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

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

• 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.

• 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

• 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

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

• 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)

• 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