By Elizabeth A. Murray, Mount St. Joseph University
The major motor branch off the cervical plexus is called the phrenic nerve. It is a vital nerve that travels from the cervical plexus down through the thorax, by clinging to the pericardial sac around the heart. Together, the right and left phrenic nerves supply the respiratory diaphragm.
Anatomy of the Diaphragm
The diaphragm is a shelf of skeletal muscle that separates the thoracic cavity from the abdominopelvic cavity and so is a rare example of skeletal muscle that’s inside of and spans our ventral body cavity, rather than being outside of, or in the walls of, a body cavity.
The respiratory diaphragm is also unusual in that it’s a muscle that inserts on itself. It has fibers originating from inside the rib cage, all around the thoracic wall, almost like the spokes of a wheel. Its insertion is a large median area of the muscle known as its central tendon.
Dome-shaped Diaphragm
When the diaphragm is at rest, it is dome shaped, and the muscle fibers are relaxed, but when our brain stimulates the phrenic nerves, and they in turn stimulate the diaphragm, the muscle fibers shorten and the dome shape of the diaphragm goes flat, sort of like tightening a drumhead.
This increases the vertical dimension of the thoracic cavity, and as Boyle’s law states that when you increase volume, you decrease pressure, so air rushes into the lungs, following this high- to low-pressure gradient. How much air? About 500 milliliters in the average person, which is the volume of a standard, disposable—or better yet, recyclable—water bottle.
Rhythmic Muscle
Another unusual aspect of the diaphragm is its rhythmicity—it is skeletal muscle, but it’s programmed for regular contractions and relaxations—whether we’re asleep, or even unconscious. Yet, it is still voluntary skeletal muscle, since we can choose to inhale or exhale at any time.
We don’t have other skeletal muscles that have such rhythmic contractions and relaxations like the diaphragm. This normal, quiet, diaphragmatic breathing is known as our tidal volume, since that 500 milliliters of air comes in and out like the tide, with barely perceptible movements of the chest wall.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
The Intercostal Muscles
But what about when we want to take a deep breath—perhaps to hold our breath and swim across the pool? That kind of breath takes in more than the approximately 500 milliliters of air taken in during a normal, diaphragmatic breath. Deep breaths—whether in or out—require chest wall muscles known as the intercostals.
As the name implies, the intercostal muscles travel between and connect the ribs. There are three main layers to these muscles of the rib cage: the external intercostals, internal intercostals, and the innermost complex of chest wall muscles.
The External Intercostals
The external intercostals have their fibers oriented down. In anatomical terms, we would describe that as from superolateral to inferomedial. The external intercostals can elevate the ribs for forceful inhalation. When this happens, the rib cage expands in a lateral or horizontal direction, further increasing its volume, and thereby decreasing its pressure, so even more air rushes in. This rib motion is often called a bucket-handle action, since the ribs move like lifting the handle on a bucket.
How much more air? It somewhat depends on a person’s size, but it also depends on how they train themselves—whether like Houdini holding his breath for his great escapes, or a swimmer or other athlete who trains their lungs and heart, along with their muscles.
The Internal Intercostals
The internal intercostals have their muscle fibers perpendicular to the external intercostals; in other words, the internal intercostal fibers are oriented in a superomedial to inferolateral direction. Because of this, the internal intercostals can pull the ribs together during exhalation or expiration.
So, during forced exhalation, like to blow out the last few birthday candles on a cake, your ribs are pulled together, decreasing the volume of the chest. When the chest volume decreases, the air inside the lungs is put under increasing pressure, so air is forcefully expelled.
Then, laterally are the innermost intercostals, which have roughly the same fiber direction and function as the internal intercostals.
Breathing and Chest Motions
During normal quiet breathing, when only the diaphragm is used, the chest circumference really does not change; in fact, normal, quiet breathing doesn’t cause any observable surface motion at all. This is because the diaphragm is an internal shelf of muscle. However, if you take in a very deep breath, you can see, feel, and measure your rib cage expanding in a lateral or horizontal direction.
If you want to try this, put a tape measure around your chest at armpit level. Measure first, during relaxed, quiet breathing, and then take in a big, deep breath, hold it, and measure again. External intercostals increase the horizontal dimension of the chest for deep inspiration.
Movement of Neck Muscles
Even some neck muscles, like the sternocleidomastoid and the scalenes, can help when someone takes a really deep breath. Try this yourself, watch in a mirror, and take note of the muscles in your neck. The first 500 mils taken in is from the stimulation and flattening of the diaphragm, then we recruit the external intercostals to elevate the ribs in that bucket-handle action, which expands the horizontal dimensions of the chest.
As inhalation continues, some of the neck muscles can even raise the upper ribs and sternum to bring air into the topmost areas of the lungs. This final action is sometimes called a pump-handle action, because the sternum and upper ribs are elevated like raising the handle on an old water pump. Even the pectoralis minor muscles—that normally move the scapula—can be recruited to help raise the upper ribs.
Common Questions about the Muscles that Let Your Diaphragm Breathe
The diaphragm is a shelf of skeletal muscle that separates the thoracic cavity from the abdominopelvic cavity and so is a rare example of skeletal muscle that’s inside of and spans our ventral body cavity, rather than being outside of, or in the walls of, a body cavity.
There are three main layers to the intercostal muscles of the rib cage: the external intercostals, internal intercostals, and the innermost complex of chest wall muscles.
The major motor branch off the cervical plexus is called the phrenic nerve. It is a vital nerve that travels from the cervical plexus down through the thorax, by clinging to the pericardial sac around the heart.
