Types of muscle cells
Muscle cells, commonly known as myocytes, are the cells that make up muscle tissue. There are 3 types of muscle cells in the human body; cardiac, skeletal, and smooth. Cardiac and skeletal myocytes are sometimes referred to as muscle fibers due to their long and fibrous shape. Cardiac muscle cells, or cardiomyocytes, are the muscle fibers comprise the myocardium, the middle muscular layer, of the heart. Skeletal muscle cells make up the muscle tissues connected to the skeleton and are important in locomotion. Smooth muscle cells are responsible for involuntary movement, like that of the intestines during peristalsis (contraction to propel food through the digestive system).
Skeletal Muscle cells
Skeletal muscle cells are long, cylindrical, and striated. They are multi-nucleated meaning that they have more than one nucleus. This is because they are formed from the fusion of embryonic myoblasts. Each nucleus regulates the metabolic requirements of the sarcoplasm around it. Skeletal muscle cells have high energy requirements, so they contain many mitochondria in order to generate sufficient ATP.
Skeletal muscle cells, a striated muscle cell type, form the muscle that we use to move, and are compartmentalized into different muscle tissues around the body, such as that of the biceps. Skeletal muscles are attached to bones by tendons and can be as long as 30 cm, although they are usually 2 to 3 cm in length.
The anatomy of muscle cells differs from that of other body cells and biologists have applied specific terminology to different parts of these cells. The cell membrane of a muscle cell is known as the sarcolemma and the cytoplasm is called sarcoplasm. The sarcoplasm contains myoglobin, an oxygen storage site, as well as glycogen in the form of granules in the cytosol, which both provide an energy supply.
Sarcoplasm also contains many tubular protein structures called myofibrils, which are made up of myofilaments. There are 3 types of myofilament; thick, thin, and elastic. Thick myofilaments are made from myosin, a type of motor protein, whilst thin myofilaments are made from actin, another type of protein used by cells for structure. Elastic myofilaments are composed of a springy form of anchoring protein known as titin. Together these myofilaments work to create muscle contractions by allowing the myosin protein heads to walk along the actin filaments creating a sliding action. The basic unit of striated (striped) muscle is a sarcomere comprised of actin (light bands) and myosin (dark bands) filaments.
The muscle contractions of striated muscle cells are regulated by calcium ion concentration, which is in turn regulated by a structure known as the sarcoplasmic reticulum. This structure is similar to the smooth endoplasmic reticulum of other types of cell. To produce contractile force, myosin associates with actin filaments, rotating a little and then pulling the filaments across each other, like oars propelling a boat. Skeletal muscle cells also contain two regulatory proteins, known as troponin and tropomyosin. These prevent myosin head binding site of actin from associating with myosin. The myosin head binding site on the actin filament remains covered until calcium ions are released from the sarcoplasmic reticulum (SR). The calcium ions being released from the SR is an end result of a chain of events in the contraction cycle started by an action potential triggering the release of acetylcholine (ACh), a neurotransmitter.
This process is enhanced by structures known as transverse tubules or T-tubules, which are invaginations of the sarcolemma, allowing depolarization to reach the inside of the cell more quickly. A T-tubule, flanked by enlarged sarcoplasmic reticulums called terminal cisternae, form a structure called a triad. This is involved in depolarization and activation of the muscle cell, resulting in contraction. As contraction requires energy, striated muscle cells contain many large mitochondria, which in muscle cells are referred to as sarcosomes.
Cardiac Muscle Cells
Cardiomyocytes are short and narrow, and fairly rectangular in shape. They are around 0.02 mm wide and 0.1 mm (millimeters) long. Cardiomyocytes contain many sarcosomes, which provide the required energy for contraction. Unlike skeletal muscle cells, cardiomyocytes normally contain a single nucleus. Cardiomyocytes generally contain the same cell organelles as skeletal muscle cells, although they contain more sarcosomes.
Cardiomyocytes are large and muscular, and are structurally connected by intercalated discs which have gap junctions for diffusion and communication. The discs appear as dark bands between cells and are a unique aspect of cardiomyocytes. They result from membranes of adjacent myocytes being very close together, and form a kind of glue between cells. This allows the transmission of contractile force between cells as electrical depolarization propagates from cell to cell. The key role of cardiomyocytes is to generate enough contractile force for the heart to beat effectively. They contract together in unison, causing enough pressure to force blood around the body.
Cardiomyocytes can not divide effectively, meaning that if heart cells are lost, they cannot be replaced. The result of this is that each individual cell must work harder to produce the same output. In response to the body’s need for a greater cardiac output, cardiomyocytes can grow larger (this process is known as hypertrophy). If the cells still cannot produce the amount of contractile force that the body requires, heart failure will occur. However, satellite cells (nurse cells) are present in cardiac muscle. These are myogenic cells which act to replace damaged muscle, although their numbers are limited. Satellite cells are also present in skeletal muscle cells.
Smooth Muscle Cells
Smooth muscle cells are spindle-shaped and contain a single central nucleus. They range from 10 to 600 μm (micrometers) in length, and are the smallest type of muscle cell. They are elastic and therefore important in the expansion of organs such as the kidneys, lungs, and vagina. The myofibrils of smooth muscle cells are not aligned like in cardiac and skeletal muscle meaning that they are not striated, hence, the name smooth. Smooth muscle cells are arranged together in sheets and this organisation means that they can contract simultaneously. They have poorly developed sarcoplasmic reticulums and do not contain T-tubules, due to the restricted size of the cells. However, they do contain other normal cell organelles such as sarcosomes but in lower numbers.
Location and functions
Smooth muscle cells are responsible for involuntary contractions and are found in the walls of blood vessels and hollow organs such as the gastrointestinal tract, uterus , and bladder. They are also present in the eye and contract changing the shape of the lens causing the eye to focus. Smooth muscle is also responsible for waves of contraction throughout the digestive system, forcing food to move through the body (peristalsis).
As with cardiac and skeletal muscle cells, smooth muscle cells contract as a result of depolarization of the sarcolemma. In smooth muscle cells this is facilitated by gap junctions. Gap junctions are tunnels which allow impulses to be transmitted between them, so that depolarization can spread, causing the myocytes to contract together in unison.