Skeletal Muscle Structure and Function PDF

🐵 Skeletal muscle structure and function Created @October 30, 2024 12:01 PM Module PHOL0001 Type Practical Stage? Done...

🐵 Skeletal muscle structure and function Created @October 30, 2024 12:01 PM Module PHOL0001 Type Practical Stage? Done Reviewed 📌 Aim: Class Notes: Skeletal muscle structure A single skeletal muscle contains many muscle fibers. Different components of the skeletal muscle are surrounded by layers of connective tissue: the endomysium, perimysium, and epimysium Each individual muscle fiber is surrounded by a delicate layer of connective tissue called endomysium. Muscle fibers are grouped into bundles called fascicles. Each fascicle is surrounded by perimysium. An entire muscle is surrounded by epimysium Each skeletal muscle fiber is long (up to 30cm; 12 inches) and striated in appearance. The striations in skeletal muscle are a result of many repeating units called sarcomeres, which are the functional units of the muscle. Skeletal muscle structure and function 1  Sarcomeres are found in myofibrils. Skeletal muscle function The properties of each muscle cell facilitate its functions. Contractile; it has the ability to shorten in response to adequate electrical stimulation. Elastic; it has the ability to recoil and regain the resting length after being stretched. Excitable; it has the ability to receive and respond to a stimulus. Extensible; it has the ability to stretch when not contracted. Motor units Muscles contract in response to electrical stimulation (action potentials) Groups of muscle fibers within a muscle are innervated by motor neurons; one motor neuron and all the muscle fibers that it innervates is collectively referred to as a motor unit. An action potential in a single motor neuron will cause subsequent activation and contraction of all of the muscle fibers innervated by that motor neuron. Motor units vary greatly in size. Skeletal muscle structure and function 2  A single motor neuron may innervate only a few muscle fibers (small motor unit), or thousands of muscle fibers (large motor unit). A single muscle will often contain motor units of varying sizes, and several motor units may work together to coordinate the movement of a single muscle Neuromuscular transmission The motor neuron (of the motor unit) and the muscle fibers that it innervates are separated by a space called the synaptic cleft. Signals from the motor neuron must be transferred across this space to muscle fibers, to elicit a response in the muscle. The specialized synapse between the motor neuron (of the motor unit) and muscle fiber is called the neuromuscular junction (NMJ) → made up of the axon terminal, synaptic cleft, and motor endplate of the muscle fiber. Skeletal muscle structure and function 3 Skeletal muscle structure and function 4  Fibers, fibrils, and sarcomeres Muscle fibers and myofibrils Muscle fibers are surrounded by a sarcolemma (muscle membrane). At regular points along the muscle fiber, the sarcolemma extends deep into the center of the fiber to form structures called t-tubules. Local depolarization of the motor endplate initiates an action potential that travels along the sarcolemma and down t-tubules Skeletal muscle structure and function 5  T-tubules contain many voltage-sensitive ion channels, including L-type Ca2+ channels. In close association with t-tubules is a membrane-bound structure called the sarcoplasmic reticulum (SR) that stores Ca2+. Ca2+-release channels called ryanodine receptors (RyRs) are embedded in the SR membrane L-type Ca2+ channels in the t-tubules are physically linked to RyRs in the SR, and activation of these channels directly activates the RyRs channels The SR membrane also contains an ATP-fueled pump called SERCA (sarcoendoplasmic reticulum calcium ATPase) Depolarization causes a conformational change in L-type Ca2+ channels, and RyRs in the SR opens as a consequence. Stored Ca2+ flows out of the SR into the sarcoplasm and Skeletal muscle structure and function 6  intracellular Ca2+ concentration in the muscle fiber increases. This initiates a sequence of events that leads to muscle contraction. These events will be explained in further detail on the following pages. The muscle relaxes when nerve stimulation to the muscle stops, and Ca2+ is pumped back into the SR by SERCA. Sarcomeres joined together at their ends by structures called Z-lines, to which actin (thin filaments) and myosin (thick filaments) are tethered. Myosin overlaps with actin within the sarcomere Skeletal muscle structure and function 7  Crossbridge cycle When the muscle contracts, myosin repeatedly forms and breaks crossbridges with actin, actin slides past myosin, and the sarcomere shortensm→ formation and breaking generates the force of contraction Requires the presence of Ca2+ and energy in the form of adenosine triphosphate (ATP). In the relaxed state, myosin cannot interact with actin due to the presence of tropomyosin that blocks the actin binding site. Muscle depolarization results in the release of stored Ca2+ and some of this Ca2+ binds to troponin → conformational change in troponin and a consequent shift in the troponin-tropomyosin complex, which exposes the actin binding site → allows actin and myosin to bind and form crossbridges excitation-contraction coupling: the conversion of an electrical stimulus into an mechanical response → release of Ca2+ provides the link between electrical excitation of muscle, and the mechanical event of cross-bridge formation The steps of the cycle: 1. ATP binds An ATP molecule attaches to the myosin head and breaks the link between actin and myosin Skeletal muscle structure and function 8  2. ATP hydrolysis; myosin head becomes “cocked” The enzyme ATPase hydrolyzes (breaks down) ATP → ADP + Pi, but ADP and Pi remain attached to myosin. The myosin head rotates to a "cocked" position so that it is aligned with a new actin binding site (further along the actin filament) 3. Pi dissociates from myosin; new crossbridge forms. Pi dissociates from the myosin head, which allows myosin to bind with high affinity to actin, a new crossbridge is formed. ADP is still attached to myosin. 4. Powerstroke occurs; muscle shortens The myosin neck rotates around the myosin head which is attached to actin. This bending action results in a Skeletal muscle structure and function 9  "powerstroke" where myosin pulls actin closer to the center of the sarcomere. The muscle shortens and force is generated in the muscle. 5. ADP dissociates from myosin; myosin awaits another ATP molecule Actin and myosin remain attached and in a rigid state. Another ATP molecule is required to break the link between actin and myosin, and for another crossbridge cycle to start. Sliding filament theory The formation and breakdown of crossbridges allows actin filaments to slide over myosin and the muscle shortens. When nerve impulses to the muscle stop, Ca2+ reuptake by SR (been removed), the troponin-tropomyosin complex prevents the interaction of actin and myosin. Crossbridges can no longer form → muscle relaxes. Skeletal muscle structure and function 10  Recruitment and frequency Recruitment process of progressive activation of motor units motor units recruited at different stimulus strengths, based on their size: smaller motor units, recruited at lesser stimulus strengths larger motor units, recruited at greater stimulus strengths → more motor units are recruited, the strength of muscle contraction increases examine this through electrical stimulation of the peripheral nerve. At low stimulus strengths, no motor nerves are depolarized and no muscle contraction is seen. As stimulus strength is increased, some motor neurons are brought above their threshold potential and an action potential is fired. This results in contraction of all the muscle fibers in that motor unit, which causes a twitch (one complete cycle of contraction and relaxation) in the muscle. Skeletal muscle structure and function 11  The force of the twitch increases progressively as more motor nerve fibers are excited. This occurs until a maximum response is reached when all of the motor units that supply the muscle have been activated. Frequency refers to the number of action potentials delivered to a muscle within a set period of time. at low stimulation frequencies, the tension developed in the muscle decreases back to a resting level in between stimulations, Ca2+ is released with each stimulation The muscle relaxes when nerve stimulation to the muscle stops, and Ca2+ is pumped back into the SR (cleared from the sarcoplasm). Skeletal muscle structure and function 12  An action potential over before Ca2+ has been released, and the muscle starts to contract → it is possible to generate many action potentials during a muscle contraction. If many action potentials are fired in quick succession (the frequency of stimulation is increased), the level of Ca2+ that is released into the sarcoplasm will rise, and more Ca2+ will be available for crossbridge formation. In this way, by increasing frequency, we can increase the strength and duration of muscle contraction. Skeletal muscle structure and function 13  Twitch, summation, and tetanus Twitch one complete cycle of contraction and relaxation in a muscle electrical activity (EMG) only last for a short of time, muscle contraction last longer twitch force can be increased by: recruiting more motor units to fire simultaneously increasing the frequency of action potentials in motor axons Summation additive effect of muscle stimulation Tetanus a state of sustained maximal muscle contraction at high stimulation frequencies, the muscle has no time to relax between successive stimuli → result in a smooth contraction that is much stronger than a single twitch: a tetanic contraction Skeletal muscle structure and function 14  Incomplete tetanus (unfused tetanus) occurs if stimulation frequency is high, but at a rate that still allows partial relaxation of muscle between twitches Type of contraction Skeletal muscle structure and function 15