Lecture 7.1: Muscle Flashcards

Question

Skeletal Muscle: Parallel Muscles

Answer 1

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

Answer 2

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

Answer 3

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

Answer 4

Those within the tongue Originate in fibrous connective tissue rather than bone They allow the tongue to change shape, but not position

Answer 5

The extrinsic muscles protrude, retract and move the tongue from side to side These muscle originate from bone (mandible or hyoid)

Answer 6

A sarcomere

Answer 7

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

Answer 8

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

Answer 9

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

Answer 10

[look up image]

Answer 11

Contraction results from the interaction of thick and thin myofilaments

Answer 12

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

Answer 13

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

Answer 14

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

Answer 15

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

Answer 16

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

Answer 17

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

Answer 18

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

Answer 19

Found in the walls of passageways and cavities

Answer 20

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

Answer 21

Smooth muscle forms sheets, bundles or layers containing thousands of cells, supported by endomysium Not striated: no sarcomeres or T-tubules

Answer 22

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

Answer 23

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

Answer 24

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

Answer 25

Myoepithelial Cells Myofibroblasts Myoid Cells

Answer 26

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

Answer 27

Produce collagenous matrix, but also contract at sites of wound healing Also important in tooth eruption

Answer 28

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