OBJECTIVES At the end of this lecture the student should be able to: Define different types of muscles Enumerate cellular organization of human skeletal muscular system Enumerate molecular organization of human skeletal muscular system

Muscular System Functions Body movement Maintenance of posture Respiration Production of body heat Communication Constriction of organs and vessels Heart beat

Properties of Muscle Contractility – Ability of a muscle to shorten with force Excitability – Capacity of muscle to respond to a stimulus Extensibility – the ability to be stretched or to increase in length  Elasticity – the ability to return to normal length after extension or contraction.

Muscle Tissue Types Skeletal – Attached to bones – Nuclei multiple and peripherally located – Striated, Voluntary Smooth – Walls of hollow organs, blood vessels, eye, glands, skin – Single nucleus centrally located – Not striated, involuntary, Cardiac – Heart – Single nucleus centrally located – Striations, involuntary, intercalated disks

Skeletal Muscle Most skeletal muscles are connected to at least two bones – Muscles attach directly to bone Or muscles attach indirectly to bone through tendons Muscles produce movement by producing tension between its ends Skeletal Muscle Structure – Cellular Level – Molecular Level

Skeletal Muscle Structure – Cellular Level A Skeletal muscle fiber is an individual muscle cell Muscle fibers are long and narrow in shape – Sarcolemma The plasma membrane of the muscle cell Surrounds the sarcoplasm – Many nuclei (multi-nucleated) Located in the periphery of the muscle cell just beneath the sarcolemma

Skeletal Muscle Structure – Cellular Level Each muscle fiber contains various organelles specifically designed to meet the needs of the contractile skeletal muscle fiber – Abundant mitochondria High demand for energy (ATP) required for muscle contraction – Myoglobin Protein with a high affinity for oxygen Transfers oxygen from the blood to the mitochondria of the muscle cell

Skeletal Muscle Structure – Cellular Level Each muscle fiber contains: Myofibrils – a cylindrical bundle of contractile proteins, which are called Myofilaments, within a muscle fiber – Located in the sarcoplasm of the muscle cell Myofilaments – the contractile protein filaments that make up the Myofibrils – Actin – thin filament – Myosin – thick filament

Skeletal Muscle Structure – Cellular Level Sarcoplasmic reticulum (SR) – Saclike membranous network of tubules Elaborate form of smooth endoplasmic reticulum – Surrounds each myofibril – Contains terminal cisternae Located where the SR ends, which is near the area where actin and myosin overlap – The SR tubules and terminal cisternae store high concentrations of calcium, which is important in the process of skeletal muscle contraction

Skeletal Muscle Structure – Cellular Level Transverse tubules (T-tubules) – Closely associated with SR – Connected to the sarcolemma – Penetrate the sarcolemma into the interior of the muscle cell (invaginations) – Bring extracellular materials into close proximity of the deeper parts of the muscle fiber SR and T-tubules Function – Activate skeletal muscle contraction when the muscle cell is stimulated by a nerve impulse – Transmit nerve impulses from the sarcolemma to the myofibirls

Skeletal Muscle Structure – Molecular Level Sarcomere – Smallest contractile unit of the muscle fiber – Arrangement of Myofilaments Alternating bands of light and dark areas Due to the organization of the actin and myosin – Striated appearance

The Sarcomere A band I band Z disc sarcomere thick filament (myosin) thin filament (actin) titin (filamentous structural protein) M line H zone

Sarcomere Components Z-lines = borders of the sarcomere – Perpendicular to long axis of the muscle fiber – Anchor thin myofilaments (actin) M-lines – Perpendicular to long axis of the muscle fiber – Anchor thick myofilaments (myosin)

Sarcomere Components A-Bands Dark area where actin and myosin overlap Equal to the length of the thick myofilaments (myosin) Contains the H-Zone – Lighter area within the A-Band that contains only myosin – The M-Line is located with in the H-zone I-Bands Light area composed of actin only Contains the Z line, which is the boarder of the sarcomere – Actin is directly attached the Z-Line – Appears as a darker line through the I-Band.

Skeletal Muscle Structure – Molecular Level Actin – G-actin (globular actin) = the basic component of each actin myofilament Contains myosin binding sites – The actin myofilament consists of two strands of G-actin molecules The two strands of G-action molecules are twisted together with two regulatory proteins: – tropomyosin – troponin

Skeletal Muscle Structure – Molecular Level Tropomyosin – Rod-shaped protein that occupies the groove between the twisted strand of actin molecules – Blocks the myosin binding sites on the G-actin molecules Troponin – A complex of three globular proteins. One is attached to the actin molecule One is attached to tropomyosin One contains a binding site for calcium

Skeletal Muscle Structure – Molecular Level Myosin – Crossbridges Composed of a rod-like tail and two globular heads – The tails form the central portion of the myosin myofilament – The two globular headsface outward and in opposite directions Interact with actin during contraction. Contain binding sites for both actin and ATP – The enzyme ATP-ase is located at the ATP binding site for hydrolysis of ATP