10.2 Skeletal Muscle - Anatomy and Physiology | OpenStax (2023)

Skeletal Muscle Fibers

Because skeletal muscle cells are long and cylindrical, they are commonly called muscle fibers. Skeletal muscle fibers can be quite large for human cells, with diameters of up to 100eum and lengths up to 30 cm (11.8 in) on the upper leg Sartorius. During early development, embryonic myoblasts, each with its own nucleus, fuse with hundreds of other myoblasts to form multinucleated skeletal muscle fibers. Multiple nuclei mean multiple copies of genes, allowing the production of large amounts of proteins and enzymes needed for muscle contraction.

Some other terminology associated with muscle fibers is rooted in the Greeksarco, which means "flesh". The plasma membrane of muscle fibers is calledsarcolema, the cytoplasm is referred to assarcoplasma, and the specialized smooth endoplasmic reticulum, which stores, releases, and retrieves calcium ions (Ca⁺⁺) it's calledsarcoplasmic reticulum (SR)(Figure 10.4). As will be described shortly, the functional unit of a skeletal muscle fiber is the sarcomere, a highly organized arrangement of contractile myofilaments.actin(fine strand) emyosin(thick filament), along with other supporting proteins.

Figure10.4 muscle fiber A skeletal muscle fiber is surrounded by a plasma membrane called the sarcolemma, which contains sarcoplasm, the cytoplasm of muscle cells. A muscle fiber is made up of many fibrils, which give the cell its striated appearance.

(Video) BIO 246 A&P 10.1-10.2b Gross and Microscopic Muscle Anatomy

O Sarcômero

The striated appearance of skeletal muscle fibers is due to the arrangement of actin and myosin myofilaments in sequential order from one end of the muscle fiber to the other. Each bundle of these microfilaments and their regulatory proteins,troponinetropomiosina(along with other proteins) is calledsarcômero.

The sarcomere is the functional unit of the muscle fiber. The sarcomere itself is clustered within the myofibril that runs the length of the muscle fiber and attaches to the sarcolemma at its end. As the myofibrils contract, the entire muscle cell contracts. Because myofibrils are only about 1.2eum in diameter, hundreds to thousands (each with thousands of sarcomeres) can be found within a muscle fiber. Each sarcomere has approximately 2eum long with a three-dimensional arrangement similar to a cylinder and is bounded by structures called Z disks (also called Z lines, because the images are two-dimensional), to which the actin myofilaments are anchored (Figure 10.5). Because actin and its troponin-tropomyosin complex (projecting from the Z disks toward the center of the sarcomere) form thinner filaments than myosin, it is calledfine filamentof the sarcomere. Likewise, because the myosin filaments and their multiple heads (projecting from the center of the sarcomere, towards the Z disks, but not all towards them) have more mass and are thicker, they are calledthick filamentdo sarcômero.

Figure10,5 O Sarcômero The sarcomere, the region from one Z line to the next Z line, is the functional unit of a skeletal muscle fiber.

the neuromuscular junction

Another specialization of skeletal muscle is the location where the motor neuron terminal meets the muscle fiber – called theneuromuscular junction (JNM). It is here that the muscle fiber first responds to motor neuron signaling. Each skeletal muscle fiber in each skeletal muscle is innervated by a motor neuron in the JNM. Neuron excitation signals are the only way to functionally trigger fiber contraction.

alternation excitation-contraction

All living cells have membrane potentials, or electrical gradients across their membranes. The inside of the membrane is usually around -60 to -90 mV relative to the outside. This is known as the cell membrane potential. 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 protein channels are very small, they form the basis of both neural signaling and muscle contraction.

Both neurons and skeletal muscle cells are electrically excitable, meaning they are capable of generating action potentials. An action potential is a special type of electrical signal that can travel along a cell membrane like a wave. This allows a signal to be transmitted quickly and reliably over long distances.

although the termalternation excitation-contractionconfuses or frightens some students, it boils down to this: for a skeletal muscle fiber to contract, its membrane must first be “excited” – in other words, it must be stimulated to fire an action potential. The action potential of the muscle fiber, which travels through the sarcolemma like a wave, is “coupled” to the actual contraction through the release of calcium ions (Ca⁺⁺) of RS. Once released, Ca⁺⁺interacts with the protective proteins, forcing them apart so that the actin-binding sites are available for attachment by the myosin heads. 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. In other words, the “arousal” step in skeletal muscles is always triggered by nervous system signaling (Figure 10.6).

Figure10.6 Motor plate and innervation In JNM, the axon terminal releases ACh. The motor endplate is the location of ACh receptors in the sarcolemma of the muscle fiber. When ACh molecules are released, they diffuse across a tiny gap called the synaptic cleft and bind to receptors.

Motor neurons that instruct 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, that are specialized for transmitting action potentials over long distances – in this case, from the spinal cord to the muscle itself (which can be up to a meter away). The axons of several neurons group together to form nerves, like wires grouped together in a cable.

Signaling begins when a neuronalaction potentialtravels along the axon of a motor neuron and then along individual branches to terminate at the JNM. In JNM, the axon terminal releases a chemical messenger, orneurotransmitter, calledacetylcholine (ACh). ACh molecules diffuse through a tiny space calledsynaptic cleftand bind to ACh receptors located within theengine end plateof the sarcolemma on the other side of the synapse. Once ACh binds, a channel on the ACh receptor opens and positively charged ions can pass into the muscle fiber, causing it to shift.depolarize, which means that the muscle fiber membrane potential becomes less negative (closer to zero).

As the membrane depolarizes, another set of ion channels calledvoltage gated sodium channelsare triggered to open. Sodium ions enter the muscle fiber and an action potential rapidly spreads (or “fires”) across the entire membrane to initiate excitation-contraction coupling.

Things happen very quickly in the world of excitable membranes (think how fast you can snap your fingers once you decide to do so). Immediately after the membrane depolarizes, it repolarizes, restoring the negative membrane potential. Meanwhile, ACh in the synaptic cleft is degraded by the enzyme acetylcholinesterase (AChE), so that ACh cannot rebind to a receptor and reopen its channel, which would cause excitation and unwanted prolonged muscle contraction.

The propagation of an action potential along the sarcolemma is the excitation portion of the excitation-contraction coupling. Recall that this excitation actually triggers the release of calcium ions (Ca⁺⁺) from its storage in the cell's SR. For the action potential to reach the SR membrane, periodic invaginations occur in the sarcolemma, calledTubules T(“T” stands for “across”). You must remember that the diameter of a muscle fiber can reach 100eum, so these T tubules ensure that the membrane can get close to the SR in the sarcoplasm. The arrangement of a T tubule with the RS membranes on either side is called atriad(Figure 10.7). The triad surrounds the cylindrical structure calledmyofibrils, which contains actin and myosin.

Figure10.7 The T tubule Narrow T tubules allow the conduction of electrical impulses. The SR functions to regulate intracellular calcium levels. Two terminal cisternae (where the enlarged RS connects to the T-tubule) and a T-tubule make up a triad—a “three” of membranes, with those of the RS on two sides and the T-tubule sandwiched between them.

T tubules carry the action potential into the cell, which triggers the opening of calcium channels in the adjacent SR membrane, causing Ca⁺⁺diffuse out of the SR and into the sarcoplasm. It's the arrival of Ca⁺⁺in the sarcoplasm that initiates muscle fiber contraction by its contractile units, or sarcomeres.