DR. AYISHA QURESHI ASSISTANT PROFESSOR MBBS, MPhil MUSCLE DR. AYISHA QURESHI ASSISTANT PROFESSOR MBBS, MPhil

MUSCLE: Chemical energy ↓Muscle Mechanical energy Muscle forms about 50% of the total body weight: 40% skeletal muscle 10% smooth & cardiac muscle Simply put, Muscles perform the following functions: They contract… They generate heat They generate motion They generate force They provide support Muscles contract & change Potential (chemical) energy into Kinetic (mechanical energy)

TYPES of MUSCLE (According to appearance or movement) Skeletal Muscle (Striated) (Voluntary) Smooth Muscle (Smooth) (Involuntary) Cardiac Muscle Skeletal muscle makes up the muscular system. Smooth muscle appears throughout the body systems as components of hollow organs and tubes. Cardiac muscle is present in the heart only….The cardiac muscle contracts rhythmically & automatically throughout life to pump blood into circulation.

Skeletal muscle:

Characteristics of Skeletal Muscles: Attach to the bone Move appendages Support the body Antagonistic pairs: Flexors & extensors

SKELETAL MUSCLE CELL STRUCTURE A single skeletal muscle cell is also called a MUSCLE FIBER b/c of its greater length than width. LENGTH: upto 75,000 µm or 2.5 feet. DIAMETER: from 10 to 100 micrometres. SHAPE: elongated & cylindrical. OUTER MEMBRANE: called sarcolemma. Nucleus & Organelles: present. Mitochondria, microsomes & ER What is the chemical composition of the muscle? Proteins (20%) (either as enzymes or for muscle Cont.) Lactic Acid (in muscle that has undergone fatigue) ATP, ADP Myoglobin (stores O2 & gives colour to the muscle)

Skeletal Muscle Organization Whole Muscle (an organ) ↓ Muscle Fiber (a single cell) Myofibrils (a specialized structure) Thick & Thin filaments Myosin & Actin (protein molecules)

SKELETAL MUSCLE ORGANIZATION

PROTEINS OF MUSCLE:

(6-7 nm long polymerized G-actin, double stranded in structure) ACTIN & THIN FILAMENTS G-actin is the monomer which will form the thin filament. It is a protein with a molecular weight of 43,000. It has a prominent site for cross-linkage with myosin. G-actin ↓ F-actin (6-7 nm long polymerized G-actin, double stranded in structure) Thin filaments

Regulatory Proteins of the Muscles TROPOMYOSIN TROPONIN Rod-like protein Under resting conditions, it covers the site for myosin attachment on F-actin molecule. Forms part of Thin filaments Globular protein complex made of 3 polypeptides Forms part of thin filaments Binds to Ca2+ Troponin C Inhibitory in function Troponin I Attached to Tropomyosin Troponin T

Formation of thin filaments……

MYOSIN & THICK FILAMENTS: Thick filaments consist of 2 symmetrical halves that are mirror images of each other. Chief constituent is MYOSIN, with a mol. weight of 480,000. Its molecule has 2 ends, a globular end having 2 heads & a rod-like tail. It has 6 peptide chains: - 2 identical heavy chains (200,000 each) - 4 light chains ( 20,000 each)

Binding sites on Myosin molecule: The myosin molecule has 2 binding sites: Binding site for ACTIN ATPase site

A SARCOMERE:

A myofibril displays alternating dark & light bands. Dark bands (A bands) Anisotropic Thick & thin filaments Light bands (I bands) Isotropic Thin filaments only Actually, the alternate stacked sets of thick & thin filaments that strictly overlap one another are responsible for the light & dark bands.

A sarcomere model:

A SARCOMERE The area between 2 consecutive Z discs/ lines is called A Sarcomere. It is the functional unit of a muscle. It has a length of 2.3 µm. It has the following important features: Z-disc M-line I-band A-band H-zone Titin Nebulin

Sarcomere: Organization of Fibers

Z-disc: are dense thin membranes made up of special lattice-like proteins present transversely. Dark or A-band: Thick filaments present overlapped by the thin filaments at the ends only. Light or I band: area present b/w the ends of the 2 thick filaments. It consists of thin filaments only. H-Zone: The lighter area in the middle of the A-band, where the thin filaments do not reach. It consists of thick filaments only. M-Line: A line that extends vertically down the middle of the A-band in the center of the H-zone. Pseudo H-zone: M-line+ H-zone.

THE SARCOTUBULAR SYSTEM

Sarcotubular System The sarcoplasm of the myofibril is filled with a system of membranes, vesicles and tubules which are collectively termed as The Sarcotubular system. It is made up of: T-Tubules Sarcoplasmic Reticulum

The Sarcotubular system…

Transverse System of Tubules Sarcoplasmic Reticulum SARCOTUBULAR SYSTEM Transverse System of Tubules (T-Tubules) Sarcoplasmic Reticulum (SR) It is a fine network of interconnected compartments which run in the longitudinal axis of a myofibril embedded in the I and A bands, & surround them. They are surrounded by the sarcoplasm & are NOT connected to the outside of the cell. At their both ends they show dilated ends called as Terminal cisterns or sacs. They contain a protein called as Calsequestrin, which binds and holds CALCIUM. It is a system of tubules that runs transverse to the long axis of the muscle. They enter the myofibrils at the junction b/w the A and I bands. The T-tubules open onto the sarcolemma. It is an invagination of the cell membrane & thus communicates with the ECF. It functions to rapidly transmit the AP from the sarcolemma to all the myofibrils.