Muscle Properties Irritability - A muscle irritability refers to the ability of the muscle to respond to a stimulus. Contractility - A muscle contractility refers to the muscle’s ability to shorten in length. Elasticity - This refers to the muscles ability to stretch and return to normal length. Extensibility - This refers to the muscle’s ability to extend in length. Conductivity - This refers to a muscle’s ability to transmit nerve impulses.

Sarcomeres separated by narrow zones of dense material called Z lines within a sarcomere is a dark area called the A band (thick myofilaments) ends of the A band are darker because of overlapping thick and thin myofilaments the light coloured area is called the I band (thin myofilaments) the combination of alternating dark A bands and light I bands gives the muscle fibre its striated appearance

Muscle Contraction Muscle structure under a microscope Muscle fibres skeletal muscle viewed under a microscope contains thousands of these elongated, cylindrical cells Sarcolemma the plasma membrane that covers each muscle fibre Myofibrils found within each skeletal muscle fibre cylindrical structures which run longitudinally through the muscle fibre consist of two smaller structures called myofilaments Myofilaments thin myofilaments and thick myofilaments do not extend the entire length of a muscle fibre they are arranged in compartments called sarcomeres

Myofilaments Thin myofilaments thin myofilaments are anchored to the Z lines composed mostly of the protein actin actin is arranged in two single strands that entwine like a rope each actin molecule contains a myosin- binding site thin myofilaments contain two other protein molecules that help regulate muscle contraction (tropomyosin and troponin) Thick myofilaments composed mostly of the protein myosin which is shaped like a golf club the heads of the golf clubs project outward these projecting heads are called cross bridges and contain an actin- binding site and an ATP binding site

Sliding Filament Theory during muscle contraction, thin myofilaments slide inward toward the centre of a sarcomere sarcomere shortens, but the lengths of the thin and thick myofilaments do not change myosin cross bridges of the thick myofilaments connect with portions of actin on thin myofilaments myosin cross bridges move like the oars of a boat on the surface of the thin myofilaments thin and thick myofilaments slide past one another as thin myofilaments slide inward, the Z lines are drawn toward each other and the sarcomere is shortened myofilament sliding and sarcomere shortening result in muscle contraction this process can only occur in the presence of sufficient calcium (Ca++) ions and an adequate supply of energy (ATP)

Contractile Machinery: Sarcomeres Contractile units Organized in series ( attached end to end) Two types of protein myofilaments: - Actin: thin filament - Myosin: thick filament Each myosin is surrounded by six actin filaments Projecting from each myosin are tiny contractile myosin bridges Longitudinal section of myofibril a) at rest

High microscope magnification of a single sarcomere within a single myofibril

Contractile Machinery: Crossbridge formation and movement Cross bridge formation: - a signal comes from the motor nerve activating the fibre - the heads of the myosin filaments temporarily attach themselves to the actin filaments Cross bridge movement: - similar to the stroking of the oars and movement of rowing shell - movement of myosin filaments in relation to actin filaments - shortening of the sarcomere - shortening of each sarcomere is additive Longitudinal section of myofibril b) contraction

Contractile Machinery: Optimal Crossbridge formation Longitudinal section of myofibril Sarcomeres should be optimal distance apart For muscle contraction: optimal distance is (0.0019-0.0022 mm) At this distance an optimal number of cross bridges is formed If the sarcomeres are stretched farther apart than optimal distance: - fewer cross bridges can form  less force produced If the sarcomeres are too close together: - cross bridges interfere with one another as they form  less force produced c) Powerful stretching d) Powerful contraction

Contractile Machinery: Optimal muscle length and optimal joint angle The distance between sarcomeres is dependent on the stretch of the muscle and the position of the joint Maximal muscle force occurs at optimal muscle length (lo) Maximal muscle force occurs at optimal joint angle Optimal joint angle occurs at optimal muscle length

Muscle tension during elbow flexion at constant speed