Muscle system

Naming muscles

The muscle system includes all the skeletal muscle and is therefore responsible for voluntary movement. It is not the intent of this book to cover all the muscles, but rather to give an overview of many of the major muscles involved in movement.

Generally, the name of the muscle gives consider­able information about its location or role. Muscles are named by several criteria:

1. Location. Sometimes, a bone or body region is included in the name, giving an indication of its location (e.g., temporalis is located over the tem­poral bone and the intercostal muscles are found between the ribs). The word deep (e.g., deep digital flexor) indicates that the muscle is not found at a superficial level.

2. Action. The action performed by the muscle may be included in the name. For example, the exten­sor digitorum extends a digit while the pronator teres pronates a limb.

3. Size. Whether the muscle is large or small may be indicated in the name (e.g., pectoralis major, adductor longus). The name may include such terms as Iongus (long), brevis (short), maximus (largest), minimus (smallest).

4. Shape. Sometimes muscles are named according to their shape (e.g., deltoid or trapezius muscles).

5. Direction of the fibers. Terms, such as rectus, indi­cate that the muscle fibers run parallel or straight relative to the body axis, whereas transverse muscles run at right angles to the same axis. Oblique fibers run at some other angle relative to that axis.

6. Number of origins/bellies. The name may include the number of heads (e.g., biceps brachii) or bellies (digastricus).

7. Attachment. Many times, the origin or attachment site is included in the name of the muscle (e.g., Cleidomastoid is attached to the clavical, or its remnant, and mastoid process).

Muscles can have several types of attachments.

Arrangements of fascicles

As stated previously, skeletal muscles are arranged in fascicles (Fig. 7.1). The fascicle arrangement can vary (Fig. 7.21). Fascicles are sometimes found to be arranged in a straight line and are referred to as paral­lel muscles (an example is the abductor digiti minimi). If the arrangement is circular, the fascicles appear as concentric rings. Such an arrangement is typically

Fig. 7.21. Fascicle arrangement. Muscle fascicles can have varying arrangements allowing for different functions.

found around an orifice such as the orbicularis oris muscle that surrounds the mouth. In a convergent muscle, the fascicles converge to the tendon for inser­tion. Therefore, the muscle begins wide, similar to the shape of a fan, and then narrows, as is seen in the pectoralis major muscle. In the pennate muscle, such as the gastrocnemius muscle, the fascicles are short and attach obliquely (penna = feather) to the tendon. In a unipennate muscle, the tendon runs along one side of the tendon, whereas in multipennate, the tendon branches within the muscle and looks like many feathers laying side by side.

Muscles as levers

The muscular and skeletal systems work together to function as a lever system. A lever consists of a rigid structure (i.e., bone) that moves around a fixed point called the fulcrum. Muscle contraction acts as the force to move a load, or bone.

Three different classes of levers exist, depending on the relationship of the force, fulcrum, and load (Fig. 7.23). In a first-class lever, the fulcrum is between the force and load, which are at either end of the lever. This is similar to a playground seesaw, and it provides a mechanical advantage. The act of an animal lifting its head functions as a first-class lever.

A second-class lever exists when the force is applied at one end and the fulcrum is found at the other end, while the load is placed in between. Such levers create a mechanical disadvantage, and are thus rarely found

Fig. 7.22. Mechanical advantage versus disadvantage. A lever system can provide a mechanical advantage or a mechanical disadvantage. The equation shown at the top explains the relationship between force and distance. (A) With this lever system, a mechanical advantage is provided because of the position of the fulcrum relative to the force and load. Twenty kilograms of force is able to lift 1OOOkg of load. (B) With the fulcrum and load placed at opposite ends of the lever, and a force provided in the middle, it requires 100 times the amount of force needed in the example shown in A to move the same load. This is a mechanical disadvantage.

Fig. 7.23. Anatomical examples of lever systems. (A) In a first-class lever, the force and load are on opposite sides of the fulcrum. (B) In a second-class lever, the load is between the force and the fulcrum. (C) In a third-class lever, the force is between the load and the fulcrum.

in the body. A wheelbarrow acts as such a lever. The calf muscles act as a second-class lever when causing plantar flexion.

In a third-class lever, the fulcrum and load are at either end of the lever, and the force is applied between the two.

Muscle terminology

Origins and insertions

Muscles are always attached to other structures at either end. Generally, one end is in a fixed position while the other end moves toward it when the muscle is contracted. The fixed attachment site is called the origin, and the movable end is called the insertion (Fig. 7.24). While the origin is normally proximal or superior to the insertion, this is not always the case. The sternocleidomastoid muscle originates at the sternum, and inserts at the mastoid process of the temporal bone.

Actions

The movements of the skeleton caused by muscle contraction involve flexion or extension, adduction or abduction, protraction or retraction, elevation or depression, rotation, circumduction, pronation or supination, inversion or eversion. These actions are defined in Table 7.3.

Muscles typically work in groups rather than indi­vidually. For example, the biceps branchii and bra­chialis cause flexion of the elbow, and the triceps brachii, tensor fasciae anterbrachii, and anconeus cause extension of the elbow. The agonist is the muscle primarily responsible for producing a certain move­ment, and the antagonist is the muscle whose action opposes that movement. In the case of the elbow, the

Fig. 7.24. Muscle functions and attachments. Using the thoracic limb of the horse, this figure demonstrates the origin and insertion of muscles, as well as the agonist and antagonist functions with regards to the elbow. (Figure modified from Getty, 1964.)

Muscles of the head and neck

Table 7.4. Muscles of the head and neck.

biceps brachii is the agonist for flexion of the elbow and the triceps branchii is the antagonist. A synergist is a muscle that helps the agonist work more effi­ciently. Synergists may provide additional force to move the joint, or they may help stabilize the joint.

Muscle attachments

Muscles can make three types of attachments. In a fleshy attachment, there is an apparent direct attach­ment of the muscle to the bone, as in the case of muscle attachments to the scapula. In actuality, the muscles are attached to the bones by very short tendons. In a

tendinous attachment, the muscle is attached to the bone via dense connective tissue. In an aponeurotic attachment, there is a flat, tendinous sheet attaching the muscle, as is seen in the abdominal wall.