Mechanics of Skeletal Muscle Contraction PDF

Mechanics of Skeletal Muscle Contraction Dr. Amna Mohammed Work Output During Muscle Contraction When a muscle contracts against a load, it performs work W=L*D in which W is the work output, L is the load, and D is the distance of movement against the load....

Mechanics of Skeletal Muscle Contraction Dr. Amna Mohammed Work Output During Muscle Contraction When a muscle contracts against a load, it performs work W=L*D in which W is the work output, L is the load, and D is the distance of movement against the load. The energy required to perform the work is derived from the chemical reactions in the muscle cells during Preload & afterload  Stretching a muscle to optimize actin and myosin interaction is known as preloading. Sarcomere length is optimal in resting skeletal muscle, but, in cardiac muscle, increasing preload will further increase force production  The load that a muscle is asked to lift determines how much tension a muscle must develop in order to move or lift it and is known as the afterload. Afterload also determines how fast the muscle contracts during a lift. Minimal loads allow the muscle to contract at a maximal velocity, whereas very heavy loads are lifted slowly TYPES OF CONTRACTION Functionally, there two types of contraction: An isometric contraction occurs when the afterload is too heavy to lift, and the muscle cannot shorten even though crossbridge cycling continues. A muscle shortens undergoing isotonic contraction but maintains a constant tension.  In practice, most contractions are a mix of both types Motor Unit  Each motor neuron that leaves the spinal cord innervates multiple muscle fibers, the number depending on the type of muscle. All the muscle fibers innervated by a single nerve fiber are called a motor unit.  small muscles that react rapidly have more nerve fibers for fewer muscle fibers (for instance, as few as two or three muscle fibers per motor unit in some of the laryngeal muscles).  large muscles that do not require fine control, such as the soleus muscle, may have several hundred muscle fibers in a motor unit. THE MUSCLE TWITCH A single action potential causes a brief contraction followed by relaxation. This response is called a muscle twitch. The duration of the twitch varies with the type of muscle being tested. “Fast” muscle fibers, primarily those concerned with fine, rapid, precise movement, have twitch durations as short as 7.5 ms. “Slow” muscle fibers, principally those involved in strong, gross, sustained movements, have twitch durations up to 100 ms.  Muscle Contractions of Different Force  Summation: Summation means the adding together of individual twitch contractions to increase the intensity of overall muscle contraction.  Summation occurs in two ways:  (1) by increasing the number of motor units contracting simultaneously ( multiple fiber summation or recruitment)  (2) by increasing the frequency of contraction, (frequency summation or tetanization ) Recruitment Summation is an imprecise method of controlling muscle tension. Some tasks require a finer degree of control than can be achieved using summation alone, and this is made possible by dividing muscles into motor units. The multiunit approach allows some units to be tetanic even as others are relaxing. This permits contractile force to be maintained at a constant level and then ramped up more smoothly than can be achieved by summation. It also allows the central nervous system to choose which muscle units to activate depending on their speed and the task at hand.  Full contraction requires additional action potentials. Electrical events are very fast compared with mechanical events, so a second spike can be delivered within a few milliseconds of the first, even as Ca2 is pouring out of the SR. The second Ca2 burst adds to the first, and Ca2 concentration rises further, and a third spike increases Ca2 to an even greater extent. Because Ca2 concentration equates with crossbridge cycling and contraction, muscle tension rises in parallel with Ca2, (summation ) Tetanus  - Tetanus is summation of contractions into one continuous contraction  Tetanus may be complete (with no any relaxation between contractions) or incomplete (with some incomplete relaxations between contractions).  - The frequency of stimulation that results in tetanus is determined by the duration of the single muscle twitch.  - Treppe or staircase phenomenon :With a frequency of stimulation just below the frequency of summation, the tension developed is increased by each new stimulus until a uniform tension per contraction is reached. This is called treppe or staircase phenomenon.  - it is due to increased availability of calcium to troponin C. types of muscle fiber Some muscles must sustain a contraction for prolonged periods, but response latency is not a concern. The muscles that control posture (muscles in the legs and the back) are classic examples. Conversely, other muscles must respond rapidly but are only ever used for short bursts of activity (e.g., muscles that control eye movement). Traditionally, muscle fibers have been placed in one of two groups. In practice, most muscles are a mix of slow and fast fiber types. Slow twitch (type I) Slow-twitch fibers contract slowly, but they do not readily fatigue. They are used in maintaining posture, for example. Their energy is derived primarily from oxidative metabolism, facilitated by a rich vascular system, high levels of oxidative enzymes, and an abundance of myoglobin and mitochondria. The myoglobin gives the slow-twitch fibers a red appearance.  B. Fast twitch (type II) Fast-twitch fibers contain a myosin isoform that is specialized for rapid movement, but they fatigue more easily. They rely more heavily on glycolysis as a source of ATP than do slow-twitch fibers. Fast-twitch fibers are a diverse group that can be further subdivided into type IIA and type IIB fibers  1. Type IIA: Type IIA fibers resemble slow-twitch fibers in that they rely primarily on oxidative metabolism, supported by high numbers of mitochondria and the presence of myoglobin (which gives them their red color), yet they also have well-developed glycolytic pathways. These properties allow for wide range of activity types. 2. Type IIB: Type IIB fibers are specialized for high-speed contractions of the type used in rapid sprints. They are primarily glycolytic and fatigue easily. Muscle Fatigue. Prolonged and strong contraction of a muscle leads to the well- known state of muscle fatigue. depletion of muscle glycogen. Decrease of nerve signal through the neuromuscular Interruption of blood flow through a contracting muscle leads to muscle fatigue within 1 or 2 minutes. ] Rigor Mortis When the body dies, ATP are depleted rapidly, and the pumps that maintain ion gradients across the membrane stop working. Intracellular Ca2 concentrations rise causing actin and myosin to bind and lock into a state of rigor mortis.  time of onset depende on ambient temperatures (2 to 6 hours of death). The rigor state persists for 1 or 2 days then digestive enzymes released from lysosomal lysis the crossbridges, allowing the muscles to relax. ELECTROMYOGRAPHY recording the electrical activity of muscle on an oscilloscope. This may be done by skin electrodes or by hypodermic needle electrodes. The record obtained with such electrodes is the electromyogram (EMG). The needle electrodes can pick up the activity of single muscle fibers.  The measured EMG depicts the potential difference between the two electrodes, which is altered by the activation of muscles in between the electrodes. It has been shown by electromyography that little if any spontaneous activity occurs in the skeletal muscles of normal individuals at rest. With minimal voluntary activity a few motor units discharge, and with increasing voluntary effort, more and more are brought into play to monitor the recruitment of motor units. EMGs can be used to monitor abnormal electrical activity associated with muscle responses.