Skeletal Muscle Contraction as a Whole Human Anatomy & Physiology

Energy Sources for Contraction CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. Energy for contraction comes from molecules of ATP. This chemical is in limited supply and so must often be regenerated Creatine phosphate, which stores excess energy released by the mitochondria, is present to regenerate ATP from ADP and phosphate. Whenever the supply of ATP is sufficient, creatine phosphokinase promotes the synthesis of creatine phosphate. As ATP decomposes, the energy from creatine phosphate can be transferred to ADP molecules, converting them back to ATP.

Oxygen Supply and Cellular Respiration CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. The early phase of cellular respiration yields few molecules of ATP, so muscle has a high requirement for oxygen, which enables the complete breakdown of glucose in the mitochondria. Hemoglobin in red blood cells carries oxygen to muscle. The pigment myoglobin stores oxygen in muscle tissue.

Oxygen Debt CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. During rest or moderate activity, there is enough oxygen to support aerobic respiration. Oxygen deficiency may develop during strenuous exercise, and lactic acid accumulates as an end product of anaerobic respiration. Lactic acid diffuses out of muscle cells and is carried in the bloodstream to the liver. Oxygen debt refers to the amount of oxygen that liver cells require to convert the accumulated lactic acid into glucose, plus the amount that muscle cells need to resynthesize ATP and creatine phosphate to their original concentrations. Repaying an oxygen debt may take several hours. After exercise, the oxygen debt is repaid by deep breathing

Muscle Fatigue CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. When a muscle loses its ability to contract during strenuous exercise, it is referred to as fatigue. Muscle fatigue usually arises from the accumulation of lactic acid in the muscle. A lowered pH as a result of accumulated lactic acid prevents the muscle from contracting.

Muscle Cramp A muscle cramp occurs due to a lack of ATP required to return calcium ions back to the sarcoplasmic reticulum so muscle fibers can relax.

Heat Production CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. Contraction of skeletal muscle represents an important source of heat for the body.

Muscular Responses CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. One method of studying muscle function is to remove a single fiber and connect it to a device that records its responses to electrical stimulation.

Threshold Stimulus A muscle fiber remains unresponsive to stimulation unless the stimulus is of a certain strength, called the threshold stimulus.

All-or-None Response CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. When a muscle fiber contracts, it contracts to its full extent (all-or-none response); it cannot contract partially.

Recording a Muscular Contraction A myogram is the recording of an electrically- stimulated muscle contraction. A single, short contraction involving only a few motor units is referred to as a twitch. The time delay between when the stimulus is applied and when the muscle contracts is called the latent period, which is less than two milliseconds. The latent period is followed by a period of contraction and a period of relaxation.

Summation CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. A muscle fiber receiving a series of stimuli of increasing frequency reaches a point when it is unable to relax completely and the force of individual twitches combine by the process of summation. If the sustained contraction lacks any relaxation, it is called a tetanic contraction. An increase in the number of activated motor units within a muscle at higher intensities of stimulation is called recruitment. Individual muscle cells contract completely when adequately stimulated, but a muscle responds to different degrees depending on HOW many muscle cells are stimulated Summation and recruitment together can produce a sustained contraction of increasing strength.

a. Series of twitches b. Summation c. Tetanic contraction

Muscle Tone Muscle tone is achieved by a continuous state of sustained contraction of motor units within a muscle. It is a result of a staggered series of nerve impulses delivered to different cells within the muscle. Muscle tone keeps muscles healthy and ready to react If the nerve supply is destroyed, the muscle loses tone, becomes paralyzed, and atrophies

Effect of Exercise on Muscles Inactive muscles atrophy. Muscles challenged by resistance exercise to respond beyond their ability increase in size and strength. Muscles subjected to regular aerobic exercise become more efficient and stronger and can work longer without tiring. Aerobic exercise also benefits other body systems.

Two Types of Contractions