Muscles I: Introduction & Overview Flashcards

1
Q

Muscle Types

A

SKELETAL
SMOOTH
CARDIAC

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2
Q

Skeletal muscle

A

is attached to bones and its contraction makes possible locomotion, facial expressions, posture, and other voluntary movements of the body. Forty percent of your body mass is made up of skeletal muscle. Skeletal muscles generate heat as a byproduct of their contraction and thus participate in thermal homeostasis. Shivering is an involuntary contraction of skeletal muscles in response to perceived lower than normal body temperature. The muscle cell, or myocyte, develops from myoblasts derived from the mesoderm. Myocytes and their numbers remain relatively constant throughout life. Skeletal muscle tissue is arranged in bundles surrounded by connective tissue. Under the light microscope, muscle cells appear striated with many nuclei squeezed along the membranes. The striation is due to the regular alternation of the contractile proteins actin and myosin, along with the structural proteins that couple the contractile proteins to connective tissues. The cells are multinucleated as a result of the fusion of the many myoblasts that fuse to form each long muscle fibre.

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3
Q

Smooth muscle tissue

A

contraction is responsible for involuntary movements in the internal organs. It forms the contractile component of the digestive, urinary, and reproductive systems as well as the airways and arteries. Each cell is spindle shaped with a single nucleus and no visible striations.

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4
Q

Cardiac muscle

A

forms the contractile walls of the heart. The cells of cardiac muscle, known as cardiomyocytes, also appear striated under the microscope. Unlike skeletal muscle fibres, cardiomyocytes are single cells typically with a single centrally located nucleus. A principal characteristic of cardiomyocytes is that they contract on their own intrinsic rhythms without any external stimulation. Cardiomyocyte attach to one another with specialized cell junctions called intercalated discs. Intercalated discs have both anchoring junctions and gap junctions. Attached cells form long, branching cardiac muscle fibres that are, essentially, a mechanical and electrochemical syncytium allowing the cells to synchronize their actions. The cardiac muscle pumps blood through the body and is under involuntary control. The attachment junctions hold adjacent cells together across the dynamic pressures changes of the cardiac cycle.

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5
Q

Flexion

A

refers to a movement that decreases the angle between two body parts. Flexion at the elbow is decreasing the angle between the ulna and the humerus. When the knee flexes, the ankle moves closer to the buttock, and the angle between the femur and tibia gets smaller.

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6
Q

Extension

A

refers to a movement that increases the angle between two body parts. Extension at the elbow is increasing the angle between the ulna and the humerus. Extension of the knee straightens the lower limb.

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7
Q

Abduction

A

is a movement away from the midline – just as abducting someone is to take them away. For example, abduction of the shoulder raises the arms out to the sides of the body.

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8
Q

Adduction

A

is a movement towards the midline. Adduction of the hip squeezes the legs together.
In fingers and toes, the midline used is not the midline of the body, but of the hand and foot respectively. Therefore, abducting the fingers spreads them out.

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9
Q

Medial rotation

A

is a rotational movement towards the midline. It is sometimes referred to as internal rotation. To understand this, we have two scenarios to imagine. Firstly, with a straight leg, rotate it to point the toes inward. This is medial rotation of the hip. Secondly, imagine you are carrying a tea tray in front of you, with elbow at 90 degrees. Now rotate the arm, bringing your hand towards your opposite hip (elbow still at 90 degrees). This is internal rotation of the shoulder.

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10
Q

Lateral rotation

A

is a rotating movement away from the midline. This is in the opposite direction to the movements described above.

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11
Q

Pronation

A

at the forearm is a rotational movement where the hand and upper arm are turned so the thumbs point towards the body. When the forearm and hand are supinated, the thumbs point away from the body. Pronation of the foot is turning of the sole outwards, so that weight is borne on the medial part of the foot.

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12
Q

Supination

A

of the forearm occurs when the forearm or palm are rotated outwards. Supination of the foot is turning of the sole of the foot inwards, shifting weight to the lateral edge.

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13
Q

Protraction

A

Moving in a forward direction.

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14
Q

Retraction

A

Movement in a backward direction.

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15
Q

Two major groupings of muscles:

A

Forelimb
Including Thorax and Neck.

Hindlimb
Including Abdomen.

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16
Q

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

A

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

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17
Q

Each skeletal muscle has three layers of connective tissue called

A

“mysia” that enclose it and provide structure to the muscle as a whole, and also compartmentalize the muscle fibers within the muscle.

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18
Q

Epimysium

A

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.

collagenous sheath that binds the fascicles into a single muscle.

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19
Q

Inside each skeletal muscle, muscle fibers are organized into

A

individual bundles, each called a fascicle, by a middle layer of connective tissue called the perimysium

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20
Q

fascicular organization

A

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.

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21
Q

endomysium

A

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.

consists mainly of reticulin fibres and a small amount of collagen, conveys numerous small blood vessels, lymphatics and nerves throughout the muscle

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22
Q

Describe skeletal muscles that work with tendons to pull on bones,

A

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.

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23
Q

In other places, the mysia may fuse with

A

a broad, tendon-like sheet called an aponeurosis, or to fascia, the connective tissue between skin and bones.

24
Q

The broad sheet of connective tissue in the lower back that the latissimus dorsi muscles (the “lats”) fuse into is an example of an

A

aponeurosis

25
Q

Every skeletal muscle is also richly supplied by blood vessels for

A

nourishment, oxygen delivery, and waste removal. In addition, every muscle fiber in a skeletal muscle is supplied by the axon branch of a somatic motor neuron, which signals the fiber to contract.

26
Q

Unlike cardiac and smooth muscle, the only way to functionally contract a skeletal muscle is through

A

signaling from the nervous system.

27
Q

perimysium

A

P, composed of collagen and through which larger vessels and nerves run. The epimysium E is a collagenous sheath that binds the fascicles into a single muscle

28
Q

Skeletal Muscle: Fascicular Organisation

A

Micrograph (b) demonstrates the characteristic histological features of skeletal muscle fibres in longitudinal section. Skeletal muscle fibres are extremely elongated, unbranched cylindrical cells with numerous flattened nuclei located at fairly regular intervals just beneath the sarcolemma (plasma membrane). Each muscle fibre has multiple nuclei arranged at the cell periphery. In transverse section, as in micrograph (a), most muscle fibre profiles appear to contain only a single nucleus, while some do not include any because the plane of section has cut between the zones containing a nucleus. In routine histological preparations stained with H&E, it is often possible to see the striations in skeletal muscle when cut in longitudinal section. Special stains are required for better resolution of these structures (see Fig. 6.6a).

29
Q

Skeletal muscle can be divided into four types, based on patterns of fascicular organisation:

A

Parallel
Convergent
Pennate
Circular

30
Q

Parallel fascicular organisation

A

fascicles are arranged parallel to the long axis of the muscle; when it contracts it shortens & gets larger in diameter.

31
Q

Convergent fascicular organisation

A

fascicles extend over a broad area & come together/converge at a common attachment site; adaptive to diff. activities bc the stimulation of diff. portions of the muscle can change the direction it pulls.

32
Q

Pennate fascicular organisation

A

Pennate: fascicles form a common angle w/ the tendon; muscle fibers pull at an angle & produce more tension

33
Q

Circular Fascicular Organisation

A

fascicles arranged concentrically around an opening; muscle contracts & opening becomes smaller

34
Q

Skeletal Muscle: Fibrilar Organisation

A

Because skeletal muscle cells are long and cylindrical, they are commonly referred to as muscle fibers. Skeletal muscle fibers can be quite large for human cells, with diameters up to 100 μm and lengths up to 30 cm (11.8 in) in the Sartorius of the upper leg. During early development, embryonic myoblasts, each with its own nucleus, fuse with up to hundreds of other myoblasts to form the multinucleated skeletal muscle fibers. Multiple nuclei mean multiple copies of genes, permitting the production of the large amounts of proteins and enzymes needed for muscle contraction.
Some other terminology associated with muscle fibers is rooted in the Greek sarco, which means “flesh.” The plasma membrane of muscle fibers is called the sarcolemma, the cytoplasm is referred to as sarcoplasm, and the specialized smooth endoplasmic reticulum, which stores, releases, and retrieves calcium ions (Ca++) is called the sarcoplasmic reticulum (SR). As will soon be described, the functional unit of a skeletal muscle fiber is the sarcomere, a highly organized arrangement of the contractile myofilaments actin (thin filament) and myosin (thick filament), along with other support proteins.

35
Q

sarcolemma

A

The plasma membrane of muscle fibers

36
Q

sarcoplasm

A

The cytoplasm muscle fibres

37
Q

sarcoplasmic reticulum (SR).

A

the specialized smooth endoplasmic reticulum, which stores, releases, and retrieves calcium ions (Ca++)

38
Q

sarcomere

A

, a highly organized arrangement of the contractile myofilaments actin (thin filament) and myosin (thick filament), along with other support proteins.

39
Q

sliding filament theory.

A

Mitochondria Mt and numerous glycogen granules G provide a rich energy source in the scanty cytoplasm between the myofibrils. The mature muscle cell contains little rough endoplasmic reticulum; it contains, however, a smooth membranous system S which is involved in activation of the contractile mechanism.

40
Q

The striated appearance of skeletal muscle fibers is due to

A

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.

41
Q

Skeletal Muscle: Contraction

A

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 μm in diameter, hundreds to thousands (each with thousands of sarcomeres) can be found inside one muscle fiber. Each sarcomere is approximately 2 μm 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. 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.

42
Q

Skeletal Muscle: Tendon

A

A tendon or sinew is a tough band of fibrous connective tissue that connects muscle to bone and is capable of withstanding tension.
Tendons are similar to ligaments; both are made of collagen. Ligaments connect one bone to another, while tendons connect muscle to bone.

Histologically, tendons consist of dense regular connective tissue. The main cellular component of tendons are specialized fibroblasts called tenocytes. Tenocytes synthesize the extracellular matrix of tendons, abundant in densely packed collagen fibers. The collagen fibers are parallel to each other and organized into fascicles. Individual fascicles are bound by the endotendineum, which is a delicate loose connective tissue containing thin collagen fibrils[1][2] and elastic fibres.[3] Groups of fascicles are bounded by the epitenon, which is a sheath of dense irregular connective tissue. The whole tendon is enclosed by a fascia. The space between the fascia and the tendon tissue is filled with the paratenon, a fatty areolar tissue.[4] Normal healthy tendons are anchored to bone by Sharpey’s fibres.

43
Q

Standard muscle contraction,

A

muscles change size.

44
Q

Isometric muscular contraction

A

, tendons change size.

45
Q

Where is smooth muscle found

A

Smooth muscle (so-named because the cells do not have striations) is present in the walls of hollow organs like the urinary bladder, uterus, stomach, intestines, and in the walls of passageways, such as the arteries and veins of the circulatory system, and the tracts of the respiratory, urinary, and reproductive systems. Smooth muscle is also present in the eyes, where it functions to change the size of the iris and alter the shape of the lens; and in the skin where it causes hair to stand erect in response to cold temperature or fear.

46
Q

Smooth Muscle: Organisation

A

Smooth muscle fibers are spindle-shaped (wide in the middle and tapered at both ends, somewhat like a football) and have a single nucleus; they range from about 30 to 200 μm (thousands of times shorter than skeletal muscle fibers), and they produce their own connective tissue, endomysium. Although they do not have striations and sarcomeres, smooth muscle fibers do have actin and myosin contractile proteins, and thick and thin filaments. These thin filaments are anchored by dense bodies. LA dense body is analogous to the Z-discs of skeletal and cardiac muscle fibers and is fastened to the sarcolemma. Calcium ions are supplied by the SR in the fibers and by sequestration from the extracellular fluid through membrane indentations called calveoli.

smooth muscle fibres are elongated, spindle-shaped cells with tapered ends which may occasionally be bifurcated. Smooth muscle fibres are generally much shorter than skeletal muscle fibres and contain only one nucleus which is elongated and centrally located in the cytoplasm at the widest part of the cell; however, depending on the contractile state of the fibres at fixation, the nuclei may sometimes appear to be spiral-shaped. Smooth muscle fibres are bound together in irregular branching fasciculi and these fasciculi, rather than individual fibres, are the functional contractile units. Within the fasciculi, individual muscle fibres are arranged roughly parallel to one another with the thickest part of one cell lying against the thin parts of adjacent cells. The contractile proteins of smooth muscle are not arranged in myofibrils, as in skeletal and cardiac muscle, and thus visceral muscle cells are not striated. Between the individual muscle fibres and between the fasciculi, there is a network of supporting collagenous tissue.

47
Q

Smooth Muscle: Cellular Anatomy

A

At low magnification, micrograph (a) demonstrates the spindle-shaped and elongated central nuclei N of smooth muscle cells. The cells at the lower right are cut longitudinally, and those at the upper left transversely. Between them is a band of supporting tissue S containing the cytoplasmic processes of fibroblasts F. Note the relative sparsity of mitochondria M and other intracellular organelles. At high magnification in micrograph (b), details of the plasma membrane and endomembrane system can be seen. The plasma membrane contains numerous flask-shaped invaginations. In some areas, these are irregular in shape and size and may be involved in pinocytosis. In other areas, the invaginations are regular in shape and distribution and are called caveolae C. The endomembrane system contains some elements which represent a poorly developed Golgi and endoplasmic reticulum ER. Other vesicular and tubular structures T are seen near the plasma membrane, often in association with caveolae; these probably constitute a system analogous to the sarcoplasmic reticulum of skeletal muscle, with caveolae being analogous to the T tubule system. Thick and thin filaments Fi of myosin and actin criss-cross the cytoplasm of each cell and are anchored to the cell membrane at attachment junctions (focal adhesion densities) J. Filaments are also attached within the cytoplasm to focal densities D which are believed to hold filaments in register. The narrow intercellular spaces are of almost uniform width, but at numerous sites the plasma membranes of adjacent cells form specialised cell junctions. Nexus (gap) junctions G mediate spread of excitation throughout visceral muscle.

48
Q

Smooth Muscle: Contraction

A

Because smooth muscle cells do not contain troponin, cross-bridge formation is not regulated by the troponin-tropomyosin complex but instead by the regulatory protein calmodulin. In a smooth muscle fiber, external Ca++ ions passing through opened calcium channels in the sarcolemma, and additional Ca++ released from SR, bind to calmodulin. The Ca++-calmodulin complex then activates an enzyme called myosin (light chain) kinase, which, in turn, activates the myosin heads by phosphorylating them (converting ATP to ADP and Pi, with the Pi attaching to the head). The heads can then attach to actin-binding sites and pull on the thin filaments. The thin filaments also are anchored to the dense bodies; the structures invested in the inner membrane of the sarcolemma (at adherens junctions) that also have cord-like intermediate filaments attached to them. When the thin filaments slide past the thick filaments, they pull on the dense bodies, structures tethered to the sarcolemma, which then pull on the intermediate filaments networks throughout the sarcoplasm. This arrangement causes the entire muscle fiber to contract in a manner whereby the ends are pulled toward the center, causing the midsection to bulge in a corkscrew motion.

49
Q

Smooth Muscle: Control

A

Smooth muscle is not under voluntary control; thus, it is called involuntary muscle. The triggers for smooth muscle contraction include hormones, neural stimulation by the ANS, and local factors. In certain locations, such as the walls of visceral organs, stretching the muscle can trigger its contraction (the stress-relaxation response).
Axons of neurons in the ANS do not form the highly organized NMJs with smooth muscle, as seen between motor neurons and skeletal muscle fibers. Instead, there is a series of neurotransmitter-filled bulges called varicosities as an axon courses through smooth muscle, loosely forming motor units.

50
Q

varicosity

A

releases neurotransmitters into the synaptic cleft. Also, visceral muscle in the walls of the hollow organs (except the heart) contains pacesetter cells

51
Q

pacesetter cell

A

can spontaneously trigger action potentials and contractions in the muscle.

52
Q

Cardiac Muscle

A

Cardiac muscle or myocardium exhibits many structural and functional characteristics intermediate between those of skeletal and visceral muscle. Like the former, its contractions are strong and utilise a great deal of energy; like the latter, the contractions are continuous and initiated by inherent mechanisms, although they are modulated by external autonomic and hormonal stimuli. Cardiac muscle fibres are essentially long, cylindrical cells with one or at most two nuclei which are centrally located within the cell. The ends of the fibres are split
longitudinally into a small number of branches, the ends
of which abut onto similar branches of adjacent cells,
giving the impression of a continuous three-dimensional cytoplasmic network; this was formerly described as a syncytium before the discrete intercellular boundaries were recognised. Between the muscle fibres, delicate collagenous tissue analogous to the endomysium of skeletal muscle supports the extremely rich capillary network necessary to meet the high metabolic demands of strong, continuous activity. Cardiac muscle fibres have an arrangement of contractile proteins similar to that of skeletal muscle and are consequently striated in a similar manner. However, this is often difficult to see with light microscopy due to the
irregular branching shape of the cells and their myofibrils. Cardiac muscle fibres also have a system of T tubules
and sarcoplasmic reticulum analogous to that of skeletal muscle. In the case of cardiac muscle, however, there is a slow leak of Ca2+ ions into the cytoplasm from the sarcoplasmic reticulum after recovery from the preceding contraction; this causes a succession of automatic contractions independent of external stimuli. The rate of this inherent rhythm is then modulated by external autonomic and hormonal stimuli. Between the ends of adjacent cardiac muscle cells are specialised intercellular junctions called intercalated discs which not only provide points of anchorage for the myofibrils but also permit extremely rapid spread of contractile stimuli from one cell to another. Thus, adjacent fibres are triggered to contract almost simultaneously, thereby acting as a functional syncytium. In addition, a system of highly modified cardiac muscle cells constitutes the pacemaker regions of the heart and ramifies throughout the organ as the Purkinje system, thus coordinating contraction of the myocardium as a whole in each cardiac cycle.

53
Q

gap junction

A

forms channels between adjacent cardiac muscle fibers that allow the depolarizing current produced by cations to flow from one cardiac muscle cell to the next

54
Q

electric coupling

A

in cardiac muscle it allows the quick transmission of action potentials and the coordinated contraction of the entire heart. This network of electrically connected cardiac muscle cells creates a functional unit of contraction called a syncytium.

55
Q

desmosome

A

The remainder of the intercalated disc is composed of desmosomes. A desmosome is a cell structure that anchors the ends of cardiac muscle fibers together so the cells do not pull apart during the stress of individual fibers contracting

56
Q

Intercalated discs

A

are part of the sarcolemma and contain two structures important in cardiac muscle contraction: gap junctions and desmosomes.

57
Q

Contractions of the heart (heartbeats)

A

are controlled by specialized cardiac muscle cells called pacemaker cells that directly control heart rate. Although cardiac muscle cannot be consciously controlled, the pacemaker cells respond to signals from the autonomic nervous system (ANS) to speed up or slow down the heart rate. The pacemaker cells can also respond to various hormones that modulate heart rate to control blood pressure.
The wave of contraction that allows the heart to work as a unit, called a functional syncytium, begins with the pacemaker cells. This group of cells is self-excitable and able to depolarize to threshold and fire action potentials on their own, a feature called autorhythmicity; they do this at set intervals which determine heart rate. Because they are connected with gap junctions to surrounding muscle fibers and the specialized fibers of the heart’s conduction system, the pacemaker cells are able to transfer the depolarization to the other cardiac muscle fibers in a manner that allows the heart to contract in a coordinated manner.
Another feature of cardiac muscle is its relatively long action potentials in its fibers, having a sustained depolarization “plateau.” The plateau is produced by Ca++ entry though voltage-gated calcium channels in the sarcolemma of cardiac muscle fibers. This sustained depolarization (and Ca++ entry) provides for a longer contraction than is produced by an action potential in skeletal muscle. Unlike skeletal muscle, a large percentage of the Ca++ that initiates contraction in cardiac muscles comes from outside the cell rather than from the SR.