Nerve-Muscle Interaction Skeletal muscle activation is initiated through neural activation NS can be divided into central (CNS) and peripheral (PNS) The NS can be divided in terms of function: motor and sensory activity Sensory: collects info from the various sensors located throughout the body and transmits the info to the brain Motor: conducts signals to activate muscle contraction

Activation of motor unit and its innervation systems 1.Spinal cord 2. Cytosome 3. Spinal nerve 4. Motor nerve 5. Sensory nerve 6. Muscle with muscle fibres

Motor Unit Motor nerves extend from the spinal cord to the muscle fibres Each fibre is activated through impulses delivered via motor end plate Motor unit: a group of fibres activated via the same nerve All muscle fibres of one particular motor unit are always of the same fibre type Muscles needed to perform precise movements generally consist of a large number of motor units and few muscle fibres Less precise movements are carried out by muscles composed of fewer motor units with many fibres per unit

All-or-none Principle Whether or not a motor unit activates upon the arrival of an impulse depends upon the so called all-or-none principle An impulse of a certain magnitude (or strength) is required to cause the innervated fibres to contract Every motor unit has a specific threshold that must be reached for such activation to occur

Intra-muscle Coordination The capacity to apply motor units simultaneously is known as intra-muscle coordination Many highly trained power athletes, such as weightlifters, wrestlers, and shot putters, are able to activate up to 85% of their available muscle fibres simultaneously (untrained: 60%) Force deficit: the difference between assisted and voluntarily generated maximal force (trained: 10%, untrained: 20-35%)

Intra-muscle Coordination cont. Trained athletes have not only a larger muscle mass than untrained individuals, but can also exploit a larger number of muscle fibres Athletes are more restricted in further developing strength by improving intra-muscular coordination Trained individuals can further increase strength only by increasing muscle diameter

Inter-muscle Coordination The interplay between muscles that generate movement through contraction (agonists) and muscles responsible for opposing movement (antagonists) is called inter- muscle coordination The greater the participation of muscles and muscle groups, the higher the importance of inter-muscle coordination To benefit from strength training the individual muscle groups can be trained in relative isolation Difficulties may occur if the athlete fails to develop all the relevant muscles in a balanced manner

Inter-muscle Coordination cont. High-level inter-muscle coordination greatly improves strength performance and also enhances the flow, rhythm, and precision of movement Trained athlete is able to translate strength potential to enhance inter-muscle coordination

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 b) contraction Longitudinal section of myofibril

Contractile Machinery: Optimal Crossbridge formation Sarcomeres should be optimal distance apart For muscle contraction: optimal distance is ( 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 Longitudinal section of myofibril 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

How Can Muscle Change Size? Actin and myosin (more so myosin) filaments change size The number of myofibrils change The number of blood capiliaries within a fibre change –Why is this important? The amount of connective tissue in a fibre will change