Electromyography Lab 3 Types of Muscle Primary function of muscle: Conversion of chemical energy into mechanical energy • Skeletal – “Voluntary”; striated, attached to the skeleton – Locomotion, heat – Contracts in response to somatic motor neuron only • Cardiac – Involuntary; striated – Only in heart – Autonomic control – Endocrine modulation • Smooth – Involuntary; no striations – Internal organs: stomach, bladder, blood vessels – Autonomic control – Endocrine modulation Motor Unit A single motor neuron and all of the muscle fibers it controls is a motor unit. Motor unit recruitment: The increase in the number of simultaneously active motor units within a muscle based on demand. Control of skeletal muscle contraction - the number of motor units within a muscle that are activated - the frequency of motor neuron impulses in each motor unit Skeletal Muscle Structure Each skeletal muscle contains numerous muscle fibers -Each muscle fiber encloses a bundle of 4 to 20 elongated structures called myofibrils -Each myofibril in turn is composed of thick and thin myofilaments Skeletal Muscle Structure A bands = Stacked thick & thin myofilaments -Dark bands H bands = Center of the A band, consisting of thick bands only I bands = Consist only of thin myofilaments -Light bands -Divided into two halves by a disc of protein called the Z line Sarcomere = Distance between two Z lines -Smallest subunit of muscle contraction 6 Skeletal Muscle Contraction A muscle contracts and shortens because the myofibrils contract and shorten -Myofilaments themselves do not shorten -Instead, the thick and thin filaments slide relative to each other -Sliding filament mechanism -Z lines move closer together, as the I and H bands become shorter -A band does not change in size 7 Skeletal Muscle Contraction A thick filament is composed of several myosin subunits packed together -Myosin consists of two polypeptide chains wrapped around each other -Each chain ends with a globular head A thin filament is composed of two chains of actin proteins twisted together in a helix Cross Bridge Cycle Muscle contraction involves a series of events called the cross-bridge cycle -Hydrolysis of ATP by myosin, activates the head for the later power stroke -The ADP and Pi remain bound to the head, which binds to actin forming a cross-bridge -During the power stroke, myosin returns to its original shape, releasing ADP and Pi -ATP binds to the head which releases actin 11 Skeletal Muscle Contraction When a muscle is relaxed, its myosin heads cannot bind to actin because the attachment sites are blocked by tropomyosin -In order for muscle to contract, tropomyosin must be removed by troponin -This process is regulated by Ca2+ levels in the muscle fiber sarcoplasm 12 Skeletal Muscle Contraction -In low Ca2+ levels, tropomyosin inhibits crossbridge formation -In high Ca2+ levels, Ca2+ binds to troponin -Tropomyosin is displaced, allowing the formation of actin-myosin cross-bridges 13 Skeletal Muscle Contraction A muscle fiber is stimulated to contract by motor neurons, which secrete acetylcholine at the neuromuscular junction -The membrane becomes depolarized -Depolarization is conducted down the transverse tubules (T tubules) -Stimulate the release of Ca2+ from the sarcoplasmic reticulum (SR) 14 Skeletal Muscle Contraction 15 Skeletal Muscle A motor unit consists of a motor neuron and all of the muscle fibers it innervates -All fibers contract together when the motor neuron produces impulses Muscles that require precise control have smaller motor units Muscles that require less precise control but exert more force, have larger motor units Recruitment is the cumulative increase in motor unit number and size leading to a stronger contraction 16 Skeletal Muscle Contraction A muscle stimulated with a single AP quickly contracts and relaxes in a response called a twitch Summation is a cumulative response when a second twitch “piggybacks” on the first Tetanus occurs when there is no relaxation between twitches -A sustained contraction is produced Muscle Contraction • • • • • Muscle tension: force created by muscle Load: weight that opposes contraction Contraction: creation of tension in muscle Relaxation: release of tension Steps leading up to muscle contraction: – Events at the neuromuscular junction – Excitation-contraction coupling – Contraction-relaxation cycle Vocabulary Tonus: Slight tension constantly present to maintain the readiness of the muscle. Results from alternate periodic activation of a small number of motor units within the muscle by motor centers in the brain and spinal cord. Asynchronous Recruitment: Recruitment of different motor units within the same to carry out the same task. The number may stay the same but the actual motor units might be different. Grading: Changes in the strength of muscle contraction or the extent of contraction in proportion to the load on the muscle. Grading is responsible for the smooth movements like swimming, walking etc.. Electromyography: The detection, amplification, and recording of changes in skin voltage produced by underlying skeletal muscle contraction is electromyography. Motor unit activation causes muscle fibers to generate and conduct electrical impulses resulting in contraction of the muscle fibers. Many fibers simultaneously conducting electrical signals induce voltage differences that can be measured by surface electrodes. Electromyogram : The reading obtained from electromyography. Dynamometry: Measurement of power. The graphic record derived from the use of a dynamometer is known as a dynamogram. Fatigue: Decrease in the muscle’s ability to generate force. Reversible. Electromyography Lab Experimental Objectives : EMG I 1. To observe and record skeletal tonus. 2. To record maximum clench strength for right and left hands 3. To observe, record and correlate motor unit recruitment with increased power of skeletal muscle contraction 4. To listen to EMG “sounds” and correlate sound intensity with motor unit recruitment EMG II 1. To determine the maximum clench strength for right and left hands and compare differences between male and female subjects. 2. To observe and record, and correlate motor unit recruitment with increased power of skeletal muscle contraction. 3. To record the force produced by clench muscles, EMG when inducing fatigue.
