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The human body is made up of hundreds of skeletal muscles, which are one of the three types of vertebrate muscle tissue, the others being cardiac and smooth muscle. Skeletal muscle cells, also known as muscle fibers, are long and multinucleated. They are attached to bones by tendons and are responsible for producing movement, maintaining body posture, controlling body temperature, and stabilizing joints. The direction of muscle fibers can be parallel, perpendicular, or oblique relative to the midline. Perpendicular muscle fibers, also known as transverse fibers, are those that are perpendicular to the midline. They are found in muscles such as the transversus abdominus, a deep muscle in the abdomen.
| Characteristics | Values |
|---|---|
| Definition | Perpendicular muscle fibers are also known as transverse muscle fibers |
| Muscle type | Skeletal muscle |
| Muscle composition | Actin and myosin filaments called myofilaments, repeated in units called sarcomeres |
| Muscle shape | Strap, fusiform, convergent, triangular, rhomboidal, pennate, unipennate, bipennate, multipennate, hydrostats |
| Muscle function | Producing movement, maintaining body posture, controlling body temperature, stabilizing joints |
| Muscle architecture | The physical arrangement of muscle fibers at the macroscopic level that determines a muscle's mechanical function |
| Muscle contraction | Before a skeletal muscle fiber can contract, it has to receive an impulse from a nerve cell |
| Muscle force | The force produced by a given muscle is proportional to the cross-sectional area, or the number of parallel sarcomeres present |
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What You'll Learn
Skeletal muscle
Each skeletal muscle contains multiple fascicles, or bundles of muscle fibres. Each individual fibre and each muscle is surrounded by a type of connective tissue layer of fascia. Skeletal muscle fibres are striated, multinucleated cells ranging from 10 to 100 micrometers in diameter and several centimetres long. The nuclei are located in the cell's periphery, adjacent to the sarcolemma. The sarcolemma is a tubular sheath that encases and defines each muscle fibre, forming a barrier between extracellular and intracellular compartments.
Each muscle fibre is composed of several hundred to several thousand myofibrils. Myofibrils are composed of actin (thin filaments), myosin (thick filaments), and support proteins. The arrangement of actin and myosin gives skeletal muscle its microscopic striated appearance and creates functional units called sarcomeres. The two most significant myofilaments are actin and myosin filaments arranged distinctively to form various bands on the skeletal muscle.
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Muscle architecture
There are several types of muscle architecture, including parallel, pennate, and hydrostats. The type is determined by the direction in which the muscle fibers are oriented relative to the force-generating axis. Parallel muscles, like the rectus abdominus, have fibers that run parallel to the force-generating axis. Strap muscles, such as the laryngeal muscles, have fibers that run longitudinally to the contraction direction, allowing them to control the fundamental frequency in speech and singing.
Pennate muscles, on the other hand, have fibers that are oriented at an angle to the force-generating axis. These muscles can be further categorized into unipennate, bipennate, and multipennate. Unipennate muscles, like the lateral gastrocnemius, have fibers oriented at an angle to the force-generating axis and are all on the same side of a tendon. Bipennate muscles, such as those found in the limbs, have fibers attached to both sides of the tendon.
Convergent or triangular muscles, like the pectoralis major, have fibers that converge at one end, typically at a tendon, and spread out in a fan-shape at the other end. These muscles are versatile due to their ability to change the direction of pull based on fiber contraction. Fusiform muscles, like the biceps brachii, are wider in the center and taper off at the ends, resembling a spindle. Their line of action runs in a straight line between the attachment points, often tendons, resulting in a concentrated force.
Muscular hydrostats, such as those found in invertebrates, function independently of a hardened skeletal system. They are supported by a membrane of connective tissue that maintains a constant volume, enabling the fibers to stabilize the muscle structure. These muscles can contract along three lines of action relative to the long axis: parallel, perpendicular, and helical. The balance of forces along these lines allows for complex movements.
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Muscle contraction
The Sliding Filament Theory explains the process of muscle contraction. It suggests that the interaction between myosin and actin proteins is central to muscle contraction. Myosin binds to actin, and the sliding interaction between these two proteins results in sarcomere shortening, leading to a change in muscle length. This process is powered by the energy released from the hydrolysis of adenosine triphosphate (ATP) molecules, which are produced through the oxidation of fats and carbohydrates.
There are several types of muscle contractions, including isometric, isotonic, and eccentric contractions. An isometric contraction generates tension without changing the length of the muscle, such as when gripping an object. In isotonic contraction, the muscle tension remains constant despite changes in muscle length, occurring when the force of contraction matches the load on the muscle. Isotonic contractions can be further categorised into concentric and eccentric contractions. Concentric contractions involve muscle tension overcoming the load, resulting in the muscle shortening, like when lifting a heavy weight. On the other hand, eccentric contractions occur when the tension is insufficient to overcome the load, leading to muscle lengthening, such as when slowly lowering a weight.
The time between a stimulus to the motor nerve and the subsequent contraction of the muscle is called the latent period, typically lasting about 10 milliseconds. This delay is due to the time required for nerve action potential propagation, chemical transmission, and other steps in excitation-contraction coupling. Summation, achieved through frequency summation and multiple fibre summation, can increase muscle force by producing a summation of muscle twitches.
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Muscle naming conventions
Shape: The shape of a muscle can be reflected in its name. For instance, the deltoid muscle is a large, triangular-shaped muscle covering the shoulder. The name deltoid comes from the Greek letter delta, which is shaped like a triangle. Another example is the orbicularis muscle, which is named after its orbital shape.
Size: In certain muscle groups, such as the buttocks, the size of the muscles influences their names. For instance, the gluteus maximus is the largest, gluteus medius is medium-sized, and the gluteus minimus is the smallest.
Fiber Direction: The direction of muscle fibers can also be indicated in the muscle's name. There are three primary directions: parallel, perpendicular (also called transverse), and oblique. For example, the rectus abdominis indicates that the fibers are parallel to the midline, the transverse abdominis refers to perpendicular fibers, and the oblique muscles denote fibers running at an angle.
Location and Attachments: The location of a muscle often determines its name, particularly in relation to specific bones or structures. For example, the frontalis muscle is located on the frontal bone of the skull. The name of a muscle can also indicate its attachments, with the origin always named first. The sternocleidomastoid muscle, for instance, originates from the sternum and clavicle and inserts on the mastoid process of the temporal bone.
Number of Origins or Muscles in a Group: Some muscle names indicate the number of origins or muscles in a group. The prefix "quad" in quadriceps indicates four muscles in the group, while "bi" in biceps brachii indicates two origins, and "tri" in triceps brachii indicates three origins.
Action: Muscle names may also reflect the movement or action they produce. For example, flexors decrease the angle at a joint, extensors increase the angle, and abductors move the bone away from the midline.
Additionally, muscle names often have Latin or Greek roots, providing clues about their function, shape, action, or location.
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Muscle types
On the other hand, pennate muscles have fibres that are oriented at an angle to the force-generating axis, resulting in greater force production compared to parallel muscles. Examples of pennate muscles include the quads (quadriceps femoris) and the lateral gastrocnemius. Within pennate muscles, there are subtypes: unipennate, bipennate, and multipennate. Unipennate muscles have fibres oriented at an angle to the force-generating axis and are all on the same side of a tendon, while bipennate muscles have fibres attached to both sides of a tendon.
Hydrostats, such as strap muscles, have fibres that run longitudinally to the contraction direction. These muscles can shorten to about 40-60% of their resting length. The sartorius muscle is an example of a strap muscle. Another type of muscle architecture is the convergent or triangular muscle, where fibres converge at one end and spread out in a fan-shape at the other. The pectoralis major is an example of a convergent muscle. Rhomboidal muscles, such as the rhomboids or the gluteus maximus, have expansive proximal and distal attachments, making them well-suited for stabilisation or force generation.
Additionally, muscle fibres can be classified into two types: Type I (slow) and Type II (fast). Type II is further divided into type IIA (oxidative) and type IIX (glycolytic). These fibre types have distinct metabolic, contractile, and motor unit properties. The direction of muscle fibres relative to the midline can also be used for naming muscles. For example, perpendicular fibres are called transverse, as seen in the transversus abdominus.
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Frequently asked questions
Perpendicular muscle fibers are muscle fibers that are positioned at a 90-degree angle relative to the midline. They are also known as transverse muscle fibers.
Examples of perpendicular muscle fibers include the transversus abdominis, external and internal obliques, and the biceps brachii.
Perpendicular muscle fibers, also known as transverse fibers, enable muscles to contract and change shape. These contractions can apply or resist compressive forces, allowing for diverse and complex movements.
Perpendicular muscle fibers are different from parallel muscle fibers, which are arranged in a straight line and parallel to the midline. Perpendicular fibers are also distinct from oblique muscle fibers, which are positioned diagonally to the midline.
Perpendicular muscle fibers, as part of skeletal muscles, play a crucial role in producing movement, maintaining body posture, controlling body temperature, and stabilizing joints. They also contribute to the overall mechanical function and performance of the muscular system.
