Muscle Physiology

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 Long cylindrical cells Many nuclei per cell Striated Voluntary Rapid contractions

Skeletal Muscle Produce movement Maintain posture & body position Support Soft Tissues Guard entrance / exits Maintain body temperature Store nutrient reserves

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 Structure

Skeletal Muscle Fiber

Sarcomere

Z line

Sarcomere Relaxed

Sarcomere Partially Contracted

Sarcomere Completely Contracted

A sarcomere model:

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

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.

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.

Molecular events during muscular contraction

Muscle Contraction Types Isotonic contraction Isometric contraction

Muscle Contraction Types Isotonic contraction Isometric contraction

Muscle Contraction Types Isotonic contraction Isometric contraction

Tetany Sustained contraction of a muscle Result of a rapid succession of nerve impulses

Tetanus

Muscle Fatigue –When muscles can no longer perform a required activity, they are fatigued Results of Muscle Fatigue –Depletion of metabolic reserves –Damage to sarcolemma and sarcoplasmic reticulum –Low pH (lactic acid) –Muscle exhaustion and pain

Energy metabolism during muscle contraction ATP as Energy Source

Creatine Molecule capable of storing ATP energy Creatine + ATPCreatine phosphate + ADP ADP + Creatine phosphate ATP + Creatine

Metabolism Aerobic metabolism –95% of cell demand –Kreb’s cycle –1 pyruvic acid molecule  17 ATP Anaerobic metabolism –Glycolysis  2 pyruvic acids + 2 ATP –Provides substrates for aerobic metabolism –As pyruvic acid builds converted to lactic acid

Motor Unit: The Nerve-Muscle Functional Unit A motor unit is a motor neuron and all the muscle fibers it supplies The number of muscle fibers per motor unit can vary from a few (4-6) to hundreds ( ) Muscles that control fine movements (fingers, eyes) have small motor units Large weight-bearing muscles (thighs, hips) have large motor units

Motor Unit: The Nerve-Muscle Functional Unit Figure 9.12 (a)

Fast twitch Slow twitch Motor Unit: The Nerve-Muscle Functional Unit

Neuromuscular Junction

Cardiac Muscle Branching cells One/two nuclei per cell Striated Involuntary Medium speed contractions

Cardiac Muscle Found only in heart where it forms a thick layer called the myocardium Fibers joined by intercalated disks –IDs are composites of desmosomes and gap junctions –Allow excitation in one fiber to spread quickly to adjoining fibers Under control of the ANS (involuntary) and endocrine system (hormones) Some cells are autorhythmic –Fibers spontaneously contract (aka Pacemaker cells)

Cardiac Muscle Tissue Figure 10.10a

Mechanism of Cardiac Muscle Excitation, Contraction & Relaxation Excitation-contraction coupling and relaxation in cardiac muscle

The Action Potential Types

Smooth Muscle Fibers smaller than those in skeletal muscle Spindle-shaped; single, central nucleus More actin than myosin No sarcomeres –Not arranged as symmetrically as in skeletal muscle, thus NO striations. Caveolae: indentations in sarcolemma; –May act like T tubules Dense bodies instead of Z disks –Have noncontractile intermediate filaments

Smooth Muscle Figure 9.24 Grouped into sheets in walls of hollow organs Longitudinal layer – muscle fibers run parallel to organ’s long axis Circular layer – muscle fibers run around circumference of the organ Both layers participate in peristalsis

Smooth Muscle Is innervated by autonomic nervous system (ANS) Visceral or unitary smooth muscle –Only a few muscle fibers innervated in each group –Impulse spreads through gap junctions –Who sheet contracts as a unit –Often autorhythmic Multiunit: –Cells or groups of cells act as independent units –Arrector pili of skin and iris of eye

SkeletalCardiacSmooth Appearance ControlvoluntaryunvoluntaryUnvoluntary Neural inputsomaticANS Hormone0EpiEpi/others Ca++ protTroponin Calmodulin Gap junctionsNoYes PacemakerNoYesNo