Skeletal Muscle Fiber Structure

Each skeletal muscle fiber is a skeletal muscle cell. These cells are incredibly large, with diameters of up to 100 µm and lengths of up to 30 cm. Within each muscle fiber are myofibrils—long cylindrical structures that lie parallel to the muscle fiber. Myofibrils run the entire length of the muscle fiber, and because they are only approximately 1.2 µm in diameter, hundreds to thousands can be found inside one muscle fiber. When the myofibrils shorten, the entire muscle cell contracts and generates force, pulling on the tendons that attach the muscle to the bone and moving the bone. (Figure 20.2).

Myofibrils are composed of smaller structures called myofilaments. There are two main types of filaments: thick filaments and thin filaments; each has different compositions and locations. Thick filaments (composed of the protein myosin) and thin filaments (composed of the protein actin) (see Figure 20.3).

20.2 Muscle Actions

Synovial joints allow the body a tremendous range of movements. Each movement at a synovial joint results from the contraction or relaxation of the muscles that are attached to the bones on either side of the articulation. Movement types are generally paired, with one being the opposite of the other. Body movements are always described in relation to the anatomical position of the body: upright stance, with upper limbs to the side of body and palms facing forward. Refer to Figure 20.4 as you go through this section.

Figure 20.4 Movements of the Body, Part 1 Synovial joints give the body many ways in which to move. (a)–(b) Flexion and extension motions are in the sagittal (anterior–posterior) plane of motion. These movements take place at the shoulder, hip, elbow, knee, wrist, metacarpophalangeal, metatarsophalangeal, and interphalangeal joints. (c)–(d) Anterior bending of the head or vertebral column is flexion, while any posterior-going movement is extension. (e) Abduction and adduction are motions of the limbs, hand, fingers, or toes in the coronal (medial–lateral) plane of movement. Moving the limb or hand laterally away from the body, or spreading the fingers or toes, is abduction. Adduction brings the limb or hand toward or across the midline of the body, or brings the fingers or toes together. Circumduction is the movement of the limb, hand, or fingers in a circular pattern, using the sequential combination of flexion, adduction, extension, and abduction motions. Adduction/abduction and circumduction take place at the shoulder, hip, wrist, metacarpophalangeal, and metatarsophalangeal joints. (f) Turning of the head side to side or twisting of the body is rotation. Medial and lateral rotation of the upper limb at the shoulder or lower limb at the hip involves turning the anterior surface of the limb toward the midline of the body (medial or internal rotation) or away from the midline (lateral or external rotation). (credit: Openstax Anatomy and Physiology)

Table 20.1

Facial Muscles

The orbicularis oris is a circular muscle that moves the lips and allows you to kiss. The orbicularis oculi is a circular muscle that closes the eye. The zygomaticus muscle pulls the lips up into a smile.

Chewing Muscles

In anatomical terminology, chewing is called mastication. Muscles involved in chewing must be able to exert enough pressure to bite through and then chew food before it is swallowed. The massetermuscle is the main muscle used for chewing because it elevates the mandible (lower jaw) to close the mouth, and it is assisted by the temporalismuscle, which retracts the mandible (Figure 20.6). You can feel the temporalis move by putting your fingers to your temple as you chew.

Figure 20.6 Muscles That Move the Lower Jaw The muscles that move the lower jaw are typically located within the cheek and originate from processes in the skull. This provides the jaw muscles with the large amount of leverage needed for chewing. (credit: Openstax Anatomy and Physiology)

Neck Muscles

The head, attached to the top of the vertebral column, is balanced, moved, and rotated by the neck muscles. When these muscles act unilaterally, the head rotates. When they contract on both sides, the head flexes or extends. The major muscle that flexes (ex- the head flexes when you look down) and rotates the head is the sternocleidomastoid (Figure 20.7). Place your fingers on both sides of the front of your neck and turn your head to the left and to the right. You will feel the movement originate there.

The trapezius muscle is located on the back of the neck, and it extends the head (ex- the head extends when you look up).

Figure 20.7 Posterior and Lateral Views of the Neck The superficial and deep muscles of the neck are responsible for moving the head, cervical vertebrae, and scapulas. (credit: Openstax Anatomy and Physiology)

The change in volume of the thoracic cavity during breathing is due to the alternate contraction and relaxation of the diaphragm (Figures 20.8 and 20.9). It separates the thoracic and abdominal cavities, and is dome-shaped at rest. The superior surface of the diaphragm is convex, creating the elevated floor of the thoracic cavity. The inferior surface is concave, creating the curved roof of the abdominal cavity.

Figure 20.9 Diaphragm The diaphragm separates the thoracic and abdominal cavities. (credit: Openstax Anatomy and Physiology)

The external intercostal (inter- means between and -costal means rib) muscles are located in between the ribs (Figure 20.10). The principal role of the intercostal muscles is to assist in breathing by changing the dimensions of the rib cage. The external intercostal muscles aid in inspiration of air during breathing because when they contract, they raise the rib cage, which expands it.

Figure 20.10 Intercostal Muscles The external intercostals are located laterally on the sides of the body. (credit: Openstax Anatomy and Physiology)

Abdominal Wall Muscles

It is a complex job to balance the body on two feet and walk upright. The muscles of the vertebral column, thorax, and abdominal wall extend, flex, and stabilize different parts of the body’s trunk.

The rectus abdominis muscles (Figure 20.11) are a pair of long, linear muscles that extend the length of the abdomen. The rectus abdominis muscles flex the trunk (ex- when you do sit ups). Each muscle is segmented by three transverse bands of collagen fibers called the tendinous intersections. This results in the look of “six-pack abs,” as each segment hypertrophies on individuals at the gym who do many sit-ups.

On the lateral wall of the abdomen, the external oblique muscle is located closest to the surface, and the internal oblique muscle is underneath it (Figure 20.11). Both of these muscles flex the trunk laterally (bend to the side).

Figure 20.11Muscles of the Abdomen (a) The anterior abdominal muscles include the medially located rectus abdominis, which is covered by a sheet of connective tissue called the rectus sheath. On the flanks of the body, medial to the rectus abdominis, the abdominal wall is composed of three layers. The external oblique muscles form the superficial layer, while the internal oblique muscles form the middle layer, and the transverses abdominus forms the deepest layer. (b) The muscles of the lower back move the lumbar spine but also assist in femur movements. (credit: Openstax Anatomy and Physiology)

Shoulder Muscles

The pectoralis major is thick and fan-shaped, covering much of the top portion of the anterior (upper) thorax (Figure 20.12a). It flexes the shoulder (ex- throw a softball underhand or pick up a child). The deltoid is a thick muscle that creates the rounded lines of the shoulder (Figure 20.12b). The deltoid abducts the arm (the arm moves away from the body as when you reach out to the side).

Figure 20.12 Muscles That Move the Humerus (a, c) The muscles that move the humerus anteriorly are generally located on the anterior side of the body and originate from the sternum (e.g., pectoralis major) or the anterior side of the scapula (e.g., subscapularis). (b) The muscles that move the humerus superiorly generally originate from the superior surfaces of the scapula and/or the clavicle (e.g., deltoids). The muscles that move the humerus inferiorly generally originate from middle or lower back (e.g., latissiumus dorsi). (d) The muscles that move the humerus posteriorly are generally located on the posterior side of the body and insert into the scapula (e.g., infraspinatus). (credit: Openstax Anatomy and Physiology)

Arm Muscles

The biceps brachii is located on the anterior (front) side of the arm and crosses the shoulder and elbow joints to flex the elbow and forearm (Figure 20.13). The triceps brachii is located on the posterior (back) side of the arm, and it extends the elbow and forearm (Figure 20.13).

Figure 20.13 Muscles That Move the Forearm The muscles originating in the upper arm flex, extend, pronate, and supinate the forearm. The muscles originating in the forearm move the wrists, hands, and fingers. (credit: Openstax Anatomy and Physiology)

Forearm Muscles

Their are several different forearm flexor and extensor muscles, but we will group them together because they have the same actions. The forearm flexors flex the wrist and fingers while the forearm extensors extend the wrist and fingers (Figure 20.13).

Hip Muscles

The psoas major and iliacus are located on the anterior (front) side of the hip and make up the iliopsoas muscle (Figure 20.14), which flexes the hip (ex- when you move your leg forward to kick a ball or when you lift your leg up to go up a step). The gluteus maximus is located on the posterior (back) side of the hip and makes up your butt (Figure 20.14 and 20.15). It extends the hip (ex- when you swing your leg back behind you).

Figure 20.14 Hip and Thigh Muscles The large and powerful muscles of the hip that move the femur generally originate on the pelvic girdle and insert into the femur. The muscles that move the lower leg typically originate on the femur and insert into the bones of the knee joint. The anterior muscles of the femur flex the hip. The posterior muscles of the femur extend the hip. A combination of gluteal and thigh muscles also adduct, abduct, and rotate the thigh and lower leg. (credit: Openstax Anatomy and Physiology)

Thigh Muscles

Located on the anterior (front) side of the thigh, the quadriceps consists of four muscles (rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius) (Figures 20.14 and 20.16). The tendon common to all four is the quadriceps tendon (patellar tendon), which inserts into the patella and continues below it as the patellar ligament. The patellar ligament attaches to the tibia. The quadriceps extends the knee (Figure 20.17).

On the posterior (back) side of the thigh, the hamstring includes three long muscles (biceps femoris, semitendinosus, and semimembranosus) (Figures 20.14 and 20.18). The hamstrings flex the knee (Figure 20.17).

Lower Leg (Calf) Muscles

The tibialis anterior is a long and thick muscle on the lateral surface of the tibia (Figure 20.19) that flexes the ankle (ex- when you point your toes up towards your knee). On the posterior (back) side of the calf, the most superficial and visible muscle of the calf is the gastrocnemius (Figures 20.19 and 20.20). The gastrocnemius muscle extends the ankle (ex- when you point your toes down).

Figure 20.19 Muscles of the Lower Leg The muscles of the anterior compartment of the lower leg are generally responsible for dorsiflexion, and the muscles of the posterior compartment of the lower leg are generally responsible for plantar flexion. The lateral and medial muscles in both compartments invert, evert, and rotate the foot. (credit: Openstax Anatomy and Physiology)

Adapted from Openstax Human Biology and Anatomy and Physiology