EDU2EXP Exercise & Performance 1 The Exercising Muscle Structure, function and control

EDU2EXP Exercise & Performance 2 Types of Muscles Smooth Involuntary muscle; controlled by the autonomic nervous system – Located in the walls of blood vessels and throughout internal organs Cardiac Controlled by the autonomic nervous and endocrine systems – Located only in the heart Skeletal Voluntary muscle; controlled consciously by the somatic nervous system – More than 600 different skeletal muscles located throughout the body

EDU2EXP Exercise & Performance 3 Microscopic Images of Muscle Microscopic photographs of (a) skeletal, (b) cardiac, and (c) smooth muscle a b c

EDU2EXP Exercise & Performance 4 The Basic Structure of Skeletal Muscle

EDU2EXP Exercise & Performance 5 Structure of a Single Muscle Fiber

EDU2EXP Exercise & Performance 6 Structure of the Sarcomere The sliding filament theory is the explanation for how muscles produce force (or, usually, shorten). It explains that the thick and thin filaments within the sarcomere slide past one another, shortening the entire length of the sarcomere. In order to slide past one another, the myosin heads will interact with the actin filaments and, using ATP, bend to pull past the actin.

EDU2EXP Exercise & Performance 7 Neural Control of Exercising Muscle

EDU2EXP Exercise & Performance 8 Neurons Nerve Impulse A nerve impulse—an electrical charge—is the signal that passes from one neuron to the next and finally to an end organ, such as a group of muscle fibers, or back to the CNS.

EDU2EXP Exercise & Performance 9 What Is an Action Potential? An action potential is the first step in the chain of events leading to contraction. In an action potential the electrical potential of the cell rises and falls rapidly Action potentials only occur when the electrical difference (inside to outside the cell) reaches a threshold value Once threshold is met or exceeded, the all-or-none principle applies and an action potential results A neuron that emits an action potential is said to fire

EDU2EXP Exercise & Performance 10 The Synapse A synapse is the site of an impulse transmission from one neuron to another The junction between the axon terminals of a neuron and the receiving cell is called a synapse

EDU2EXP Exercise & Performance 11 The Neuromuscular Junction The site where an alpha-motor neuron communicates with a muscle fiber Nerve impulses (action potentials) travelling down the motor neurons of the nervous system cause the skeletal muscle fibers at which they terminate to contract. The junction between the terminal of a motor neuron and a muscle fiber is called the neuromuscular junction. It is simply one kind of synapse. The terminals of motor axons contain thousands of vesicles filled with acetylcholine (Ach) Ach is released on a membrane on the muscle fibre causing Na+ ions to diffuse in. This creates a change in electrical charge resulting in an action potential being created in the muscle fibre. This leads to muscular contraction.

EDU2EXP Exercise & Performance 12 Alpha Motor Neurons One alpha-motor neuron innervates many muscle fibers, -> collectively called the motor unit The action potential arrives at the dendrites and travels down the axon to the axon terminal

EDU2EXP Exercise & Performance 13 Control of Small vs. Large Motor Responses Muscles controlling fine movements, such as those controlling the eyes, have a small number of muscle fibers per motor neuron (about 1 neuron for every 15 muscle fibers). Muscles with more general function, such as those controlling the calf muscle in the leg, have many fibers per motor neuron (about 1 neuron for every 2,000 muscle fibers). A motor unit is defined as all of the muscle fibers supplied by a single motoneuron, and therefore, by a single axon and its branches. Motor Units Vary in Size !

EDU2EXP Exercise & Performance 14 Muscle Biopsy The muscle biopsy allows us to study muscle fibers and the effects of acute exercise and chronic training on muscle fiber composition A hollow Bergstrom needle is inserted into the muscle belly to take the sample The sample is mounted, frozen, thinly sliced, and examined under a microscope

EDU2EXP Exercise & Performance 15 Muscle Fibre Types Type 1 = Slow twitch –Generates energy aerobically –For endurance exercise Type 2 = fast twitch 2a - some aerobic power / anaerobic 2x - predominantly anaerobic –Generates energy anaerobically –For short intense exercise

EDU2EXP Exercise & Performance 16 Typical Muscle Fibre Composition for Elite Athletes Sport% slow twitch% fast twitch Distance Runners Track Sprinters Non athletes

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EDU2EXP Exercise & Performance 18 Single Muscle Fiber Physiology Peak power is different between muscle fiber types

EDU2EXP Exercise & Performance 19 Types of Muscle Contraction Concentric contraction: Force is developed while the muscle is shortening Isometric contraction: Force is generated but the length of the muscle is unchanged Eccentric contraction: Force is generated while the muscle is lengthening

EDU2EXP Exercise & Performance 20 Muscle Force Generation Key Points 3 types of muscle contraction –Concentric –Isometric –Eccentric Force production is increased by recruitment of more motor units and through increased frequency of stimulation Force production is maximized at the muscle’s optimal length Speed of contraction also affects the amount of force produced