Structure and action of skeletal muscle Mechanisms of contraction CHAPTER 12 Structure and action of skeletal muscle Mechanisms of contraction Neural control of skeletal muscle contraction Cardiac and smooth muscle

Types of Muscle in the Body Muscle is involved in all movements of parts of our body. Muscle create movements by converting chemical energy to mechanical energy. Skeletal muscle: muscle mainly attached to the skeleton and used for movement. “muscle of locomotion and exercise” Smooth muscle: muscle mainly lining internal organs of the body, such as blood vessels, gut, urinary bladder, etc. “muscle of regulatory movements, closure & opening, constriction & dilation” Cardiac muscle: muscle of the heart. Its contraction and relaxation causes pumping action of the heart.

Types of Muscle in the Body Skeletal Smooth Cardiac

Skeletal Muscle Movements Tendon Flexor muscle Adductor muscle Extensor muscle Abductor muscle

Skeletal Muscle Structure

Skeletal Muscle Structure A single muscle cell is called muscle fiber. Each muscle fiber contains several hundreds to several thousands of structures called myofibrils. Diameter of a muscle fiber ranges from 10-80 mm, but the length may extend the entire length of the muscle (e.g the thigh muscle fiber is about 35 cm long). Plasma membrane of muscle fiber is called sarcolemma. Each muscle fiber is ultimately attached to a bone via a hard connective tissue called tendon. Cytoplasm of muscle fiber is called sarcoplsam. The endoplasmic reticulum is called sarcoplasmic reticulum (storage site for calcium). Transverse tubules are structures that: pass laterally through the muscle fiber, Transmit nerve signals closer to individual myofibrils, Provide access of oxygen and glucose to inner parts of muscle fiber. Transverse Tubules

Skeletal Muscle Structure = Thin filament A Band = Thick filament

Skeletal Muscle Contraction Contraction (shortening in length) of a muscle fiber is due to sliding of thick and thin filaments against each other at opposite directions. This is called sliding filament theory. The size of filament remains the same but they slide against each other resulting in shortening of sarcomere. The sliding occurs by formation of cross bridges between the filaments. During the shortening of the sarcomere the A band remain the same length while the H and I bands shorten causing the Z discs to come closer to each other.

Actin Thin Filament

Molecular Basis of Contraction Thick Filament

Molecular Basis of Contraction

Molecular Basis of Contraction

Excitation-Contraction Coupling When a nerve impulse is received by a muscle fiber this leads to its contraction. The sequences which lead to contraction are as follow: 1- Nerve impulse causes the release of Ach from axon terminal into the neuromuscular junction. 2- The ACh binds with a receptor on the muscle fiber and generates action potential on the muscle fiber. 3- The action potential spreads over the sarcolemma as well as the T tubuls. 4- Depolarization of the T tubuls cause release of calcium from the sarcoplasmic reticulum into the sarcoplasm. 5- Calcium binds to troponin which leads to muscle contraction. As long as the level of calcium in sarcoplasm is above 1 mM the contraction will continue. For the contraction to stop (relaxation) the calcium in the Sarcoplasm is actively removed back into sarcoplasmic reticulum and stored there for next contraction.

Excitation-Contraction Coupling T-tubule Sarcoplasmic reticulum Diffusion Active

Neuromuscular Junction

Structure of Muscle Fibers

Motor Unit Each somatic motor neuron with all its muscle fibers that it innervates is called motor unit.

Properties of Skeletal Muscle Skeletal muscle can be studied in vitro: e.g Gastronemius muscle of frog. This muscle can be stimulated with electric shock and contraction can be recorded by force transducer. Using this preparation, the followings can be studied: Muscle twitch : applying a single electric shock causes the muscle to contract and relax. Summation : if a second electric shock immediately follows the first, another muscle twitch will be produced which will ride on the first one. Tetanus : high frequency stimulation of the muscle causes continuous contractions which fuse together. Series-Elastic Component : connective tissue and tendons show elasticity (resist distension). This helps in muscle relaxation. Muscle twitch is graded

Types of Skeletal Muscle Contractions Isotonic contraction: a type of contraction that results in muscle shortening. In this type of contraction the tension on the muscle fibers remain constant. iso = same, tonic = strength Isometric contraction: when a muscle remains the same length during contraction while the tension changes. iso = same, metric = length

Smooth Muscle The arrangement of the actin and myosin filaments is different. Therefore showing no striation. Fibers are smaller than skeletal muscle fibers. Same chemical substances (actin and myosin) cause contraction. Sliding filament mechanism applies here. Smooth muscle has slower rate of contraction and relaxation than skeletal and cardiac muscle. Less energy is required for smooth muscle contractions. Smooth muscle can contract without action potential, using local factors. Level of cytoplasmic Ca++ is crucial for contraction, however the Ca ++ mainly comes from outside the cell (has less developed SR).

Cardiac Muscle Like skeletal muscle the cardiac muscle is striated. Is contracted by the sliding filament mechanism. Unlike skeletal muscle, the cardiac muscle cells are interconnected by gap junctions. The cells are joined electrically, this is called myocardium (single functional unit). Contractility (ability to contract) is increased by epinephrine and stretch. Can produce action potential automatically.