Muscle contraction power point presentation I need to do a 25 slide of PowerPoint about Muscle Contraction. I must submit 5 slides each week, so I sh -

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Muscle Contraction

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Muscle contraction power point presentation I need to do a 25 slide of PowerPoint about Muscle Contraction. I must submit 5 slides each week, so I sh

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Chapter 17
1

Learning Objectives
Lesson 17.1: Skeletal Muscle Tissue, Cells, Fibers, and Microfilaments
List and discuss the three generalized functions of skeletal muscle tissue.
Discuss the three characteristics of skeletal muscle cells that allow them to function as they do.
List and discuss the structural parts of skeletal muscle fibers that are also found in other types of cells and the structural parts of skeletal muscle fibers that are not found in other cells.
Discuss the structure and function of myofilaments.
2

Introduction
Muscular system is responsible for moving the framework of the body
In addition to movement, muscle tissue performs various other functions
3

The muscular system performs other functions vital to maintaining a constant internal environment.
3

General Functions
Movement of the body as a whole or movement of its parts
Heat production
Posture
4

Skeletal muscle contractions produce movement of the body as a whole or movement of its parts.
Muscle cells, like all cells, produce heat by the process known as catabolism.
The heat produced by just one cell is inconsequential, but because skeletal muscle cells are both highly active and numerous, together they produce a major share of total body heat.
The continued partial contraction of many skeletal muscles makes possible standing, sitting, and maintaining a relatively stable position of the body while walking, running, or performing other movements.
4

Characteristics of Skeletal Muscle Cells
Excitability (irritability): Ability to be stimulated
Contractility: Ability to contract, or shorten, and produce body movement
Extensibility: Ability to extend, or stretch, thereby allowing muscles to return to their resting length
5

Because skeletal muscle cells are excitable, they can respond to regulatory mechanisms such as nerve signals.
The term contraction, when applied to muscles, is meant in a broad sense of pulling the ends together, regardless of whether the cell actually gets shorter.
Muscles may extend while still exerting force, as when lowering a heavy object in your hand.
All of these characteristics of muscle cells are related to the microscopic structure of skeletal muscle cells.
5

Overview of the Muscle Cell
Muscle cells are called fibers because of their threadlike shape
Sarcolemma: Plasma membrane of muscle fibers
Sarcoplasmic reticulum (SR)
T tubules: Network of tubules and sacs found within muscle fibers
Membrane of the SR continually pumps calcium ions from the sarcoplasm and stores the ions within its sacs for later release
6

Some adult muscle fibers have one of these tiny precursor cells hugging their outer boundary.
These satellite cells are stem cells that fuse with myocytes during strength training to make bigger muscle fibers.
The satellite cells can also become active after a muscle injury to produce more muscle fibers.
Muscle cells contain networks of tubules and sacs, known as the sarcoplasmic reticulum, which is the muscle fibers version of smooth endoplasmic reticulum.
See Figure 17-3 of the textbook for a diagram of the storage and release of calcium ions.
6

T Tubules
Transverse tubules extend across the sarcoplasm at right angles to the long axis of the muscle fiber
Membrane has ion pumps that continually transport Ca++ ions inward from the sarcoplasm
Allow electrical impulses traveling along the sarcolemma to move deeper into the cell
7

T tubules are formed by inward extensions of the sarcolemma.
7

Structure of the Skeletal Muscle

Figure 17-1 shows structure of skeletal muscle. A, Skeletal muscle organ composed of bundles of contractile muscle fibers held together by connective tissue. B, Greater magnification of a single fiber showing smaller fibersmyofibrilsin the sarcoplasm. Note the sarcoplasmic reticulum and T tubules forming a three-part structure called a triad. C, Myofibril magnified further to show a sarcomere between successive Z disks (Z lines). Cross striae are visible. D, Molecular structure of a myofibril showing thick myofilaments and thin myofilaments.
8

Features of the Skeletal Muscle Cell

Figure 17-2 shows features of the skeletal muscle fiber. Note especially the T tubules, which are extensions of the plasma membrane, or sarcolemma, and the sarcoplasmic reticulum (SR), a type of smooth endoplasmic reticulum that forms networks of tubular canals and sacs containing stored calcium ions. A triad is a triplet of adjacent tubules: a terminal (end) sac of the SR, a T tubule, and another terminal sac of the SR.
9

Overview of the Muscle Cell
Muscle fibers contain many mitochondria and several nuclei
Myofibrils: Numerous fine fibers packed close together in sarcoplasm
Sarcomere
Contractile unit of muscle fibers
Each myofibril consists of many sarcomeres
10

A typical muscle fiber may have somewhere in the neighborhood of a thousand or more myofibrils tightly packed inside it.
Myofibrils, in turn, are made up of still finer fibers called myofilaments.
There are two types of myofilaments in the myofibril: thick filaments and thin filaments.
A sarcomere is a segment of myofibril between two successive Z disks.
10

Striated Muscle and Triad
Striated muscle
Dark stripes called A bands; light H band runs across the midsection of each dark A band
Light stripes called I bands; dark Z disk extends across the center of each light I band
Triad
Triplet of tubules; a T tubule sandwiched between two sacs of sarcoplasmic reticulum
11

Because of its cross striations, skeletal muscle is sometimes called striated muscle.
Electron microscopy of skeletal muscle reveals details that help us understand concepts of its structure and function.
The triad allows an electrical impulse traveling along a T tubule to stimulate the membranes of adjacent sacs of the sarcoplasmic reticulum.
11

Skeletal Muscle Striations

A: Courtesy Dr. J.H. Venable, Department of Anatomy, Colorado State University, Fort Collins, CO. B: Courtesy Dr. H.E. Huxley.
12

Figure 17-4 shows skeletal muscle striations. Color-enhanced scanning electron micrographs (SEMs) showing longitudinal views of skeletal muscle fibers. B shows detail of A at greater magnification. Note that the myofilaments of each myofibril form a pattern that, when viewed together, produces the striated (striped) pattern typical of skeletal muscle.
12

Myofilaments
Each myofibril contains thousands of thick and thin myofilaments
Four different kinds of protein molecules make up myofilaments
Myosin
Makes up almost all the thick filaments
Myosin heads are known as cross bridges when attached to actin
13

Myosin molecules are shaped like two golf clubs twisted together.
Myosin heads are chemically attracted to the actin molecules of the nearby thin filaments, so they angle toward the thin filaments.
13

Structure of Myofilaments

Figure 17-5 shows structure of myofilaments. A, Thin myofilament. B, Thick myofilament. C, Cross-section of several thick and thin myofilaments showing the relative positions of myofilaments and the myosin heads that will form cross bridges between them.
14

Cross Section of Myofilaments

From Leeson CR, Leeson T, Paparo A: Text/atlas of histology, St Louis, 1988, Saunders.
15

Figure 17-6 shows cross-section of myofibrils. Color-enhanced SEMs showing a cross-section from a skeletal muscle fiber. Note the dense arrangement of thick and thin filaments, seen here in cross-section as mere dots. Note also the dark glycogen granules and sarcoplasmic reticulum tubules sandwiched between the myofibrils.
15

Myofilaments
Actin: Globular protein that forms two fibrous strands twisted around each other to form the bulk of the thin filament
Tropomyosin: Protein that blocks the active sites on actin molecules
Troponin: Protein that holds tropomyosin molecules in place
16

Actin and myosin molecules have a chemical attraction for one another, but at rest, the active sites on the actin molecules are covered by long tropomyosin molecules.
16

Excitation and Contraction
of a Muscle Fiber
A skeletal muscle fiber remains at rest until stimulated by a motor neuron
Neuromuscular junction: Motor neurons connect to the sarcolemma at the motor endplate
Acetylcholine (ACh): The neurotransmitter released into the synaptic cleft that diffuses across the gap, stimulates the receptors, and initiates an impulse in the sarcolemma
17

See Box 17-2 for a complete list of all the major events that occur during muscle excitation and contraction.
The neuromuscular junction (NMJ) is a synapse where neurotransmitter molecules transmit signals.
When nerve impulses reach the end of a motor neuron fiber, small vesicles release a neurotransmitter, ACh, into the synaptic cleft.
17

Neuromuscular Junction
A: Courtesy of Don Fawcett, Harvard Medical School, Boston, Massachusetts. In Pollard TD: Earnshaw W: Cell biology, ed 2, St. Louis, 2007, Saunders
18

Figure 17-7 shows neuromuscular junction (NMJ). A, Scanning electron micrograph showing several NMJs. N, Nerve fibers; M, muscle fibers. B, This sketch shows a cutaway side view of the NMJ. Note how the distal end of a motor neuron fiber forms a synapse, or chemical junction, with an adjacent muscle fiber. Neurotransmitter molecules (specifically, acetylcholine [ACh]) are released from the neurons synaptic vesicles and diffuse across the synaptic cleft. There they stimulate receptors in the motor endplate region of the sarcolemma.
18

Excitation of a Muscle Fiber

Figure 17-8 shows effects of excitation on a muscle fiber. Excitation of the sarcolemma by a nerve impulse initiates an impulse in the sarcolemma. The impulse travels across the sarcolemma and through the T tubules, where it triggers adjacent sacs of the sarcoplasmic reticulum to release a flood of calcium ions (Ca++) into the sarcoplasm. Ca++ is then free to bind to troponin molecules in the thin filaments. This binding, in turn, initiates the chemical reactions that produce a contraction. ACh, Acetylcholine; NMJ, neuromuscular junction.
19

Learning Objectives
Lesson 17.2: Muscle
Contraction and Relaxation
Explain the series of steps in muscle contraction and the sliding-filament theory.
Explain the series of steps in muscle relaxation.
Identify and explain the energy sources for muscle contraction, including aerobic and anaerobic respiration.
Define a motor unit and myography.
Describe the following types of skeletal muscle contractions: twitch, treppe, tetanic, and tonic.
20

Excitation and Contraction
Nerve impulse travels over the sarcolemma and inward along the T tubules, which triggers the release of calcium ions
Calcium binds to troponin, which causes tropomyosin to shift and expose active sites on actin
21

Once the active sites are exposed, energized myosin heads of the thick filaments bind to actin molecules in the nearby thin filaments.
The myosin head temporarily forms a cross bridge between the thick and thin filaments.
After forming cross bridges, the myosin heads bend with great force, literally pulling the thin filaments past them; this is often called the power stroke of myosin.
21

Molecular Basis of Muscle Contraction

Figure 17-9 shows the molecular basis of muscle contraction.
22

Role of Calcium in Muscle Contraction

From Lodish H: Molecular cell biology, ed 4, New York, 2000, WH Freeman.
23

Figure 17-10 shows the role of calcium in muscle contraction. Color-enhanced SEM of a thin filament. When calcium is absent, the active myosin binding sites on actin are covered by tropomyosin. However, after calcium becomes available and binds to troponin, the tropomyosin is pulled out of its blocking position and reveals the active binding sites on actin.
23

Cross Bridges

From Lodish H: Molecular cell biology, ed 4, New York, 2000, WH Freeman.
24

Figure 17-11 shows cross bridges. Color-enhanced SEM showing the myosin heads functioning as cross bridges that connect the thick filaments to the thin filaments, pulling on the thin filaments and causing them to slide.
24

Sliding-Filament Model
(Slide 1 of 2)
When active sites on actin are exposed, myosin heads bind to them
Myosin heads bend and pull the thin filaments past them
Each head releases, binds to the next active site, and pulls again
The entire myofibril shortens
25

Perhaps a better name for the sliding-filament model might be the ratcheting-filament model because the myosin heads actively ratchet the thin filaments toward the center of the sarcomere with great force.
Muscle heads usually contract to about 80% of their starting length.
25

26
B: Courtesy H.E. Huxley, Brandeis University, Waltham, Ma.
Sliding-Filament Model
(Slide 2 of 2)

Figure 17-12 shows sliding-filament model. A, During contraction, myosin cross bridges pull the thin filaments toward the center of each of two sarcomeres, thus shortening the myofibril and the entire muscle fiber. B, Color-enhanced transmission electron micrographs (TEMs) showing the shortening of a single sarcomere caused by the sliding of filaments during muscle contraction.
26

Contracting Sarcomere

Figure 17-13 shows simplified contracting sarcomere. This diagram illustrates the concept of muscle contraction as a sort of tug-of-war game in which the myosin heads (shown here as little people) hold onto thin filament ropesthus forming cross bridges. As the myosin heads pull on the thin filaments, the Z disks (Z lines) get closer togetherthus shortening the sarcomere. Likewise, the short length of a sarcomere may be held in position by the continued effort of the myosin heads.
27

Muscle Relaxation
Immediately after the Ca++ ions are released, the sarcoplasmic reticulum begins actively pumping them back into the sacs
Ca++ ions are removed from the troponin molecules, thereby shutting down the contraction
28

Because the active transport carriers of the SR have greater affinity for calcium than the troponin molecules do, the calcium ions are stripped off the troponin molecules and returned to the sacs of the SR.
Troponin without its bound calcium allows the tropomyosin to once again block actins active sites.
Myosin heads reaching for the next active site on actin are blocked; therefore, the thin filaments are no longer being held or pulled by the thick filaments.
28

Energy Sources for Muscle Contraction
(Slide 1 of 2)
Hydrolysis of adenosine triphosphate (ATP) yields the energy required for muscular contraction
ATP binds to the myosin head and then transfers its energy to the myosin head to perform the work of pulling the thin filament during contraction
Muscle fibers continually resynthesize ATP from the breakdown of creatine phosphate (CP)
29

Before contraction occurs, each myosin cross-bridge head moves into a resting position when an ATP molecule binds to it.
The ATP molecule breaks its outermost high-energy bond, thereby releasing inorganic phosphate and transferring the energy to the myosin head.
If a cell runs out of ATP completely and cannot resynthesize more, contraction stops, possibly resulting in stiffness caused by the inability of myosin heads to disengage from actin.
29

30
Energy Sources for Muscle Contraction
(Slide 2 of 2)

Figure 17-14 shows energy sources for muscle contraction. A, A basic model of two high-energy molecules in the sarcoplasm: adenosine triphosphate (ATP) and creatine phosphate (CP). B, This diagram shows how energy released during the catabolism of nutrients can be transferred to the high-energy bonds of ATP directly or, instead, stored temporarily in the high-energy bond of CP. During contraction, ATP is hydrolyzed and the energy of the broken bond is transferred to a myosin head. ADP, Adenosine diphosphate.
30

Energy Sources for Muscle Contraction: Catabolic Pathways
Aerobic pathway
Occurs when adequate O2 is available from blood
Slower than anaerobic pathway
Anaerobic pathway
Very rapid, providing energy during first minutes of maximal exercise
May occur when low levels of O2 are available
Production of an oxygen debt is sometimes called excess postexercise oxygen consumption (EPOC)
31

Aerobic pathways supply energy for the long term rather than the short term.
Anaerobic pathways result in the formation of lactic acid, which requires oxygen to convert back to glucose.
31

Blood Supply of Muscle Fibers

From Lodish H: Molecular cell biology, ed 4, New York, 2000, WH Freeman.
32

Figure 17-15 shows blood supply of muscle fibers. Tiny exchange vessels called capillaries branch out longitudinally (L) from small arteries (arrow) to form a network that supplies muscle fibers with glucose and oxygen and removes carbon dioxide and lactate. In this micrograph, the muscle fibers appear translucent and the blood vessels appear reddish.
32

Aerobic and Anaerobic Pathways

Figure 17-16 shows aerobic and anaerobic pathways during muscular activity. A, The aerobic pathway (dark bars) can supply energy for muscle contraction for a longer time than can the anaerobic pathway (light bars). B, The lines show that 100% of the energy used at the beginning of maximal exercise comes from anaerobic processes. However, the muscles soon switch to aerobic sources of energy, so after 1 hour of maximal exercise, nearly 100% of the energy comes from aerobic respiration in the muscle fibers.
33

Energy Sources for Muscle Contraction: Skeletal Muscle
Skeletal muscle contraction produces waste heat that can be used to help maintain the setpoint body temperature
34

Heat production, or thermogenesis, is an important function of skeletal muscles, especially in adults.
Temperature sensors in the skin and other parts of the body feed this information back to the hypothalamus, which compares the actual value with the setpoint value.
Reference Figure 17-17 in the textbook.
34

Motor Units
Some motor units consist of only a few muscle fibers, whereas others consist of numerous fibers
Generally, the smaller the number of fibers in a motor unit, the more precise are the available movements; the larger the number of fibers in a motor unit, the more powerful the contraction available
Myography: A method of graphing the changing tension of a muscle as it contracts
35

Muscles are composed of bundles of muscle fibers held together by fibrous connective tissue.
The motor neuron, often called a somatic motor neuron, is one of several nerve cells that enter a muscle organ together in a bundle called a motor nerve.
One of these motor neurons, plus the muscle fibers to which it attaches, constitutes a functional unit called a motor unit.
35

Motor Unit

36
B: Courtesy Dr. Paul C. Letourneau, Department of Anatomy, Medical School, University of Minnesota, MN.

Figure 17-18 shows a motor unit. A motor unit consists of one somatic motor neuron and the muscle fibers supplied by its branches. A, Sketch showing a single motor unit. B, Photomicrograph showing a nerve (black) branching to supply several dozen individual muscle fibers (red). C, Diagram showing several motor units within the same muscle organ.
36

Myography

Figure 17-19 shows myography. This classic type of myograph, called a kymograph, records muscle contractions as graphs showing changes in length. An isolated muscle moves the pen upward during contraction, and the weight pulls the muscle and pen downward as the muscle relaxes. A change in tension produces this change in length. Electrical voltage simulates a nerve impulse to stimulate the fibers in the muscle. Modern myography often uses computer-based systems that record similar muscle tension graphs.
37

Twitch Contraction
(Slide 1 of 2)
A quick jerk of a muscle that is produced as a result of a single, brief threshold stimulus (generally occurs only in experimental situations)
The twitch contraction has three phases
Latent phase
Contraction phase
Relaxation phase
38

To get the muscle to contract, an electrical stimulus of sufficient intensity (the threshold stimulus) is applied to the muscle.
During the latent phase of the twitch contraction, nerve impulses travel to the sarcoplasmic reticulum to trigger the release of calcium.
The binding of calcium to troponin and the sliding of filaments occur in the contraction phase.
In the relaxation phase, the sliding stops.
38

Twitch Contraction
(Slide 2 of 2)

Figure 17-20 shows the twitch contraction. Three distinct phases are apparent: (1) the latent period, (2) the contraction phase, and (3) the relaxation phase.
39

Treppe: The Staircase Phenomenon
Gradual, steplike increase in the strength of contraction seen in a series of twitch contractions that occur 1 second apart
Eventually, the muscle responds with less forceful contractions, and the relaxation phase becomes shorter
If the relaxation phase disappears completely, a contracture occurs
40

In warm muscle fibers, calcium ions diffuse through the sarcoplasm more efficiently and more actin-myosin reactions occur; in addition, calcium ions accumulate in the sarcoplasm of muscles that have not had time to relax and pump much of the calcium back into their SR.
Thus, up to a point, a warm fiber contracts more strongly than does a cool fiber; therefore, after the first few stimuli, muscle responds to successive stimuli with maximal contractions.
In time, repeated stimulation of muscle lessens its excitability and contractility and may result in muscle fatigue, a condition in which the muscle does not respond to the strongest stimuli.
40

Muscle Contractions
(Slide 1 of 2)

Figure 17-21 shows myograms of various types of muscle contractions. A, A single twitch contraction. B, The treppe phenomenon, or staircase effect, is a steplike increase in the force of contraction over the first few in a series of twitches.
41

42
Muscle Contractions
(Slide 2 of 2)

Figure 17-21 shows myograms of various types of muscle contractions. C, Incomplete tetanus occurs when a rapid succession of stimuli produces twitches that seem to add together (wave summation) to produce a rather sustained contraction. D, Complete tetanus is a smoother sustained contraction produced by the summation of twitches that occur so close together that the muscle cannot relax at all.
42

Tetanus Contractions
Multiple wave summation
Incomplete tetanus
Complete tetanus
The availability of calcium determines whether a muscle will contract; if calcium is continuously available, a contraction will be sustained
43

The multiple wave summation is produced when multiple twitch waves are added together to sustain muscle tension for a longer time.
An incomplete tetanus is produced when very short periods of relaxation occur between peaks of tension.
A complete tetanus occurs when the stimulation is such that the twitch waves fuse into a single, sustained peak.
43

Twitch and Tetanus

Adapted from Pollard T, Earnshaw W: Cell biology, ed 2, Philadelphia, 2008, Saunders.
44

Figure 17-22 shows the role of calcium in twitch and tetanus. A, A single, sudden increase in calcium (Ca++) availability triggers the twitch contraction. B, Repeated stimuli maintain a high level of calcium, permitting sustained (tetanic) contraction.
44

Muscle Tone
Tonic contraction: Continual, partial contraction of a muscle
Muscles with less tone than normal are flaccid
Muscles with more tone than normal are spastic
Muscle tone is maintained by negative feedback mechanisms
45

At any one time, a small number of muscle fibers within a muscle contract and produce a tightness, or muscle tone.
Muscle tone is particularly important for maintaining posture.
45

Learning Objectives
Lesson 17.3: Isotonic and Isometric Contractions, Muscle Types, and Muscular Disorders
Explain the graded strength principle.
Describe isotonic and isometric muscle contractions.
Describe the anatomical and functional characteristics of cardiac and smooth muscle.
Discuss major muscular disorders.
46

Graded Strength Principle
(Slide 1 of 2)
Factors that contribute to the phenomenon of graded strength
Metabolic condition of individual fibers
Number of muscle fibers contracting simultaneously
Number of motor units recruited
Intensity and frequency of stimulation
47

The graded strength principle is based on the fact that skeletal muscles contract with varying degrees of strength at different times.
The metabolic condition of individual fibers influences their capacity to generate force; thus, if many fibers of a muscle organ are unable to maintain a high level of ATP and become fatigued, the entire muscle organ suffers some loss in its ability to generate maximum force of contraction.
The greater the number of fibers contracting, the stronger the contraction.
47

Strength of Contraction Compared With Strength of Stimulus

Figure 17-23 shows the strength of muscle contraction compared with the strength of the stimulus. After the threshold stimulus is reached, a continued increase in stimulus strength produces a proportional increase in muscle strength until the maximal level of contraction strength is reached.
48

Maximal strength that a muscle can develop bears a direct relationship to the initial length of its fibers
A shortened muscles sarcomeres are compressed; therefore, the muscle cannot develop much tension
Strongest maximal contraction is possible only when the skeletal muscle has been stretched to its optimal length
49
Graded Strength Principle
(Slide 2 of 2)

An overstretched muscle cannot develop much tension because the thick myofilaments are too far from the thin myofilaments.
49

Length-Tension Relationship

Figure 17-24 shows the length-tension relationship. As this graph of muscle tension shows, the maximum strength that a muscle can develop is directly related to the initial length of its fibers. At a short initial length, the sarcomeres are already compressed, and thus the muscle cannot develop much tension (position A). Conversely, the thick and thin myofilaments are too far apart in an overstretched muscle to generate much tension (position C). Maximum tension can be generated only when the muscle has been stretched to a moderate, optimal length (position B).
50

Stretch Reflex
The load imposed on a muscle influences the strength of a skeletal contraction
Stretch reflex: The body tries to maintain constancy of muscle length in response to increased load
51

The stretch reflex maintains a relatively constant length as load is increased up to a maximum sustainable level.
When the load becomes too heavy and thus threatens to cause injury to the muscle or skeleton, the body abandons this reflex and forces you to relax and drop the load.
Refer to Figure 17-25, which shows a diagram of the stretch reflex.
51

Strength of Muscle Contraction

Figure 17-26 shows the factors that influence the strength of muscle contraction.
52

Isotonic and Isometric Contraction
(Slide 1 of 2)
Isotonic contraction
Contraction in which the tone or tension in a muscle remains the same as the length of the muscle changes
Isometric contraction
Contraction in which muscle length remains the same while muscle tension increases
53

Isotonic literally means same tension.
Because the muscle is moving against its resistance in an isotonic contraction, the energy of contraction is used to pull on the thin myofilaments and thus change the length of a fibers sarcomeres.
Concentric contractions are those in which the muscle shortens as it contracts; eccentric contractions are those in which the muscle lengthens while contracting.
Isometric literally means same length.
Most body movements occur as a result of both types of contractions.
53

Isotonic and Isometric Contraction
(Slide 2 of 2)

Figure 17-27 shows isotonic and isometric contraction. A, In isotonic contraction the muscle shortens and produces movement. Concentric contractions occur when the muscle shortens during the movement. Eccentric contractions occur when the contracting muscle lengthens. B, In isometric contraction the muscle pulls forcefully against a load but does not shorten because it cannot overcome the resistance.
54

Cardiac Muscle
(Slide 1 of 2)
Found only in the heart
Also known as striated involuntary muscle
Cardiac muscle resembles skeletal muscle but has unique features related to its role in continuously pumping blood
Each cardiac muscle contains parallel myofibrils
Syncytium: Continuous, electrically coupled mass
55

See Table 17-1 for a list of characteristics of muscle tissues.
Cardiac muscle forms the bulk of the wall of each chamber of the heart.
Cardiac muscle contracts rhythmically and continuously to provide the pumping action needed to maintain constant blood flow.
Cardiac muscle fibers form strong, electrically coupled junctions with other fibers; individual cells also exhibit branching.
55

Cardiac Muscle Fiber

Figure 17-28 shows cardiac muscle fiber. Unlike other types of muscle fibers, cardiac muscle fiber is typically branched and forms junctions, called intercalated disks, with adjacent cardiac muscle fibers. Like skeletal muscle fibers, cardiac muscle fibers contain sarcoplasmic reticula and T tubulesalth

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