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The human body is a complex system, and its muscular structure is no exception. In this topic, we will be delving into the specific muscle highlighted and exploring its role in the intricate network of human physiology. By understanding the function, anatomy, and interactions of this muscle, we can gain valuable insights into how our bodies work and move. This knowledge is not just academic; it has practical applications in fields such as sports, medicine, and even our understanding of the human condition. So, let's begin our exploration and identify the muscle in question.
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Muscles of respiration
The muscles of respiration, also known as the 'breathing pump muscles', are a group of muscles that work together to facilitate inhalation and exhalation. These muscles form a complex arrangement around the lungs, similar to semi-rigid bellows. The primary function of these muscles is to expand and contract the thoracic cavity, allowing air to move in and out of the lungs.
The diaphragm is the major muscle responsible for breathing and is considered the chief muscle of inspiration. It is a thin, dome-shaped muscle located between the thoracic and abdominal cavities. During inhalation, the diaphragm contracts, moving downward and causing the ribs to move outward and upward, thereby expanding the thoracic cavity and drawing air into the lungs. When the diaphragm relaxes during exhalation, the elastic recoil of the lungs causes the thoracic cavity to contract, forcing air out of the lungs and returning to its dome shape.
In addition to the diaphragm, the intercostal muscles also play a crucial role in respiration. These muscles are attached between the ribs and help manipulate the width of the rib cage. There are three layers of intercostal muscles, with the external intercostal muscles being the most important in respiration. The contraction of these fibres raises each rib upward and forward, expanding the rib cage and assisting in inhalation. The internal intercostal muscles, on the other hand, pull the ribs downward and inward, reducing the size of the thoracic cavity during exhalation.
Accessory muscles of respiration are those that assist in breathing but do not play a primary role. Examples of accessory inspiratory muscles include the sternocleidomastoid, scalenus anterior, medius, and posterior, pectoralis major and minor, and serratus anterior, among others. These muscles help elevate the rib cage and assist in inhalation. The accessory expiratory muscles, such as the abdominal muscles (rectus abdominis, external oblique, internal oblique, and transversus abdominis), depress the rib cage and aid in exhalation.
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Contractility
The preload and afterload of ventricular muscle also influence contractility. The preload refers to the degree of stretch of the ventricular muscle, and the afterload refers to the resistance against which the muscle works. By increasing preload, contractility is increased. Factors that increase contractility are known as positive inotropic factors, while those that decrease it are called negative inotropic factors. For example, sympathetic stimulation is a positive inotrope, while parasympathetic stimulation is a negative inotrope. Higher concentrations of intracellular calcium also increase contractility.
The contractile state of the myocardium is influenced by multiple forces. Calcium and drugs such as norepinephrine can increase the rate at which tension develops in the cardiac muscle. However, contractility assessment is complex and challenging to measure in the intact heart. An assessment of contractility should ideally be load-independent, but obtaining such a measurement remains controversial.
Spinobulbar muscular atrophy (SBMA) is a neuromuscular disease characterised by muscle weakness and atrophy. It is caused by a trinucleotide repeat expansion in the androgen receptor gene on the X chromosome. Studies have shown that contractility is reduced in SBMA patients, with a decrease in muscle contractility potentially relating to motor neuron degeneration and changes in fibre type distribution and muscle architecture.
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Myosin and actin
The interaction of the protein filaments myosin and actin is fundamental to muscular contraction in all animals. Myosin molecules interact with actin filaments in muscle cells, producing muscular movement. This interaction was first described in groundbreaking papers published in 1954 by Huxley and Niedergerke, and Huxley and Hanson, who used high-resolution microscopy to observe the position of myosin and actin filaments during various stages of contraction in muscle fibres.
The sliding filament theory, proposed by these scientists, states that muscle tension is generated by the sliding of actin past myosin. Actin filaments are tethered to structures called z discs or "z bands", located at the lateral ends of each sarcomere. When the actin filament length shortens, so does the sarcomere and, consequently, the muscle. Huxley and Niedergerke, and Huxley and Hanson, observed that the "A band", a zone of the repeated sarcomere arrangement containing thick filaments of myosin, remained relatively constant in length during contraction. Meanwhile, the "I band", rich in thinner actin filaments, changed length along with the sarcomere.
The myosin-actin interaction can be likened to a person standing between two bookcases (representing the z discs) and pulling them in via ropes (representing actin). The person's arms are analogous to the myosin molecules. The myosin molecules remain centred during normal muscle contraction, just as the person remains centred between the bookcases. The myosin S1 region, the globular end nearest to actin, has multiple hinged segments that enable bending and facilitate contraction. The slimmer and typically longer "tail" region of myosin (S2) also exhibits flexibility, rotating in concert with the S1 contraction.
The myosin-actin cycling process involves myosin reaching forward, binding to actin, contracting, releasing actin, and then reaching forward again to bind actin in a new cycle. This cyclic rowing action, powered by ATP-hydrolysis, produces the macroscopic muscular movements we are familiar with.
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Masseter insertion
The masseter muscle is a powerful, thick, rectangular muscle with two divisions: superficial and deep. It is one of the muscles of mastication (chewing) and is involved in movements of the mandible. The masseter muscle originates from the zygomatic arch and inserts along the angle and lateral surface of the mandibular ramus.
The insertion of the masseter muscle along the angle and lateral surface of the mandibular ramus allows it to aid in the protrusion of the mandible, enabling the anterior motion of the jaw. This means that the masseter muscle helps to push the jaw forward. The masseter muscle is involved in elevating and protracting the mandible, which occurs during the closing of the jaws.
The superficial head of the masseter muscle originates at the maxillary process of the zygomatic bone (cheekbone) and the inferior (lower) border of the zygomatic arch. It then inserts into the lateral surface of the ramus and the angle of the mandible. The deep head of the masseter muscle originates at the deep or inferior surface of the zygomatic arch and inserts into the upper half of the ramus as high as the coronoid process of the mandible.
The masseter muscle is a large muscle located on both sides of the face, extending from the cheekbone down to the angle of the mandible. It is important for mastication, as it helps to elevate and protrude the lower jaw, allowing a person to bite down or chew their food. The masseter muscle can become enlarged in patients who habitually clench or grind their teeth, chew gum, or have bruxism.
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Temporalis
The temporalis muscle is a large, thin, fan-shaped muscle situated within the temporal fossa of the skull. It is one of the four primary muscles of mastication (chewing of food). The temporalis muscle is the strongest muscle of the temporomandibular joint and the primary retractor of the mandible. The contraction of the posterior fibres of the temporalis muscle results in the backward movement of the mandible (retrusion). The contraction of its anterior fibres moves the mandible dorsocranially (elevation). In unison, these actions facilitate the closing of the mouth and the approximation of the teeth. Additionally, the unilateral contraction of the temporalis muscle plays an important role in the side-to-side movements of the jaw.
The temporalis muscle is divided into the anterior, middle and posterior parts. The anterior fibres run inferiorly, in an almost vertical direction, while its posterior fibres are directed almost horizontally. The middle portion contains oblique fibres, which pass through the lateral aspect of the skull. Both anterior and posterior fibres converge onto a narrow tendon that runs medial to the zygomatic arch. The tendon inserts onto the apex and medial surface of the coronoid process of the mandible. The temporalis muscle covers the temporal fossa with its deep surface. Superficially, the muscle is covered by the temporal fascia, masseter muscle, subcutaneous tissue and skin. The temporalis muscle is the third most commonly involved of the masticatory muscles after the masseter and lateral pterygoid in myofascial TrP pain syndrome.
The temporalis muscle is supplied by the deep temporal nerves of the anterior division of the mandibular nerve. It may also receive branches from the middle temporal nerve. The blood supply is furnished by the middle and deep temporal arteries, branches of the superficial temporal artery, and internal maxillary artery, respectively.
Tension of the temporal muscle can induce pain in the temporal area. Common causes include vasculitides, such as giant cell arteritis, which can cause swelling and massive pain in the temporal area. Clinically, it is important to rule out an inflammation of the superficial temporal artery, which runs in front of the ear along the zygomatic arch to the temporal area.
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Frequently asked questions
The masseter.
The temporalis.
The pectoralis minor.
