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NSCA CPT Chapter 1 - Structure and function of the muscular, nervous, and skeletal systems

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    Chapter Objectives:

    • Discuss the function and the structures of skeletal muscles.
    • List the steps of the sliding filament theory.
    • Explain concepts of fiber type and the application to exercise performance.
    • Discuss the function and structure of the nervous system as it applies to the skeletal muscle control.
    • Describe the role exercise plays in bone health and the function of tendons and ligaments.

    The Muscular System

    Muscles generate force whenever they are activated. We call this muscle contraction or muscle action.

    There are three muscle types: Skeletal, cardiac, and smooth. 

    Skeletal muscles attach to bones and are used to rotate joints. These muscles allow us to do running, jumping and lifting we do every day. 

    Gross Anatomy of Skeletal Muscle

    Each skeletal muscle has three levels of connective tissue.

    Epimysium is the outer layer of connective tissue surrounding the whole muscle.

    A bundle of muscle fibers is known as a fascicle. 

    These fascicles are covered by perimysium: the connective tissue covering for this second layer.

    Endomysium is the last layer of connective tissue. It surrounds the individual muscle fibers. 

    Microscopic Anatomy of Skeletal Muscle

    Each fiber is a cell, like any other cell it has the same structural components. The skeletal muscle cells have more than one nucleus. 

    Myofibrils

    The myofibrils are bundles of myofilaments. He myofilaments are made of myosin and actin. Their arrangement gives muscle its striated appearance.

    Two protein structures are associated with actin. Tropomyosin and Troponin. These are regulatory proteins that take care of regulating the interaction of both actin and myosin.    

    Sarcomere

    The most basic contractile unite of a muscle. 

    Consists of an M line, Z line, H zone, I band, and A band. 

    Neuromuscular junction

    This is the site where the muscle receives stimulus from the nervous system. 

    The motor endplate, the space between the axon terminal and the motor endplate, also known as the synaptic cleft, are the other parts of this neuromuscular junction.

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    Sliding Filament Theory

    This theory states that muscles shorten or lengthen when filaments slide past one another without any change in length in the filaments.

    These are the steps for the sliding filament theory:

    The Resting Phase: The myofibril has a lot of calcium within it, and because of this we don’t have many myosin cross bridges that are bound to the actin.

    The Excitation Contraction Coupling Phase: The SR, or sarcoplasmic reticulum, will release calcium in this stage when it becomes stimulated. These new calcium ions that are released will bond with the troponin. These events cause a shift in tropomyosin, and the myosin cross bridges will form much quicker to the actin in the cell.

    The Contraction Phase: This is the third phase. Here we have the hydrolysis of ATP occurring and this causes the fibers to contract. 

    The Recharge Phase: This fourth phase begins when the calcium is available in the cell. 

    The Relaxation Phase: Here the stimulation stops, and the calcium is now pumped in the SR for use at a later time. Thus, altogether actin and myosin cannot link together. 

    ATP: Adenosine Triphosphate is the energy source of muscular actions. 

    ATPase: Adenosine Triphosphatase causes the splitting of the ATP molecules. 

    Types of Muscle Actions

    Concentric Muscle Action: This occurs when the force produced in the muscle overcomes the external resistance in the opposing direction. In a bicep curl, this would be bringing the weight up.

    Eccentric Muscle Action: This is the lengthening action. When the force produced by the muscle is less than the force opposing it. So, in a bicep curl, this would be the negative portion, or the lowering down of the weight.

    Isometric (static) Muscle Action: This is when the muscle force is equal to the opposing force. These actions lead to no movement. A typical example is a plank, but with the bicep curl example, it would be leaving the weigh at one spot in the movement.

    Delayed-Onset Muscle Soreness (DOMS) and Eccentric Muscle Actions

    This is muscle pain that lasts 24 – 48 hours following an exercise program starting or even some new movement patterns. It is a combination of connective and muscle tissue damage along with an inflammatory action activating the pain receptors.

    Exercise is seen as the best way to reduce these pains.

    Muscle Fiber Types 

    All fibers are not the same in terms of contractile performance and their basic characteristics physiologically. 

    Type I fibers: Slow oxidative or slow twitch fibers. These have high oxidative capacities and are very resistant to fatigue. They also contract and relax more slowly.

    Type IIa fibers: The fast oxidative glycolytic fibers. Large and powerful fibers with moderate to high anaerobic metabolic capabilities. These have moderate oxidative and anaerobic capacities.

    Type IIx fibers: The fast glycolytic fibers. Large and powerful fibers with moderate to high anaerobic metabolic capabilities. These are purely anaerobic and fatigue very quickly. 

    It is possible to transition some fibers.

    The Nervous System:

    In charge of directing and controlling voluntary movement.

    Organization of the Nervous System:

    The nervous system is broken down into the Central nervous system and the Peripheral nervous system. Also, the CNS and PNS.

    The CNS is made up of the brain and the spinal cord.

    The peripheral nervous system is the rest of the nervous system outside of the brain and the spinal cord.

    Neurons:

    The most basic unit of the nervous system is the nerve cell, also called a neuron.

    Motor neurons conduct nerve impulses to the muscles from the central nervous system.

    The synapse is the site of communication between two neurons or a neuron and a muscle or gland cell. This is the neuromuscular junction we talked about earlier, too.

    The nerve cell is made of a cell body, dendrites, axon, and myelin sheath.

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