The Muscular System PowerPoint Presentation PDF

The Muscular System PARA 1033 - Fall 2022 Dan Aitken Functional Overview Producing body movements All body movements are controlled by muscles Stabilizing body positions Maintaining posture, standing, sitting Storing and moving substances within the body Sphincter control,...

The Muscular System PARA 1033 - Fall 2022 Dan Aitken Functional Overview Producing body movements All body movements are controlled by muscles Stabilizing body positions Maintaining posture, standing, sitting Storing and moving substances within the body Sphincter control, pumping of blood, digestive system Generating heat Thermogenesis, regulation of core temperature, shivering Different Types of Muscular Tissue Skeletal Muscle Move the bones of the skeleton - produce body movements Smooth Muscle Walls of hollow organs, blood vessels, digestive tract Cardiac Muscle Contraction of the heart chambers Properties of Muscular Tissue Electrical Excitability Production and propagation of action potentials Contractility Contract and relax to produce force/movement Extensibility Ability to stretch (within limits) Elasticity Ability to return to its original length following stretch Skeletal Muscles Skeletal muscle tissue is composed of cells called muscle fibres (aka myocytes), connective tissue, blood vessels and nerves Remember that the subcutaneous layer of the skin (hypodermis) is what separates the muscles from the skin Composed of adipose (fat) tissue and connective tissue Adipose tissue stores energy, insulates from heat loss and reduces physical trauma Provides a pathway for nerves, blood vessels and lymph vessels in and out of the muscle tissue Skeletal Muscles Fascia is a broad band of irregular connective tissue that lines the body walls and limbs and supports and surrounds muscles and organs Holds muscles with similar functions together Three layers of connective tissue extend from the fascia to protect and strengthen the skeletal muscle Epimysium - outer layer Perimysium - middle layer. Surrounds groups of muscle fibres and bundles them into fascicles Endomysium - inner layer Skeletal Muscles Tendons Epimysium, perimysium, endomysium band together and form a rope like band of tissue Connects skeletal muscle to bone Nerve and Blood Supply One artery and two veins typically accompany every nerve that penetrates skeletal muscle Rich capillary bed within muscle tissue due to high metabolic demand Neurons that stimulate skeletal muscles are somatic motor neurons Microscopic Anatomy of Skeletal Myocytes The plasma membrane of a muscle fibre is called the sarcolemma Within the sarcolemma exist tunnels called transverse tubules (T tubules) that are filled with interstitial fluid Myocyte action potentials travel along the sarcolemma and through the T-tubules, ensuring the excitation (and contraction) of the entire muscle fibre simultaneously The cytoplasm of a myocyte is called sarcoplasm and contains: Glycogen - to convert to glucose when needed Myoglobin - protein that binds to and releases oxygen when needed Microscopic Anatomy of Skeletal Myocytes Myofibrils are the contractile organelles within the sarcoplasm Extend the entire length of the muscle fibre and give the cell its striped (striated) appearance Sarcoplasmic Reticulum (SR) encircles each myofibril Similar in appearance to endoplasmic reticulum in non- muscular cells Stores and releases calcium ions (Ca2+) to trigger the muscle contraction Microscopic Anatomy of Skeletal Myocytes Filaments or myofilaments are small protein structures within myofibrils that are directly responsible for the contractile process Thick filament - composed of myosin (protein) Thin filament - composed of actin (protein) Sarcomeres are compartments made up of two thin filaments and one thick filament Separated from the next sarcomere by a Z-disc Microscopic Anatomy of Skeletal Myocytes Muscle Proteins There are many different proteins within a skeletal myocyte - contractile, regulatory and structural Actin and Myosin are the primary contractile proteins Troponin is a regulatory protein that aids in muscle contraction When a muscle is damaged, troponin can leak out of the cell and into circulation showing high levels of Troponin on a blood test - Myocardial Infarction Contraction and Relaxation of Skeletal Myocytes During skeletal muscle contraction, the size and length of the filaments does NOT change Rather, the thin and thick filaments slide past each other pulling on the z-disc - thus, shortening the length of the sarcomere Shortening of the sarcomeres causes shortening of the whole muscle fibre - which leads to shortening of the entire muscle Contraction and Relaxation of Skeletal Myocytes Muscle contraction occurs due to an increase in sarcoplasm Ca 2+ levels and relaxes when the Ca2+ levels decrease Muscle action potentials travelling along the sarcolemma (and through the T-tubules) stimulates the release of calcium ions from the Sarcoplasmic Reticulum (SR) - voltage-gated Ca 2+ channels Calcium binds to troponin causing a cascading reaction that eventually leads to a muscle contraction While stimulated, calcium is continually pumped back into the SR via Ca -ATPase pumps 2+ Contraction and Relaxation of Skeletal Myocytes As long as the calcium release channels remain open (action potentials continue to propagate), Ca2+ flows into the sarcoplasm faster then the Ca2+-ATPase pumps it back to the SR When action potentials cease, the voltage-gated calcium release channels close the the remaining calcium ions in the sarcoplasm are pumped back into the sarcoplasmic reticulum Rigor Mortis (aka rigidity of death) - after death, cell membranes become leaky and Ca leaks out of the SR. Due to the lack of 2+ metabolic process (ATP), the calcium is not pumped back into the SR. Neuromuscular Junction (NMJ) The origin of muscle cell action potential The synapse between a somatic motor neuron and a skeletal muscle fibre Within the axon terminal of the motor neuron is contained thousands of acetylcholine (ACh) molecules - cholinergic neurotransmitter The region of sarcolemma opposite the motor neuron contains millions of acetylcholine receptors Neuromuscular Junction (NMJ) A nerve impulse elicits a muscle action potential as follows: 1. Release of ACh - nerve impulse stimulates the release of ACh across the synapse 2. Activation of ACh receptors - Na , and other ions flow + across the cell membrane 3. Production of muscle action potential - inflow of Na makes + the inside of a muscle fibre more positively charged. This change in charge triggers the action potential 4. Termination of ACh activity - acetylcholinesterase (AChE) breaks down any ACh remaining in the synaptic cleft Muscle Metabolism Skeletal muscle fibres have a very wide range of energy needs dependant on activity A large amount of ATP is needed for many metabolic processes within a myocyte The ATP stored within muscle fibres only lasts for a few seconds of contraction. Beyond that, the cell must produce ATP via one of 3 methods: Creatine Phosphate - unique to muscle fibres Anaerobic Glycolysis Aerobic Respiration Muscle Metabolism Creatine Phosphate While relaxed, muscle fibres produce more ATP then they need for resting metabolism This excess ATP is converted and stored as creatine phosphate When muscle contraction begins, the enzyme creatine kinase (CK) catalyses the process that converts creatine back into ATP These energy stores can last for about 15 seconds of maximal muscle contraction Muscle Metabolism Anaerobic Glycolysis Anaerobic - without the use of oxygen. Glycolysis - catabolism of glucose into pyruvic acid In the presence of oxygen, pyruvic acid is converted to ATP During anaerobic conditions, pyruvic acid is converted into lactic acid and ATP via anaerobic glycolysis (lactic acid fermentation) Anaerobic glycolysis provides about 2 minutes of maximal muscle activity Faster process then aerobic respiration but produces less ATP Muscle Metabolism Aerobic Respiration As previously stated, aerobic conditions allow pyruvic acid to by converted to ATP in the mitochondria of the cell The Krebs Cycle - discussed in GREAT detail next term Oxygen required for muscle fibre metabolism is obtained via diffusion from the blood and release from myoglobin Provides enough energy for light to moderate exercise from several minutes to an hour or more Requires oxygen, glucose, fatty acids, and proteins Muscle Metabolism Muscle Fatigue The inability to maintain force of contraction after prolonged activity Likely caused by: Inadequate release of Ca2+ from the SR, depletion of creatine phosphate, insufficient oxygen, depletion of glycogen, buildup of lactic acid Muscle Metabolism Oxygen Debt (recovery oxygen uptake) Additional oxygen required above and beyond the resting oxygen consumption following exercise During exercise, the RR and HR increase to increase the amount of oxygen delivery to muscle cells (and other tissues) This extra oxygen is used to aid in the restoration of baseline metabolic conditions (homeostasis) Convert lactic acid back to glycogen, resynthesize creatine phosphate, replace oxygen released from myoglobin Muscle Tension A single somatic motor neuron synapses with an average of 150 skeletal muscle fibres - this is called a motor unit All the muscle fibres in a motor unit contract in unison Even while at rest, skeletal muscles exhibit muscle tone Small amount of tautness or tension due to weak, involuntary contractions of a motor unit Very important in smooth muscle (digestion, BP) When a motor neuron becomes damaged, the effecting muscle becomes flaccid - a state of limpness in which muscle tone is lost Muscle Tension Isotonic Contraction The force of contraction (tension) developed in the muscle remains almost constant while the muscle changes its length Body movements and moving objects Isometric Contractions The tension generated is not enough to exceed the resistance of the object to be moved Holding an object steadily with an outstretched arm Body posture, stabilizing of joints and extremities Cardiac Muscle Tissue Several differences from skeletal muscle fibres Contain intercalated discs (irregular transverse thickening of the sarcolemma) that connect one cardiac myocyte to the next Remain contracted much longer than skeletal muscles Contains a plateau period during its action potential Contracts when stimulated by its own autorhythmic muscle fibres as opposed to motor neuron impulse Largely dependant on aerobic respiration Smooth Muscle Tissue Several differences from skeletal muscle fibres Filaments are not arranged in orderly sarcomeres - no striations (hence the “smooth” appearance) Lack transverse tubules and have very little sarcoplasmic reticulum Slower and longer lasting contractions Can shorten and stretch to greater extents Sustains muscle tone long term How Skeletal Muscles Produce Movement Skeletal muscles produce movements by exerting force on tendons, which in turn, pull on bones or other structures like skin To produce movement, bones act as levers and joints function as fulcrums Lever - a rigid structure that can move around a fixed point Fulcrum - the fixed point A lever is acted on at two different points by two different forces: The effort - causes the movement The load - opposes the movement (resistance) Major Muscle Groups Sternocleimastoid Scalene Trapezius Deltoid Biceps Brachii Rectus Abdominis Quadriceps group Major Muscle Groups Sternocleimastoid Trapezius Latissimus Dorsi Gluteus Maximus Triceps Brachii Major Muscle Groups Deltoid Anterior, lateral, posterior Main site for prehospital IM injection Major Muscle Groups Quadriceps Rectus femoris Vastus lateralis Vastus medialis Vastus intermedius  Major Muscle Groups Neck Muscles Accessory Respiratory Muscles Sternocleimastoid Scalene Trapezius Building and Losing Muscle Mature skeletal muscle fibres cannot undergo cell division Growth of skeletal muscle after birth is due to hypertrophy - the enlargement of existing cells Exercise causes skeletal muscle hypertrophy Between the ages of 30-50, humans undergo a progressive loss of skeletal muscle mass (atrophy) that is replaced by fibrous connective tissue and adipose tissue Accompanying this loss is a decrease in maximal strength, slowing of muscle reflexes and loss of flexibility