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Module 5 Notes.docx

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Western University
Physiology 2130

Physiology 2130 Module 5 Online Notes (Sec 5.1 to 5.24) Muscles Introduction  Muscles are biological machines that utilize chemical energy from the breakdown and metabolism of food to perform useful work  There are three kinds of muscle cells: o Skeletal muscle – used primarily for voluntary motion o Smooth muscle – found within the walls of blood vessels, airways, various ducts, urinary bladder, uterus, and the digestive tract o Cardiac muscle – found in the heart  The body contains over 600 different muscles, these muscles perform three principal functions: Movement, Heat Production and Body Support and Posture A Whole Look at the Structure of Muscle  Breakdown of the Structure of Muscles: o Whole muscles are made up of bundles of fasciculi. o Each fascicle is made up of groups of muscle cells or fibers. o Each muscle cell contains many bundles of myofibrils. o Each myofibril contains thin and thick myofilaments. o Thin myofilaments contain mostly the protein actin along with troponin and tropomyosin. o Thick myofilaments contain the protein myosin. o The interaction of thin and thick myofilaments results in muscle contraction. Structure of a Skeletal Muscle  A whole muscle, like the biceps muscle of the upper arm is composed of groups of fasciculi surrounded by a white connective tissue (perimysium)  Each fascicle is made up of bundles of muscle cells or muscle fibers  Within each cell there are cylindrical bundles of myofibrils  Myofibrils are composed of two types of myfilaments which are the actual contractile elements of the cell Structure of a Muscle Cell  Muscle Cells (or fibers) are one of the only cells in the body with more than one nucleus  They are surrounded by the sarcolemma – the muscle cell membrane – over which the action potential is transmitted  The sarcolemma has small tube-like projections called transverse (T) tubules that extend down into the cell  These T tubules conduct the action potential deep into the cell where the contractile proteins are located  Within the muscle cell are long cylindrical myofibrils that contain the contractile proteins of the muscle – the think and thick myofilaments  Myofibrils are surrounded by the Sarcoplasmic reticulum (SR)  This is a mesh like network of tubes containing calcium ions (essential for contraction)  At the end of the SR are the terminal cisternae – membranous enlargement of the SR, close to the T tubule (where the action potential travel) Thin Myofilament  Thin myofilaments are composed of mainly the globular protein actin  Each actin molecule contains a special binding site for the other contractile protein myosin  Many actin molecules are strung together (like beads on a necklace) and then twisted to form the backbone of the thin myofilaments  Also found on the thin myofilaments are long protein strands called tropomyosin  When the muscle is at rest, these proteins cover the binding sites for myosin  A 3 regulatory protein, troponin, is made up of three sub units o Troponin A – binds to actin o Troponin T – binds to tropomyosin ++ o Troponin C – binds with Ca  At rest, the troponin complex holds the tropomyosin over the myosin binding sites  When Ca bind to the troponin C unit, the tropomyosin is pulled off the myosin binding sites by the troponin Thick Myofilament  The Thick Myofilament is made up of the protein myosin  Myosin has a long, bendable tail and two heads that can each attach to the myosin binding sites on actin  The heads also have a site that can bind and split ATP  The splitting of ATP releases energy to the myosin that powers the contraction of the muscle  Many myosin molecules are arranged to form one thick filament Actin/Myosin Relationship  Groups of thin (actin) myofilaments and groups of thick (myosin) microfilaments are arranged in repeating pattern (thick, thin, thick, & so on) along the length of one end of the muscle cell to the other  Each group of thin myofilaments extends outward in opposite direction from a central Z disk (or Z line), where they are anchored  Similarly groups of thick myofilaments extend outwards to attach to a central M line  Each myofilament is parallel to the length of the myofibril and the muscle cell  The region from one Z disk to another is called a sarcomere – the smallest functional contractile unit of the muscle cell  Under a microscope, the repeating pattern of thick and thin myofilaments gives the muscle cell a banded or striated appearance which is why skeletal muscle is often referred to as striated muscle  Thick filaments appear as dark bands (A bands)  Regions containing only thin filaments appear lighter (I bands) Muscle Contraction – The Sliding Filament Theory The interaction between actin and myosin leads to muscle contraction. When the head of a myosin molecule attaches to the binding site on actin and forms a crossbridge, the myosin undergoes a change in shape. This change in shape causes the myosin head to swing, producing the power stroke. This power stroke is much like the stroke of an oar in the water—it propels the boat past the water. In the muscle, the power stroke slides the actin past the myosin. It is very important to realize that neither the thin nor the thick filaments shorten during a muscle contraction. Excitation-Contraction Coupling and Muscle Contraction  Excitation-contraction coupling refers to the process by which an action potential in the cell membrane (sarcolemma) excites the muscle cell to produce a muscle contraction  The action potential that was generated at the neuromuscular junction will spread out over the sarcolemma and down the T-tubules into the core of the muscle cell  This action
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