Topic 13: Muscle Cells

Question 1

What are the three types of contractile cells in mammals?

Answer 1

skeletal muscle: bipolar, striated actomyosin arrays

smooth muscle: actomyosin cortical contraction

cardiac muscle: bipolar, striated actomyosin arrays

all rely on actin-myosin based movement for contraction

have different arrangements of actin and myosin and differ in the signals that cause contraction

Question 2

What controls contraction in cardiac muscle?

Answer 2

involuntary control at pacemaker

Question 3

What controls contraction in skeletal muscle?

Answer 3

voluntary control at a neuromuscular junction

Question 4

What controls contraction in smooth muscle?

Answer 4

controlled involuntary

stimulated by both hormonal and neural inputs

Question 5

What is the tissue structure of skeletal muscle?

Answer 5

muscle consists of bundles of parallel muscle fibers (single cell)

full skeletal muscle is a network of bundled muscle cells

Question 6

What is the cell structure in skeletal muscle?

Answer 6

muscle cells from by fusion of muscle precursor cells

multinucleate cells each called a muscle fiber

multinucleate cells share a common cytoplasm and are called a syncytium

Question 7

What is the cytoskeletal arrangement of skeletal muscle?

Answer 7

sarcomere composed of interdigitated thick and thin filaments

referred to as striated muscle

Question 8

What are the components of the scarcomere?

Answer 8

thick filaments: contain many molecules of Type II myosin, tail-tail dimer allows formation of bipolar filaments

thin filaments: F actin complexes including structural and regulatory factors (troponin and tropomyosin)

structural proteins: e.g. alpha actinin (actin bundling)

myomesin: bundles myosin into thick filaments, stabilizes tail-tail-dimer

titin: tethers actin and myosin to sacromere

Question 9

How is the activation of skeletal muscle dependent on calcium?

Answer 9

myosin binding sites on actin are normally blocked by tropomyosin therefore in order to have contraction, the tropomyosin must be moved out of the way

troponin C binds calcium and causes a conformation change that is transmitted to the tropomyosin freeing up the myosin binding sites on actin

Question 10

What is tropomyosin?

Answer 10

wraps around thin filaments and blocks myosin binding

Question 11

What is troponin?

Answer 11

Ca++ sensing protein

when Ca++ is present, it binds to troponin and causes a conformational change which displaces tropomyosin, allows myosin to bind

Question 12

What is the regulation of calcium levels in skeletal muscles?

Answer 12

motor neurons initiate contractile cycle at the neuromuscular junction

causes change in intracellular calcium concentration to initiate contraction

a motor neuron synapsed with a muscle is called a neuromuscular junction and releases the neurotransmitter acetylcholine

Question 13

What are the steps of activation of a skeletal muscle?

Answer 13

1. AP opens voltage gated Ca++ channels
2. ACh released into synapse
3. ACh binding to receptors causes influx of Na+ = depolarization of muscle cell
4. AP transmitted down T-tubule into muscle
5. AP causes release of Ca++ from sarcoplasmic reticulum (SER)

Question 14

What is the sliding filament model?

Answer 14

contraction is due to thin filaments sliding past thick filaments, with no change in the length of either type of filament

thin filaments slide inwards, bring Z disk closer together

contraction = shortening of sarcomere

Question 15

What is the cell structure of smooth muscle?

Answer 15

non-striated

unicellular

Question 16

What is the cytoskeletal arrangement of smooth muscle?

Answer 16

actin arranged in a crisscross pattern across the cell anchored in dense bodies

actin and myosin form a mesh on intracellular face = cortex

uses non-muscle myosin (no true filaments)

contraction of myosin causes shortening of entire cell

Question 17

What is the process of activation of smooth muscle?

Answer 17

neural or hormonal impulses cause opening of Ca channels and an influx of calcium into the cell

calmodulin: Ca++ sensory molecule that is activated when bound to Ca++

active calmodulin binds MLCK (myosin light chain kinase)

MLCK phosphorylate rMLC

phosphorylation of rMLC causes activation of myosin (non-muscle myosin Type II)

Question 18

What is the contractile cycle of smooth muscle?

Answer 18

actin and myosin bundles contract and pull on dense bodies producing a shortening of the smooth muscle cell

dense body: intersection between MF and IF cytoskeletons

Question 19

What is the cell structure of cardiac muscle?

Answer 19

striated (contain sarcomeres)

unicellular: connected by intercalated disks

intercalated disks are high in desmosomes and gap junctions

impulse for contraction comes from a set of auto-rhythmic cells = pacemaker

Ca++ is the contractile signal

Question 20

What is the cytoskeletal arrangement of cardiac muscle?

Answer 20

sarcomeres

actin and myosin

cardiac myosin

Question 21

What causes the activation of cardiac muscle?

Answer 21

activated by the cardiac pacemaker

spread by gap junctions

longer depolarization allows chamber emptying

opening of voltage gated Ca++ channels prolongs depolarization

Ca++ spreads between cells via gap junctions