Patterns of Fascicle Organization
Skeletal muscle is enclosed in connective tissue scaffolding at three levels. Each muscle fiber (cell) is covered by endomysium and the entire muscle is covered by epimysium. When a group of muscle fibers is “bundled” as a unit within the whole muscle it is called a fascicle. Fascicles are covered by a layer of connective tissue called perimysium (see Figure 10.3). Fascicle arrangement is correlated to the force generated by a muscle and affects the muscle’s range of motion. Based on the patterns of fascicle arrangement, skeletal muscles can be classified in several ways. What follows are the most common fascicle arrangements.
Parallel muscles have fascicles that are arranged in the same direction as the long axis of the muscle (Figure 11.2.1). The majority of skeletal muscles in the body have this type of organization. Some parallel muscles are flat sheets that expand at the ends to make broad attachments such as the sartorius (see Figure 11.2.2). Other parallel muscles have a larger central region called a muscle belly tapering to tendons on each end. This arrangement is called fusiform such as the biceps brachii (see Figure 11.2.2).
Circular muscles are also called sphincters (see Figure 11.2.1). When they relax, the sphincters’ concentrically arranged bundles of muscle fibers increase the size of the opening, and when they contract, the size of the opening shrinks to the point of closure. The orbicularis oris muscle is a circular muscle that goes around the mouth. When it contracts, the oral opening becomes smaller, as when puckering the lips for whistling. Another example is the orbicularis oculi, one of which surrounds each eye. Consider, for example, the names of the two orbicularis muscles (orbicularis oris and oribicularis oculi), where part of the first name of both muscles is the same. The first part of orbicularis, orb (orb = “circular”), is a reference to a round or circular structure; it may also make one think of orbit, such as the moon’s path around the earth. The word oris (oris = “oral”) refers to the oral cavity, or the mouth. The word oculi (ocular = “eye”) refers to the eye.
When a muscle has a widespread expansion over a sizable area and the fascicles come to a single, common attachment point, the muscle is called convergent. The attachment point for a convergent muscle could be a tendon, an aponeurosis (a flat, broad tendon), or a raphe (a very slender tendon). The large muscle on the chest, the pectoralis major, is an example of a convergent muscle because it converges on the intertubercular groove and greater tubercle of the humerus via a tendon (see image 11.3).
Pennate muscles (penna = “feathers”) blend into a tendon that runs through the central region of the muscle for its whole length, somewhat like the quill of a feather with the muscle fascicles arranged similar to the feathers. Due to this design, the muscle fibers in a pennate muscle can only pull at an angle, and as a result, contracting pennate muscles do not move their tendons very far. However, because a pennate muscle generally can hold more muscle fibers within it, it can produce relatively more tension for its size. There are three subtypes of pennate muscles.
In a unipennate muscle, the fascicles are located on one side of the tendon. The extensor digitorum of the forearm is an example of a unipennate muscle. A bipennate muscle such as the rectus femurs has fascicles on both sides of the tendon as in the arrangement of a single feather. Multipennate muscles have fascicles that insert on multiple tendons tapering towards a common tendon, like multiple feathers converging on a central point. A common example is the deltoid muscle of the shoulder, which covers the shoulder but has a single tendon that inserts on the deltoid tuberosity of the humerus.
The Lever System of Muscle and Bone Interactions
Skeletal muscles do not work by themselves. Muscles are arranged in pairs based on their functions. For muscles attached to the bones of the skeleton, the connection determines the force, speed, and range of movement. These characteristics depend on each other and can explain the general organization of the muscular and skeletal systems.
The skeleton and muscles act together to move the body. Have you ever used the back of a hammer to remove a nail from wood? The handle acts as a lever and the head of the hammer acts as a fulcrum, the fixed point that the force is applied to when you pull back or push down on the handle. The effort applied to this system is the pulling or pushing on the handle to remove the nail, which is the load, or “resistance” to the movement of the handle in the system. Our musculoskeletal system works in a similar manner, with bones being stiff levers and the articular endings of the bones—encased in synovial joints—acting as fulcrums. The load would be an object being lifted or any resistance to a movement (your head is a load when you are lifting it), and the effort, or applied force, comes from contracting skeletal muscle.
Chapter Review 11.1 and 11.2
Skeletal muscles each have an origin and an insertion. The end of the muscle that attaches to the bone being pulled is called the muscle’s insertion and the end of the muscle attached to a fixed, or stabilized, bone is called the origin. The muscle primarily responsible for a movement is called the prime mover, and muscles that assist in this action are called synergists. A synergist that makes the insertion site more stable is called a fixator. Meanwhile, a muscle with the opposite action of the prime mover is called an antagonist. Several factors contribute to the force generated by a skeletal muscle. One is the arrangement of the fascicles in the skeletal muscle. Fascicles can be parallel, circular, convergent, pennate, fusiform, or triangular. Each arrangement has its own range of motion and ability to do work.
Critical Thinking Questions
1. What effect does fascicle arrangement have on a muscle’s action?<
2. Movements of the body occur at joints. Describe how muscles are arranged around the joints of the body.
3. Explain how a synergist assists an agonist by being a fixator.
- move away from midline in the sagittal plane
- (also, prime mover) muscle whose contraction is responsible for producing a particular motion
- muscle that opposes the action of an agonist
- bulky central body of a muscle
- pennate muscle that has fascicles that are located on both sides of the tendon
- (also, sphincter) fascicles that are concentrically arranged around an opening
- fascicles that extend over a broad area and converge on a common attachment site
- muscle fibers bundled by perimysium into a unit
- synergist that assists an agonist by preventing or reducing movement at another joint, thereby stabilizing the origin of the agonist
- movement that decreases the angle of a joint
- muscle that has fascicles that are spindle-shaped to create large bellies
- end of a skeletal muscle that is attached to the structure (usually a bone) that is moved when the muscle contracts
- pennate muscle that has a tendon branching within it
- end of a skeletal muscle that is attached to another structure (usually a bone) in a fixed position
- fascicles that extend in the same direction as the long axis of the muscle
- fascicles that are arranged differently based on their angles to the tendon
- prime mover
- (also, agonist) principle muscle involved in an action
- muscle whose contraction helps a prime mover in an action
- pennate muscle that has fascicles located on one side of the tendon
Answers for Critical Thinking Questions
- Fascicle arrangements determine what type of movement a muscle can make. For instance, circular muscles act as sphincters, closing orifices.
- Muscles work in pairs to facilitate movement of the bones around the joints. Agonists are the prime movers while antagonists oppose or resist the movements of the agonists. Synergists assist the agonists, and fixators stabilize a muscle’s origin.
- Agonists are the prime movers while antagonists oppose or resist the movements of the agonists. Synergists assist the agonists, and fixators stabilize a muscle’s origin.