Review Sheet Exercise 12 Microscopic Anatomy and Organization of Skeletal Muscle Answers

x.2 Skeletal Muscle

Learning Objectives

Describe the construction and function of skeletal muscle fibers

Past the end of this section, you lot volition be able to:

• Describe the connective tissue layers surrounding skeletal muscle
• Define a musculus fiber, myofibril, and sarcomere
• List the major sarcomeric proteins involved with contraction
• Identify the regions of the sarcomere and whether they change during contraction
• Explicate the sliding filament process of muscle contraction

Each skeletal musculus is an organ that consists of various integrated tissues. These tissues include the skeletal musculus fibers, blood vessels, nervus fibers, and connective tissue. Each skeletal musculus has iii layers of connective tissue that enclose it, provide structure to the muscle, and compartmentalize the musculus fibers within the muscle (Figure 10.ii.1). Each muscle is wrapped in a sheath of dense, irregular connective tissue called the epimysium, which allows a muscle to contract and movement powerfully while maintaining its structural integrity. The epimysium also separates muscle from other tissues and organs in the area, allowing the muscle to motion independently.

Figure 10.2.1 – 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 bundles, called fascicles, surrounded by a centre layer of connective tissue called the perimysium. This fascicular organization is common in muscles of the limbs; information technology allows the nervous system to trigger a specific motion of a muscle past activating a subset of muscle fibers inside a 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 surrounds the extracellular matrix of the cells and plays a role in transferring force produced past the muscle fibers to the tendons.

In skeletal muscles that work with tendons to pull on basic, the collagen in the three connective tissue layers intertwines with the collagen of a tendon. At the other cease of the tendon, it fuses with the periosteum blanket the bone. The tension created by contraction of the musculus fibers is then transferred though the connective tissue layers, 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 wide, tendon-like sheet called an aponeurosis, or to fascia, the connective tissue between skin and bones. The broad canvass of connective tissue in the lower back that the latissimus dorsi muscles (the "lats") fuse into is an example of an aponeurosis.

Every skeletal muscle is also richly supplied past claret vessels for nourishment, oxygen delivery, and waste removal. In addition, every muscle fiber in a skeletal musculus is supplied by the axon branch of a somatic motor neuron, which signals the fiber to contract. Different cardiac and shine muscle, the only way to functionally contract a skeletal muscle is through signaling from the nervous organization.

Skeletal Muscle Fibers

Because skeletal musculus cells are long and cylindrical, they are commonly referred to as muscle fibers (or myofibers). Skeletal musculus fibers can exist quite large compared to other cells, with diameters up to 100 μyard and lengths up to xxx cm (xi.8 in) in the Sartorius of the upper leg. Having many nuclei allows for production of the large amounts of proteins and enzymes needed for maintaining normal part of these big poly peptide dense cells.  In improver to nuclei, skeletal muscle fibers too incorporate cellular organelles found in other cells, such as mitochondria and endoplasmic reticulum.  Howver, some of these structures are specialized in musculus fibers.  The specialized smooth endoplasmic reticulum, called the sarcoplasmic reticulum (SR), stores, releases, and retrieves calcium ions (Ca⁺⁺).

The plasma membrane of muscle fibers is chosen the sarcolemma (from the Greek sarco, which means "flesh") and the cytoplasm is referred to as sarcoplasm (Effigy 10.two.2). Within a muscle cobweb, proteins are organized into structures called myofibrils that run the length of the cell and contain sarcomeres connected in serial. Because myofibrils are only approximately i.two μm in bore, hundreds to thousands (each with thousands of sarcomeres) can be found within one musculus fiber.  The sarcomere is the smallest functional unit of a skeletal muscle fiber and is a highly organized organization of contractile, regulatory, and structural proteins. Information technology is the shortening of these individual sarcomeres that lead to the wrinkle of individual skeletal musculus fibers (and ultimately the whole muscle).

Effigy x.2.two – Muscle Cobweb: A skeletal muscle fiber is surrounded by a plasma membrane chosen the sarcolemma, which contains sarcoplasm, the cytoplasm of muscle cells. A muscle fiber is composed of many myofibrils, which contain sarcomeres with low-cal and dark regions that give the cell its striated advent.

The Sarcomere

A sarcomere is defined every bit the region of a myofibril contained betwixt 2 cytoskeletal structures chosen Z-discs (also called Z-lines), and the striated appearance of skeletal muscle fibers is due to the arrangement of the thick and thin myofilaments within each sarcomere (Figure 10.ii.2).  The dark striated A band is composed of the thick filaments containing myosin, which bridge the center of the sarcomere extending toward the Z-dics.  The thick filaments are anchored at the middle of the sarcomere (the Yard-line) by a protein chosen myomesin.  The lighter I band regions contain sparse actin filaments anchored at the Z-discs past a protein chosen α-actinin.  The sparse filaments extend into the A band toward the M-line and overlap with regions of the thick filament.  The A ring is dark because of the thicker mysoin filaments likewise equally overlap with the actin filaments.  The H zone in the middle of the A band is a fiddling lighter in color, because the thin filaments exercise not extend into this region.

Because a sarcomere is defined by Z-discs, a single sarcomere contains 1 dark A ring with half of the lighter I band on each end (Effigy ten.2.ii).  During contraction the myofilaments themselves do not change length, merely actually slide across each other so the distance between the Z-discs shortens. The length of the A band does non change (the thick myosin filament remains a constant length), but the H zone and I band regions shrink.  These regions represent areas where the filaments do not overlap, and as filament overlap increases during contraction these regions of no overlap subtract.

Myofilament Components

The thin filaments are composed of two filamentous actin chains (F-actin) comprised of private actin proteins (Figure x.ii.3).  These thin filaments are anchored at the Z-disc and extend toward the center of the sarcomere.  Within the filament, each globular actin monomer (G-actin) contains a mysoin binding site and is likewise associated with the regulatory proteins, troponin and tropomyosin.  The troponin protein complex consists of three polypeptides.  Troponin I (TnI) binds to actin, troponin T (TnT) binds to tropomyosin, and troponin C (TnC) binds to calcium ions.  Troponin and tropomyosin run forth the actin filaments and control when the actin binding sites volition exist exposed for binding to myosin.

Thick myofilaments are equanimous of myosin protein complexes, which are composed of six proteins: two myosin heavy chains and iv light concatenation molecules.  The heavy bondage consist of a tail region, flexible swivel region, and globular head which contains an Actin-binding site and a bounden site for the high energy molecule ATP.  The lite chains play a regulatory role at the hinge region, but the heavy chain caput region interacts with actin and is the most important factor for generating force.  Hundreds of myosin proteins are arranged into each thick filament with tails toward the Chiliad-line and heads extending toward the Z-discs.

Other structural proteins are associated with the sarcomere but practise not play a straight function in active force production.  Titin, which is the largest known poly peptide, helps align the thick filament and adds an rubberband element to the sarcomere.  Titin is anchored at the Thousand-Line, runs the length of myosin, and extends to the Z disc.  The sparse filaments also take a stabilizing protein, called nebulin, which spans the length of the thick filaments.

Figure 10.2.3 – The Sarcomere: The sarcomere, the region from one Z-line to the adjacent Z-line, is the functional unit of a skeletal muscle fiber.

External Website

Watch this video to learn more about macro- and microstructures of skeletal muscles. (a) What are the names of the "junction points" between sarcomeres? (b) What are the names of the "subunits" within the myofibrils that run the length of skeletal muscle fibers? (c) What is the "double strand of pearls" described in the video? (d) What gives a skeletal muscle fiber its striated appearance?

The Sliding Filament Model of Contraction

The arrangement and interactions between thin and thick filaments allows for the shortening of the sarcomeres which generates strength. When signaled by a motor neuron, a skeletal musculus cobweb contracts as the sparse filaments are pulled and slide past the thick filaments within the fiber's sarcomeres. It is important to notation that while the sarcomere shortens, the private proteins and filaments practise non change length just only slide next to each other.  This process is known as the sliding filament model of muscle contraction (Effigy 10.ii.4).

Figure x.2.iv – The Sliding Filament Model of Muscle Contraction: When a sarcomere contracts, the Z lines move closer together, and the I ring becomes smaller. The A band stays the same width. At full wrinkle, the thin and thick filaments overlap.

The filament sliding process of wrinkle tin only occur when myosin-binding sites on the actin filaments are exposed by a series of steps that begins with Ca⁺⁺ entry into the sarcoplasm.  Tropomyosin winds around the chains of the actin filament and covers the myosin-bounden sites to prevent actin from bounden to myosin. The troponin-tropomyosin complex uses calcium ion binding to TnC to regulate when the myosin heads form cross-bridges to the actin filaments.  Cross-span germination and filament sliding will occur when calcium is present, and the signaling process leading to calcium release and musculus contraction is known as Excitation-Wrinkle Coupling.

Chapter Review

Skeletal muscles contain connective tissue, blood vessels, and nerves. There are iii layers of connective tissue: epimysium, perimysium, and endomysium. Skeletal muscle fibers are organized into groups called fascicles. Blood vessels and nerves enter the connective tissue and branch in the cell. Muscles attach to bones straight or through tendons or aponeuroses. Skeletal muscles maintain posture, stabilize bones and joints, control internal move, and generate heat.

Skeletal musculus fibers are long, multinucleated cells. The membrane of the cell is the sarcolemma; the cytoplasm of the cell is the sarcoplasm. The sarcoplasmic reticulum (SR) is a grade of endoplasmic reticulum. Musculus fibers are composed of myofibrils which are equanimous of sarcomeres linked in serial. The striations of skeletal muscle are created by the organisation of actin and myosin filaments resulting in the banding pattern of myofibrils.  These actin and myosin filaments slide over each other to crusade shortening of sarcomeres and the cells to produce force.

Interactive Link Questions

Spotter this video to acquire more virtually macro- and microstructures of skeletal muscles. (a) What are the names of the "junction points" between sarcomeres? (b) What are the names of the "subunits" inside the myofibrils that run the length of skeletal muscle fibers? (c) What is the "double strand of pearls" described in the video? (d) What gives a skeletal muscle cobweb its striated advent?

(a) Z-lines. (b) Sarcomeres. (c) This is the arrangement of the actin and myosin filaments in a sarcomere. (d) The alternate strands of actin and myosin filaments.

Every skeletal muscle cobweb is supplied by a motor neuron at the NMJ. Watch this video to learn more than near what happens at the neuromuscular junction. (a) What is the definition of a motor unit? (b) What is the structural and functional departure betwixt a large motor unit and a small motor unit of measurement? Can you give an example of each? (c) Why is the neurotransmitter acetylcholine degraded after binding to its receptor?

(a) It is the number of skeletal muscle fibers supplied by a single motor neuron. (b) A large motor unit has one neuron supplying many skeletal muscle fibers for gross movements, like the Temporalis muscle, where grand fibers are supplied by one neuron. A small motor has one neuron supplying few skeletal musculus fibers for very fine movements, like the extraocular middle muscles, where six fibers are supplied past 1 neuron. (c) To avert prolongation of musculus contraction.

Review Questions

Critical Thinking Questions

1. What would happen to skeletal muscle if the epimysium were destroyed?

2. Describe how tendons facilitate body movement.

3. What causes the striated appearance of skeletal muscle tissue?

Glossary

acetylcholine (ACh)

neurotransmitter that binds at a motor terminate-plate to trigger depolarization

actin

protein that makes up virtually of the thin myofilaments in a sarcomere musculus fiber

activeness potential

change in voltage of a cell membrane in response to a stimulus that results in manual of an electrical signal; unique to neurons and muscle fibers

aponeurosis

broad, tendon-like sheet of connective tissue that attaches a skeletal musculus to another skeletal muscle or to a bone

depolarize

to reduce the voltage deviation between the inside and outside of a cell's plasma membrane (the sarcolemma for a muscle fiber), making the within less negative than at rest

endomysium

loose, and well-hydrated connective tissue roofing each musculus cobweb in a skeletal muscle

epimysium

outer layer of connective tissue effectually a skeletal muscle

excitation-wrinkle coupling

sequence of events from motor neuron signaling to a skeletal muscle fiber to wrinkle of the fiber'due south sarcomeres

fascicle

packet of muscle fibers inside a skeletal muscle

motor stop-plate

sarcolemma of muscle fiber at the neuromuscular junction, with receptors for the neurotransmitter acetylcholine

myofibril

long, cylindrical organelle that runs parallel within the muscle cobweb and contains the sarcomeres

myosin

poly peptide that makes upwards most of the thick cylindrical myofilament inside a sarcomere musculus fiber

neuromuscular junction (NMJ)

synapse betwixt the axon last of a motor neuron and the section of the membrane of a muscle cobweb with receptors for the acetylcholine released by the concluding

neurotransmitter

signaling chemical released by nerve terminals that demark to and activate receptors on target cells

perimysium

connective tissue that bundles skeletal muscle fibers into fascicles within a skeletal muscle

sarcomere

longitudinally, repeating functional unit of measurement of skeletal muscle, with all of the contractile and associated proteins involved in contraction

sarcolemma

plasma membrane of a skeletal muscle cobweb

sarcoplasm

cytoplasm of a musculus cell

sarcoplasmic reticulum (SR)

specialized smooth endoplasmic reticulum, which stores, releases, and retrieves Ca⁺⁺

synaptic cleft

space between a nervus (axon) concluding and a motor end-plate

T-tubule

project of the sarcolemma into the interior of the cell

thick filament

the thick myosin strands and their multiple heads projecting from the center of the sarcomere toward, merely not all to way to, the Z-discs

thin filament

thin strands of actin and its troponin-tropomyosin circuitous projecting from the Z-discs toward the middle of the sarcomere

triad

the group of one T-tubule and two concluding cisternae

troponin

regulatory poly peptide that binds to actin, tropomyosin, and calcium

tropomyosin

regulatory protein that covers myosin-binding sites to prevent actin from binding to myosin

voltage-gated sodium channels

membrane proteins that open sodium channels in response to a sufficient voltage change, and initiate and transmit the action potential as Na⁺ enters through the aqueduct

Solutions

Answers for Disquisitional Thinking Questions

1. Muscles would lose their integrity during powerful movements, resulting in muscle damage.
2. When a muscle contracts, the force of motility is transmitted through the tendon, which pulls on the bone to produce skeletal movement.
3. Dark A bands and light I bands repeat forth myofibrils, and the alignment of myofibrils in the cell cause the unabridged prison cell to announced striated.

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Source: https://open.oregonstate.education/aandp/chapter/10-2-skeletal-muscle/

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