Muscular System – Types, Diagram, Functions, Structure

Interactions of Skeletal Muscles

Skeletal muscles interact to produce movements by way of anatomical positioning and the coordinated summation of innervation signals.

Skeletal muscle contractions can be grouped based on the length and frequency of contraction.

Twitch

When stimulated by a single action potential muscle contracts and then relaxes. The time between the stimulus and the initiation of contraction is termed the latent period, which is followed by the contraction period. At peak contraction, the muscle relaxes and returns to its resting position. Taken all together these three periods are termed a twitch.

Summation

If an additional action potential were to stimulate a muscle contraction before a previous muscle twitch had completely relaxed then it would sum onto this previous twitch increasing the total amount of tension produced in the muscle. This addition is termed summation. Within a muscle, summation can occur across motor units to recruit more muscle fibers, and also within motor units by increasing the frequency of contraction.

Multiple fiber summation

When a weak signal is sent by the central nervous system to contract a muscle, the smaller motor units, being more excitable than the larger ones, are stimulated first. As the strength of the signal increases, more (and larger) motor units are excited. The largest motor units have as much as 50 times the contractile strength as the smaller ones; thus, as more and larger motor units are activated, the force of muscle contraction becomes progressively stronger. A concept known as the size principle allows for a gradation of muscle force during weak contraction to occur in small steps, which become progressively larger as greater amounts of force are required.

Frequency summation

For skeletal muscles, the force exerted by the muscle can be controlled by varying the frequency at which action potentials are sent to muscle fibers. Action potentials do not arrive at muscles synchronously, and, during a contraction, only a certain percentage of the fibers in the muscle will be contracting at any given time. In a typical circumstance, when a human is exerting as much muscular force as they are consciously able, roughly one-third of the fibers in that muscle will be contracting at once. This relatively low level of contraction is a protective mechanism to prevent damage to the muscle tissue and attaching tendons and structures.

Tetanus

If the frequency of action potentials generated increases to such a point that muscle tension has reached its peak and plateaued and no relaxation is observed then the muscle contraction is described as a tetanus.

How Skeletal Muscles Are Named

The anatomical arrangement of skeletal muscle fascicles can be described as parallel, convergent, pennate, or sphincter.

Skeletal muscle can be categorized into four groups based on its anatomical arrangement.

Parallel

Parallel muscles are characterized by fascicles that run parallel to one another, and contraction of these muscle groups acts as an extension of the contraction of a single muscle fiber. Most skeletal muscles in the body are parallel muscles; although they can be seen in a variety of shapes such as flat bands, spindle shaped, and some can have large protrusions in their middle known as the belly of the muscle.

Parallel muscles can be divided into fusiform and non-fusiform types based on their shape. Fusiform muscles are more spindle shaped (their diameter at the center is greater than at either end), whereas, non-fusiform muscles are more rectangular with a constant diameter.

The biceps brachii is an example of a  fusiform parallel muscle, and is responsible for flexing the forearm.

Convergent

Convergent muscles have a common point of attachment, from which the muscle fascicles extend outward, not necessarily in a specific spatial pattern, allowing the muscle to cover a broad surface. These muscles do not tend to exert as much force on their tendons. Muscle fibers can often exert opposing effects during contraction, such as not pulling in the same direction depending on the location of the muscle fiber. Covering a broad surface these fibers allow for more versatile types of movement. Because the fascicles pull on the tendons at an angle, they do not move the tendon as far as their parallel muscle counterparts. Despite this they generate greater tension because they possess a greater amount of muscle fibers than similarly sized parallel muscles.

The pectoralis major found in the chest is an example of a convergent muscle, and is responsible for flexing the upper arm.

Pennate

In Pennate muscles, the tendon runs through the length of the muscle. Fascicles pull on the tendon at an angle, thus not moving as far at the parallel muscles during a contraction. However, these muscles tend to have relatively more muscle fibers than similarly sized parallel muscles, and thus carry more tension.

If all the fascicles of a pennate muscle are on the same side of the tendon, the pennate muscle is called unipennate. If the fascicles lie to either side of the tendon the muscle is called bipennate. If the central tendon branches within a pennate muscle, the muscle is called multipennate.

The rectus femoris found in the thigh, and responsible for its flexion, is an example of a bipennate muscle.

Circular

The fibers of the circular or sphincter muscles are arranged concentrically around an opening or recess. As the muscle contracts, the opening it circumvents gets smaller. For this reason, these muscles are often found at the entrances and exits of external and internal passageways. Skeletal circular muscles are different from smooth muscle equivalents due to their structure and because they are under voluntary control

The orbicularis oris which controls the opening of the mouth is an example of a circular muscle.

How Skeletal Muscles Produce Movements

Muscles are arranged in groupings of agonist, antagonist, and synergists that produce and modulate movement.

Muscles exist in groupings that work to produce movements by muscle contraction. Muscles are classified according to their actions during contractions as agonists, antagonists, or synergists.

For muscle pairings referred to as antagonistic pairs, one muscle is designated as the extensor muscle, which contracts to open the joint, and the flexor muscle, which acts opposite to the extensor muscle. These pairs exist in places in the body in which the body cannot return the limb back to its original position through simple lack of contraction. Typical muscle pairings include the biceps brachii and triceps brachii, which act to flex or extend the forearm.

Agonist Muscles

Agonist’s muscles are those we typically associate with the movement itself and are thus sometimes referred to as prime movers. Agonist’s muscles produce the primary movement or series of movements through their own contractions. To generate a movement, agonist muscles must physically be arranged so that they cross a joint by way of the tendon. The contraction will move limbs associated with that joint. In this sense, the bone acts as a lever with the attached muscle fiber’s contraction, driving movement.

During flexing of the forearm, the biceps brachii is the agonist muscle, pulling the forearm up towards the shoulder.

Antagonist Muscles

The majority of muscles are grouped in pairs, with an antagonist to each agonist’s muscle. Exceptions include those muscles such as sphincter muscles that act to contract in a way that is opposite to the resting state of the muscle. Antagonist’s muscles act as opposing muscles to agonists, usually contracting as a means of returning the limb to its original, resting position.

During flexing of the forearm, the triceps brachii is the antagonist muscle, resisting the movement of the forearm up towards the shoulder.

Synergist Muscles

Synergist muscles act around a moveable joint to produce motion similar to or in concert with agonist’s muscles. They often act to reduce the excessive force generated by the agonist’s muscle and are referred to as neutralizers. Synergists are useful because they fix certain joints to allow a range of contractions, in contrast with the sheer power of an agonist contraction that limits the range of possible movements.

During flexing of the forearm, the brachioradialis and brachialis act as synergist muscles, aiding the biceps brachii in pulling the forearm up towards the shoulder. The muscles of the rotator cuff are also synergists in that they fix the shoulder joint allowing the biceps brachii to exert a greater force.

Muscle Attachment Sites

Tendons are composed of connective tissue that attaches muscle to bone.

The most skeletal muscle attaches to the bone in order to produce movement. However, some skeletal muscle attaches directly to other muscles, fascia, or tissues such as the skin.

Tendons

A tendon is a cord-like, fibrous connective tissue that connects muscle to bone and is capable of withstanding tension. At either end of the tendon, its fibers intertwine with the fascia of a muscle or the periosteum (a dense fibrous covering of a bone), allowing force to be dissipated across the bone or muscle.

Tendons mainly consists of closely-packed collagen fibers running parallel to the force generated by the muscle to which they are attached. Intertwined with the collagen fibers are elastin molecules, which improve the tendons’ elasticity, and various proteoglycans, proteins to which many carbohydrate molecules are attached. These proteins play a key role in maintaining the organization of the tendon, especially during compression and extension.

Tendons were once thought to play only a passive connective role. However, research into their elastic properties has demonstrated that they can also act as springs. The elasticity of tendons allows them to passively store energy for later release. The most widely-researched example is the Achilles tendon which stores and releases elastic energy during walking, improving efficiency and reducing muscle load.

Aponeuroses

Not all muscle attaches via tendons. Aponeuroses are large, sheet-like layers of connective tissue with a similar composition to tendons. Aponeuroses can also attach to bone, as in the scalp aponeuroses, and to the fascia of other muscles or tissues, such as the anterior abdominal aponeuroses. Their large form and shape provides structure and distributes tension across a wider area or large number of muscle groups.

Other Attachments

Muscles can also attach directly to other tissues, which is most evident in the face. The skeletal muscles involved in controlling expression attach directly onto the fascia of the skin.

Arrangement of Fascicles

Skeletal muscles are grouped into fascicles, which are bunches of muscle fibers surrounded by a perimysium.

Muscle Fascia

Skeletal muscle tissue is composed of numerous muscle fibers which are separated from adjacent muscles and other tissues by a layer of dense, elastic connective tissue termed the fascia. This fascia can project beyond the end of the muscle and attach to bones, other muscles, and other tissues. Key muscle groups and the associated vascular and nervous systems can also be separated from other tissue, such as in the upper arm. These groupings are called fascial compartments.

This fascia is interlinked with a serious of fascia found throughout the body, including the superficial fascia which is the lowermost layer of the skin and the visceral fascia which surrounds internal organs. The fascia surrounding a muscle or muscle group does not contain many blood vessels, but is rich with sensory receptors.

Muscle fascia is predominately composed of cross-linked collagen and elastin fibers oriented parallel to the direction of muscle force, making them able to resist high-tension forces while remaining somewhat elastic.

Fascicles

Beneath the fascia in skeletal muscle is another layer of connective tissue termed the epimysium which is closely associated with the fascia. It extends inwards and becomes the perimysium, then into the muscle separating muscle fibers into small bundles termed fascicles. Fascicles can be arranged in a variety of anatomical positions within a muscle, producing different movements.

Each individual fiber within a fascicle is surrounded by a thin connective layer termed the endomysium, which helps maintain close association between the muscle fiber and associated vascular and nervous systems.

The organization of connective tissue throughout and around a muscle provides strength and flexibility while distributing the force evenly. It also maintains the close association of the vascular and nervous system with the muscle, which is required to deliver necessary metabolites and nerve impulses.

Cardiac and Smooth Muscle Tissue

Whilst both cardiac and smooth muscles are also wrapped in connective tissue, they are not differentiated in the same way as skeletal muscles.

Lever Systems

Arrangement of muscles allows them to move relative to one another, while the insertion joint acts as the pivot point for a lever system.

Skeletal muscle is usually attached to a relatively immovable part of the body at one end and more mobile region on the other end of a joint. The attachment at the immovable end is referred to as the origin and at the moveable end, the insertion. Upon contraction, the insertion is pulled towards the origin. This movement can be described using a simple lever system. Muscles can have multiple origins and insertions which modulate the type of movement they produce.

Levers

A lever is composed of three parts: a fixed rod which is attached to a fulcrum (pivot) and a load. Depending on the relative position of the three components, levers can move heavy loads or move loads further or faster when a force is applied.

If a load is close to a pivot and the force is applied far from the pivot, then the lever is said to operate at mechanical advantage. A large but relatively small force can move a heavy object. The classic example of such a lever is a car jack. With big movements of the lever, the heavy car is lifted in small increments off the ground.

If a load is far from a pivot and a force is applied near to the pivot, then the lever is said to operate at a mechanical disadvantage. A large force is required to move a relatively small load, but the speed and distance at which that load can be moved is greatly increased. An example of this is a spade combined with forceful movements of the muscles in the arm, which results in large movements of the spade head.

In muscles, the joints are the pivots and the bones are the fixed rods. The load is the weight of the bone, associated tissues, and other objects being moved, and the force is applied by the muscle at its insertion point.

Classes of Levers

Levers can also vary based on the relative position of the load, pivot and point of force application. Classes of levers include:

First-class Lever

In a first class lever, the load and force sit on either side of the pivot like a seesaw. First-class levers are relatively uncommon in the body, but one example is the triceps brachii muscle of the upper arm which acts to extend the forearm. The force is applied at its point of insertion on the ulna in the forearm, the elbow is the pivot, and the load is the humerus in the upper arm. Thinking of the relative distance between the points of attachment, the triceps brachii can be said to act at a mechanical disadvantage.

Second-class Lever

In a second-class lever, the force is applied at one end with the pivot at the other and the load in between. Second-class levers are also relatively uncommon in the body. One example is raising yourself up on your toes. The pivot is based at the front of the foot, the load is the weight of the body, and the force is applied through the Achilles tendon in the heel. All second-class levers in the body act at a mechanical advantage since the force is always applied closer to the load than to the pivot.

Third-class Lever

In a third-class lever the force is applied between the load and the pivot. The majority of muscles in the body are third-class levers and all act at a mechanical disadvantage as the force is applied closer to the pivot than the load. An example of a third-class lever in the body is the biceps brachii, which flexes the forearm towards the shoulder. Originating at the scapula, the pivot is the elbow, with the force applied immediately after the elbow at the point of insertion on the radius of the forearm. The load is the forearm and any objects a person carries.

Types of Movement

Working together using the lever principles discussed above, skeletal muscles can induce a wide range of movements.

Flexors and Extensors

Flexors and extensors adjust and change the angle between two body parts. Flexion decreases the angle and extension increases the angle. For example, extension of the arm opens the angle of the elbow joint, while flexion allows for the bending of the arm. Flexion can also move inwards towards the body or forward, such as with hips or shoulders. Extension in this case moves the limbs towards the posterior side of the body lever.

Abduction and Adduction

Abduction and adduction are movements relative to the midline of the body. Abduction is the movement away from the midline of the body and adduction is movement towards this line. For example, moving arms or legs laterally away from the body is abduction, and bringing the limbs back to the midline is adduction.

Internal Rotation

Internal or medial rotation is specific to the shoulder or hip and brings the distal portions of the limbs inwards towards the midline. Internal rotation can also move the humerus and femur inward. External or lateral rotation is the opposite movement, pointing the distal portion of limbs as well as the humerus and femur away from the mid-line.

Depression and Elevation

A limb or a body part can be moved upward (or in a superior direction) through elevation. For example, the trapezius elevates the apex of the shoulder upward. Depression is the opposite of elevation, or moving body parts in an inferior direction.