Wrapping, Levers, and the Axial Muscles

Claire Brooks

Last episode we lived at the molecular level. Filaments sliding, cross bridges, motor units. Today we zoom back out to the whole muscle, and then to the trunk and head, the axial core. Marcus, you gave the wrapping a quick mention last time, the unwrapping a gift idea.

Marcus Hayes

I did, and then I abandoned it for the sarcomere. So let's go back to the wrapping, because it decides a lot. Each muscle fiber sits in a thin sleeve of loose connective tissue called endomysium. The perimysium is thicker. It wraps about 20 to 60 fibers into a bundle called a fascicle, and you can see fascicles with the naked eye. The perimysium also carries the larger vessels and nerves, plus muscle spindles, which are stretch receptors. Then the epimysium wraps the whole muscle and grades into fascia on its outer surface.

Claire Brooks

So the layers are plumbing and sensors as well as packaging. And the way the fascicles are arranged changes how the muscle behaves.

Marcus Hayes

Right. Five patterns, and each has a signature example. Fusiform muscles are thick in the middle and taper at both ends, like the biceps brachii. Parallel muscles keep a uniform width, like the rectus abdominis. Triangular, or convergent, muscles fan out from a narrow end, like the pectoralis major. Circular muscles are rings, the sphincters, like the orbicularis oculi around the eye. And then pennate muscles.

Claire Brooks

The ones shaped like a feather.

Marcus Hayes

Yes. The fascicles run at an angle into a tendon that runs the length of the muscle. Unipennate, like the palmar interosseous in the hand. Bipennate, like the rectus femoris in the thigh. Multipennate, like the deltoid. The practical point is that angling the fascicles packs more fibers into a given space, so you get more tension from a muscle of the same size. What you give up is how far it can shorten.

Claire Brooks

Now, how do these muscles actually hook onto things? Because most attach to bone at both ends.

Marcus Hayes

Two ways. A direct, or fleshy, attachment has almost no gap between muscle and bone. Just a narrow space spanned by collagen fibers. The margins of the brachialis and the lateral head of the triceps brachii are examples. An indirect attachment uses a tendon, a cord or sometimes a broad sheet called an aponeurosis. The two ends of the biceps brachii work that way, with the thick middle part called the belly. One more thing: you may have learned origin for the stationary end and insertion for the moving end. The slides call those terms inconsistent and sometimes misleading, and they say no single naming system works across the whole body.

Claire Brooks

Which brings in levers. Bone is the beam, the joint is the pivot, the muscle supplies the effort. The slides work through the elbow.

Marcus Hayes

This is my favorite number in the chapter. The effort arm, from the elbow to where the muscle pulls, is about 5 centimeters. The resistance arm, from the elbow to a load in your palm, is 33 centimeters. Divide 5 by 33 and the mechanical advantage is about 0.15. The slide labels that low force, but high speed and distance. So the elbow trades strength for reach. Your hand moves a long way fast, and the muscle works hard to do it.

Claire Brooks

Let's start with the face, because facial muscles break a pattern. Most muscles pull on bones across a joint. Facial muscles attach to soft tissue like the dermis and fascia, which is why they can move skin.

Marcus Hayes

And they do more than expressions. They contribute to speech and chewing too. The occipitofrontalis overlies the dome of the cranium. The orbicularis oculi is the sphincter of the eyelid and closes the eye. The orbicularis oris encircles the mouth. The buccinator is a cheek muscle used in chewing, sucking, and blowing. The platysma is a thin superficial sheet over the upper chest and lower face. It helps open the mouth and tenses the skin of the chin and neck. And a group acts on the mouth, including the zygomaticus major and minor and the risorius.

Claire Brooks

Then there's the group that does the heavy lifting for eating. Temporalis, masseter, and the pterygoid muscles move the mandible during chewing.

Marcus Hayes

And the tongue has both intrinsic and extrinsic muscles, which is how it gets such a range of movement. Around the hyoid bone, the little bone floating in your neck, there are two groups. The suprahyoid group is four muscle pairs above it, and they elevate the hyoid and assist mouth movements. The infrahyoid group is four pairs below it, and they depress it. Then comes the pharyngeal constrictors. Three pairs encircle the pharynx and form a muscular funnel that aids swallowing.

Claire Brooks

So a single bite recruits the face, jaw, tongue, hyoid, and throat. Now the neck. Sternocleidomastoid.

Marcus Hayes

Say it three times fast. It rotates and flexes the neck. The scalenes do lateral flexion. The trapezius is the largest and most superficial neck extensor. The slides also list deeper extensors, the splenius and semispinalis muscles, but they're marked as not needed for the quiz, so I'll leave them there.

Claire Brooks

The skull is a nice lever too. In the slides' first class lever picture, the weight of the face pulls down in front, the pivot sits behind the ear, and muscles at the back of the skull provide the effort.

Claire Brooks

Now the trunk, starting with something you're doing right now. Breathing. The diaphragm is a muscular dome between the chest and abdominal cavities. When it contracts, it flattens, and that causes air intake.

Marcus Hayes

And between the ribs sit three layers of intercostal muscles. The external intercostals are the most superficial, and their fibers run down and forward. The internal intercostals lie deeper, with fibers running the opposite way. The innermost intercostals vary in number and run the same direction as the internals. Opposite fiber directions, layer on layer. And the abdominal wall takes that idea even further.

Claire Brooks

Walk me through it.

Marcus Hayes

Three layers enclose the lumbar region and extend halfway across the front of the abdomen. The external oblique is most superficial, fibers down and forward. The internal oblique is the middle layer, fibers up and forward. The transversus abdominis is deepest, with horizontal fibers. Think plywood, where each sheet is grained in a different direction, so the whole is strong and supportive. That's my analogy, not the slide's. Then the rectus abdominis, a pair of vertical muscles from sternum to pubis, each enclosed in a rectus sheath. The sheaths meet in the middle at the linea alba.

Claire Brooks

Around the back, the latissimus dorsi and trapezius are the big superficial muscles. Underneath, the erector spinae runs along each side of the vertebral column for its whole length, in three columns. From lateral to medial they're iliocostalis, longissimus, and spinalis. Though the slides strike those three names through, so check what your instructor wants.

Marcus Hayes

Same with the pelvic floor, which is marked not on the unit quiz. It's a multilayered muscular sheet that supports the abdominal and pelvic organs, built from the levator ani and ischiococcygeus muscles.

Claire Brooks

Which leads to what happens when a wall has a weak point. A hernia is when viscera protrude through a weak point in the wall of the abdominopelvic cavity.

Marcus Hayes

The most common type needing treatment is the inguinal hernia. The slide's image shows a loop of small intestine pushing through the inguinal canal into the upper scrotum. The canal is a passage in the lower abdominal wall, and the external inguinal ring is its end. So even with all that layered, crossgrained muscle, there are spots where the design has a gap.

Claire Brooks

And that's the whole arc from this episode. Wrapping, fascicle patterns, levers, then the face, the neck, the breathing muscles, and the trunk wall. The slides' study tips are good: feel the muscle on yourself when you can, find its attachments on a skeleton, learn where the name comes from, and say the names out loud.

Marcus Hayes

Names like sternocleidomastoid, which describe exactly where it attaches. Once you can say it, you've got a map.