Muscle Physiology - Anatomy

Skeletal muscle is the only muscle type that is multinucleated. Both cardiac and smooth muscle cells have only one nucleus.

Smooth muscle is under involuntary control, innervated by the autonomic nervous system, and contains mononucleated cells. Skeletal muscle is striated, multinucleated, and under voluntary control. Cardiac muscle is striated, mononucleated, and under involuntary control.

Cardiac muscle also uses intercalated discs, specialized cellular junctions, to facilitate electrical conduction between cardiomyocytes. This helps coordinate the contraction of the heart.

Cardiac muscle is physiologically and morphologically distinct from skeletal and smooth muscle. Instead of using myosin light chain kinase (like smooth muscle), cardiac muscle uses the same sarcomere pattern of skeletal muscle. This explains the presence of striations in both types of tissue.

Cardiac muscle is unique, however, in that it has gap junctions that allow the exchange of ions between individual cells. This allows the myocardium, or muscular portion of heart tissue, to beat in a coordinated fashion, as cells are depolarizing alongside one another. Additionally, intercalated discs are present at the ends of sarcomeres, but are not present in skeletal muscle.

These two characteristics allow us to conclude that muscle cell type 2 is cardiac muscle, and will be found in the myocardium.

The presence of gap junctions within the intercalated discs of contractile cardiac myocytes allows for the rapid passage of ions from one cell to another. Once pacemaker cells in the sinoatrial node of the heart spontaneously generate action potentials, this wave of depolarization spreads into neighboring contractile myocytes via gap junctions. These gap junction connections are crucial to the heart operating in a unified and coordinated fashion, and are responsible for the characteristic wavelike contraction of the heart from the apex to the base.

The pericardial membrane is the tissue that surrounds the heart. The easiest way to determine the answer in this problem is to understand that "cardial" indicates pertinence heart and that "peri" is a prefix meaning "around."

Cardiac output (CO) is defined as:

Rearrange to solve for heart rate.

Slow twitch fibers are located within muscles used for endurance and posture. They have a red coloring due to an abundance of myoglobin and participate in aerobic glycolysis. Slow twitch fibers contain large amounts of mitochondria and fewer glycolytic enzymes in order to enhance ATP production and maintain contractions for longe periods of time. Slow twitch fibers are associated with endurance fitness, such as running a marathon, and prolonged postural contractions, such as the muscles that keep the spine and neck erect.

Fast twitch fibers are located within muscles used for spurt and explosive excercises. They have a white coloring due to a lack in myoglobin and participate in anaerobic glycolysis. Fast twitch fibers contain fewer mitochondria and more glycolytic enzymes, aiding in rapid (but unsastainable) energy production. Fast twitch fibers are associated with speed and agility fitness, such as a sprinting a short distance.

Most skeletal muscles have origins closer to the midline than their insertions. The bone from which the muscle originates is typically held steady, while the bone onto which the muscle inserts is pulled toward the origin point of the muscle during flexion. For example, the biceps brachii originate on the scapula and insert on the radius. When activated, the biceps brachii pull the radius toward the scapula.

Gap junctions only occur in skeletal and smooth muscle. Through gap junctions, the signal from the neuron to contract may diffuse throughout the muscle, causing wave-like contraction of smooth and/or cardiac muscle.

Intercalated discs are found only in cardiac muscle; they add structural stability to cardiac muscle, and help the propagation of action potentials due to gap junctions. The other features listed are features common to both skeletal and cardiac muscle.

Type I muscle (slow oxidative) consists of slow-twitch muscle fibers. Its fibers are red due to high concentrations of myoglobin, and it has a high density of mitochondria (and thus increased oxidative phosphorylation). This leads to a sustained contraction, and resistance to fatigue. Think "one slow red ox."

Individual skeletal muscle fibers are multinucleate and behave as single units. They contain bundles of myofibrils which are surrounded by a sarcoplasmic reticulum and is invaginated by transverse tubules. Each myofibril contains interdigitating thick and thin filaments arranged longitudinally in sarcomeres. Repeating units of sarcomeres account for the unique banding pattern in striated muscle.

The perimysium surrounds the fascicles in a skeletal muscle fiber. From largest to smallest, the sequence goes as follows: the Epimysium surrounds the entire muscle; the Perimysium surrounds the fascicles inside the muscle; the Endomysium surrounds the myofiber (muscle cell).

The vastus medialis, one of the quadricep muscles is located in the anterior compartment of the thigh and is supplied by the femoral artery.

The term pennate describes muscles whose fibers insert obliquely into the tendon. The vastus medialis, rectus femoris, and deltoid are all examples of this type of muscle.

The fibularis longus is supplied by the fibular artery, a branch of the posterior tibial artery. All other statements are true.

The extensor digitorum longus attaches from the proximal anterior fibula to the dorsal surface of toes 2 through 5.

Characteristics of smooth muscle include: striated, somatic innervation, multinucleated, and requiring calcium for contraction. Furthermore, similar characteristics are seen in cardiac muscle such as: striations, autonomic innervation, 1-2 nuclei, requires calcium for contraction. Finally, smooth muscle is nonstriated, has autonomic innervation, mononucleated, and requiring calcium for contraction.

Calcium is released from the sarcoplasmic reticulum into the the sarcoplasm. It binds the troponin molecules on the thin filaments, causing the strands of tropomyosin to shift, exposing the myosin-binding sites on the thin filaments.

Calcium is stored in the sarcoplasmic reticulum at high concentrations. When neurons signal the contraction of muscle, calcium is released from the sarcoplasmic reticulum, and facilitates the contraction of muscle fibers and ATP hydrolysis, to generate muscle contraction force.