Anatomy and Chemistry Basics
Muscle Tissue
Muscle contraction is described by the sliding-filaments model:
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ATP binds to a myosin head and forms ATP + Pi. When ATP binds to a myosin head, it is converted to ADP and Pi, which remain attached to the myosin head.
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Ca2+ exposes the binding sites on the actin filaments. Ca2+ binds to the troponin molecule causing tropomyosin to expose positions on the actin filament for the attachment of myosin heads.
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Cross bridges between myosin heads and actin filaments form. When attachment sites on the actin are exposed, the myosin heads bind to actin to form cross bridges.
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ADP and Pi are released, and sliding motion of actin results. The attachment of cross bridges between myosin and actin causes the release of ADP and Pi. This, in turn, causes a change in shape of the myosin head, which generates a sliding movement of the actin toward the center of the sacromere. This pulls the two Z discs together, effectively contracting the muscle fiber to produce a power stroke.
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ATP causes the cross bridges to unbind. When a new ATP molecule attaches to the myosin head, the cross bridge between the actin and myosin breaks, returning the myosin head to its unattached position.
Without the addition of a new ATP molecule, the cross bridges remain attached to the actin filaments. This is why corpses become stiff with rigor mortis (new ATP molecules are unavailable).
Stimulation of muscle contraction
Neurons, or nerve cells, are stimulated when the polarity across their plasma membrane changes. The polarity change, called an action potential, travels along the neuron until it reaches the end of the neuron. A gap called a synapse or synaptic cleft separates the neuron from a muscle cell or another neuron. If a neuron stimulates a muscle, then the neuron is a motor neuron, and its specialized synapse is called a neuromuscular junction. Muscle contraction is stimulated through the following steps:
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Action potential generates release of acetylcholine. When an action potential of a neuron reaches the neuromuscular junction, the neuron secretes the neurotransmitter acetylcholine (Ach), which diffuses across the synaptic cleft.
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Action potential is generated on the motor end plate and throughout the T tubules. Receptors on the motor end plate, a highly folded region of the sarcolemma, initiate an action potential. The action potential travels along the sarcolemma throughout the transverse system of tubules.
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Sarcoplasmic reticulum releases Ca2+. As a result of the action potential throughout the transverse system of tubules, the sarcoplasmic reticulum releases Ca2+.
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Myosin cross bridges form. The Ca2+ released by the sarcoplasmic reticulum binds to troponin molecules on the actin helix, prompting tropomyosin molecules to expose binding sites for myosin cross-bridge formation. If ATP is available, muscle contraction begins.
Phases of a muscle contraction
A muscle contraction in response to a single nerve action potential is called a twitch contraction. A myogram, a graph of muscle strength (tension) with time, shows several phases, shown in Figure 1 ):
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The latent period is the time required for the release of Ca2+.
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The contraction period represents the time during actual muscle contraction.
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The relaxation period is the time during which Ca2+ is returned to the sarcoplasmic reticulum by active transport.
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The refractory period is the time immediately following a stimulus during which the muscle fiber will not respond to a second stimulus. For skeletal muscle fibers, this period typically ends during the early part of the contraction period.
Figure 1 The phases of a myogram.
Quality of a muscle contraction
The following factors contribute to the strength and maximum duration of a muscle contraction:
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Frequency of stimuli. If stimuli are repeatedly applied to a muscle fiber, Ca2+ may not be completely transported back into the sarcoplasmic reticulum before the next stimulus occurs. Depending upon the frequency of stimuli, Ca2+ may accumulate. In turn, the extra Ca2+ results in more power strokes and a stronger muscle contraction. Depending upon the frequency of stimuli, several effects are observed.
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A staircase effect (treppe) is produced if each successive stimulus occurs after the relaxation period of the previous stimulus. Each successive muscle contraction is greater than the previous one, up to some maximum value. In addition to the accumulation of Ca2+, other factors, such as increases in temperature and changes in pH, may contribute to this “warming up” effect commonly employed by athletes.
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Wave (temporal) summation occurs if consecutive stimuli are applied during the relaxation period of each preceding muscle contraction. In this case, each subsequent contraction builds upon the previous contraction before its relaxation period ends.
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Incomplete (unfused) tetanus occurs when the frequency of stimuli increases. Successive muscle contractions begin to blend, almost appearing as a single large contraction.
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Complete (fused) tetanus occurs when the frequency of stimuli increases still further. In this case, individual muscle contractions completely fuse to produce one large muscle contraction.
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Strength of stimulus. Muscle contractions intensify when more motor neurons stimulate more muscle fibers. This effect, called recruitment or multiple motor unit summation, is also responsible for fine motor coordination because by continually varying the stimulation of specific muscle fibers, smooth body movements are maintained.
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Length of muscle fiber contraction. Because a muscle is attached to bones, muscle contraction is restricted to lengths that are between 60 percent and 175 percent of the length that produces optimal strength. This range of muscle lengths limits myosin cross bridges and actin only to positions where they overlap and thus can generate contractions.
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Type of contraction. Muscle contraction implies that movement occurs between myosin cross bridges and actin. However, this movement does not necessarily result in shortening of the muscle. As a result, two kinds of muscle contractions are defined.
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Isotonic contractions occur when muscles change length during a contraction. Picking up a book is an example.
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Isometric contractions occur when muscles do not change length during a contraction. When holding a book in midair, muscle fibers produce a force, but no motion is generated.
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Type of muscle fiber. Muscle fibers are classified into two groups.
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Slow fibers contract slowly, have a high endurance, and are red from their rich blood supply. However, they do not produce much strength. These fibers are used for long distance running.
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Fast fibers contract rapidly, fatigue rapidly, and are white because the blood supply is limited. They generate considerable strength. These fibers are used for short distance running.
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Muscle tone. In any relaxed skeletal muscle, a small number of contractions continuously occur. Observed as firmness in a muscle, these contractions maintain body posture and increase muscle readiness.
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Muscle fatigue. Muscle fibers stop contraction when inadequate amounts of ATP are available. Lack of oxygen and glycogen and the accumulation of lactic acid (a byproduct of ATP production in the absence of oxygen), together with the lack of ATP, all contribute to muscle fatigue.
Muscle metabolism
In order for muscles to contract, ATP must be available in the muscle fiber. ATP is available from the following sources:
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ATP from within the cell. ATP available within the muscle fiber can maintain muscle contraction for several seconds.
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ATP from creatine phosphate. Creatine phosphate, a high-energy molecule stored in muscle cells, transfers its high-energy phosphate group to ADP to form ATP. The creatine phosphate in muscle cells is able to generate enough ATP to maintain muscle contraction for about 15 seconds.
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ATP from glucose stored within the cell. Glucose within the cell is stored in the carbohydrate glycogen. Through the metabolic process of glycogenolysis, glycogen is broken down to release glucose. ATP is then generated from glucose by cellular respiration.
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ATP from glucose and fatty acids obtained from the bloodstream. When energy requirements are high, glucose from glycogen stored in the liver and fatty acids from fat stored in adipose cells and the liver are released into the bloodstream. Glucose and fatty acids are then absorbed from the bloodstream by muscle cells. ATP is then generated from these energy-rich molecules by cellular respiration.
Cellular respiration is the process by which ATP is obtained from energy-rich molecules. Several major metabolic pathways are involved, some of which require the presence of oxygen. Here's a summary of the important pathways:
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In glycolysis, glucose is broken down to pyruvic acid, and two ATP molecules are generated. Because no oxygen is used during the various metabolic steps of this pathway, glycolysis is called an anaerobic process.
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In anaerobic respiration, pyruvic acid (from glycolysis) is converted to lactic acid. No ATP is generated and, as its name indicates, no oxygen is required. The importance of this process is that it regenerates certain coenzymes necessary for glycolysis to continue. Thus, in the absence of oxygen, anaerobic respiration is indirectly responsible for the production of two ATPs (during glycolysis).
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Advantages of anaerobic respiration: Anaerobic respiration is relatively rapid, and it does not require oxygen.
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Disadvantages of anaerobic respiration: Anaerobic respiration generates only two ATPs, and lactic acid is produced. Most lactic acid diffuses out of the cell and into the bloodstream and is subsequently absorbed by the liver. Some of the lactic acid remains in the muscle fibers, where it contributes to muscle fatigue. Because both the liver and muscle fibers must convert the lactic acid back to pyruvic acid when oxygen becomes available, anaerobic respiration is said to produce oxygen debt.
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In aerobic respiration, pyruvic acid (from glycolysis) and fatty acids (from the bloodstream) are broken down, producing H2O and CO2 (carbon dioxide) and regenerating the coenzymes for glycolysis. A total of 36 ATP molecules is produced (including the two from glycolysis). However, oxygen is required for this pathway.
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Advantages of aerobic respiration: Aerobic respiration generates a large amount of ATP.
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Disadvantages of aerobic respiration: Aerobic respiration is relatively slow and requires oxygen.
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When the ATP generated from creatine phosphate is depleted, the immediate requirements of contracting muscle fibers force anaerobic respiration to begin. Anaerobic respiration can supply ATP for about 30 seconds. If muscle contraction continues, aerobic respiration, the slower ATP-producing pathway, begins and produces large amounts of ATP as long as oxygen is available. Eventually, oxygen is depleted, and aerobic respiration stops. However, ATP production by anaerobic respiration may still support some further muscle contraction. Ultimately, the accumulation of lactic acid from anaerobic respiration and the depletion of resources (ATP, oxygen, and glycogen) lead to muscle fatigue, and muscle contraction stops.
