Muscle Physiology Lecture Notes PDF

Lec.6 Muscles Physiology 1  Muscles Physiology The muscles are excitable cells; they are machines to convert the chemical energy to mechanical energy.It differs from the nervous system by the fact that it has a contractile mechanism which is activated by adenosine phosphate....

Lec.6 Muscles Physiology 1  Muscles Physiology The muscles are excitable cells; they are machines to convert the chemical energy to mechanical energy.It differs from the nervous system by the fact that it has a contractile mechanism which is activated by adenosine phosphate. The muscle can be excited electrically, mechanically, chemically → action potential (A.p.). Types of muscle Skeletal muscles: These are voluntary muscles attach to bone. Smooth muscles: Involuntary muscle. It is Muscle of the viscera (e.g., in walls of blood vessels, intestine, & other 'hollow' structures and organs in the body). Cardiac muscles: Muscle of the heart. Involuntary. Properties of muscle & Functions of muscle Properties of muscle: 1-excitability - responds to stimuli (e.g., nervous impulses) 2-contractility - able to shorten in length 3-extensibility - stretches when pulled 4-elasticity - tends to return to original shape & length after contraction or extension 2  Functions of muscle: 1-Body movement 2-Maintenance of posture 3-Heat production 4-Respiration 5-Communication Nerve and Blood Vessel Supply Motor neurons – stimulate muscle fibers to contract – Neuron axons branch so that each muscle fiber (muscle cell) is innervated. – Form a neuromuscular junction (= myoneural junction) Capillary beds surround muscle fibers – Muscles require large amounts of energy – Extensive vascular network delivers necessary oxygen and nutrients and carries away metabolic waste produced by muscle fibers. The Skeletal Muscle It is that type of the muscles that is attached to bones & moves skeleton, also called striated muscle. Composed of muscle cells, (fibers), connective tissue, blood vessels, nerves Fibers are long, cylindrical, and multinucleated. Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length Develop from myoblasts; numbers remain constant Striated appearance Nuclei are peripherally located 3 Microanatomy of Skeletal Muscle 4 Muscle Fiber Anatomy Sarcolemma - cell membrane – Surrounds the sarcoplasm (cytoplasm of fiber) Contains many of the same organelles seen in other cells. – Punctuated by openings called the transverse tubules (Ttubules) Narrow tubes that extend into the sarcoplasm at right angles to the surface Filled with extracellular fluid Myofibrils -cylindrical structures within muscle fiber – Are bundles of protein filaments (=myofilaments) Two types of myofilaments 1. Actin filaments (thin filaments) 2. Myosin filaments (thick filaments) – At each end of the fiber, myofibrils are anchored to the inner surface of the sarcolemma – When myofibril shortens, muscle shortens (contracts). 5 6 7 8 Actin (Thin) Myofilaments Thin Filament: composed of 3 major proteins 1. F (fibrous) actin 2. Tropomyosin 3. Troponin Two strands of fibrous (F) actin form a double helix extending the length of the myofilament; attached at either end at sarcomere. – Composed of G actin monomers each of which has a myosin-binding site (see yellow dot) – Actin site can bind myosin during muscle contraction. Tropomyosin: an elongated protein winds along the groove of the F actin double helix. Troponin is composed of three subunits: – Tn-A : binds to actin – Tn-T :binds to tropomyosin, – Tn-C :binds to calcium ions. 9 Now, putting it all together to perform the function of muscle: Contraction 10  Sarcomere Relaxed Sarcomere Partially Contracted Sarcomere Completely Contracted 11 Sarcotubular System Sarcotubular system of membranous structures in the form of vesicles and tubules in the Sarcoplasm of the muscle fiber. It surrounds the myofibrils embedded in the Sarcoplasm. The Sarcotubular system is formed mainly by two types of structure: 1. “T” tubules 2. “L” tubules of Sarcoplasmic reticulum. 12 neuromuscular junction neuromuscular junction, also called myoneural junction, site of chemical communication between a nerve fibre and a muscle cell. The neuromuscular junction is analogous to the synapse between two neurons. A nerve fibre divides into many terminal branches; each terminal ends on a region of muscle fibre called the end plate. Embedded in the end plate are thousands of receptors, which are long protein molecules that form channels through the membrane. Upon stimulation by a nerve impulse, the terminal releases the chemical neurotransmitter acetylcholine from synaptic vesicles. Acetylcholine then binds to the receptors, the channels open, and sodium ions flow into the 13  end plate. This initiates the end-plate potential, the electrical event that leads to contraction of the muscle fiber. Synaptic cleft 14 15