What are myofibrils and myofilaments? | Socratic
Actin filaments, usually in association with myosin, are responsible for many types Most of the cytoplasm consists of myofibrils, which are cylindrical bundles of. Myofilaments are bundled together to make a myofibril. The search returns a description to the keyword and an associated image if available. If the search does not return a results, a link to a Google search is presented. acetylcholine · myofibrils · I-band · A-band · Z-line · H-band · M-line · myofilament · thick The myofibril contains several important histological landmarks.
Each muscle is surrounded by a connective tissue sheath called the epimysium. Fascia, connective tissue outside the epimysium, surrounds and separates the muscles. Portions of the epimysium project inward to divide the muscle into compartments. Each compartment contains a bundle of muscle fibers.
Each bundle of muscle fiber is called a fasciculus and is surrounded by a layer of connective tissue called the perimysium. Within the fasciculus, each individual muscle cell, called a muscle fiber, is surrounded by connective tissue called the endomysium.
Skeletal muscles have an abundant supply of blood vessels and nerves. Before a skeletal muscle fiber can contract, it has to receive an impulse from a neuron.
Generally, an artery and at least one vein accompany each nerve that penetrates the epimysium of a skeletal muscle. Branches of the nerve and blood vessels follow the connective tissue components of the muscle of a nerve cell and with one or more minute blood vessels called capillaries Source: The cell membrane of a muscle cell is called the sarcolemma, and this membrane, like that of neurons, maintains a membrane potential.
So, impulses travel along muscle cell membranes just as they do along nerve cell membranes. However, the 'function' of impulses in muscle cells is to bring about contraction.
To understand how a muscle contracts, you need to know a bit about the structure of muscle cells. Skeletal muscle is the muscle attached to the skeleton. Hundreds or thousands of muscle fibers cells bundle together to make up an individual skeletal muscle.
Muscle cells are long, cylindrical structures that are bound by a plasma membrane the sarcolemma and an overlying basal lamina and when grouped into bundles fascicles they make up muscle.
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- SKELETAL MUSCLE ORGANIZATION
- Muscle Contraction
The sarcolemma forms a physical barrier against the external environment and also mediates signals between the exterior and the muscle cell. The sarcoplasm is the specialized cytoplasm of a muscle cell that contains the usual subcellular elements along with the Golgi apparatus, abundant myofibrils, a modified endoplasmic reticulum known as the sarcoplasmic reticulum SRmyoglobin and mitochondria. Transverse T -tubules invaginate the sarcolemma, allowing impulses to penetrate the cell and activate the SR.
Myofibrils are contractile units that consist of an ordered arrangement of longitudinal myofilaments. Myofilaments can be either thick filaments comprised of myosin or thin filaments comprised primarily of actin. Upon depolarization, the pre-synaptic vesicles containing the neurotransmitter acetylcholine fuse with the membrane, releasing acetylcholine into the cleft. Acetylcholine binds to receptors on the post-synaptic membrane and causes depolarization of the muscle fiber, which leads to its contraction.
Typically, one action potential in the neuron releases enough neurotransmitter to cause one contraction in the muscle fiber. In muscle cells, the sarcolemma or plasma membrane extends transversely into the sarcoplasm to surround each myofibril, establishing the T-tubule system.
These T-tubules allow for the synchronous contraction of all sarcomeres in the myofibril. The T-tubules are found at the junction of the A- and I- bands and their lumina are continuous with the extracellular space. At such junctions, the T-tubules are in close contact with the sarcoplasmic reticulum, which forms a network surrounding each myofibril.
The part of the sarcoplasmic reticulum associated with the T-tubules is termed the terminal cisternae because of its flattened cisternal arrangement. When an excitation signal arrives at the neuromuscular junction, the depolarization of the sarcolemma quickly travels through the T-tubule system and comes in contact with the sarcoplasmic reticulum, causing the release of calcium and resulting in muscle contraction. Smooth Muscle Smooth muscle forms the contractile portion of the wall of the digestive tract from the middle portion of the esophagus to the internal sphincter of the anus.
It is found in the walls of the ducts in the glands associated with the alimentary tract, in the walls of the respiratory passages from the trachea to the alveolar ducts, and in the urinary and genital ducts. The walls of the arteries, veins, and large lymph vessels contain smooth muscle as well. Smooth muscle is specialized for slow and sustained contractions of low force.
Instead of having motor units, all cells within a whole smooth muscle mass contract together. Smooth muscle has inherent contractility, and the autonomic nervous system, hormones and local metabolites can influence its contraction. Since it is not under conscious control, smooth muscle is involuntary muscle.
Smooth muscle fibers are elongated spindle-shaped cells with a single nucleus. In general, they are much shorter than skeletal muscle cells. The nucleus is located centrally and the sarcoplasm is filled with fibrils. The thick myosin and thin actin filaments are scattered throughout the sarcoplasm and are attached to adhesion densities on the cell membrane and focal densities within the cytoplasm.
Since the contractile proteins of these cells are not arranged into myofibrils like those of skeletal and cardiac muscle, they appear smooth rather than striated. Smooth muscle fibers are bound together in irregular branching fasciculi that vary in arrangement from organ to organ. These fasciculi are the functional contractile units. There is also a network of supporting collagenous tissues between the fibers and the fasciculi.
Cardiac Muscle Cardiac muscle shares important characteristics with both skeletal and smooth muscle.
Functionally, cardiac muscle produces strong contractions like skeletal muscle. However, it has inherent mechanisms to initiate continuous contraction like smooth muscle. The rate and force of contraction is not subject to voluntary control, but is influenced by the autonomic nervous system and hormones.
Skeletal Muscle Fibers
Histologically, cardiac muscle appears striated like the skeletal muscle due to arrangement of contractile proteins. It also has several unique structural characteristics: Let's discuss each myofilament in turn. Each actin molecule is composed of two strands of fibrous actin F actin and a series of troponin and tropomyosin molecules. Each F actin is formed by two strings of globular actin G actin wound together in a double helical structure, much like twisting two strands of pearls with each other.
Each G actin molecule would be represented by a pearl on our hypothetical necklace.
Each G actin subunit has a binding site for the myosin head to attach to the actin. Tropomyosin is a long string-like polypeptide that parallels each F actin strand and functions to either hide or expose the "active sites" on each globular actin molecule.
Actin, Myosin, and Cell Movement - The Cell - NCBI Bookshelf
Each tropomyosin molecule is long enough to cover the active binding sites on seven G-actin molecules. These proteins run end-to-end the entire length of the F actin. Associated with each tropomyosin molecule is a third polypeptide complex known as troponin. Troponin complexes contain three globular polypeptides Troponin I, Troponin T, and Troponin C that have distinct functions. Troponin I binds to actin, Troponin T binds to tropomyosin and helps position it on the F actin strands, and Troponin C binds calcium ions.
There is one troponin complex for each tropomyosin. When calcium binds to Troponin C, it causes a conformational change in the entire complex that results in exposure of the myosin binding sites on the G actin subunits.
More on this later. The thick myofilaments are composed chiefly of the protein myosin, and each thick myofilament is composed of about myosin molecules bound together. Each myosin is made up of 6 proteins subunits, 2 heavy chains and 4 light chains. The heavy chains have a shape similar to a golf club, having a long shaft-like structure, to which is connected the globular myosin head. The shafts, or tails wrap around each other and interact with the tails of other myosin molecules, forming the shaft of the thick filament.
The globular heads project out at right angles to the shaft. Half of the myosin molecules have their heads oriented toward one end of the thick filament, and the other half are oriented in the opposite direction. It is the myosin heads that bind to the active sites on the actin.
The connection between the head and the shaft of the myosin molecules functions as a hinge, and as such is referred to as the hinge region. The hinge region can bend, and as we shall see later, creates the power stroke when the muscle contracts. The center of the thick filaments are composed only of the shaft portions of the heavy chains. It is the ATP that provides the energy for muscle contraction.
Each of the myosin heads is associated with two myosin light chains that play a role in regulating the actions of the myosin heads, but the exact mechanism is not fully understood. The three dimensional arrangement of the myosin heads is very important. Imagine that you were looking at a thick filament from the end, and that there is a myosin head sticking straight up. As you moved around the circumference of the thick filament, you would see myosin heads every 30 degrees.
This allows each thick filament to interact with 6 thin filaments. Likewise, each thin filament can interact with three thick filaments. This arrangement requires that there be two thin filaments for every thick filament in the myofibril see image below. Adapted from the following image: During muscle contraction, the myosin heads link the thick and thin myofilaments together, forming cross bridges that cause the thick and thin myofilaments to slide over each other, resulting in shortening of each sarcomere, each skeletal muscle fiber, and the muscle as a whole-much like the two parts of an extension ladder slide over each other.
To summarize, in order for the shortening of the muscle to occur, the myosin heads have three important properties: