twitch can’t aid a muscle in performing a task. for a muscle to perform any kind of work, many fibers must contract at the same time. in addition, the muscle needs to stay contracted for longer than a split second.
also, muscles are often called upon to contract at different strengths. for example, lifting a pencil requires an entirely different amount of contraction than does, say, lifting a sofa. the force of contraction is affected by a number of things, including the size of the muscle, the degree of stretch, and the number of muscle fibers contracting.
stimulus frequency
the frequency of stimuli can alter contraction strength. when impulses reach muscle fibers even faster, the fibers don’t have a chance to relax completely before the next impulse arrives.
this condition of rapid contraction with only partial relaxation is called incomplete tetanus.
if the impulses arrive so fast that the muscle can’t relax at all between stimuli, the twitches merge into one prolonged contraction called complete tetanus.
energy source for contraction
all muscle contraction requires energy in the form of atp. however, muscles store only very small amounts of atp. in fact, just a few seconds of activity will completely deplete the atp within a muscle fiber. consequently, the constant synthesis of atp is a necessity. depending upon activity level, muscles obtain their energy supply in several ways.
? at rest, muscles obtain most of their energy by metabolizing fatty acids. because oxygen is plentiful, it uses the process of aerobic respiration to break down fatty acids for energy. (the term aerobic means “with oxygen.”)
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? when beginning to exercise, the demand for oxygen suddenly increases. the heart and lungs work harder to meet this demand, but, in the short term, the supply of oxygen dropings. when this happens, muscles quickly restock their waning supply of atp by breaking down a compound called creatine phosphate (cp), which is stored in muscle. this high-energy compound can furnish the muscle with fuel for about 20 seconds of high-energy activity or a minute of more moderate activity.
? if exercise continues, the supply of cp is exhausted before the supply of oxygen has reached an acceptable level. at this point, muscles switch to anaerobic (meaning “without oxygen”) respiration of glucose. muscles receive much of their glucose through the bloodstream however, some is stored within muscle in the form of glycogen. anaerobic respiration can generate energy quickly therefore, it’s useful for intense bursts of activity. however, it also produces a byproduct called lactic acid, which, as it accumulates in muscle, leads to muscle fatigue.
? after about 10 minutes of more moderate activity, the heart and lungs have had a chance to increase the supply of oxygen to the muscles. this allows muscles to shift back to aerobic respiration. aerobic respiration produces more atp than anaerobic respiration. also, its byproducts are carbon dioxide and water, which, unlike lactic acid, aren’t toxic to muscle.
disorders of the neuromuscular junction
interference with any of the steps necessary for skeletal muscle contraction will result in muscle weakness or paralysis. a number of toxins and diseases target the neuromuscular junction. following are just a few:
? • botulism: this is a form of food poisoning usually acquired from eating improperly canned foods. the bacteria clostridium botulinum blocks release of ach, inhibiting nerve transmission so muscles can’t contract. death results from paralysis of respiratory muscles.
? • myasthenia gravis: in this disease, the body produces antibodies against receptors for ach. as a result, not all ach can find a receptor. nerve transmission is poor, and profound muscular weakness results.
? • tetanus (“lockjaw”): this disease results from the bacterium clostridium tetani, which causes motor neurons to fire excessively. this leads to overstimulation of muscles, resulting in severe muscle spasms and sustained contractions. jaw muscles are typically affected first, hence the name lockjaw.
? • curare: once used to poison arrows, curare is now used in anesthesia to relax skeletal muscles. curare binds to ach receptor sites, stopping nerve transmission and causing paralysis. because the diaphragm is paralyzed, patients receiving curare must be mechanically ventilated.

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