Revised 10/3/2007
The MUSCULAR SYSTEM refers to the SKELETAL MUSCLE SYSTEM, which is made up of skeletal muscle organs. We discussed the structure of skeletal muscle organs in the previous set of notes (Chapter 9).
ORIGIN & INSERTION: FIG. 11.1
Skeletal muscles produce movement by exerting force on tendons, which then pull on bones. Most muscles cross a joint and attach to the articulating bone that forms a joint. When the muscle contracts, it pulls one of the articulating bones toward the other. The attachment of the muscle tendon to the stationary bone is called the muscle's ORIGIN. The other muscle tendon attaches to the muscle's INSERTION, the bone that is moved. In limbs, the origin is usually proximal and the insertion is usually distal. When a muscle contracts, it pulls its insertion toward its origin. The fleshy portion between the origin and insertion is called the BELLY or GASTER of the muscle. Notice how the belly shortens and thickens during the contraction.
ARRANGEMENT OF FASCICLES: (TABLE 11.1)
Skeletal muscle fibers are arranged within the skeletal muscles in bundles called FASCICLES (fasciculi). The muscle fibers are arranged in a parallel fashion within each fascicle, but the fascicle arrangements vary:
In a PARALLEL arrangement, the fascicles are arranged parallel to the longitudinal axis of the muscle and they end in flat tendons. An example is the strap-like sartorius muscle (FIG. 11.22, page 359) of the thigh.
Another type of fascicle arrangement is called FUSIFORM. In this arrangement, the fascicles are nearly parallel with the longitudinal axis of the muscle, but the ends of the muscle taper toward the tendons. The belly of the muscle (the fleshy part in the middle) is more expanded than the ends. An example is the biceps brachii (FIG. 11.1).
In a CONVERGENT (triangular) pattern, the muscle has a broad origin and its fascicles converge toward a single tendon. The muscle is essentially triangular. EXAMPLE: pectoralis major (FIG. 11.12, page 319).
In the PENNATE fascicle arrangement, the muscle fibers are short and the fascicles are directed obliquely (diagonally) toward a central tendon that runs the length of the muscle. If the fascicles insert into only one side of the tendon, the muscle is UNIPENNATE. EXAMPLE: gluteus maximus (FIG. 11.3, page 296). If the fascicles insert into the tendon from both sides, so that the muscle resembles a feather, the arrangement is BIPENNATE. EXAMPLE: rectus femoris (Table 11.1). Fascicles that attach obliquely (at a diagonal) from many directions to several tendons are called MULTIPENNATE. EXAMPLE: deltoid (Table 11.1).
In the CIRCULAR arrangement, the fascicles are arranged to form a circle of muscle fibers. This arrangement is always found surrounding an external body opening. EXAMPLE: orbicularis oculi and orbicularis oris (FIG. 11.4, page 299).
The PATTERN OF FASCICLE ARRANGEMENT influences both the muscle's RANGE OF MOTION and its POWER. Skeletal muscle fibers shorten to about 70% of their resting length when contracted. So, the longer the muscle fibers, the greater the range of movement. Muscles with a parallel fascicle arrangement provide the greatest degree of shortening, so they provide the greatest range of motion. Muscle strength is more dependent on the number of muscle fibers in the muscle; the greater the number of muscle fibers, the greater the muscle power. Pennate muscles have a large number of short fascicles distributed over their tendons; this gives them greater power, but a shorter range of motion. Parallel muscles can shorten quite a bit because the fascicles are longer, but they have a smaller number of muscle fibers. That gives parallel muscles a greater range of motion with less power compared to pennate muscles.
INTERACTIONS OF SKELETAL MUSCLES & GROUP ACTIONS:
Muscles that assume the major responsibility for producing a specific movement are called PRIME MOVERS (AGONISTS). The biceps brachii is the prime mover of elbow flexion (see FIG. 11.1)
Muscles that oppose, or reverse, a particular movement by a prime mover are called ANTAGONISTS. When a prime mover is active, the antagonist muscle fibers are relaxed, yielding to the movement of the prime mover. Antagonists can also help to regulate the action of the prime mover by partially contracting to provide some resistance or to slow or stop the action. During elbow flexion, the triceps brachii is the antagonist (see FIG. 11.1).
Most movements also involve muscles called SYNERGISTS, which reduce undesirable or unnecessary extra movements that might result as the prime mover contracts. For example, when a muscle crosses two or more joints, its contraction causes movement at all of the joints unless other mucles act as muscle stabilizers.
Muscles that help to maintain posture are also synergists.
FIXATORS are synergists which immobilize the origin of the prime mover so that the prime mover can act more efficiently. For example, the deltoid is the prime mover of arm abduction. The deltoid muscle originates on the scapula, which is a moveable bone. The pectoralis minor, rhomboids, and trapezius muscles attach the scapula to the axial skeleton. When the deltoid contracts to abduct the arm, these other muscles hold the scapula still and therefore act as fixators.
Depending on the movement, muscles can act as prime movers, antagonists, synergists, or fixators at any one time.
NAMING SKELETAL MUSCLES
Skeletal muscles are named according to structural and functional criteria. On your own, read about how muscles are named in Table 11.2. This will help you in learning muscle names. You will be given a hand-out in class that lists the muscles and their actions that you need to learn.