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Tissues of the Body > Lecture 7.1: Muscle > Flashcards

Lecture 7.1: Muscle Flashcards

1
Q

What is Myalgia?

A

Muscle Pain

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2
Q

What is Myasthenia?

A

Weakness of the muscles

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3
Q

What is the Myocardium?

A

It is the muscular component of the heart

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4
Q

What is Myopathy?

A

Any disease of the muscles

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5
Q

What is a Myoclonus?

A

A sudden spasm of the muscles

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6
Q

Sarcolemma

A

The outer membrane of a muscle cell

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7
Q

Sarcoplasm

A

The cytoplasm of a muscle cell

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8
Q

Sarcoplasmic Reticulum

A

The smooth endoplasmic reticulum of a muscle cell

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9
Q

Types of Muscle Tissues (3)

A

Skeletal Muscle (Striated)
Cardiac Muscle (Striated)
Smooth Muscle (Non-striated)

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10
Q

How much of total body mass is muscle?

A

c.40%

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11
Q

What is the function of muscle?

A

Muscles generate motile forces through contraction to allow movement to support bodily functions

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12
Q

How do muscles work- key aspects?

A

Assemblies of contractile muscle cells are ‘machines’ that convert chemical energy to power mechanical work

Actin and myosin filaments interact to facilitate the contraction of whole muscle cells in each case

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13
Q

Smooth Muscle [Str, Contra, Nuc, Sha, Ctrl, Mech]

A

Striations: Absent
Contractions: Slow, sustained or rhythmic
Nuclei: Single central cigar shaped nucleus
Shape: Spindle shaped/Fusiform (up to 10 x 100 µm)
Control: Intrinsic, hormonal, autonomic, local factors (NO)
Mechanism: Contractile filaments rotate dense bodies causing “corkscrewing” in response to Ca2+ release

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14
Q

Cardiac Muscle [Str, Contra, Nuc, Sha, Ctrl, Mech]

A

Striations: Present
Contractions: Variable, rhythmic
Nuclei: One to two centrally located nuclei per cell
Shape: Branched cylindrical cells (up to 20 x 100 µm) joining at intercalated discs, branching facilitates synchronous contraction
Control: Intrinsic, but with autonomic influence
Mechanism: Contractile myofilaments shorten sarcomeres in response to Ca2+ release from SR

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15
Q

Skeletal Muscle [Str, Contra, Nuc, Sha, Ctrl, Mech]

A

Striations: Present (discrete A and I bands)
Contractions: Rapid, forceful
Nuclei: Multinucleated, peripheral nuclei
Shape: Large cylindrical fibres (up to 0.1 x 20 cm) arranged into fascicles
Control: Somatic innervation (voluntary)
Mechanism: Contractile myofilaments shorten sarcomeres in response to Ca2+ release from SR

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16
Q

How does Skeletal Muscle Develop? (4)

A

Mesodermally-derived, multipotent myogenic stem cells give rise to myoblasts

Nearsynchronous fusion of myoblasts forms a primary myotube with a chain of multiple central nuclei

These fuse to form myofibres in which nuclei are gradually displaced to the periphery by newly-synthesised actin and myosin myofilaments

Skeletal muscle retains a stem cell population (satellite cells) that allow for hypertrophy and repair

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17
Q

Red Fibres in Muscle

A

Muscles that work to resist gravity tend to contain red oxidative fibres, with abundant myoglobin

ATP is generated by aerobic respiration

Muscles that resist gravity, but also require bursts of movement are rich in intermediate (type IIa) fibres

These are capable of rapid contraction, but also appear red as they contain abundant myoglobin

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18
Q

White Fibres in Muscle

A

Muscles required for darting movements have less myoglobin

Hence fast glycolytic fibres appear white

They depend on anaerobic respiration for short bursts

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19
Q

Organisation of Skeletal Muscle

A

[look up image]

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20
Q

What does the form of Skeletal Muscle depend on?

A

Form depends on the orientation of constituent fibres

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21
Q

Skeletal Muscle Forms

A

Sphincter Muscles
Convergent Muscle
Pennate
Parallel Muscles

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22
Q

Skeletal Muscle: Sphincter Muscles

A

Organised into circles

23
Q

Skeletal Muscle: Convergent Muscle

A

The origin is wider than the point of insertion

24
Q

Skeletal Muscle: Pennate

A

In pennate (feather-like) muscles, fibres lie in a different plane to the point of insertion

25
Q

Skeletal Muscle: Parallel Muscles

A

Fibres in parallel muscles lie in the same plane as their tendons, and are termed fusiform if they are wider at the belly

26
Q

Skeletal Muscle Functions (4)

A

Movement
Posture
Stability of joints (e.g. muscles of the rotator cuff of the shoulder)
Thermoregulation: 40% of body mass and generates considerable heat

27
Q

Myotendinous Junctions

A

These are the points of force transmission from myofilaments to tendons

Tendon fibrils interdigitate with folds in the sarcolemma, and are continuous with the connective tissue layers of muscle (e.g. endomysium)

These are the weakest part of the muscle/ tendon complex and are susceptible to tears

28
Q

Intrinsic Muscles of the Tongue

A

Those within the tongue

Originate in fibrous connective tissue rather than bone

They allow the tongue to change shape, but not position

29
Q

Extrinsic Muscles of the Tongue

A

The extrinsic muscles protrude, retract and move the tongue from side to side

These muscle originate from bone (mandible or hyoid)

30
Q

What is the smallest functional unit of skeletal muscle?

A

A sarcomere

31
Q

Skeletal Muscle Contraction: Neuromuscular Junction

A

A motor nerve impulse (action potential) causes the release of the neurotransmitter acetylcholine (ACh) at axon terminals supplying muscle fibres at synaptic cleft

Activation of nicotinic ACh receptors on muscle fibre membranes opens ligand-gated Na+ channels allowing an influx of Na+ in the sarcolemma

APs are propagated deep into muscle fibres through invaginations in the
sarcolemma (called T-tubules) as voltage-gated Na+ channels open

The activation of a single motor neurone leads to weak, but distributed contraction

The activation of more motor neurons will activate more muscle fibres, increasing the force of contraction

32
Q

Raised intracellular Ca2+ stimulates skeletal muscle contraction

A

All things at the neuromuscular junction occur

T-tubule depolarisation activates ryanodine receptors in the membrane of the SR, releasing stored Ca2+ into the cytoplasm

Ca2+ stimulates muscle contraction by binding to troponin subunit C (TnC)

This allows myosin binding sites to be revealed

Ca2+ is returned to the SR via ATPase pumps

33
Q

Sliding Filament Mechanism of Contraction

A

Upon stimulation, thin (actin) filaments are pulled towards the M-line

This shortens the sarcomere

When the impulse to contract ceases, a titin “spring” allows sarcomeres to passively return to their original size

34
Q

Sarcomere Structure

A

[look up image]

35
Q

What is contraction the result of?

A

Contraction results from the interaction of thick and thin myofilaments

36
Q

Thick Myofilaments

A

Myosin is the main component of thick filaments:

• Each myosin molecule is shaped like two golf clubs twisted together
• Myosin tails point toward the M line in centre of the sarcomere
• Myosin heads project outward from shaft in spiralling fashion
• Myosin hydrolyses ATP to provide energy to adopt a “cocked” position

37
Q

Thin Myofilaments

A

Actin is main component of thin filaments:
• Globular actin (G actin) polymerises to form helical actin filaments
• Each actin subunit has a binding site for myosin heads, but these are hidden by tropomyosin
• Ca2+ binding to troponin C (TnC) in thin filaments causes a conformational shift in tropomyosin, revealing a myosin binding site

38
Q

Skeletal Muscle Contraction (5)

A

When a nerve impulse stimulates a myofibre to contract, Ca2+ released from the SR binds to troponin C, promoting a conformational shift in tropomyosin

This reveals a myosin binding site on actin

The myosin head hydrolyses ATP to adopt a “cocked” configuration

When bound to actin, the head flexes, pulling actin towards the M-line

ADP and Pi are then released, and the cycle begins again

39
Q

What in skeletal muscle form a triad?

A

T-tubules
Sarcoplasmic Reticulum
In skeletal muscle, triads run along the junction between the A- and I-bands

40
Q

Mechanism of Cardiac Muscle Contraction

A

The mechanism of cardiac muscle contraction is similar to skeletal muscle,

Although APs are generated by pacemaker cells located within the heart, such as the sino-atrial (SA) node

Heart rate and force of contraction can be modulated by the ANS

41
Q

What do the SR and T-tubules form in cardiac muscle?

A

A diad

42
Q

Where do T-tubules lie in cardiac muscle in comparison to skeletal muscle?

A

In contrast to skeletal muscle, the T tubules of cardiac muscle lie in register with the Z bands and not with the A-I band junction

43
Q

Purkinje Fibres: What are they? Structure? Function?

A

Purkinje fibres are specialised cardiac myocytes that lie adjacent to the endocardium

They are large cells with abundant glycogen, sparse myofilaments, and extensive gap junctions

Purkinje fibres conduct action potentials more rapidly than cardiac muscle fibres, allowing the ventricles to contract in a synchronous manner

44
Q

Smooth Muscle: Locations

A

Found in the walls of passageways and cavities

45
Q

Smooth Muscle: Function

A

Smooth muscle supports vascular structures, as well as the gut, respiratory tract and genitourinary system

46
Q

Smooth Muscle: Mass Structure

A

Smooth muscle forms sheets, bundles or layers containing thousands of cells, supported by endomysium

Not striated: no sarcomeres or T-tubules

47
Q

Control of Smooth Muscle Contraction

A

Smooth muscle is not under voluntary control, and responds to stimuli in form of innervation, hormones, drugs, or local concentrations of blood gases

Smooth muscle from different sites may respond differently to the same stimulus

It requires less ATP, but still relies on actin-myosin interactions

48
Q

Mechanism of Smooth Muscle Contraction

A

Thick (myosin) and thin (actin and tropomyosin) filaments are arranged diagonally within smooth muscle cells, and are anchored at dense bodies

Filaments spiral down the long axis like stripes on a barbers pole, so the smooth muscle cell twists as it contracts

Ca2+ release stimulates contraction via sliding filaments

Smooth muscle lacks troponin, and myosin is activated by phosphorylation rather than by hydrolysing ATP directly

49
Q

Time taken for smooth muscle to contract as compared to skeletal muscle

A

Smooth muscle takes 30 times longer to contract than skeletal muscle, but can maintain tension for long periods (using 1% of the energy)

50
Q

Types of Modified Smooth Muscle Cells (3)

A

Myoepithelial Cells
Myofibroblasts
Myoid Cells

51
Q

Myoepithelial Cells

A

Stellate cells forming a basketwork around the secretory units of some exocrine glands

Contraction assists secretion into secretory ducts

Myoepithelial cells in the ocular iris contract to dilate the pupil

52
Q

Myofibroblasts

A

Produce collagenous matrix, but also contract at sites of wound healing

Also important in tooth eruption

53
Q

Myoid Cells

A

Surround seminiferous tubules

Their contraction helps move immature sperm towards the efferent ducts (which are surrounded by smooth muscle)

Tissues of the Body (18 decks)

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