Muscle tissues always have work to do. They pull things up and push things down and around. They move things inside you and outside you. And they move you, too, of course. They work together with all the other systems in your body, but more than any other organ system, the muscular system specializes in movement: of food and air into and out of your body; of the circulation of your blood within your body; of different parts of your body relative to one another, as when you change position; and of your body through space — what you normally think of as “movement.”
All muscle tissue is strong. Most is enduring, some of it astoundingly so. Its cells are crowded with mitochondria, thousands of little factories constantly turning out molecules of ATP, a refined fuel. The muscle cells use the fuel to manufacture strong and flexible proteins, with which they build and repair themselves and do work.
All the work that muscle tissues do is done by the coordinated contraction and release of millions of sarcomeres, tiny structures within the muscle cells. Muscle activity accounts for most of the body’s energy consumption.
In this chapter, we give you an overview of some of the things muscles do and how they do it, and then we name the muscles. At the end of the chapter, we list some common muscle ailments, one of which you’ve probably experienced.
Functions of the Muscular System
This section focuses mainly on the functions of the skeletal muscles, the muscles that move your bones. The skeletal muscles comprise a substantial portion of your body mass, and most of what you eat goes to fuel their metabolism. In this section, you’ll find out something about what they do with all that energy.
Supporting your structure
Muscles are attached to bones on the inside of your body and skin on the outside, with various types of connective tissue between the layers. Thus, they hold your body together. Along with your skin and your skeleton, your muscles shield your internal organs from injury from impact or penetration.
Moreover, and don’t take this personally, but your body is heavy. As it does with everything else, gravity pulls your weight downward (toward the planet’s center). But gravity doesn’t only pull on the soles of your feet — it pulls on all your weight. If gravity had its way, you’d be lying on the floor right now. Your muscles pull your weight up (“oppose” the pull of gravity) and hold you upright. Gravity is a relentless cosmic force, and eventually, gravity will win. But while you’re still fighting, your muscles need fuel and rest.
Moving you
Contracting and releasing a muscle moves the bone it’s attached to relative to the rest of the body. The movement of the bone, in turn, moves all the tissue attached to it through space, as when you raise your arm. Certain combinations of these types of movements move the entire body through space, as when you walk, run, swim, skate, or dance.
Muscle contraction is responsible for little movements, too, like blinking your eyes, dilating your pupils, and smiling.
Poised positioning
A very close interaction outside of your conscious control between some muscle cells and the nervous system keeps you not just upright, but in balance. Nerve impulses throughout the muscular system cause muscles to contract or relax to oppose gravity in a more subtle way when, say, you’re shifting your weight from one side to the other as you step. This interaction is called muscle tone, and it’s what is enabling you to hold your head up right now.
When you step down a steep incline in rough terrain, your muscle tone brings your abs and your back muscles into action in a different way than when you step across your living room rug. The mechanisms of muscle tone may move your arms up and away from your body to counterbalance the pull of gravity with an accuracy and precision you could never calculate cognitively. Below your conscious level, the mechanisms of muscle tone are active every minute of every day, even when you’re asleep.
Muscle tone relies on muscle spindles — specialized muscle cells that are wrapped with nerve fibers. (See the “Skeletal muscle” section later in this chapter for information about spindle fibers.) The central nervous system stays in contact with the muscles through the muscle spindles. (Turn to Chapter 7 for the structures of the central nervous system.) Spindles send messages about your body position through the spinal cord to the brain; to initiate the fine adjustments, the brain sends signals through the spinal cord and nerves to the muscle spindles about which muscles to contract and which to release.
Maintaining body temperature
Muscle contributes to homeostasis (see Chapter 2) by generating heat to balance the loss of heat from the body surface. Muscle contraction uses energy from the breakdown of ATP and generates heat as a byproduct. Shivering is a series of muscle contractions that generate extra heat to keep your temperature up in cold situations. If the heat generated by the muscles raises body temperature too high, other thermoregulatory processes, such as sweating, are activated.
Pushing things around inside
The other two types of muscle tissues, smooth muscle and cardiac muscle, have their own important functions, discussed in other chapters. Here, we give you an overview of some of the muscles that keep things moving within your body, all without any thought from you.
Cardiac muscle
The cardiac muscle that makes up the walls of the heart contracts rhythmically, pumping the blood into the arteries. It hits with so much force that the artery walls briefly stretch — detectable as your pulse. However, there must be enough force from the blood pushing on the walls by the time it reaches the capillaries to push out the plasma. This is why blood pressure is so important. Smooth muscle in the walls of the arteries help control this by contracting to constrict the vessel or relaxing to dilate it. Damage to this layer is one cause of arteriosclerosis (hardening of the arteries), which takes away the arteries’ subtle control of blood pressure. See Chapter 9 for more about the heart and blood vessels.
Diaphragm
The diaphragm is a skeletal muscle whose contraction and release forces air in and out of the lungs. Chapter 10 covers the respiratory system in all its breathtaking glory.
Digestive smooth muscle
The organs of the digestive tract have walls containing smooth muscle that contracts in pulsating waves, pushing ingested material along. Think of this muscular lining as a conveyor belt on a disassembly line. Refer to Chapter 11 for details.
Controlling release
Sphincter muscles are essentially valves: rings of smooth muscle that are fully contracted in their resting state, holding some material in one place, and then relaxing only briefly to allow the material to move through. You find sphincters at various places in the digestive system (refer to Chapter 11), from the very beginning to the very end, and in other parts of the body as well.
Most sphincters aren’t under conscious control. Two of them — the urinary sphincter, which holds urine in the bladder, and the anal sphincter, which holds feces in the colon — come under conscious control usually at around 2 years of age. This control allows the release of these bodily wastes under culturally appropriate circumstances. Its acquisition is considered a milestone in infant development.
By the way, human males have much stronger urinary sphincter muscles than do females, meaning that they can retain about twice as much urine in the bladder (up to 800mL or 1.69 pints) for twice as long. Kindly keep that in mind, fellas, when you’re on a road trip with girls.
Talking about Tissue Types
A “muscle tissue type” is not the same as “a muscle.” Your left bicep is a muscle; in all, you have hundreds of named muscles. (Refer to the “Naming the Skeletal Muscles” section later in the chapter and to the “Muscular System” color plate in the center of the book.) There are only three muscle tissue types: skeletal muscle tissue, cardiac muscle tissue, and smooth muscle tissue.
Defining unique features of muscle cells
Your muscle tissue is made up of cells that are different from the other cells of your body. These cells are so unique that they’re even different from each other, based on the type of muscle tissue they belong to. The three muscle types are distinguishable anatomically by their characteristic cells and structures and physiologically as voluntary or involuntary.
Muscle cells feature these characteristics:
Single or multiple nuclei: Cardiac muscle cells and smooth muscle cells have one nucleus apiece, like most other cells. Skeletal muscle cells (referred to as fibers) are multinucleate, meaning numerous nuclei are found within one cell membrane. Skeletal muscle cells don’t grow extra nuclei; during the development of skeletal muscle tissue, numerous skeletal muscle cells merge into one large cell, and most of the nuclei are retained within one continuous cell membrane, along with most of the mitochondria.
Striation: Skeletal muscle is striated, meaning that, under a microscope, alternating light and dark bands are visible in the fiber (muscle cell). Striation is the result of the structures inside the skeletal muscle cells (see the “Skeletal Muscle” section later in the chapter) that carry out the mechanism of contraction called the sliding filament model. (See the “Getting a Grip on the Sliding Filament” section later in the chapter). Cardiac muscle cells are striated as well, and they also contract by a variation of the sliding filament model. Smooth muscle cells are not striated in appearance but do follow a version of the sliding filament model.
See Figure 6-1 to get an idea of how muscle cells and tissues are similar and different.
Muscle cells can also be categorized by the type of contraction they perform. Smooth and cardiac muscle cells are involuntary, meaning their contraction is initiated and controlled by parts of the nervous system that are far from the conscious level of the brain (the autonomic nervous system). You have no practical way to consciously control, or even become aware of, the smooth muscle contractions in your stomach that are grinding up this morning’s muffin. The involuntary contractions that cause your heartbeat aren’t even under the nervous system’s control, as we discuss in Chapter 9.
Skeletal muscle is classified as voluntary because you make a decision at the conscious level to move the muscle. At least you do sometimes — you decide to reach for a doorknob and turn it, for example, and your muscles carry out the command from your brain to do so.
But note this: If the doorknob is charged with static electricity, your arm pulls your hand away before you’re even consciously aware of being zapped. This somatic reflex arc is still classified as voluntary movement, however, because it involves skeletal muscle, controlled by the somatic (voluntary) nervous system.
Not everything in anatomy and physiology makes sense at first. Just remember that skeletal muscle is classified as voluntary.
Table 6-1 sums up the characteristics and classifications of muscle cells.
Skeletal muscle tissue is, essentially, bundles of fibers bundled together. Like fibrous material of every kind, skeletal muscle tissue gets its strength from assembling individual fibers together into strands, and then bundling and rebundling the strands. Two properties make this particular fibrous material very special: The strands are made of protein, and they renew and repair themselves constantly.
At the cellular level
Individual muscle cells, which physiologists call fibers, are slender cylinders that sometimes run the entire length of a muscle. Each fiber (cell) has many nuclei located along its length and close to the cell membrane, which is called the sarcolemma in skeletal muscle fibers. Outside the sarcolemma is a lining called the endomysium, a type of connective tissue, which houses capillaries and nerves.
Muscle spindles are specialized skeletal muscle fibers that are wrapped with nerve fibers. Figure 6-2 shows how skeletal muscle is connected to the nervous system. Spindles are distributed throughout the muscle tissue and provide sensory information to the central nervous system. Motor neurons transmit impulses to trigger a muscle fiber to contract. Each fiber must be stimulated individually by a neuron at its motor end plate. However, a single motor neuron can stimulate numerous fibers, forming a motor unit. Large motor units (one neuron, numerous fibers) allow for gross motor skills like walking and lifting. Small motor units (one neuron, few fibers) provide fine motor skills like grasping and handwriting.
Bundled within the muscle fibers are myofibrils (refer to Figure 6-2). The myofibrils are composed of sarcomeres, which are distinct units arranged linearly (end to end) along the length of the myofibril. A sarcomere is the functional unit of muscle contraction. (Refer to the “Getting a Grip on the Sliding Filament Model” section later in the chapter for more on muscle contraction within sarcomeres.)
At the tissue level
Muscle fibers are bound together into bundles called fascicles. Each fascicle is bound by a connective-tissue lining called a perimysium. Spindle fibers are distributed throughout each fascicle. The fascicles are then bound together to form a muscle, a discrete assembly of skeletal muscle tissue, like the biceps brachii (your biceps), with a connective-tissue wrapper called an epimysium holding the whole package together.
Tendons — ropy extensions of the connective tissue covering the skeletal bones — weave into the epimysium, holding the muscle firmly to the bone (see Chapter 5 for more on connectivity in the skeletal system). Muscles connect to other muscles using aponeuroses, a connective tissue similar to a tendon but broad and flat.
How many ways can you say “fiber”? Anatomists need them all when they’re talking about the muscular system. Make sure you’re thinking at the right level of organization (subcellular, cellular, or tissue) when you see these terms: filament, myofibril, fiber, and fascicle.
Working together: Synergists and antagonists
Groups of skeletal muscles that contract simultaneously to move a body part are said to be synergistic. The muscle that does most of the moving is the prime mover. The muscles that help the prime mover achieve a certain body movement are synergists. When you move your elbow joint, the bicep is the prime mover and the brachioradialis stabilizes the joint, thus aiding the motion.
Antagonistic muscles also act together to move a body part, but one group contracts while the other releases, a kind of push-pull. One example is flexing your arm. When you bend your forearm up toward your shoulder, your biceps muscle contracts, performing a concentric contraction. In the meantime, the triceps muscle in the back of your arm relaxes, performing an eccentric contraction. The actions of the biceps and triceps muscles are opposite, but you need both actions to allow you to flex your arm, which is why they are both, confusingly, referred to as contractions. Antagonistic actions lower your arm, too: The biceps relaxes, and the triceps contracts.
Cardiac muscle
The heart has its own very special type of muscle tissue, called cardiac muscle. The cells (fibers) in cardiac muscle contain one nucleus (they’re uninucleated) and are cylindrical; they may be branched in shape. Unlike skeletal muscle, where the fibers lie alongside one another, cardiac muscle fibers interlock, which promotes the rapid transmission of the contraction impulse throughout the heart. Cardiac muscle cells are striated, like skeletal muscle cells, and cardiac muscle contraction is involuntary, like smooth muscle contraction. Cardiac muscle fibers contract in a way very similar to skeletal muscle fibers, by a sliding filament mechanism (more on that in a minute).
Cardiac muscle tissue is on the job, day and night, from before birth to the moment of death. The cardiac muscle cells contract regularly and simultaneously hundreds of millions of times throughout your lifetime. When cardiac muscle tissue gives up, the game is over.
Unlike skeletal muscle and smooth muscle, contraction of the heart muscle is autonomous, which means it occurs without stimulation by a nerve. In between contractions, the fibers relax completely (see Chapter 9).
Smooth muscle
Smooth muscle tissue is found in the walls of organs and structures of many organ systems, including the digestive system, the urinary system, the respiratory system, the cardiovascular system, and the reproductive system. Smooth muscle tissue is fundamentally different from skeletal muscle tissue and cardiac muscle tissue in terms of cell structure and physiological function. However, smooth muscle sarcomeres are similar.
Smooth muscle fibers (cells) are fusiform (thick in the middle and tapered at the ends) and arranged to form sheets of tissue. Smooth muscle cells aren’t striated. However, smooth muscle contractions utilize the same sliding filament mechanism as skeletal muscle cells (see the next section for more).
Smooth muscle contraction is typically slow, strong, and enduring. Smooth muscle can hold a contraction longer than skeletal muscle. In fact, some smooth muscles, notably the sphincters, are in a constant state of contraction. Childbirth is among the few occasions in life when humans (some humans, anyway) consciously experience smooth muscle contraction (although they don’t consciously control it).
Getting a Grip on the Sliding Filament
A muscle contracts when all the sarcomeres in all the myofibrils in all the fibers (cells) contract all together. The sliding filament model describes the fine points of how this happens.
The key to the sliding filament model is the distinctive shapes of the protein molecules myosin and actin, and their partial overlap in the sarcomere. The special chemistry of ATP supplies the energy for the filaments’ movement. The following sections explain how sarcomeres create muscle contraction.
The sarcomere is the functional unit within the myofibril. Sarcomeres line up end to end along the myofibril.
Assembling a sarcomere
The sarcomere is composed of thick filaments and thin filaments. The thick filaments are molecules of the protein myosin, which is dense and rubbery. The thin filaments are made up of two strands of the lighter (less dense) protein actin, which is springy. The thin and thick filaments line up together in an orderly way to form a sarcomere. One end of a thin filament touches and adjoins the end of another thin filament forming the Z line that runs perpendicular to the filament axis. The sarcomere begins at one Z line and ends at the next Z line. The thick filaments line up precisely between the thin filaments. Sarcomeres and Z lines are shown in Figures 6-2 and 6-3.
Illustration by Kathryn Born, MA
FIGURE 6-3: Sarcomere structure and shortening: a) sarcomere before contraction; b) close-up view of a power stroke; c) contracted sarcomere showing Z lines closer together.
The two types of filaments overlap only partially when the sarcomere is at rest. The partial overlap gives skeletal and cardiac muscle cells their striations: where thick and thin filaments overlap, the tissue appears dark (dark band); where only thin filaments are present, the tissue appears lighter (light band).
The myosin filaments have numerous club-shaped heads that point away from its center (toward the two Z lines). These heads rest nearly touching the myosin binding sites on the actin. Why, then, do they not link up? Wrapping around the actin is the troponin-tropomyosin complex. When a muscle is at rest, this protein covers up the binding sites. Until the myosin heads can link up with the actin, the fiber cannot contract. (See Figure 6-3a for a sarcomere at rest.)
Telling the fiber to contract
Motor neurons provide the stimulus for a muscle fiber to contract. From the end of its axon, a motor neuron releases acetylcholine (ACh), which binds to the muscle cell in a designated area called the motor end plate. This triggers the fiber to generate an impulse, which spreads through the sarcolemma (for more on nervous communication and impulse, see Chapter 7). The impulse is spread deep into the cell by tunnels called T-tubules. When the impulse reaches a sarcoplasmic reticulum (SR), it is triggered to release the calcium ions stored there. The ions move to the sarcomeres, allowing contraction to begin. Triggering relaxation is a passive process. The neuron simply stops releasing ACh. Muscle impulse stops and the calcium ions return to the SR.
Contracting and releasing the sarcomere
After calcium is released from the SR, it attaches to the troponin. In doing so, it is pulled away from the actin — like pulling a push pin out of a bulletin board. Because troponin is still tightly bound to the tropomyosin, the whole protein complex slides around the actin, exposing the binding sites. Now the myosin heads can link up with the actin forming cross bridges, all without the use of energy. However, to actually cause a contraction, we must shorten the sarcomere.
When the cross bridges form, the myosin heads immediately bend, pulling the ends of the sarcomere (Z lines) closer together. ATP will then bind to the myosin, causing it to let go of the actin. The energy in ATP causes the heads to cock back into their original position. They will then form a new cross bridge except it is now further down the actin (similar to how a ratchet wrench works). These power strokes continue — grab, pull in, let go, cock back — as long as the binding sites are uncovered and ATP is present. Refer to Figure 6-3 to get a picture of this process.
The sarcomere shortens as the filaments slide past each other. Because the myofibrils are made of sarcomeres that share Z lines (refer to Figure 6-2), and this shortening is occurring in all of them, the two ends of the myofibril are pulled noticeably closer together. Because a muscle fiber is packed full of myofibrils, it is shortened as its ends are pulled together, too. A skeletal muscle is made of numerous fibers bundled in parallel to each other so its ends are pulled together as well. The muscle contracts, pulling whatever it’s attached to closer together, all with the force generated by the overlapping of these microscopic filaments (but multiplied thousands of times over).
The filaments (actin and myosin) do not shrink. They maintain their length at all times. The myosin filaments also do not move. Actin is pulled toward the center, progressively increasing the amount of overlap with the myosin (refer to Figure 6-3c).
In order to relax, the fiber simply stops being told to contract (the motor neuron halts its stimulation). This causes the SR to call back the calcium ions like a mother calling her kids back home for dinner. They happily oblige, letting go of the troponin, which will pin back into the actin. This slides the tropomyosin over the top of the binding sites again, knocking off any of the myosin heads that were attached (breaking the cross bridges). The actin will then slowly slide back restoring the length of the sarcomere to its original position.
Naming the Skeletal Muscles
Get ready, because we’re about to tell you the muscle names from head to toe — literally. Check out the “Muscular System” color plate in the center of the book as you go through this section.
To name muscles, anatomists had to come up with a set of rules to follow so the names would make sense. They chose to focus on certain characteristics to derive each muscle’s Latin name. As you go through the following sections, refer to Chapter 1 if necessary for the names of the body regions. Examples of characteristics in muscle names are given in Table 6-2.
The largest muscle in the buttocks is the gluteus maximus (maximus means large in Latin); a smaller muscle in the buttocks is the gluteus minimus (minimus means small in Latin).
Muscle location
The frontalis muscle lies on top of the skull’s frontal bone.
Muscle shape
The deltoid muscle, shaped like a triangle, comes from delta — the Greek alphabet’s fourth letter, which is also shaped like a triangle.
Muscle action
The extensor digitorum is a muscle that extends the fingers or digits.
Number of muscle attachments
The biceps brachii attaches to bone in two locations, whereas the triceps brachii attaches to bone in three locations.
Muscle fiber direction
The rectus abdominis muscle runs vertically along your abdomen (rectus means straight in Latin).
Starting at the top
Your head contains muscles that perform three basic functions: chewing, making facial expressions, and moving your neck. Ear wiggling falls into this category, too.
To chew, you use the muscles of mastication (a big, fancy word that means “chewing”). The masseter, a muscle that runs from the zygomatic bone (your cheekbone) to the mandible (your lower jaw), is the prime mover for mastication, so its name is based on its action (masseter, mastication). The fan-shaped temporalis muscle works with the masseter to allow you to close your jaw. It lies on top of the skull’s temporal bone, so its name is based on its location. Figure 6-4 shows the muscles of the head and neck.
To smile, frown, or make a funny face, you use several muscles. The frontalis muscle along with a tiny muscle called the corrugator supercilii raises your eyebrows and gives you a worried or angry look when wrinkling your brow. (Think of the appearance of corrugated cardboard, and then feel the skin between your eyebrows when you wrinkle your brow.) The orbicularis oculi muscle surrounds the eye (the word orbit, as in orbicularis, means “to encircle”; oculi refers to the eye). This muscle allows you to blink your eyes and close your eyelids, but it also gives you those little crow’s feet at the corners of your eyes. The orbicularis oris surrounds the mouth. (Or refers to mouth, as in “oral.”) You use this muscle to pucker up for a kiss. Figure 6-5 shows the facial muscles.
If you play the trumpet or another instrument that requires you to blow out, you’re well aware of what your buccinator muscle does. This muscle is in your cheek. (Bucc means “cheek,” as in the word buccal, which refers to the cheek area.) It allows you to whistle and also helps keep food in contact with your teeth as you chew. Remember that your zygomatic is your cheekbone? Well, the zygomaticus muscle is a branched muscle that runs from your cheekbone to the corners of your mouth. This muscle pulls your mouth up into a smile when the mood strikes you.
When you want to nod yes, no, or tilt your head into a maybe so, your neck muscles come into play. You have two sternocleidomastoid muscles, one on each side of your neck. We know this is a long name, but the name reflects the locations of its attachments: the sternum, clavicle, and mastoid process of the skull’s temporal bone. When both sternocleidomastoid muscles contract, you can bring your head down toward your chest and flex your neck. When you turn your head to the side, one sternocleidomastoid muscle contracts — the one on the opposite side of the direction your head is turned. So if you turn your head to the left, your right sternocleidomastoid muscle contracts, and vice versa. If you lean your head back to look up at the sky or to shrug your shoulders, your trapezius muscle allows you to do so.
The trapezius is an antagonist to the sternocleidomastoid muscle. If you remember basic geometry, you can see that the trapezius is shaped like a trapezoid. It runs from the base of your skull to your thoracic vertebrae and connects to your scapula (shoulder blades). Therefore, the trapezius and sternocleidomastoid muscles connect your head to your torso and provide a nice segue to the next section. The trapezius is shown in Figure 6-7.
Illustration by Kathryn Born, MA
FIGURE 6-6: Anterior muscles of the chest and abdomen.
Illustration by Kathryn Born, MA
FIGURE 6-7: Posterior view of the neck and torso muscles.
Twisting the torso
The torso muscles have important functions. They not only give support to your body but also connect to your limbs to allow movement, allow you to inhale and exhale, and protect your internal organs. In this section, we cover the muscles that run along the front of you (called your anterior or ventral side) and then cover the muscles of your back (your posterior or dorsal side).
In your chest (see Figure 6-6), your pectoralis major muscles connect your torso at the sternum and collarbones (clavicles) to your upper limbs at the humerus bone in the upper arm. Your “pecs” also help to protect your ribs, heart, and lungs. You can feel your pectoralis major muscle working when you move your arm across your chest. Also in your chest are the muscles between and around the ribs. The internal intercostal muscles help to raise and lower your rib cage as you breathe. However, the torso’s largest muscles are the abdominal muscles.
The abdominal muscles really form the center of your body. If the abdominal muscles are weak, the back is weak because the abdominal muscles help to flex the vertebral column. So if the vertebral column doesn’t flex easily, the muscles attached to it can become strained and weak. And the muscles of the abdomen and back join to the upper and lower limbs. Therefore, if the abdomen and back are weak, the limbs can have problems.
The muscles of the abdomen are thin, but the fact that these muscle fibers run in different directions increases their strength. This woven effect makes the tissues much stronger than they would be if they all went in the same direction. Think about how a child connects building blocks. Laying a top layer of blocks perpendicular to the blocks underneath helps the structure stay together, which is similar to how the abdominal muscle tissues provide strength and stability.
The “six-pack” muscle of the abdomen, the rectus abdominis, forms the front layer of the abdominal muscles, and it runs from the pubic bone up to the ribs and sternum. The function of the rectus abdominis muscle is to hold in the organs of the abdomino-pelvic cavity and allow the vertebral column to flex.
Other layers of abdominal muscles also help to hold in your organs on the side of your abdomen and provide strength to your body’s core. The external oblique muscles attach to the eight lower ribs and run downward toward the middle of your body (slanting toward the pelvis). The internal oblique muscles lie underneath the external oblique (makes sense, eh?) at right angles to the external oblique muscles. The internal oblique muscles extend from the top of the hip at the iliac crest to the lower ribs.
Together, the external and internal oblique muscles form an X, essentially strapping together the abdomen. The abdomen’s deepest muscle, the transversus abdominis, runs horizontally across the abdomen; its function is to tighten the abdominal wall, push the diaphragm upward to help with breathing, and help the body bend forward. The transversus abdominis is connected to the lower ribs and lumbar vertebrae and wraps around to the pubic crest and linea alba. The linea alba (“white line”) is a band of connective tissue that runs vertically down the front of the abdomen from the xiphoid process at the bottom of the sternum to the pubic symphysis (the strip of connective tissue that joins the hip bones).
The muscles in your back (refer to Figure 6-7) serve to provide strength, join your torso to your upper and lower limbs, and protect organs that lie toward the back of your trunk (such as your kidneys). The deltoid muscle joins the shoulder to the collarbone, scapula, and humerus. This muscle is shaped like a triangle (think of the Greek letter delta: Δ). The deltoid muscle helps you raise your arm up to the side (that is, laterally). The latissimus dorsi muscle is a wide muscle that’s also shaped like a triangle. It originates at the lower part of the spine (thoracic and lumbar vertebrae) and runs upward on a slant to the humerus. Your “lats” allow you to move your arm down if you have it raised, and also to reach, such as when you’re climbing or swimming.
Spreading your wings
Your upper limbs have a wide range of motions. Obviously, your upper limbs are connected to your torso. One of the muscles that provides that connection, the serratus anterior, is below your armpit (the anatomic term for armpit is axilla) and on the side of your chest. The serratus anterior muscle connects to the scapula and the upper ribs. You use this muscle when you push something or raise your arm higher than horizontal. Its action pulls the scapula downward and forward.
Although the biceps brachii and triceps brachii are muscles located in the top (anterior) part of your upper arm, their actions allow your forearm (lower arm) to move. Figure 6-8a shows an anterior view of the upper limb. The name biceps refers to this muscle’s two origins (points of attachment); it attaches to the scapula in two places. From there, it runs to the radius of the forearm (its point of insertion). The triceps brachii is the only muscle that runs along the back (posterior) side of the upper arm. Figure 6-8b shows a posterior view of the upper limb. The name triceps refers to the fact that it has three attachments: one on the scapula and two on the humerus. It runs to the ulna of the forearm. You can feel this muscle in motion when you push or punch. Other muscles of the arm include the brachioradialis, which helps you flex your arm at the elbow, and the supinator, which rotates your arm from a palm-down position to a palm-up position (remember the word supinator has the word “up” in it).
Illustration by Kathryn Born, MA
FIGURE 6-8: The muscles of the upper limb: anterior (a) and posterior (b).
Your forearm contains muscles that control the fine movements of your fingers. When you type or play the piano, you’re using your extensor digitorum and flexor digitorum muscles to raise and lower your fingers onto the keyboard and move them to the different rows of keys. As you lift your hands off the keyboard, the muscles of your wrist kick into gear. The flexor carpi radialis (attached to the radius bone) and flexor carpi ulnaris (attached to the ulna bone) allow your wrist to flex forward or downward. The extensor carpi radialis longus (which passes by the carpal bones), the extensor carpi radialis brevis, and the extensor carpi ulnaris allow your wrist to extend; that is, bend upward.
The muscles in your upper arm move your forearm. The muscles of your forearm move your wrists, hands, and fingers. There are no muscles in your fingers — only the tendons that connect to those bones.
Getting a leg up
Your lower limbs are connected to your buttocks, and your buttocks are connected to your hips. The iliopsoas connects your lower limb to your torso and consists of two smaller muscles: the psoas major, which joins the thigh to the vertebral column, and the iliacus, which joins the hipbone’s ilius to the thigh’s femur bone. Originating on the iliac spine of the hip and joining to the inside surface of the tibia (a bone in your shin), the sartorius muscle is a long, thin muscle that runs from the hip to the inside of the knee (see Figure 6-9a). These muscles stabilize your lower limbs and provide strength for them to support your body’s weight and to balance your body against the pressure of gravity.
Illustration by Kathryn Born, MA
FIGURE 6-9: The muscles of the lower limb: anterior (a) and posterior (b).
Some muscles in the lower limb allow the thigh to move in a variety of positions. The buttocks muscles allow you to straighten your lower limb at the hip and extend your thigh when you walk, climb, or jump. The gluteus maximus — the largest muscle in the buttocks — is the largest muscle in the body (see Figure 6-9b). The gluteus maximus is antagonistic to the stabilizing iliopsoas muscle, which flexes your thigh. The gluteus medius muscle, which lies behind the gluteus maximus, allows you to raise your leg to the side so that you can form a 90-degree angle with your two legs (this action is abduction of the thigh). Several muscles serve as adductors; that is, they move an abducted thigh back toward the midline. These muscles include the pectineus and adductor longus, which become injured when you “pull a groin muscle,” as well as the adductor magnus and gracilis, which run along the inside of your thigh.
Other muscles in the thigh serve to move the lower leg. Along your thigh’s front and lateral side, four muscles work together to allow you to kick. These four muscles — the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius — are better known as the quadriceps (quadriceps femoris). Quad means “four,” as in quadrilateral or quadrant. Refer to Figure 6-9a.
The hamstrings are a group of muscles that are antagonistic to the quadriceps. The hamstrings — the biceps femoris, semimembranosus, and semitendinosus — run down the back of the thigh (refer to Figure 6-9b) and allow you to flex your lower leg and extend your hip. They originate on the ischium of the hipbone and join (insert at) to the tibia of the lower leg. You can feel the tendons of your hamstring muscles behind your knee.
Your lower leg’s shin and calf muscles move your ankle and foot. The gastrocnemius, better known as the “calf muscle,” begins (originates) at the femur (thigh bone) and joins (inserts at) the Achilles tendon that runs behind your heel. You can feel your gastrocnemius muscle contracting when you stand on your toes. The antagonist of the gastrocnemius, the tibialis anterior, starts on the surface of the tibia (shinbone), runs along the shin, and connects to your ankle’s metatarsal bones. You can feel this muscle contract when you raise your toes and keep your heel on the floor. The fibular longus and fibular brevis (brevis meaning “short,” as in brevity) run along the outside of the lower leg and join the fibula to the ankle bones. In doing so, the fibular muscles help to move the foot. The extensor digitorum longus and the flexor digitorum longus muscles join the tibia to the feet and allow you to extend and flex your toes, respectively, like your fingers.
Pathophysiology of the Muscular System
The body has so much skeletal muscle tissue, and it performs so many functions that wear and tear is normal and expected, not really “pathophysiological.” Sore or strained muscles need only time to repair themselves and even become better than ever. However, many serious conditions can affect skeletal muscle and leave the sufferer disabled, in considerable pain, and even with a much-shortened life span. The following sections give you an overview of conditions that can affect skeletal muscles.
Muscular dystrophy
The muscular dystrophies (MD) are a group of more than 30 diseases characterized by progressive weakness and degeneration of the skeletal muscles. All are inherited, each in its own pattern. The disorders differ in terms of the distribution and extent of muscle weakness, age of onset of symptoms, and rate of progression. The prognosis for people with MD varies according to the disorder’s type and progression. Some cases may be mild and progress very slowly over a normal life span, while others produce severe muscle weakness and functional disability beginning at a young age. Some children with MD die in infancy, while others live into adulthood with only moderate disability.
The most common form is Duchenne muscular dystrophy (DMD). Since the mutation is on the X chromosome, it is far more common in males. Females receive two copies of the X (so they have a chance for a properly functioning gene on the second copy) and because the symptoms are so severe, the odds of a male passing on his mutated chromosome are very low.
The symptoms of DMD become evident usually before a child is 3 years old. The muscles slowly weaken, shorten, and degenerate. Fat and connective tissue replace normal muscle tissue, thus causing problems in the heart and lungs. DMD patients are often in a wheelchair by about age 12 and usually die in their teens. Some female carriers of DMD gene do experience symptoms, but they are much milder.
Myotonic muscular dystrophy affects males and females, and the onset of symptoms can occur at any age. Progressive muscle weakness and stiffness is usually evident first in the muscles of the face and neck. Turning the head becomes difficult. Eventually, those afflicted have problems with actions such as swallowing, because their muscles don’t relax after contractions. Then, the arms and legs become affected. Eventually, the patient may require a wheelchair or even be confined to bed.
There’s no specific treatment to stop or reverse any form of MD. Treatment to manage symptoms may include drug therapy, physical therapy, respiratory therapy, speech therapy, orthopedic appliances used for support, and corrective orthopedic surgery. Some patients may need assisted ventilation to treat respiratory muscle weakness and a pacemaker for cardiac abnormalities.
Muscle spasms
A muscle spasm is a sudden, strong, involuntary contraction, sometimes causing severe pain. Any muscle can go into spasm, and the effects vary according to the location and nerves that are nearby, but most often, you feel it as a cramp. Common causes of muscle spasms include overuse, insufficient stretching, and dehydration. Muscle spasms are a common cause of back and neck pain. The gastrocnemius (calf muscle) is a common place for sudden cramps to occur — the dreaded charley horse.
Not all spasms are painful. Hiccups, which are the result of spasm in the diaphragm, aren’t usually painful — annoying, but not painful. The same with facial tics, such as when your eyelid twitches when you’re stressed.
Fibromyalgia
Fibromyalgia, a chronic pain condition, isn’t exactly a muscle disease, but severe and widespread muscle pain is a major symptom of this mysterious condition. Strictly speaking, fibromyalgia isn’t a disease at all but a syndrome — a group of symptoms that appear to be closely related, although the underlying cause may not be known. Recent research has suggested a genetic component. The disorder is often seen in families, among siblings or mothers and their children.
The muscle pain can be more or less severe, more or less chronic, and more or less debilitating. Fibromyalgia patients often have more of a neurotransmitter called substance P in their spinal fluid, which is believed by some clinicians to alter the affected person’s perception of pain. A sensation that someone else may not even notice can be experienced by the fibromyalgia sufferer as excruciating. Some drug treatments are available, and some pain management techniques as well as stress reduction are helpful for some patients. Fibromyalgia is an active area of clinical research.
Seeing how your muscular system moves you
Not everything in anatomy and physiology makes sense at first. Just remember that skeletal muscle is classified as voluntary.
The abdominal muscles really form the center of your body. If the abdominal muscles are weak, the back is weak because the abdominal muscles help to flex the vertebral column. So if the vertebral column doesn’t flex easily, the muscles attached to it can become strained and weak. And the muscles of the abdomen and back join to the upper and lower limbs. Therefore, if the abdomen and back are weak, the limbs can have problems.
