Each myofibril, in turn, contains several varieties of protein molecules, called myofilaments. The larger, or thick myofilaments are made of the protein, myosin, and the smaller thin myofilaments are chiefly made of the protein, actin. 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. 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. 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.
Some skeletal muscles are broad in shape and some narrow. In some muscles the fibers are parallel to the long axis of the muscle; in some they converge to a narrow attachment; and in some they are oblique. Each skeletal muscle fiber is a single cylindrical muscle cell. An individual skeletal muscle may be made up of hundreds, or even thousands, of muscle fibers bundled together and wrapped in a connective tissue covering.
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 packet of these microfilaments and their regulatory proteins, troponin and tropomyosin along with other proteins is called a sarcomere.
Watch this video to learn more about macro- and microstructures of skeletal muscles. The sarcomere is the functional unit of the muscle fiber. The sarcomere itself is bundled within the myofibril that runs the entire length of the muscle fiber and attaches to the sarcolemma at its end.
As myofibrils contract, the entire muscle cell contracts. Because myofibrils are only approximately 1. Because the actin and its troponin-tropomyosin complex projecting from the Z-discs toward the center of the sarcomere form strands that are thinner than the myosin, it is called the thin filament of the sarcomere. Likewise, because the myosin strands and their multiple heads projecting from the center of the sarcomere, toward but not all to way to, the Z-discs have more mass and are thicker, they are called the thick filament of the sarcomere.
This is where the muscle fiber first responds to signaling by the motor neuron. Every skeletal muscle fiber in every skeletal muscle is innervated by a motor neuron at the NMJ.
Excitation signals from the neuron are the only way to functionally activate the fiber to contract. Every skeletal muscle fiber is supplied by a motor neuron at the NMJ. Watch this video to learn more about what happens at the NMJ. All living cells have membrane potentials, or electrical gradients across their membranes. The inside of the membrane is usually around to mV, relative to the outside. Neurons and muscle cells can use their membrane potentials to generate electrical signals. They do this by controlling the movement of charged particles, called ions, across their membranes to create electrical currents.
This is achieved by opening and closing specialized proteins in the membrane called ion channels. Although the currents generated by ions moving through these channel proteins are very small, they form the basis of both neural signaling and muscle contraction. Both neurons and skeletal muscle cells are electrically excitable, meaning that they are able to generate action potentials.
An action potential is a special type of electrical signal that can travel along a cell membrane as a wave. This allows a signal to be transmitted quickly and faithfully over long distances. The myosin then pulls the actin filaments toward the center, shortening the muscle fiber. In skeletal muscle, this sequence begins with signals from the somatic motor division of the nervous system. The motor neurons that tell the skeletal muscle fibers to contract originate in the spinal cord, with a smaller number located in the brainstem for activation of skeletal muscles of the face, head, and neck.
These neurons have long processes, called axons, which are specialized to transmit action potentials long distances— in this case, all the way from the spinal cord to the muscle itself which may be up to three feet away.
The axons of multiple neurons bundle together to form nerves, like wires bundled together in a cable. Signaling begins when a neuronal action potential travels along the axon of a motor neuron, and then along the individual branches to terminate at the NMJ.
At the NMJ, the axon terminal releases a chemical messenger, or neurotransmitter , called acetylcholine ACh. The ACh molecules diffuse across a minute space called the synaptic cleft and bind to ACh receptors located within the motor end-plate of the sarcolemma on the other side of the synapse.
Once ACh binds, a channel in the ACh receptor opens and positively charged ions can pass through into the muscle fiber, causing it to depolarize , meaning that the membrane potential of the muscle fiber becomes less negative closer to zero.
As the membrane depolarizes, another set of ion channels called voltage-gated sodium channels are triggered to open.
Things happen very quickly in the world of excitable membranes just think about how quickly you can snap your fingers as soon as you decide to do it. Immediately following depolarization of the membrane, it repolarizes, re-establishing the negative membrane potential. Meanwhile, the ACh in the synaptic cleft is degraded by the enzyme acetylcholinesterase AChE so that the ACh cannot rebind to a receptor and reopen its channel, which would cause unwanted extended muscle excitation and contraction.
Propagation of an action potential along the sarcolemma is the excitation portion of excitation-contraction coupling. The arrangement of a T-tubule with the membranes of SR on either side is called a triad Figure.
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