Muscular System Module 2 A + B: Skeletal Muscle · 2014. 7. 1. · Muscular System Module 2 A + B:...

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Muscular System Module 2 A + B:

Skeletal Muscle∗

Donna Browne

Based on Skeletal Muscle† by

OpenStax College

This work is produced by OpenStax-CNX and licensed under the

Creative Commons Attribution License 3.0‡

Abstract

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 �bers• Describe excitation-contraction coupling

1 Module 2A starts here ***************************

Skeletal MuscleThe best-known feature of skeletal muscle is its ability to contract and cause movement. Skeletal musclesact 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 anyposition. Muscles also prevent excess movement of the bones and joints, maintaining skeletal stability andpreventing skeletal structure damage or deformation. Joints can become misaligned or dislocated entirely bypulling on the associated bones; muscles work to keep joints stable. Skeletal muscles are located throughoutthe body at the openings of internal tracts to control the movement of various substances. These musclesallow functions, such as swallowing, urination, and defecation, to be under voluntary control. Skeletal musclesalso protect internal organs (particularly abdominal and pelvic organs) by acting as an external barrier orshield 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. Musclecontraction requires energy, and our bodies use ATP for energy. ATP When ATP is broken down, heat isproduced. This heat is very noticeable during exercise, when sustained muscle movement causes body tem-perature to rise, and in cases of extreme cold, when shivering produces random skeletal muscle contractionsto generate heat.

∗Version 1.2: Jan 6, 2014 9:05 am -0600†http://cnx.org/content/m46476/1.3/‡http://creativecommons.org/licenses/by/3.0/

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Each skeletal muscle is an organ that consists of various integrated tissues. These tissues include theskeletal muscle �bers, blood vessels, nerve �bers, and connective tissue. Each skeletal muscle has three layersof connective tissue that enclose it and provide structure to the muscle as a whole, and also compartmentalizethe muscle �bers within the muscle (Figure 1 (The Three Connective Tissue Layers )). Each muscle iswrapped in a sheath of dense, irregular connective tissue called the epimysium, which allows a muscleto contract and move powerfully while maintaining its structural integrity. The epimysium also separatesmuscle from other tissues and organs in the area, allowing the muscle to move independently.

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The Three Connective Tissue Layers

Figure 1: Bundles of muscle �bers, called fascicles, are covered by the perimysium. Muscle �bers arecovered by the endomysium.

Inside each skeletal muscle, muscle �bers are organized into individual bundles, each called a fascicle,Each fascicle in surrounded by a middle layer of connective tissue called the perimysium. Inside each fasci-

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cle, each muscle �ber is encased in a thin connective tissue layer of connective tissue called the endomysium.The endomysium contains the nutrients to support the muscle �ber. These nutrients are supplied via bloodto the muscle tissue.

In skeletal muscles that work with tendons to pull on bones, the collagen in the three tissue layersintertwines with the collagen of a tendon. At the other end of the tendon, it fuses with the periosteumcoating the bone. The tension created by contraction of the muscle �bers is then transferred though the tothe tendon, and then to the periosteum to pull on the bone for movement of the skeleton. In other places,the connective tissue may fuse with a broad, tendon-like sheet called an aponeurosis, or to fascia, theconnective tissue between skin and bones. The broad sheet of connective tissue in the lower back that thelatissimus dorsi muscles (the �lats�) fuse into is an example of an aponeurosis.

Every skeletal muscle is also richly supplied by blood vessels for nourishment, oxygen delivery, and wasteremoval. In addition, every muscle �ber in a skeletal muscle is supplied by the axon branch of a motorneuron, which signals the �ber to contract. Unlike cardiac and smooth muscle, the only way to functionallycontract a skeletal muscle is through signaling from the nervous system.

2 Skeletal Muscle Fibers

Because skeletal muscle cells are long and cylindrical, they are commonly referred to as muscle �bers.Skeletal muscle �bers can be quite large for human cells, with diameters up to 100 µm and lengths up to30 cm (11.8 in) in the Sartorius of the upper leg. During early development, myoblasts, each with its ownnucleus, fuse with up to hundreds of other myoblasts to form the multinucleated skeletal muscle �bers.Multiple nuclei permit the production of the large amounts of proteins and enzymes needed for musclecontraction.

Some other terminology associated with muscle �bers is rooted in the Greek sarco, which means ��esh.�The plasma membrane of muscle �bers is called the sarcolemma, the cytoplasm is referred to as sar-coplasm, and the specialized smooth endoplasmic reticulum, is called the sarcoplasmic reticulum (SR)(Figure 2 (Muscle Fiber )). As will soon be described, the functional unit of a skeletal muscle �ber isthe sarcomere, a highly organized arrangement of the contractile myo�laments actin (thin �lament) andmyosin (thick �lament), along with other support proteins.

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Muscle Fiber

Figure 2: A skeletal muscle �ber is surrounded by a plasma membrane called the sarcolemma, whichcontains sarcoplasm, the cytoplasm of muscle cells. A muscle �ber is composed of many �brils, whichgive the cell its striated appearance.

3 The Sarcomere

The striated appearance of skeletal muscle �bers is due to the arrangement of the myo�laments of actinand myosin in order from one end of the muscle �ber to the other. Actin is often referred to as the thin�lament, while myosin may be called the thick �lament.

The sarcomere is the functional unit of the muscle �ber. The sarcomere itself is bundled withinthe myo�bril that runs the entire length of the muscle �ber. As myo�brils contract, the entire muscle cellcontracts. Hundreds to thousands (each with thousands of sarcomeres) can be found inside one muscle �ber.Each sarcomere is bordered by structures called Z-discs (also called Z-lines), to which the actin myo�lamentsare anchored (Figure 3 (The Sarcomere )).

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The Sarcomere

Figure 3: The sarcomere, the region from one Z-line to the next Z-line, is the functional unit of askeletal muscle �ber.

4 The Neuromuscular Junction ***************************Module 2B startshere***********************************

Another specialization of the skeletal muscle is the site where a motor neuron's terminal meets the muscle�ber�called the neuromuscular junction (NMJ). This is where the muscle �ber �rst responds to sig-naling by the motor neuron. Every skeletal muscle �ber in every skeletal muscle is innervated by a motorneuron at the NMJ. Excitation signals from the neuron are the only way to functionally activate the �berto contract.

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5 Stimulation of the Skeletal Muscle

All living cells have membrane potentials. Neurons and muscle cells can use their membrane potentials togenerate electrical signals. They do this by controlling the movement of charged particles, called ions, acrosstheir membranes to create electrical currents. This is achieved by opening and closing specialized proteins inthe membrane called ion channels. Although the currents generated by ions moving through these channelproteins are very small, they form the basis of both neural signaling and muscle contraction.

In skeletal muscle, this sequence begins with signals from the somatic motor division of the nervoussystem. In other words, the �excitation� step in skeletal muscles is always triggered by signaling from thenervous system (Figure 4 (Motor End-Plate and Innervation )).

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Motor End-Plate and Innervation

Figure 4: At the NMJ, the axon terminal releases ACh. The motor end-plate is the location of theACh-receptors in the muscle �ber sarcolemma. When ACh molecules are released, they di�use across aminute space called the synaptic cleft and bind to the receptors.

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The motor neurons that tell the skeletal muscle �bers to contract originate in the spinal cord, witha 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 longdistances� in this case, all the way from the spinal cord to the muscle itself (which may be up to three feetaway). 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 thenalong the individual branches to terminate at the NMJ. At the NMJ, the axon terminal releases a chemicalmessenger, or neurotransmitter, called acetylcholine (ACh). The ACh molecules di�use across a minutespace called the synaptic cleft and bind to ACh receptors located within the motor end-plate of thesarcolemma on the other side of the synapse. Once ACh binds, a channel in the ACh receptor opens andpositively charged ions can pass through into the muscle �ber, causing it to depolarize, meaning that themembrane potential of the muscle �ber becomes less negative (closer to zero.)

As the membrane depolarizes, another set of ion channels are triggered to open. Ions enter the muscle�ber, and an action potential rapidly spreads (or ��res�) along the entire membrane to begin the process ofmuscle contraction.

Things happen very quickly in the world of excitable membranes (just think about how quickly you cansnap your �ngers as soon as you decide to do it). Immediately following depolarization of the membrane, itrepolarizes, re-establishing the normal state. Meanwhile, the ACh in the synaptic cleft is degraded by theenzyme acetylcholinesterase (AChE) so that the ACh cannot rebind to a receptor and cause unwantedextended muscle excitation and contraction.

The signal is sent along the sarcolemma which triggers the release of calcium ions (Ca++) from its storagein the cell's sarcoplasmic reticulum. T-tubules bring that signal to the SR. The arrangement of a T-tubulewith the membranes of SR on either side is called a triad (Figure 5 (The T-tubule )). The triad surroundsthe cylindrical structure called a myo�bril, which contains actin and myosin.

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The T-tubule

Figure 5: Narrow T-tubules permit the conduction of electrical impulses. The SR functions to regulateintracellular levels of calcium. Two terminal cisternae (where enlarged SR connects to the T-tubule)and one T-tubule comprise a triad�a �threesome� of membranes, with those of SR on two sides and theT-tubule sandwiched between them.

The T-tubules carry the action potential into the interior of the cell, which triggers the opening of calciumchannels in the membrane of the adjacent SR, causing Ca++ to di�use out of the SR and into the sarcoplasm.It is the arrival of Ca++ in the sarcoplasm that initiates contraction of the muscle �ber by its contractileunits, or sarcomeres.

6 Chapter Review

Skeletal muscles contain connective tissue, blood vessels, and nerves. There are three layers of connectivetissue: epimysium, perimysium, and endomysium. Skeletal muscle �bers are organized into groups calledfascicles. Blood vessels and nerves enter the connective tissue and branch in the cell. Muscles attach tobones directly or through tendons or aponeuroses. Skeletal muscles maintain posture, stabilize bones andjoints, control internal movement, and generate heat.

Skeletal muscle �bers are long, multinucleated cells. The membrane of the cell is the sarcolemma; thecytoplasm of the cell is the sarcoplasm. The sarcoplasmic reticulum (SR) is a form of endoplasmic reticulum.Muscle �bers are composed of myo�brils. The striations are created by the organization of actin and myosinresulting in the banding pattern of myo�brils.

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Glossary

De�nition 1: acetylcholine (ACh)neurotransmitter that binds at a motor end-plate to trigger depolarization

De�nition 2: actinprotein that makes up most of the thin myo�laments in a sarcomere muscle �ber

De�nition 3: action potentialchange in voltage of a cell membrane in response to a stimulus that results in transmission of anelectrical signal; unique to neurons and muscle �bers

De�nition 4: aponeurosisbroad, tendon-like sheet of connective tissue that attaches a skeletal muscle to another skeletalmuscle or to a bone

De�nition 5: depolarizeto reduce the voltage di�erence between the inside and outside of a cell's plasma membrane (thesarcolemma for a muscle �ber), making the inside less negative than at rest

De�nition 6: endomysiumloose, and well-hydrated connective tissue covering each muscle �ber in a skeletal muscle

De�nition 7: epimysiumouter layer of connective tissue around a skeletal muscle

De�nition 8: excitation-contraction couplingsequence of events from motor neuron signaling to a skeletal muscle �ber to contraction of the�ber's sarcomeres

De�nition 9: fasciclebundle of muscle �bers within a skeletal muscle

De�nition 10: motor end-platesarcolemma of muscle �ber at the neuromuscular junction, with receptors for the neurotransmitteracetylcholine

De�nition 11: myo�brillong, cylindrical organelle that runs parallel within the muscle �ber and contains the sarcomeres

De�nition 12: myosinprotein that makes up most of the thick cylindrical myo�lament within a sarcomere muscle �ber

De�nition 13: neuromuscular junction (NMJ)synapse between the axon terminal of a motor neuron and the section of the membrane of a muscle�ber with receptors for the acetylcholine released by the terminal

De�nition 14: neurotransmittersignaling chemical released by nerve terminals that bind to and activate receptors on target cells

De�nition 15: perimysiumconnective tissue that bundles skeletal muscle �bers into fascicles within a skeletal muscle

De�nition 16: sarcomerelongitudinally, repeating functional unit of skeletal muscle, with all of the contractile and associatedproteins involved in contraction

De�nition 17: sarcolemmaplasma membrane of a skeletal muscle �ber

De�nition 18: sarcoplasmcytoplasm of a muscle cell

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De�nition 19: sarcoplasmic reticulum (SR)specialized smooth endoplasmic reticulum, which stores, releases, and retrieves Ca++

De�nition 20: synaptic cleftspace between a nerve (axon) terminal and a motor end-plate

De�nition 21: T-tubuleprojection of the sarcolemma into the interior of the cell

De�nition 22: thick �lamentthe thick myosin strands and their multiple heads projecting from the center of the sarcomeretoward, but not all to way to, the Z-discs

De�nition 23: thin �lamentthin strands of actin and its troponin-tropomyosin complex projecting from the Z-discs toward thecenter of the sarcomere

De�nition 24: triadthe grouping of one T-tubule and two terminal cisternae

De�nition 25: troponinregulatory protein that binds to actin, tropomyosin, and calcium

De�nition 26: tropomyosinregulatory protein that covers myosin-binding sites to prevent actin from binding to myosin

De�nition 27: voltage-gated sodium channelsmembrane proteins that open sodium channels in response to a su�cient voltage change, and initiateand transmit the action potential as Na+ enters through the channel

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