Smooth Muscle, Striated Muscle, and Heart Function

Posted on Dec 14, 2024 in Biology

Smooth Muscle Cells

Smooth muscle cells are the simplest of the three muscle types.

  • Aspect
    • Smooth muscle cells are fusiform, elongated, with sharp ends and an enlarged central portion with an elongated nucleus.
    • Most are 5-20 µm in diameter (and up to 200 µm in the gravid uterus), and their length can vary between 20 µm and 1 mm or more (e.g., in the gravid uterus).
    • These descriptions are of isolated cells, as they usually form narrow beams and homogenous bundles, making it difficult to see their cell boundaries with an optical microscope.
  • Nucleus
    • The nucleus of smooth muscle cells is unique and takes the cell shape; therefore, it can be oval or elongated.
    • The chromatin has fine dots with one or two nucleoli.
  • Structure (see comparative table in skeletal muscle)
    • The cytoplasm (sarcoplasm) is acidophilic and homogeneous, except for a fine longitudinal striation that appears with some special techniques.
    • These striations correspond to the presence of myofibrils in a longitudinal arrangement.
    • Smooth muscle fibers are surrounded by a basement membrane rich in polysaccharides.
  • Tissue
    • Smooth muscle cell nuclei overlap and are located at different levels.
    • Cross sections dissect most of the cells through the ends, but in some planes, they pass through the nucleus in the more voluminous central region.
    • The organization of smooth muscle depends on the configuration and function of the organ.
    • Many organs have two or more layers of smooth muscle oriented in various directions.
    • They generally form bundles.
    • To this end, they are arranged parallel to the axis thereof, in compact form, and overlap each other so that the central thick portion of one of them is juxtaposed to the thin portion of the adjacent cell.
    • These booklets can also be isolated, as in the case of hair erector muscles and the iris.

Striated Muscle Tissue

Striated muscle tissue is the major component of the human body. Each muscle is individually wrapped by a sheath of connective tissue called the epimysium. This cover has extensions that penetrate into the muscle, resulting in the perimysium and the division of muscle bundles of varying sizes. Finally, each muscle fiber is wrapped in turn by a thin layer of tissue, the endomysium.

Individually, the muscle fibers are multinucleated syncytial cells, with nuclei disposed under the cell membrane, or sarcolemma. In a longitudinal section view of the microscope, two structures are featured: the myofibrils, with a longitudinal layout, and striations, with a perpendicular layout. Both are the result of the particular distribution of contractile proteins in striated muscle.

The length of muscle fibers in adults is variable and may reach 50 cm in the sartorius. The transverse diameters are also variable. In the brachial biceps, the size of Type I fibers is about 60 mm in men and about 57 mm in women, whereas type II fibers are of the order of 70 mm in men and 50 mm in women. This difference in size, higher in type II fibers in men and in type I fibers in women, is not seen until the age of fifteen. At younger ages, both types of fibers have similar diameters in both sexes.

The Human Heart

The human heart is a muscle that can stay strong and work well for a hundred years or more. If we reduce cardiovascular risk factors, we can keep the heart healthy for longer.

A heartbeat is a pumping action in two phases which takes approximately one second. As blood collects in the upper chambers (right and left atria), the heart’s natural pacemaker (SA node) sends an electrical signal that stimulates contraction of the atria. This contraction pushes blood through the tricuspid and mitral valves into the lower chambers that are at rest (left and right ventricles). This phase of the pumping action (the longest) is called diastole.

The second phase of the pumping action begins when the ventricles are full of blood. The electrical signals from the SA node travel along a path of electrical conduction to the ventricles, causing them to contract. This phase is called systole. With the tricuspid and mitral valves securely closed to prevent backflow of blood, the pulmonary and aortic valves open. While the right ventricle pushes blood to the lungs to pick up oxygen, oxygen-rich blood flows from the left ventricle to the heart and other body parts. When blood passes into the pulmonary artery and aorta, the ventricles relax, and the pulmonary and aortic valves close. By reducing the pressure in the ventricles, the tricuspid and mitral valves open, and the cycle begins again. This series of contractions is repeated constantly, increasing with effort and decreasing time at rest. But the heart does not act independently. The brain senses the conditions around us (the weather, stressors, and level of physical activity) and regulates the cardiovascular system to meet the needs of the organism under these conditions.

Heart Facts

The heart weighs between 7 and 15 ounces (200-425 grams) and is a little bigger than a fist. At the end of a long life, a person’s heart may have beat (i.e., expanded and contracted) more than 3.5 billion times. Each day, the average heart beats 100,000 times, pumping about 2,000 gallons (7,571 liters) of blood.

Your heart is found between the lungs in the middle of the chest, behind and slightly to the left of the sternum. A two-layered membrane called the pericardium surrounds your heart like a bag. The outer layer of the pericardium surrounds the roots of the major blood vessels of the heart and is attached to the spine, diaphragm, and other body parts by ligaments. The inner layer of the pericardium is attached to the heart muscle. A layer of fluid separates the two layers of the membrane, letting the heart move as it beats yet still be attached to the body.

Heart Chambers

The heart has four chambers. The upper chambers are called the “left atrium” and “right atrium,” and the lower chambers are called the “left ventricle” and “right ventricle.” A muscular wall called the septum separates the left and right atria and the left and right ventricles. The left ventricle is the largest and strongest chamber of the heart. The left ventricular walls have a thickness of only half an inch (just over one centimeter) but are strong enough to impel the blood through the aortic valve to the rest of the body.

Heart Valves

The valves that control the flow of blood through the heart are four:

  • The tricuspid valve controls blood flow between the right atrium and right ventricle.
  • The pulmonary valve controls blood flow from the right ventricle to the pulmonary arteries, which carry blood to the lungs to pick up oxygen.
  • The mitral valve allows oxygen-rich blood from the lungs to pass from the left atrium to the left ventricle.
  • The aortic valve allows oxygen-rich blood to pass from the left ventricle to the aorta, the body’s largest artery, which carries blood to the rest of the body.

The Heartbeat Conduction System

The electrical impulses generated by the heart muscle (the myocardium) cause contraction of the heart. This electrical signal begins in the sinoatrial (SA) node, located at the top of the right atrium. The SA node is also called the “natural pacemaker” of the heart. The electrical impulses of this natural pacemaker spread through the muscle fibers of the atria and ventricles, causing them to contract. Although the SA node sends electrical impulses at a certain speed, heart rate may vary with physical demands, the level of stress, or hormonal factors.

The Circulatory System

The heart and circulatory system make up the cardiovascular system. The heart acts as a pump that forces blood to the organs, tissues, and cells. Blood delivers oxygen and nutrients to every cell and removes carbon dioxide and waste products from these cells. Blood is carried from the heart to the body through a complex network of arteries, arterioles, and capillaries and back to the heart through venules and veins. If you join all the vessels of this extensive network and place them in a straight line, they would cover a distance of 60,000 miles (more than 96,500 kilometers), enough to circle the Earth more than twice.