Skeletal Muscle Physiology PDF 2024-2025

Skeletal Muscle Contraction  The Function of the Muscles Movement Maintenance of posture and muscle tone Heat production Protecting bones and internal organs Key features of muscle cells: a) Excitability respond to stimuli by producing action potentials b) Contract...

Skeletal Muscle Contraction  The Function of the Muscles Movement Maintenance of posture and muscle tone Heat production Protecting bones and internal organs Key features of muscle cells: a) Excitability respond to stimuli by producing action potentials b) Contractility shorten & thicken to generate a force c) Extensibility extend without damage d) Elasticity return to original shape Muscle Types: Peripheral 1. Skeletal muscle nucleus 2. Smooth muscle Central 3. Cardiac muscle nucleus Contractile cells of the Central body can be classified nucleus into 3 major groups based on their; ❖Shape, ❖Position of nuclei, ❖Presence of striation ❖Under voluntary or involuntary control. COMPARISION OF MUSCLE CELLS  Skeletal muscle structure Organization of skeletal muscle: Actin (thin) and myosin (thick) are the smallest contractile elements of a muscle unit. These are called myofilaments. These combine to form myofibrils. Myofibrils combine to form muscle fibers, Muscle fibers come together to form a muscle fascicle, Muscle fascicles fuse to form skeletal muscle  Skeletal muscle structure There are 3 connective tissue layers that surround the muscle units from inside to outside; ❖ Endomysium ❖ Perimysium ❖ Epimysium A muscle fiber is surrounded by endomysium (electrically isolates each muscle cells from each other), A fascicle is surrounded by perimysium, A skeletal muscle is surrounded externally by the epimysium. Starting from the innermost part, a muscle contains the following structures: Myofilaments (actin, myosin) - myofibril - muscle fiber - sarcolemma - endomysium - fascicle - perimysium - skeletal muscle - epimysium EPIMYSIUM PERIMYSIUM ENDOMYSIUM  Sarcolemma is the structure that surrounds the muscle fiber. Sarcoplasm is the cytoplasm of the muscle cell Sarcoplasmic reticulum (SR) is the endoplasmic reticulum of the muscle cell Sarcomere is the functional unit of the muscle.  Connective Tissue Structure Skeletal muscles are usually attached to bones by bundles of connective tissue consisting of collagen fibers known as tendons. (Between fibers) (Cell) (wrapped by perimysium)  Filament structure There are many mitochondria in the muscle cell for ATP. Each muscle fiber is innervated by a nerve ending ending in its middle part. Myofilaments are divided into two groups: thin and thick: There are 3 thin myofilaments; actin, troponin, tropomyosin. The thick myofilament is myosin. Each myofibril is composed of about 1500 myosin filaments and 3000 actin filaments  Filament structure Thin myofilaments: 1- Actin: has binding site for myosin 2- Tropomyosin: covers the active points of actin. 3- Troponin: It is found on tropomyosin. Thick myofilaments: 1- Myosin: the tail consists of 2 heavy chains and the myosin head consists of 4 light chains. It has ATPase properties and has a binding site for actin.  Sarcomere structure I band: thin (actin) filaments region H zone: consists only thick filament (myosin). A band: consists of both thick and thin filaments. M line: In the middle of the H band, non-contractile filaments connect myosins together Z line: It is the region between two I bands. MUSCLE PROTEINS Dystrophin, titin, actinin, desmin, nebulin are sarcomere intracellular skeletal proteins. Connecting myosin to the Z line: TITIN (Prevents excessive stretching of the sarcomere) Produces F-actin from G-actins: NEBULIN Connecting actin to the Z line: α-ACTININ DYSTROPHIN, which binds actin to dystroglycan in the outer cell membrane Dystroglycan binds to LAMININ in the extracellular matrix. Connecting the Z line to the cell membrane: DESMIN DYSTROPHIN: It creates intracellular stability. The dystrophin-glycoprotein complex ensures the integrity of muscle fibers and myofibrils. Duchenne muscular dystrophy. Becker muscular dystrophy  SARCOTUBULARY SYSTEM: There are T tubules on the sarcolemma. T tubules are inward folds of the muscle fiber membrane. The action potential travels through the muscle membrane via T tubules and reaches the sarcoplasm. Sarcoplasmic reticulum (SR) is a Ca+ store. SR has a tubular structure around the myofibrils. SR expand between the A-I bands, these expansions are known as terminal cisternas. 1 T tubule + 2 Terminal Cisterna = TRIAD (in skeletal muscle). When an action potential arrives, the triad causes the Ca+2 channels in the terminal cisternas to open and extracellular Ca+2 to enter the sarcoplasm.  Molecular Mechanisms of Skeletal Muscle Contraction The junction of an axon terminal with the motor end plate is known as a neuromuscular junction. ❖ Acetylcoline receptors are neuronal-muscle type nicotinic ligand-gated non-selective ion channels. CONTRACTION IN SKELETAL MUSCLE: Muscle contraction occurs according to the Sliding Filaments Theory. 1. In the resting muscle, the active areas of actin are covered with tropomyosin. 2. Ca+2 binds to troponin. 3. Tropomyosin changes conformationally, stretches, and actin's active sites open. 4. There is ATP in the myosin head, and ATP-->ADP + phosphate is formed by the ATPase enzyme activity of the head. 5. Myosin heads connect to the active regions of actin and acto-myosin cross bridges are established, 6. Phosphate separation initiates the power stroke, because phosphate grabs and pulls the myosin head for the power stroke; It also pushes actin in the opposite direction. This is the power pulse and the filaments slide over each other. 7. After the power pulse, ADP + phosphate in the myosin head is released 8. A new ATP binds to the myosin head again, actin- myosin cross bridges are separated 9. New cycle begins The contraction cycle continues as long as the usable ATP and Ca+2 levels are high in the sarcoplasm. When AP ends, the relaxation process begins. End of the skeletal muscle contraction (relaxation): 1. Ach is degraded by acetylcholinesterase (AChE) in the synaptic cleft. 2. SR reuptakes calcium ions through the SERCA channel, decreasing the calcium concentration in the cytosol. 3. When calcium concentrations approach resting levels, the troponin-tropomyosin complex returns to its normal position. 4. Myosin head bounds a new ATP. It turns into ATP->ADP+phosphate and is kept ready for the new cycle. 5. Active sites of actin close. 6. Cross bridge formation stops. 7. Muscle relaxation occurs and the muscle passively returns to its resting length.  ATP energy is used for muscle contraction. ATP is used specifically in these 4 stages; 1. In order for myosin heads to bind to actin and be used in the power stroke mechanism, 2. For the myosin head to separate from actin, a new ATP must be bound to the myosin head. In this way, the cycle can be repeated. 3. To pump Ca++ back into the SR by SERCA after contraction. 4. It is used to re-establish the appropriate Na-K gradient in the muscle cell and to be ready for a new action potential. SERCA (Sarcoplasmic Reticulum Calcium-ATPase) Skeletal muscle energy metabolism 1. Storated ATP 2. Creatine phosphate stores 3. Lactic acid system (in the generation of explosive power) 4. Oxidative phosphorylation First of all, ATP stored in the muscle fiber is used, it is in small amount, contraction is continued for 1-2 seconds. Then creatine phosphate stores are used, phosphorylcreatine-->converted to creatine + phosphate, contraction lasts 8-10 seconds. Then, glucose-glycogen stores are used and broken down into glucose-->pyruvate. Pyruvate-->enters the KREBS cycle in the mitochondria, this is aerobic glycolysis. (Running, walking, swimming). Motor neuron Motor neuron is a single nerve cell extends from the brain or spinal cord to a MUSCLE. When a neuron fires, all muscle cells stimulated by that neuron contract. For skeletal muscle cells to contract, each cell must be stimulated by a motor neuron. The motor unit consists of a motor neuron and the muscle fibers that innervates by the same motor neuron. Each motor unit acts independently of each other. When strong contraction is required, the nervous system stimulates more than one motor unit. Stimulation of additional motor units to increase contraction power is called RECRUITION. Motor neuron The motor unit can be small or large depending on the number of muscle fibers it stimulates. Number of cells per motor unit = MOTOR UNIT SIZE Small motor units, which innervate a few muscle cells, enable precise movements. medial rectus, lateral rectus muscle Large movements are created by large motor units. quadriceps muscle Lifting a chair and picking up an apple activates a different number of motor units. The strength of muscle contraction depends on the; 1. Intensity of the nervous system stimulus, 2. The number and size of activated motor units, 3. The type of muscle fibers stimulated.  Muscle fatigue When a skeletal muscle fiber is stimulated repeatedly, the fiber's tension eventually decreases even as stimulation continues. This decrease in muscle tension as a result of previous contractile activity is known as muscle fatigue. Additional characteristics of fatigued muscle are a reduced rate of shortening and a slower rate of relaxation. Skeletal muscle fiber types Muscle fibers show different mechanical and contractile properties. There are two types of fibers: Slow twitch muscle fibers (Type-1): twitch duration is longer than 100 ms -slow-oxidative Fast twitch muscle fibers (Type-2): twitch duration is about 7.5 ms Divided into 3; Type 2a (fast-oxidative-glycolitic) Type 2b (Type-2X) (fast-glycolitic) Thank you