Understanding Skeletal Muscle: Layers, Contraction, and Neuromuscular Junction | Summaries Acting

Dr Anjali Saxena Page 1

SKELETAL MUSCLE- NOTES I

Learning Objectives

By the end of this section, you will be able to:

• Describe the layers of connective tissues packaging skeletal muscle

• Explain how muscles work with tendons to move the body

• Identify areas of the skeletal muscle fibers

• Describe excitation-contraction coupling & Neuromuscular junction

The best-known feature of skeletal muscle is its ability to contract and cause movement.

Skeletal muscles act not only to produce movement but also to stop movement, such as

resisting gravity to maintain posture. Small, constant adjustments of the skeletal muscles are

needed to hold a body upright or balanced in any position. Muscles also prevent excess

movement of the bones and joints, maintaining skeletal stability and preventing skeletal

structure damage or deformation. Joints can become misaligned or dislocated entirely by

pulling on the associated bones; muscles work to keep joints stable. Skeletal muscles are

located throughout the body at the openings of internal tracts to control the movement of

various substances. These muscles allow functions, such as swallowing, urination, and

defecation, to be under voluntary control. Skeletal muscles also protect internal organs

(particularly abdominal and pelvic organs) by acting as an external barrier or shield to

external trauma and by supporting the weight of the organs.

Skeletal muscles contribute to the maintenance of homeostasis in the body by generating

heat. Muscle contraction requires energy, and when ATP is broken down, heat is produced.

This heat is very noticeable during exercise, when sustained muscle movement causes body

temperature to rise, and in cases of extreme cold, when shivering produces random skeletal

muscle contractions to generate heat.

Each skeletal muscle is an organ that consists of various integrated tissues. These tissues

include the skeletal muscle fibers, blood vessels, nerve fibers, and connective tissue. Each

skeletal muscle has three layers of connective tissue (called “mysia”) that enclose it and

provide structure to the muscle as a whole, and also compartmentalize the muscle fibers

within the muscle (Figure 1). Each muscle is wrapped in a sheath of dense, irregular

connective tissue called the epimysium, which allows a muscle to contract and move

powerfully while maintaining its structural integrity. The epimysium also separates muscle

from other tissues and organs in the area, allowing the muscle to move independently.

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SKELETAL MUSCLE- NOTES I

Learning Objectives

By the end of this section, you will be able to:

Describe the layers of connective tissues packaging skeletal muscle
Explain how muscles work with tendons to move the body
Identify areas of the skeletal muscle fibers
Describe excitation-contraction coupling & Neuromuscular junction The best-known feature of skeletal muscle is its ability to contract and cause movement. Skeletal muscles act not only to produce movement but also to stop movement, such as resisting gravity to maintain posture. Small, constant adjustments of the skeletal muscles are needed to hold a body upright or balanced in any position. Muscles also prevent excess movement of the bones and joints, maintaining skeletal stability and preventing skeletal structure damage or deformation. Joints can become misaligned or dislocated entirely by pulling on the associated bones; muscles work to keep joints stable. Skeletal muscles are located throughout the body at the openings of internal tracts to control the movement of various substances. These muscles allow functions, such as swallowing, urination, and defecation, to be under voluntary control. Skeletal muscles also protect internal organs (particularly abdominal and pelvic organs) by acting as an external barrier or shield to external trauma and by supporting the weight of the organs. Skeletal muscles contribute to the maintenance of homeostasis in the body by generating heat. Muscle contraction requires energy, and when ATP is broken down, heat is produced. This heat is very noticeable during exercise, when sustained muscle movement causes body temperature to rise, and in cases of extreme cold, when shivering produces random skeletal muscle contractions to generate heat. Each skeletal muscle is an organ that consists of various integrated tissues. These tissues include the skeletal muscle fibers, blood vessels, nerve fibers, and connective tissue. Each skeletal muscle has three layers of connective tissue (called “mysia”) that enclose it and provide structure to the muscle as a whole, and also compartmentalize the muscle fibers within the muscle (Figure 1). Each muscle is wrapped in a sheath of dense, irregular connective tissue called the epimysium , which allows a muscle to contract and move powerfully while maintaining its structural integrity. The epimysium also separates muscle from other tissues and organs in the area, allowing the muscle to move independently.

Figure 1. The Three Connective Tissue Layers. Bundles of muscle fibers, called fascicles, are covered by the perimysium. Muscle fibers are covered by the endomysium. Inside each skeletal muscle, muscle fibers are organized into individual bundles, each called a fascicle , by a middle layer of connective tissue called the perimysium. This fascicular organization is common in muscles of the limbs; it allows the nervous system to trigger a specific movement of a muscle by activating a subset of muscle fibers within a bundle, or fascicle of the muscle. Inside each fascicle, each muscle fiber is encased in a thin connective tissue layer of collagen and reticular fibers called the endomysium. The endomysium contains the extracellular fluid and nutrients to support the muscle fiber. These nutrients are supplied via blood to the muscle tissue. In skeletal muscles that work with tendons to pull on bones, the collagen in the three tissue layers (the mysia) intertwines with the collagen of a tendon. At the other end of the tendon, it fuses with the periosteum coating the bone. The tension created by contraction of the muscle fibers is then transferred though the mysia, to the tendon, and then to the periosteum to pull on the bone for movement of the skeleton. In other places, the mysia may fuse with a broad, tendon-like sheet called an aponeurosis , or to fascia, the connective tissue between skin and bones. The broad sheet of connective tissue in the lower back that the latissimus dorsi muscles (the “lats”) fuse into is an example of an aponeurosis.

Figure 2. 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 composed of many fibrils, which give the cell its striated appearance.

THE SARCOMERE

The striated appearance of skeletal muscle fibers is due to the arrangement of the myofilaments of actin and myosin in sequential order from one end of the muscle fiber to the other. Each packet of these microfilaments and their regulatory proteins, troponin and tropomyosin (along with other proteins) is called a sarcomere. 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 1.2 μ m in diameter, hundreds to thousands (each with thousands of sarcomeres) can be found inside one muscle fiber. Each sarcomere is approximately 2 μ m in length with a three-dimensional cylinder-like arrangement and is bordered by structures called Z-discs (also called Z-lines, because pictures are two- dimensional), to which the actin myofilaments are anchored (Figure 3). 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. Figure 3. The Sarcomere. 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 the skeletal muscle is the site where a motor neuron’s terminal meets the muscle fiber—called the neuromuscular junction (NMJ). 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.

EXCITATION-CONTRACTION

COUPLING

Figure 4. Motor End-Plate and Innervation. At the NMJ, the axon terminal releases ACh. The motor end-plate is the location of the ACh-receptors in the muscle fiber sarcolemma. When ACh molecules are released, they diffuse across a minute space called the synaptic cleft and bind to the receptors. 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. Sodium ions enter the muscle fiber, and an action potential rapidly spreads (or “fires”) along the entire membrane to initiate excitation- contraction coupling. 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. 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 membrane of the SR, there are periodic invaginations in the sarcolemma, called T- tubules (“T” stands for “transverse”). You will recall that the diameter of a muscle fiber can be up to 100 μ m, 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 membranes of SR on either side is called a triad (Figure 5). The triad surrounds the cylindrical structure called a myofibril , which contains actin and myosin.

Understanding Skeletal Muscle: Layers, Contraction, and Neuromuscular Junction, Summaries of Acting

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Download Understanding Skeletal Muscle: Layers, Contraction, and Neuromuscular Junction and more Summaries Acting in PDF only on Docsity!

SKELETAL MUSCLE- NOTES I

Learning Objectives

THE SARCOMERE

THE NEUROMUSCULAR JUNCTION

EXCITATION-CONTRACTION

COUPLING