MUSCLE DR. AYISHA QURESHI ASSISTANT PROFESSOR MBBS, MPhil

MUSCLE (1) purposeful movement of the whole body or parts of the body (such as walking or waving your hand), (2) manipulation of external objects (such as driving a car or moving a piece of furniture), (3) propulsion of contents through various hollow internal organs (such as circulation of blood or movement of a meal through the digestive tract), and (4) emptying the contents of certain organs to the external environment (such as urination or giving birth).

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

TYPES of MUSCLE (According to appearance or movement) Muscle Skeletal Muscle (Striated) (Voluntary) Smooth Muscle (Smooth) (Involuntary) Cardiac Muscle (Striated) (Involuntary)

Types of Muscle

SKELETAL MUSCLE:

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

SKELETAL MUSCLE ANATOMY:

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 micrometers. 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 O 2 & 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

A single muscle fiber

LAYERS COVERING A MUSCLE: The skeletal muscle has the following layers covering it: Epimysium Perimysium Endomysium

PROTEINS OF MUSCLE:

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 Rod-like protein Mol. Weight: 70,000 2 chains: alpha & beta chains Under resting conditions, it covers the site for myosin attachment on F-actin molecule. Forms part of Thin filaments TROPONIN Globular protein complex made of 3 polypeptides Forms part of thin filaments Binds to Ca2+ Inhibitory in function Attached to Tropomyosin

THIN FILAMENTS: Length: 1 µm Diameter: 5-8 nm No. of G-Actin mol: Other Proteins: - Nebulin: provides elasticity to the sarcomere. - Titin: is the largest known protein in the body. It connects the Z-line to the M-line in the sarcomere & contributes to the contraction of skeletal muscle.

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: 1.Binding site for ACTIN 2.ATPase sit e

A SARCOMERE:

A myofibril displays alternating dark & light bands. Myofibril Dark bands (A bands) Anisotropic Thick & thin filaments Light bands (I bands) Isotropic Thin filaments only

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 disks I band A band H Zone M line Titin Nebulin Figure 12-5: The two- and three-dimensional organization of a sarcomere

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

SARCOTUBULAR SYSTEM 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. Transverse System of Tubules (T-Tubules) 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.

THE TRIAD The cisterns of the SR & the central portion of the T- tubules give rise to a characteristic pattern called the TRIAD. Each TRIAD consists of 2 terminal sacs of SR & 1 central t-tubule. There is no physical communication between each component of the triad. In the triad, the cisterns of the SR have the Ryanodine receptors which are complimentary to the Dihydropyridine receptors on the t-tubule. They are both involved in excitation-contraction coupling.