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So from my understanding, if I maintain the contraction of my muscle, the myosin head will be bonded to the actin. Why is ATP needed to maintain this bond?
Actually, it is not. ATP releases the myosin bonded to the actin, so when it is absent, the muscle remains contracted. This is, actually, precisely what happens during rigor mortis after death.
However, as long as you are alive, on a molecular level this is a dynamic process - actin release and bonding - and maintaining it requires energy. Moreover, muscle relaxation is not achieved by stopping the ATP flow, but incurrs additional ATP cost - it happens by actively removing calcium ions required to enable the actin to bind the myosin.
Why does maintaining contraction of muscles require ATP? - Biology
Populations affected by large disasters or traumatic events like wars or earthquakes are often fertile ground for unique medical discoveries. During World War II, Nazi Germany bombed London for 57 consecutive days during the beginning of what came to be known as the Blitzkrieg, or eight-month Lightning War. Victims of the Blitz, as it is known in London, included those afflicted with a specific set of symptoms: pain and swelling with accompanying effects of depleted blood volume (shock, weakness, low blood pressure, and decreased urine output). Less obvious was acute kidney failure, which could lead quickly to death if left untreated.
What caused the Blitz victims to suffer from these symptoms? Extreme physical trauma to muscles&mdashnamely, compression&mdashdestroys skeletal muscle tissue. This condition is called rhabdomyolysis (rhabdo– refers to striation, myo– to muscle, and –lysis to breakdown).
The products of skeletal muscle dissolution, some of which are toxic, circulate in the blood until they are filtered out. Creatine kinase is one of these products in fact, rhabdomyolysis is diagnosed with a creatine kinase level five times the normal upper limit. Myoglobin is another. Much like hemoglobin, myoglobin uses heme to carry oxygen. It is not, however, housed within a red blood cell. Thus, an erythrocyte-free urine sample that tests positive for heme points compellingly toward rhabdomyolysis. Myoglobin oxygen reserves are just one of the specialized features of muscles, as we will see in this chapter.
Skeletal muscles are only able to exert an effect on the body by moving bony structures around joints. Bones are more than simply a support structure, however they also provide protection to internal organs, serve as a storage reserve of calcium and other minerals, and are the site of hematopoiesis. Further, skeletal muscle isn’t the only form of muscle in the body smooth muscle plays roles in the cardiovascular, respiratory, reproductive, and digestive systems, and cardiac muscle comprises the contractile tissue of the heart. In this chapter, we’ll explore the biology of all of these tissues, completing our tour of systems anatomy and physiology!
11.1 The Muscular System
The muscular system is composed of not only skeletal muscle, but also smooth muscle and cardiac muscle. Skeletal muscle is essential for supporting the body and allowing for movement. The contraction of skeletal muscle also compresses venous structures and helps propel blood through the low-pressure venous system toward the heart, as well as lymph through the lymphatic system. Rapid muscle contraction also leads to shivering, which is important in thermoregulation. Smooth muscle is responsible for involuntary movement, such as the rhythmic contractions of smooth muscle in the digestive system called peristalsis. Smooth muscle also aids in the regulation of blood pressure by constricting and relaxing the vasculature. Cardiac muscle is a special type of muscle that is able to maintain rhythmic contraction of the heart without nervous system input. In this section, we will discuss each type of muscle as well as the physiology of muscles.
Muscle can be divided into the three different subtypes: skeletal muscle, smooth muscle, and cardiac muscle. Each muscle type performs specific functions, although they share some similarities. All muscle is capable of contraction, which relies on calcium ions. All muscle is innervated, although&mdashas we will see&mdashthe part of the nervous system that innervates the muscle and the ability of the muscle to contract without nervous input varies from type to type.
Skeletal muscle is responsible for voluntary movement and is therefore innervated by the somatic nervous system. Due to the arrangement of actin and myosin into repeating units called sarcomeres, it appears striped or striated when viewed microscopically. Skeletal muscle is multinucleated because it is formed as individual muscle cells fuse in long rods during development.
There are multiple different types of fibers within skeletal muscle. Red fibers, also known as slow-twitch fibers, have high myoglobin content and primarily derive their energy aerobically. Myoglobin is an oxygen carrier that uses iron in a heme group to bind oxygen, imparting a red color. Red fibers also contain many mitochondria to carry out oxidative phosphorylation. White fibers, also known as fast-twitch fibers, contain much less myoglobin. Because there is less myoglobin, and therefore less iron, the color is lighter. These two types of fibers can be mixed in muscles. Muscles that contract slowly, but that can sustain activity (such as the muscles that support posture), contain a predominance of red fibers. Muscles that contract rapidly, but fatigue quickly, contain mostly white fibers.
Poultry provides a great example of the difference between red and white fibers. Most muscles of support, such as the thigh, are considered dark meat and contain a high concentration of red fibers. Active muscles, like the pectoral muscles (breast meat), are considered white meat and have a high concentration of white fibers.
Smooth muscle is responsible for involuntary action. Thus, smooth muscle is controlled by the autonomic nervous system. It is found in the respiratory tree, digestive tract, bladder, uterus, blood vessel walls, and many other locations. Smooth muscle cells have a single nucleus located in the center of the cell. Just like skeletal muscle, smooth muscle cells contain actin and myosin, but the fibers are not as well-organized, so striations cannot be seen. Compared to skeletal muscle, smooth muscle is capable of more sustained contractions a constant state of low-level contraction, as may be seen in the blood vessels, is called tonus. Smooth muscle can actually contract without nervous system input in what is known as myogenic activity. In this case, the muscle cells contract directly in response to stretch or other stimuli.
The MCAT loves to test the fact that both smooth and cardiac muscle exhibit myogenic activity. These muscle cells will respond to nervous input, but do not require external signals to undergo contraction.
Cardiac muscle has characteristics of both smooth and skeletal muscle types. Cardiac muscle is primarily uninucleated, but cells may contain two nuclei. Like smooth muscle, cardiac muscle is involuntary and innervated by the autonomic nervous system. However, like skeletal muscle, cardiac muscle appears striated.
One of the unique characteristics of cardiac muscle is how each cardiac myocyte communicates. Cardiac muscle cells are connected by intercalated discs, which contain many gap junctions. These gap junctions are connections between the cytoplasm of adjacent cells, allowing for the flow of ions directly between cells. This allows for coordinated muscle cell depolarization and efficient contraction of cardiac muscle.
Cardiac muscle cells are able to define and maintain their own rhythm through myogenic activity. Starting at the sinoatrial (SA) node, depolarization spreads using conduction pathways to the atrioventricular (AV) node. From there, the depolarization spreads to the bundle of His and its branches, and then to the Purkinje fibers. The gap junctions allow for progressive depolarization to spread via ion flow across the gap junctions between cells. The nervous and endocrine systems also play a role in the regulation of cardiac muscle contraction. The vagus nerve provides parasympathetic outflow to the heart and slows the heart rate. Norepinephrine from sympathetic neurons or epinephrine from the adrenal medulla binds to adrenergic receptors in the heart, causing an increased heart rate and greater contractility. One of the ways epinephrine does this is by increasing intracellular calcium levels within cardiac myocytes. Ultimately, cardiac contraction&mdashlike that of all types of muscle&mdashrelies on calcium.
The main characteristics of each muscle type are summarized in Table 11.1.
Sliding Filament Theory
The most widely accepted theory explaining how muscle fibers contract is called the sliding filament theory. According to this theory, myosin filaments use energy from ATP to &ldquowalk&rdquo along the actin filaments with their cross bridges. This pulls the actin filaments closer together. The movement of the actin filaments also pulls the Z lines closer together, thus shortening the sarcomere.
When all of the sarcomeres in a muscle fiber shorten, the fiber contracts. A muscle fiber either contracts fully or it doesn&rsquot contract at all. The number of fibers that contract determines the strength of the muscular force. When more fibers contract at the same time, the force is greater.
Muscles and Nerves
Muscles cannot contract on their own. They need a stimulus from a nerve cell to &ldquotell&rdquo them to contract. Let&rsquos say you decide to raise your hand in class. Your brain sends electrical messages to nerve cells, called motor neurons, in your arm and shoulder. The motor neurons, in turn, stimulate muscle fibers in your arm and shoulder to contract, causing your arm to rise. Involuntary contractions of cardiac and smooth muscles are also controlled by nerves.
Answers, BIO 2310, Muscle Tissue
1. Capable of contraction and relaxation. It functions to produce movement, maintain posture, support, guard exits/entrances (e.g. sphincter), and maintain body temperature.
2. Skeletal muscle is attached to skeleton, is striated, voluntary and causes body movement. Cardiac muscle is heart muscle, is striated with intercalated discs, is involuntary and causes heart pumping. Smooth muscle is found in the wall of tubular viscera and is not striated, is involuntary and causes mixing & movement called peristalsis.
3. Connective tissue around groups of muscles or filling spaces if fascia. Epimysium is connective tissue around a single muscle, perimysium is connective tissue around fascicles, fascicles are bundles of muscle cells, a tendon is connective tissue cord attaching muscle to (periosteum of) bone, aponeurosis is a broad sheet-like tendon.
4. Skeletal muscle must have nerve supply to function and has an excellent blood supply.
5. Sarcolemma is muscle cell membrane, myofiber is muscle cell, myofibril is the striated cylinders in the muscle cell, myofilaments are the contractile proteins. A band is the dark colored region, I band is light. Z lines separate the myofibril into sarcomeres which are comprised of thin myofilaments attached to the Z lines called actin and the thick myosin myofilaments. The sarcomere is the functional unit of muscle contraction because it squeezes together during contraction from the myosin pulling on the actin. The sarcoplasmic reticulum with its expanded regions called terminal cisternae are the muscle cell’s version of an endoplasmic reticulum. It functions to store calcium ions. Tropomyosin is a thin ribbon-like protein that wraps around actin and blocks myosin from attaching its head to the actin. It prevents contraction. Troponin is a small protein that acts like the glue holding the tropomyosin in place. Troponin has a binding site for calcium. Transverse tubules are inward extensions of the sarcolemma into the interior of the cell.
6. Stimulation of the muscle cell’s sarcolemma travels into the cell through the T-tubules causing calcium release from the sarcoplasmic reticulum. The calcium binds to troponin causing it to release the tropomyosin which can then move out of the way. Now, the myosin head can form a cross bridge binding to actin. The myosin head is energized with the binding of ATP and swivels toward the center of the sarcomere causing the power stroke. This causes the sarcomere to squeeze together. ATP is also needed for the actin & myosin to release from each other so that relaxation can occur. ATP is also needed for putting the calcium back into the sarcoplasmic reticulum because it is active transport.
7. All skeletal muscle cells need a motor neuron (movement nerve cell) to provide stimulation for contraction. There is a gap between the distal end of the neuron and the muscle cell and this is the neuromuscular junction. A chemical called acetylcholine is released from the neuron to bridge the gap and take the stimulation to the muscle cell. The motor neuron plus how ever many muscle cells it supplies is the motor unit. It may be one neuron and one muscle cells for the motor unit in areas where your movement is precise (e.g. eye movement) or one neuron for 500 muscle cells where your movement is not precise (e.g. lower back muscles).
8. It is really the ‘on-off’ switch. It binds troponin causing the physical blocker, the tropomyosin, to move out of the way.
9. A little bit of ATP is present in this state in the muscle cell. More can be quickly manufactured by converting creatine phosphate to ATP. Rapidly, but inefficiently, you can make ATP from anaerobic metabolism. As long as oxygen supply is sufficient, you can very efficiently make a lot of ATP from aerobic metabolism, a slow process.
10. Oxygen debt is to restore the ATP aerobically and to remove lactic acid (end-product from anaerobic metabolism) from muscle cells. Glycogen debt is to restore glucose stores and the best way to restore these is to eat carbohydrates.
11. Lack of ATP. Lactic acid also contributes to the soreness of these muscles.
12. Latent period, Contraction period, Relaxation period
13. All stimuli strong enough to cause a muscle twitch will cause identical muscle twitches. However, the all-or-none principle applies to the muscle CELL only, not the entire muscle.
14. For a small contraction of your biceps muscle, some (say 10%) of the muscle cells will do their “all.” For a bigger contraction of your biceps muscle (say 60%) more muscle cells contract maximally. For a maximal contraction of the whole biceps muscle, all of the muscle cells will be contracting maximally.
15. Multiple motor unit summation = spatial summation and occurs when many muscle cells or motor units contract at the same time making a bigger whole muscle contraction (as is described for number 14). Temporal summation = wave summation and is when muscle cells contract repeatedly and rapidly, so that the next contraction is occurring before the previous one has totally relaxed. Examples of temporal summation include incomplete tetanus (repeated contraction due to repeated stimuli with a little bit of contraction between each stimulus) and complete tetanus (sustained contraction with no relaxation). Treppe is the bigger muscle twitch that is achieved upon warming up for exercise. Asynchronous motor unit summation is when not all muscle cells are working at the same time so that some can rest while others are contracting. This allows posture muscles to be contracted all day without tiring, because the motor units take turns. Muscle tone is when some of the motor units are contracting making the muscle firm, but not enough are contracting to result in movement.
16. Isometric contractions occur when you pick up something that is too heavy. While your muscle is working and creating tension, it is not shortening. Isotonic contractions result in shortening, as in bending your elbow.
17. Slow fibers are fatigue resistant and are red. They have excellent blood supply and myoglobin for oxygen storage (think of dark meat of chicken). Therefore they are geared toward aerobic metabolism and while this is not fast these fibers do not run out of ATP and do not fatigue (think of the chicken walking around all day long). Fast fibers are fatiguable and are white. They do not have great blood supply and do not have myoglobin. They are geared toward anaerobic metabolism. They can make the ATP very quickly (think of the breast meat of chicken and the chicken flying quickly to a tree when being chased) but will run out of it soon and cannot endure (the chicken cannot fly long distances, but the goose has dark meat for the breast, why?). Intermediate are more fatigue-resistant fast fibers. You can get these through endurance training, but the fast and slow fibers are genetically determined.
Non-invasive techniques for the ASSESSMENT OF sites of muscle fatigue
Muscle fatigue is manifested most naturally in the intact organism. Non-invasive techniques of site-specific stimulation can now be used to evaluate the potential sites of the entire system for force production in human studies. All evoked muscle responses are recorded via electromyography (EMG) electrodes placed on the muscle.
Transcranial magnetic stimulation
Transcranial magnetic stimulation involves applying magnetic stimulation to the motor cortex and is optimized to activate the muscle of interest. 1 The stimulation-induced muscular response recorded by EMG is known as the motor-evoked potential (MEP). MEP is influenced not only by cortical excitability but also by spinal cord motor neuron excitability and muscle factors. MEP depression can occur in the relaxed muscle after a fatiguing exercise, possibly as a result of afferent input from the fatigued muscle. MEP is increased in the upper- and lower-limb muscles during sustained submaximal isometric contractions and is regarded as an augmentation of the central drive to the lower motoneuron pool that allows a constant level of force to be maintained despite the development of peripheral fatigue. During sustained MVC, MEP has been reported to increase during the first seconds and then to level off, increase linearly or remain stable, depending on the protocol used (that is, continuous vs intermittent) and the muscle investigated. 1
Cervicomedullary region electrical stimulation
Electrical stimulation in the cervicomedullary region aims to activate the corticospinal tract at a subcortical level, thereby eliminating cortical contributions to the evoked muscle response. The muscular response recorded by EMG is known as the cervicomedullary motor-evoked potential (CMEP). Comparison of MEP and CMEP is helpful for the localization of excitability at the cortical or subcortical level. During a sustained 30% MVC of the plantar flexors, a large increase in MEP and only a slight increase in CMEP have been reported, thus suggesting a small contribution of spinal factors to the increase in corticospinal excitability during submaximal fatiguing contractions. In contrast, during 50% MVC of the elbow flexors to task failure, similar MEP and CMEP kinetics has been found, thus indicating that central changes occur almost entirely at the spinal level. 62, 63, 64
Peripheral nerve low-intensity electrical stimulation
Low-intensity electrical stimulation of the peripheral nerve preferentially activates the Ia sensory fibers, which synapse with the α-motoneuron in the spinal cord. The signal is then carried along the motor neurons to the muscle, generating a response in the muscle known as the Hoffmann reflex (H-reflex). The H-reflex is used to assess spinal excitability and inhibition. Although there are several of an increase 65 or no change, 66 the general consensus is that there is an overall decline in the amplitude of the H-reflex with the development of muscle fatigue, thus indicating a decrease in spinal excitability. 67, 68 The rate and degree of decrease in H-reflex amplitude appear to be dependent on the type of fatiguing task.
Peripheral nerve high-intensity electrical stimulation
High-intensity stimulation of the peripheral nerve directly activates the α-motoneuron, evoking a motor response (m-wave) from the muscle. The m-wave is a compound action potential recorded with surface EMG and is used to assess peripheral excitability of the muscle membrane and transmission at the neuromuscular junction. A change in the twitch force without a change in the m-wave indicates a failure of excitation-contraction coupling.
Short-duration fatiguing contractions (
20 s) induce an enhancement in the amplitude and area of the m-wave. 69 A longer (4-min) sustained maximal contraction does not induce changes in the amplitude of the m-wave 70 but results in a significant decline in the central activation, thus suggesting that central factors contributing to fatigue can occur in the absence of a peripheral change in membrane excitability. However, more longer-duration contractions that induce fatigue (
17 min) can also induce a decline in the muscle membrane excitability and m-wave size. 69
Slow Twitch (Type 1)
Slow-twitch fibers are designed for endurance activities that require long-term, repeated contractions, like maintaining posture or running a long distance. The ATP required for slow-twitch fiber contraction is generated through aerobic respiration (glycolysis and Krebs cycle), whereby 30 molecules of ATP are produced from each glucose molecule in the presence of oxygen. The reaction is slower than anaerobic respiration and thus not suited to rapid movements, but much more efficient, which is why slow-twitch muscles do not tire quickly. However, this reaction requires the delivery of large amounts of oxygen to the muscle, which can rapidly become rate-limiting if the respiratory and circulatory systems cannot keep up.
Due to their large oxygen requirements, slow-twitch fibers are associated with large numbers of blood vessels, mitochondria, and high concentrations of myoglobin, an oxygen-binding protein found in the blood that gives muscles their reddish color. One muscle with many slow-twitch fibers is the soleus muscle in the leg (
80% slow-twitch), which plays a key role in standing.
Peripheral Nerve Disorders
Roy Freeman , Mark W. Chapleau , in Handbook of Clinical Neurology , 2013
Sustained isometric muscle contraction causes a reflex rise in blood pressure and HR ( Coote et al., 1971 ). Stimuli from exercising muscle, conveyed by lightly myelinated mechanosensitive group III and unmyelinated chemosensitive group IV muscle afferents, and the central nervous system (central command) are responsible for the generation of the increase in efferent sympathetic activity ( Mark et al., 1985 Gandevia and Hobbs, 1990 Winchester et al., 2000 ). The blood pressure and HR changes induced by a sustained handgrip have been used as a clinical test of sympathetic function ( Ewing et al., 1974 Mark et al., 1985 ).
The subject is instructed to grasp a dynamometer and sustain a fixed, isometric contraction for 3 minutes at 30% of maximum effort. The response to this test is subject to marked variability due in part to difficulty standardizing muscular effort. In addition, there is evidence that muscle afferent activity is reduced during exercise of the trained muscles and that this results in attenuation of the muscle SNA and the associated blood pressure increase normally seen during exercise ( Mark et al., 1985 ). Furthermore, muscle chemoreceptor afferent activity may be reduced because of a decrease in metabolite accumulation ( Sinoway et al., 1996 ) or reduced sensitivity of muscle afferents to accumulated metabolites in the trained muscle ( Mostoufi-Moab et al., 1998 ). While this is a valuable research tool, the sensitivity and specificity of this test are low.
How Long Does Rigor Mortis Last?
Rigor mortis can be used to help estimate the time of death. Muscles function normally immediately after death. The onset of rigor mortis may range from 10 minutes to several hours, depending on factors including temperature (rapid cooling of a body can inhibit rigor mortis, but it occurs upon thawing). Under normal conditions, the process sets in within four hours. Facial muscles and other small muscles are affected before larger muscles. Maximum stiffness is reached around 12-24 hours post mortem. Facial muscles are affected first, with the rigor then spreading to other parts of the body. The joints are stiff for 1-3 days, but after this time general tissue decay and leaking of lysosomal intracellular digestive enzymes will cause the muscles to relax. It is interesting to note that meat is generally considered to be more tender if it is eaten after rigor mortis has passed.
ATP: What Is It And Why Is It Important?
For your muscles—in fact, for every cell in your body—the source of energy that keeps everything going is called ATP. Adenosine triphosphate (ATP) is the biochemical way to store and use energy.
The entire reaction that turns ATP into energy is a bit complicated, but here is a good summary:
- Chemically, ATP is an adenine nucleotide bound to three phosphates.
- There is a lot of energy stored in the bond between the second and third phosphate groups that can be used to fuel chemical reactions.
- When a cell needs energy, it breaks this bond to form adenosine diphosphate (ADP) and a free phosphate molecule.
- In some instances, the second phosphate group can also be broken to form adenosine monophosphate (AMP).
- When the cell has excess energy, it stores this energy by forming ATP from ADP and phosphate.
- ATP is required for the biochemical reactions involved in any muscle contraction. As the work of the muscle increases, more and more ATP gets consumed and must be replaced in order for the muscle to keep moving.
Because ATP is so important, the body has several different systems to create ATP. These systems work together in phases. The interesting thing is that different forms of exercise use different systems, so a sprinter is getting ATP in a completely different way from a marathon runner!
ATP comes from three different biochemical systems in the muscle, in this order:
Now, let's look at each one in detail.
A muscle cell has some amount of ATP floating around that it can use immediately, but not very much—only enough to last for about three seconds. To replenish the ATP levels quickly, muscle cells contain a high-energy phosphate compound called creatine phosphate.
The phosphate group is removed from creatine phosphate by an enzyme called creatine kinase, and is transferred to ADP to form ATP.
The cell turns ATP into ADP, and the phosphagen rapidly turns the ADP back into ATP. As the muscle continues to work, the creatine phosphate levels begin to decrease. Together, the ATP levels and creatine phosphate levels are called the phosphagen system. The phosphagen system can supply the energy needs of working muscle at a high rate, but only for 8 to 10 seconds.
Glycogen Lactic Acid System
Muscles also have big reserves of a complex carbohydrates called glycogen. Glycogen is a chain of glucose molecules. A cell splits glycogen into glucose. Then the cell uses anaerobic metabolism (anaerobic means "without oxygen") to make ATP and a byproduct called lactic acid from the glucose.
About 12 chemical reactions take place to make ATP under this process, so it supplies ATP at a slower rate than the phosphagen system. The system can still act rapidly and produce enough ATP to last about 90 seconds. This system does not need oxygen, which is handy because it takes the heart and lungs some time to get their act together. It is also handy because the rapidly contracting muscle squeezes off its own blood vessels, depriving itself of oxygen-rich blood.
There is a definite limit to anerobic respiration because of the lactic acid. The acid is what makes your muscles hurt. Lactic acid builds up in the muscle tissue and causes the fatigue and soreness you feel in your exercising muscles.
By two minutes of exercise, the body responds to supply working muscles with oxygen. When oxygen is present, glucose can be completely broken down into carbon dioxide and water in a process called aerobic respiration.
The glucose can come from three different places:
- Remaining glycogen supplies in the muscles
- Breakdown of the liver's glycogen into glucose, which gets to working muscle through the bloodstream
- Absorption of glucose from food in the intestine, which gets to working muscle through the bloodstream
Aerobic respiration can also use fatty acids from fat reserves in muscle and the body to produce ATP. In extreme cases (like starvation), proteins can also be broken down into amino acids and used to make ATP. Aerobic respiration would use carbohydrates first, then fats and finally proteins, if necessary.
Aerobic respiration takes even more chemical reactions to produce ATP than either of the above systems. Aerobic respiration produces ATP at the slowest rate of the three systems, but it can continue to supply ATP for several hours or longer, so long as the fuel supply lasts.
So imagine that you start running. Here's what happens:
- The muscle cells burn off the ATP they have floating around in about 3 seconds.
- The phosphagen system kicks in and supplies energy for 8 to 10 seconds. This would be the major energy system used by the muscles of a 100-meter sprinter or weight lifter, where rapid acceleration, short-duration exercise occurs.
- If exercise continues longer, then the glycogen-lactic acid system kicks in. This would be true for short-distance exercises such as a 200- or 400-meter dash or 100-meter swim.
- Finally, if exercise continues, then aerobic respiration takes over. This would occur in endurance events such as an 800-meter dash, marathon run, rowing, cross-country skiing and distance skating.
When you start to look closely at how the human body works, it is truly an amazing machine!
- The external intercostal muscles contract. This moves the ribcage up and out.
- The diaphragm contracts. As it does so it moves down and becomes relatively flat.
- Both of these muscle contractions result in an increase in the volume of the thorax which in turn results in a drop in pressure inside the thorax.
- Pressure eventually drops below atmospheric pressure.
- Air then flow into the lungs from outside the body, through the mouth or nose, trachea, bronchi and bronchioles.
- Air continues to enter the lungs until the pressure inside the lungs rises to the atmospheric pressure.
- The internal intercostal muscles contract. This moves the ribcage down and in.
- The abdominal muscles contract. This pushes the diaphragm up, back into a dome shape.
- Both of these muscle contractions result in a decrease in the volume of the thorax.
- As a result of the decrease in volume, the pressure inside the thorax increases.
- Eventually the pressure rises above atmospheric pressure.
- Air then flows out of the lungs to outside of the body through the nose or mouth.
- Air continues to flow out of the lungs until the pressure in the lungs has fallen back to atmospheric pressure.