Properties of Skeletal Muscles PDF

Properties of skeletal muscles Dr. Ng Chin Theng Specific learning outcomes 8.1 Describe a motor unit. 8.2 Describe the characteristics of fast and slow skeletal muscle fibers. 8.3 Describe isometric and isotonic contraction, summation of contraction, and length-tension relationship. 8.4 Describe th...

Properties of skeletal muscles Dr. Ng Chin Theng Specific learning outcomes 8.1 Describe a motor unit. 8.2 Describe the characteristics of fast and slow skeletal muscle fibers. 8.3 Describe isometric and isotonic contraction, summation of contraction, and length-tension relationship. 8.4 Describe the electrical properties of skeletal muscle. Motor unit A motor unit consists of a motor neuron and the group of skeletal muscle fibers A single motor axon branch to innervate several muscle fibers； they function together as a group Motor unit All the muscle fibers are innervated by a single motor neuron Entire muscle receives input from hundreds of different motor neurons Motor unit In general, small muscles that react rapidly and require fine control have more nerve fibers for fewer muscle fibers Conversely, large muscles that do not require fine control, such as the soleus muscle, may have several hundred muscle fibers in a motor unit Fast vs slow muscle fibers Every muscle of the body is composed of a mixture of fast and slow muscle fibers Fast-twitch fibers can deliver greater amounts of powerful forces for a few seconds to a minute or so Muscles that react rapidly are composed mainly of fast muscle fibers Eg; gastrocnemius muscle has a greater number of fasttwitch fibers, which gives it the capability of forceful and rapid contraction eg jumping Fast vs slow muscle fibers Conversely, slow-twitch fibers provide endurance, delivering prolonged strength of contraction over many minutes to hours Muscles such as soleus that respond slowly but with prolonged contraction; composed mainly of slow fibers Percentages of fast-twitch versus slow-twitch fiber in the quadriceps muscles of different types of athletes Slow Fibers (Type 1, Red Muscle) The following are characteristics of slow fibers: 1. Slow fibers are smaller than fast fibers 2. Slow fibers are also innervated by smaller nerve fibers 3. Compared with fast fibers, slow fibers have a more extensive blood vessel system and more capillaries to supply extra amounts of oxygen 4. Slow fibers have greatly increased numbers of mitochondria to support high levels of oxidative metabolism Slow Fibers (Type 1, Red Muscle) 5. Slow fibers contain large amounts of myoglobin, an ironcontaining protein similar to hemoglobin in red blood cells Myoglobin combines with oxygen and stores it until needed, which also greatly speeds oxygen transport to the mitochondria The myoglobin gives the slow muscle a reddish appearance and hence the name red muscle Fast Fibers (Type II, White Muscle) The following are characteristics of fast fibers: 1. Fast fibers are large for great strength of contraction 2. An extensive sarcoplasmic reticulum is present for rapid release of calcium ions to initiate contraction 3. Large amounts of glycolytic enzymes are present for rapid release of energy by the glycolytic process 4. Fast fibers have a less extensive blood supply & mitochondria than do slow fibers because oxidative metabolism is of secondary importance 5. A deficit of red myoglobin in fast muscle gives it the name white muscle Comparison Fast-twitch fibers Slow-twitch fibers Large in diameter Small in diameter Rapid release of energy from glycolytic process Utilize oxygen for generation of aerobic energy Maximal power can be achieved for very short periods Mainly to provide endurance and prolonged contraction Lesser mitochondria Have more mitochondria Lesser myoglobin Have more myoglobin Lesser no. of capillaries Greater no. of capillaries Properties of skeletal muscles Electrical properties Excitability Conductivity—can conduct impulse All or none law, obeyed by single motor unit Mechanical properties Contractility Summation Shows tetanus Shows fatigue Excitability Muscle cells are excitable cells Respond by producing electrical signals such as action potential AP of skeletal muscles has the same characteristics of AP of nerve fibers RMP -90mV Application of threshold stimulus produces an action potential Local potential – end plate potential Excitability RMP: −80 to −90 mV in skeletal fibers Threshold value: -50 mV Duration of action potential: 1 - 5 msec Action potential is a ‘spike type’ Excitability Depolarization is caused by Na+ influx Repolarization is caused by K+ efflux AP can be elicited only by a threshold stimulus A subthreshold stimulus produces a local response called endplate potential (graded potential; does not propagate) Conductivity Propagation of action potential Propagates along the sarcolemma & down the T tubule Velocity of conduction: 3 - 5 m/sec All or none law All or none law, obeyed by single motor unit This principle states that when a motor unit receives a stimulus of sufficient intensity to bring forth a response, all the muscle fibres within the unit will contract at the same time, and to the maximum possible extent Contractility Isometric contraction is a type of contraction during which muscle length remains constant but tension changes Muscle does not do external work, eg. Contraction of antigravity muscles only for maintaining posture Isotonic contraction is the contraction of muscle in which tension remains constant but length of muscle changes Muscle does external work eg. lifting of load Durations of contraction are adapted to the functions of the respective muscles Ocular movements must be extremely rapid to maintain fixation of the eyes on specific objects to provide accuracy of vision Gastrocnemius muscle contract moderately rapidly to provide sufficient velocity of limb movement for running and jumping Soleus muscle provides slow contraction for continual, long-term support of the body against gravity Factors determining muscle tension in single muscle fibre Frequency of action potential. Greater the frequency, greater is the tension developed (due to summation) Diameter of fibre. Greater the diameter of fibre, greater is the tension developed Rate of fatigue. Greater the rate of fatigue, lesser is the tension developed Length of the sarcomere. Length-tension relationship Length-tension relationship The amount of actin and myosin filament overlap determines tension developed by the contracting muscle Length-tension relationship This diagram showing the strength of contraction & different length of sacromere Showing different degrees of overlap of the myosin and actin filaments at different sarcomere lengths Length-tension relationship : the active tension depends on the length of sarcomere/ degree of overlap of actin and myosin filaments / no. of cross-bridges Maximal tension can be generated when the length of sacromore is approximately at 2.0-2.2µm (B to C) Sacromere lengths greater or lesser than optimum muscle develops less tension Point A: ends of actin filaments overlap, actin myosin overlapping → contraction strength decreases rapidly Optimal tension (B-C): actin filaments overlap all myosin cross bridges → Max tension developed Optimal length Length of sarcomere increases, less overlapping between actin & myosin  tension decreases progressively Point D: no overlapping between actin and myosin filaments → tension zero 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, which is called multiple fiber summation (2) by increasing the frequency of contraction, which is called frequency summation and can lead to tetanization Multiple fiber summation Weak signals cause smaller motor units to stimulate As the strength of the stimulus increases, larger and larger motor units begin to be excited Larger motor units have greater contractile force compared to small units; this allows gradations of muscle force from weak contraction to strong contraction Frequency summation and tetanization Lower left: individual twitch contractions occurring one after another at low frequency of stimulation As the frequency increases, there comes a point when each new contraction occurs before the preceding one is over The second contraction is added partially to the first Total strength of contraction rises progressively with increasing frequency When the frequency reaches a critical level, the successive contractions eventually become so rapid that they fuse together and the whole muscle contraction appears to be completely smooth and continuous called tetanization Tetanus Tetanus is a sustained state of contraction obtained due to repeated stimulus application at a high rate It is a type of frequency summation Tetany occurs because high calcium ions are maintained in the muscle sarcoplasm, even between action potentials, so that full contractile state is sustained without allowing any relaxation between the action potentials Mechanism which causes tetanus During tetanic contraction, successive action potential causes release of calcium from sarcoplasmic reticulum before all calcium from previous action potential is pumped back This results in maintaining high concentration of calcium in the sarcoplasm, so muscle contraction is sustained This allows tension to rise greatly Muscle fatigue Muscle fatigue: a decline in the ability of the muscle to sustain the strength of contraction May be due to: - rapid build-up of lactic acid - decrease in oxygen supply - depletion in energy supply (ATP, glycogen) - decreased neurotransmitter at the synapse