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Department
Physiology
Course
Physiology 2130
Professor
Anita Woods
Semester
Winter

Description
Muscles Introduction  Muscles: They are biological machines that utilize chemical energy from the breakdown and metabolism of food to perform useful work  Three kinds of muscles are: o Skeletal (or striated) muscle: used primarily in voluntary motion o Smooth muscle: found within the walls of blood vessels, airways, various ducts, urinary bladder, uterus and digestive tract o Cardiac muscle: found in the heart  There are over 600 different muscles and these muscles perform 3 principal functions: o Movement o Heat production o Body support and posture Structure of muscle  Whole muscles are made up of bundles of fasciculi  Each fasciculi is made up of groups of muscle cells or fibers  Each muscle cell contains many bundles of myofibrils  Each myofibril contains thin and thick myofilaments o Thin myofilaments contain mostly protein actin along with troponin and tropomyosin o Thick myofilaments contain the protein myosin  The interaction of thin and thick myofilaments results in muscle contraction  Video: o Skeletal muscle is attached to bones and makes movement possible o Muscle structural pattern is a series of increasing small parallel units o Muscle is composed of fascicules and each fascicule is composed of fibers o Each fiber is an elongated cell with many nuclei o Within each fiber are myofibrils composed of thick and thin filaments made of protein o The regular arrangement of these filaments gives striated muscle its striped appearance o The basic functional unit of the muscle is the sarcomere – a section of a myofibril  Sarcomere is composed of thick filaments of myosin (red) and thin filaments (blue) of actin  Muscle contraction occurs when these thick and thin filaments slide past one another Structure of skeletal muscle  The diagram at below shows that a whole muscle, like the biceps muscle of the upper arm, is composed of groups of fasciculi surrounded by a white connective tissue called perimysium. Each fascicle, in turn, is made up of bundles of muscle cells (also called muscle fibers). Within each cell there are cylindrical bundles of myofibrils. These myofibrils are composed of two types of myofilaments, which are the actual contractile elements of the cell.  Muscle cells (or fibers) are one of the few 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 thin and thick myofilaments.  The myofibrils are surrounded by the sarcoplasmic reticulum (SR). This is a mesh-like network of tubes containing calcium ions (Ca ), which are essential for contraction.  At either end of, and continuous with the SR are the terminal cisternae, a membranous enlargement of the SR, which is close to the T tubule (where the action potential travels Thin Myofilaments  Thin myofilaments are composed predominantly of the globular protein actin  Each actin molecule has a special binding site for the other contractile molecule myosin  Many actin molecules are strung together and then twisted to form the backbone of the thin microfilaments  Thin myofilaments also contain long protein strands called tropomyosin o When muscle is at rest, these strands cover the binding site for myosin  A third regulatory protein called troponin is made up of 3 subunits: o Troponin A: binds to actin o Troponin T: binds to tropomyosin o Troponin C: binds to Ca++  At rest: the troponin complex holds the tropomyosin over the myosin binding sites  When Ca++ binds to the troponin C unit, the tropomyosin is pulled off the myosin binding sites by the troponin Thick Myofilament  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 have a site that can bind and split ATP o It is the splitting of ATP that releases the energy to the myosin that powers the contraction of the muscle  Many myosin molecules are arranged to form thick filament Actin/Myosin Relationship  Group of thin and groups of thick myofilaments are arranged in a repeating pattern (thin, thick, thin, thick and so on) along the length of the myofibril from one end of the muscle cell to the other  Each group of thin myofilaments extends outwards in opposite directions from a central Z disk (also called Z line)  Groups of thick myofilaments extend outwards from a central M line, where they are attached  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. This is the smallest functional contractile unit of the muscle cell  The repeating pattern of thin and thick filaments gives the muscle cell a banded or striated appearance; hence skeletal muscle is sometimes referred to as striated muscle cell  A bands: regions that contain thick filaments and appear as dark bands  I bands: regions that contain only thin filaments and appear lighter 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 o This power stroke slides the acting past the myosin o The sarcomere shortens  IMPORTANT: neither the thin not the thick filaments shorten during a contraction Excitation-Contraction Coupling and Muscle Contraction  Excitation-contraction coupling refers to the process by which an action potential in
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