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Specialist Certification Program The Endurance Events Myophysiology & Neurophysiology for the Endurance Events ATP and Energy Energy Basics. All living organisms share the characteristic of requiring a constant supply of usable energy in order to perform the functions needed for life. To perform their vital functions, cells must use energy obtained by breaking down organic substances. To be useful, that energy must be transferred from molecule to molecule or from one part of a cell to another. Energy Transfer. The useful method of energy transfer involves the creation of high-energy bonds by enzymes within cells. A high-energy bond is a covalent bond whose breakdown releases energy the cell can harvest. In a human muscle cell, a high-energy bond generally connects a phosphate group (PO43-) to an organic molecule. The resulting complex is called a high-energy compound. Most high-energy compounds are derived from nucleotides, the building blocks of nucleic acids in the cell. Phosphorylation. The attachment of a phosphate group to another molecule is called phosphorylation. This process on its own does not necessarily produce high-energy bonds. The creation of a high energy compound requires a phosphate group, enzymes capable of catalyzing the reactions involved, and suitable organic substrates to which the phosphate can be added. Phosphorylation of AMP. The most important such substrate is the nucleotide adenosine monophosphate (AMP). Attaching a second phosphate group produces adenosine diphosphate (ADP). A significant energy input is required to convert AMP to ADP, and the second phosphate is attached by a high-energy bond. Even more energy is required to add a third phosphate and thereby create the high-energy compound adenosine triphosphate (ATP). An ATP Molecule. Note the 3 Phosphates Located on the Terminal End Energy Transfer. The conversion of ADP to ATP is the most important method of energy storage in human cells. The reversion of ATP to ADP is the most important method of energy release. The relationships can be shown as follows. ADP + Phosphate Group + Energy ATP + H2O Enzymes. The conversion of ATP to ADP requires an enzyme known as adenosine triphosphatase, or ATPase. Throughout life, cells continuously generate ATP from ADP and use the energy provided by the ATP to perform vital functions, such as the synthesis of proteins or the contraction of muscles. Storage Capabilities. The human muscle cell is capable of storing very little ATP in its cytoplasm. The conversion of organic molecules (carbohydrates, lipids, proteins) in the cell to ATP from ADP is so rapid that there is little evolutionary need for storage. The amount of ATP in the human muscle cell could be depleted in one to two seconds of activity unless recharged to pre-existing levels. ATP supplies must be kept at peak concentrations and must not fall below 60% of resting levels for muscular activity to continue. Energy Pathways of the Cell. The three systems of metabolic pathways are available to replace ATP concentrations in the cell. All three pathways are continually active in the cell, and as a demand for energy increases, all three pathways will contribute to the reformation of ATP molecules in varying degrees. The degree of involvement of each is primarily determined by the rate of energy demand. These systems are as follows. o The Anaerobic Alactate (Creatine Phosphagen) Energy System o The Anaerobic Lactate (Glycolytic) Energy System o The Aerobic Energy System Movement Basics The Nervous System. The nervous system initiates movement by stimulating skeletal muscle tissue, beginning the process of muscle contraction. The nervous system is then responsible for initiating contraction. The effectiveness of the nervous system dictates much of an athlete’s speed and power capabilities and certain characteristics of the muscle. Skeletal Muscles. Skeletal muscles produce movement by contracting. These muscles are attached to bones through connective tissues called tendons. The muscular contractions then move the bones, producing movement. The muscular system is then responsible for producing locomotive forces. The Skeletal System. The skeletal system is composed primarily of bones and connective tissues called ligaments, which hold them together. In addition to supplying structure and framework to the body, the skeletal system accepts the forces produced by the muscles and transmits them appropriately. The skeletal system is then responsible for force transmission. Neurophysiology The Nervous System. The nervous system is one of the body’s two control systems. The nervous system recruits and activates muscle tissue to meet force production needs. The nervous system’s effectiveness at recruiting muscle tissue is critical, and the characteristics of the nervous system dictate to some degree the characteristics of skeletal muscle tissue. Nervous System Anatomy. The nervous system is composed of a central nervous system (CNS) and a peripheral nervous system. The CNS consists of the brain and the spinal cord. The peripheral nervous system consists of the branch nerves and the neuromuscular junctions. These branch nerves can be afferent or efferent. Efferent nerves effect action, while afferent nerves are sensory in nature. The Neuron. A neuron is a nerve cell. Its purpose is to conduct a neural impulse to muscle tissue, or to another neuron. The nerve cell is composed of a soma (cell body), and axon, and a dendrite. The axon conducts the neural impulse toward the cell body, while the dendrite conducts the impulse away from the cell body. Motor Neurons. Motor neurons are neurons that, when activated, stimulate skeletal muscle tissue and produce muscle contraction. The Neural Impulse. The neural impulse is basically composed of electrical pulses. If a neuron receives these pulses, and the pulses are of sufficient magnitude, the affected neuron is stimulated to conduct the impulse. As the impulse travels quickly along the axon, a wave of depolarization occurs causing voltage changes. The All or None Principle. The all or none principle states that there are no varying degrees of conduction, and neurons operate on an on or off basis. If the neural signal received is sufficient, the neuron will conduct the received impulse. If a motor neuron is sufficiently stimulated, all muscle fibers affected by that motor neuron will be stimulated into contraction. For these reasons control of the force of muscular contraction is not possible at the cellular level. Neurotransmitters. There are about 50 known neurotransmitters in humans. Acetylcholine and norepinephrine are the two major neurotransmitters in regulating our physiological response to exercise. Acetylcholine is the primary excitatory neurotransmitter for the neurons that innervate skeletal muscle and for most parasympathetic neurons. Control of Force. As indicated in our discussion of the all or none principle, the body cannot control force production by altering the magnitude of signal from the nervous system. Force must then be controlled in these other ways. o Recruitment. More motor units are called into play as force production needs rise. Each unit has a unique threshold of response, so those with lower thresholds are recruited first. o Rate Coding. Rate coding refers to the ability of the nervous system to increase the frequency of the neural impulses. If these impulses occur sufficiently close together, then the next impulse may begin before the previous subsides completely, allowing an aggregate increase in the magnitude of the impulse. Tetanus results when these individual impulses merge into one sustained impulse. o Temporal Patterning and Synchronization. Temporal Patterning and synchronization refer to the ability of the body to call motor units into play in a particular sequence with a particular timing. This unique pattern of timing of recruitment results in a greater total force production. Myophysiology Muscle Anatomy o The Muscle Fiber. A single muscle cell is called a muscle fiber. These cells are unique and resemble no other cell type. These fibers are individually capable of contraction. Inside the muscle cell is a specialized cytoplasm called the sarcoplasm that fills most of the volume of the cell. o Muscle Bundle Anatomy. Muscle fibers are packaged into bundles by several epithelial coverings, linings, and connective tissues. These provide structure and hold the muscle together. In order for oxygen, electrolytes, and nutrients carried in the blood vessels to reach the sarcoplasm, all of these membranes have to be crossed. Delivery efficiency can be improved through training. Endomysium. The endomysium is the portion of epithelial tissue that covers each muscle fiber or cell. Sarcolemma. The muscle cell membrane or sarcolemma is just inside and attached to the endomysium. Epimysium. Large numbers of muscle fibers are held together by an epithelial tissue called the perimysium, and the epimysium tissue encases the entire muscle crating the muscle bundle. Striated Skeletal Muscle Fiber Anatomy (Martini 2006) o Muscle Fiber Anatomy Myofibrils. Striated skeletal muscle is composed of protein strands called myofibrils. These myofibrils contain multiple units that are individually capable of contraction called sarcomeres. Sarcomeres. The sarcomere is the smallest anatomical unit of muscle tissue that is individually capable of displaying contractile characteristics. The sarcomere consists of noncontractile proteins (proteins not involved in the contraction process) that provide structure to the muscle tissue, and contractile proteins (proteins involved in the contraction process), as well as other parts essential to the contraction process. Thick Filaments. Thick filaments are composed of the protein myosin, and are found in the middle of the sarcomere and lie between the thin filaments. Each thick fiber possesses many crossbridges, which extend toward the thin filaments. There is a head at the end of each crossbridge, capable of carrying an ATP molecule. Thin Filaments. The thin filaments are composed of the proteins actin, troponin, and tropomyosin. They are attached to Z disks, which are noncontractile protein ends of the sarcomere, and extend toward the middle of the sarcomere. The Sarcoplasmic Recticulum. The sarcoplasmic reticulum is a net-like system of tubules and vesicles (T-system) surrounding each myofibril that serve as the pathway for integrated neural impulses. The longitudinal tubules run parallel to the myofibrils and terminate at the end of vesicles, while the perpendicular transverse tubules interconnect forming the net. The outer vesicles the reticulum contain large amounts of calcium ions (Ca ++). When the neural impulse travels over the Tsystem and between the outer vesicles, calcium is released. Striations. The overlapping of the thick and thin filaments give muscle fibers a striated appearance. These striations correspond to zones where only thin filaments are present, only thick filaments are present, or both are present in overlapping arrangements. Zones within a Human Skeletal Muscle Sarcomere (Martini 2006) Actin and Myosin Filaments The Sliding Filament Theory. The Sliding Filament Theory of muscular contraction was proposed by Huxley and was based on his analysis of the photograph below. He proposed that the overall length of the muscle decreases as one filament (actin) slides over the filament (myosin). Huxley’s Original Micrograph and Hypothesis o ATP. The role and importance of ATP as an energy carrier now becomes apparent in understanding the process of muscle contraction. In order for contraction to occur in a muscle, ATP must be present and readily replaced if the contraction - relaxation contraction chain of events is going to continue. o Stages of the Sliding Filament Theory Rest. ATP molecules present in the muscle cell bind to the end of the myosin cross-bridge. In the absence of free calcium, the troponin of the actin filaments inhibits the myosin cross-bridge from binding with actin. Excitation-Coupling. When an impulse from a motor nerve reaches the motor end-plate, the neurotransmitter acetylcholine is released, causing depolarization and triggering the release of calcium from the vesicles of the sarcoplasmic reticulum. Calcium is immediately bound by the troponin molecules on the actin filaments. This results in the turning on of active sites on the actin filament triggering conformational changes of both troponin and tropomyosin. Contraction. For contraction to occur, the crossbridge head must be loaded with an ATP molecule. The turning on of active sites exposes the myosin binding site that contains the enzyme myosin ATPase. This enzyme causes ATP to be reduced to ADP and a loose inorganic phosphate molecule. Since any chemical reduction releases energy, some energy will become available for each ATP reduced. This released energy allows the cross-bridge to swivel, creating movement. Recharging. The first stage of recharging is the breaking of the old bond between the actin and myosin crossbridge. Once a new ATP is reloaded, the bond between the myosin crossbridge and the active site on the actin filament is broken and the ATP cross-bridge is freed from the actin. Relaxation. When the flow of nervous impulses over the motor nerve innervating the skeletal muscle ceases, calcium is released from troponin and returned to storage in the outer vesicles of the sarcoplasmic reticulum. Fiber Types o Contraction Speeds. The time required to accomplish these stages determined the contraction speed (twitch) of a fiber. Some muscle fibers twitch faster than others, and some people have a greater ratio of faster to slower twitching muscle fibers than other people do. Enzyme availability is very specific to the different fibers, and ultimately is the reason for most of a fibers speed characteristics. Slow Twitch Fibers. Slower twitch fibers (ST) are called slow twitch, oxidative, type 1, or red, depending upon the taxonomy scheme used. The fibers show higher myoglobin content resulting in a more efficient usage of the cell’s limited oxygen supply. This reflects a greater dependence on the aerobic system (oxidative metabolism). Fast Twitch Fibers. Faster twitch fibers are classified into hundreds of types. For simplicity, physiologists classify these fast twitch varieties as FTA or FOG (oxidative-glycolytic), FTB (glycolytic). Fast twitch fibers contain more protein contractile filaments which can produce higher forces than slow twitch fibers. This larger size is accompanied by the development of a more extensive sarcoplasmic reticulum, which facilitates a much greater release of calcium and is responsible for a more robust contraction. o Functional Differences in Fiber Types Motor Units o Motor Unit Anatomy. A single motor neuron and the muscle fibers it acts upon is called a motor unit. A motor neuron can innervate from 5 to 150 or more muscle fibers. A high fiber to nerve ratio is associated with gross movements requiring considerable force, whereas a low fiber to nerve ratio exists when very precise, low force demands, are required of muscle. Motor Units (Eisenberg and Lee 2004) o Motor Unit Function. Motor units may function in these two ways. Volitional Function. In this type of operation, cognitive activity originates the transmission of impulses to the motor neuron, activating it and producing contraction in its pool of fibers. Movement is consciously originated. The Reflex Arc. In this type of operation, a signal generated by some sensory organ or proprioceptor is transmitted to the motor neuron, activating it and producing contraction in its pool of fibers. The brain is left out of the loop, and movement is not consciously originated. This type of movement is used in all reflex actions. Neuromuscular Adaptations to Training Improved Recruitment. Neuromuscular training produces strength gains resulting from heightened and improved recruitment of muscle fibers by the motorneurons. Improved Rate Coding. Neuromuscular training produces strength gains resulting from, improved rate coding capabilities. Improved Synchronization. Neuromuscular training produces strength gains resulting from improved synchronization of contraction. Improved Speeds of Contraction. Although these gains are limited, neuromuscular training may result in increased speeds of contraction due to shifts in myosin isoforms and enzymes. Other Strength Gains. Strength gains also result in response to the tension placed on muscle tissue during training. These increased tension levels are present in concentric activity, and are even greater in eccentric work. Improved Speed and Coordination. Neuromuscular training results in increased speed characteristics, improved coordination, and associated increases in running economy and efficiency. Causes of Neuromuscular Fatigue Fatigue at the Neuromuscular Junction. A limitation may occur at the neuromuscular junction, preventing nerve impulse transmission to the muscle fiber membrane. A number of fatiguing factors may be responsible. They include: o Acetycholine. A reduction in the release capability of acetylcholine may occur. o Cholinesterase. Hyperactivity in the enzyme cholinesterase which will inhibit acetylcholine function. Hypoactivity in cholinesterase which will cause acetylcholine to accumulate, paralyzing the fiber. o Threshold Changes. the muscle fiber membrane may develop a higher threshold o Receptor Inavailability. Other substances compete with acetylcholine for receptor sites. Acidity and Contractile Mechanism Fatigue. Inability of muscle filaments to contract properly may result because of an acidic condition caused by accumulation of H+ ions. The hydrogen ions concentration hinders the amount of calcium released from the sarcoplasmic reticulum and interferes with the calcium troponin binding capacity. Hydrogen out competes calcium for biding spaces. www.ustfccca.org