Lecture 11/12 - Neuromusclular Transmission, Excitation/Contraction Coupling, Muscle Contraction

Question 1

skeletal muscle definitons:
myo, sarco, myocyte, myofiber, sarcolemma, sarcoplasm

Answer 1

myo = muscle

sarco = flesh

myocyte = muscle cell with single nucleus

myofiber = multinucleated muscle cell

sarcolemma = myofiber plasma membrane (where AP propogates and causes change in muscle)

sarcoplasm = myofiber cytoplasm

Question 1

skeletal (voluntary) muscle - what is it made of, length, attachment, another name for it

Answer 1

made of cylindrical, multi-nucleated myofibers

each fiber spans the length of the muscle

most are attached to the skeleton across joints –> contraction causes gross movement

microscopically, fibers have a striped or striated appearance, hence also called striated muscle

Question 2

neuromusclular transmission - 9 brief steps

Answer 2

1. sodium influx, 2. deplarization, 3. release of calcium, 4. interaction of actin and myosin, 5. shortening of the sarcomere, 6. sodium channels close and sodium is pumped out, 7. repolarization, 8. calcium channels close and calcium is pumped into SR, 9. sarcomere relaxes

Question 3

what is the area between two z lines called

Answer 3

sarcomere

Question 4

skeletal myofibers - what do they contain, arrangement of actin and myosin, t tubules, where is the SR

Answer 4

contains myofibrils, which are bundles of myofibrils, made of the contractile proteins actin and myosin

actin and myosin arranged in regular pattern between attachment plates called Z lines

t tubules wrap around each myofibril at the Z line

SR (containing calcium) - stretches between T tubules along surface of myofibril

Question 5

what is the contractile unit? where is actin anchored and myosin located

Answer 5

sarcomere is the contractile unit of skeletal muscle

actin is anchored at the z line

myosin is located between actin

Question 6

myosin components

Answer 6

each myosin protein is a homodimer: 2 molecules coil together to form a tail region with 2 head groups

head groups are hinged

many myosin molecules combine to form a thick filament

Question 7

actin composition, binding sites, affinity

Answer 7

thin filaments are composed of: actin, troponin, tropomyosin

“F actin” (filamentous) consists of chains of globular actin (G-actin)

Actin has binding sites for myosin; they are hidden by the tropomyosin strand

troponin has high affinity for calcium

a troponin molecule is bound to each tropomyosin

Question 8

excitation-contraction coupling - what happens

Answer 8

AP in motor neuron causes voltage gated calcium channels in axon terminus to open –> triggers neurotransmitter vescicle fusion –> neurotransmitter resleased in cleft
(neurotransmitter is ACh)

-binds to nicotinic AChRs in specialized regions of muscle membrane at motor end plate
-AChRs open –> sodium enters cell –> local depolarization at MEP

MEP depolarization triggers AP in surrounding sarcolemma –> spreads into T tubules

Question 9

what is the motor end plate part of?

Answer 9

neuromuscular junction

Question 10

how does depolarization of t tubules cause muscle to contract

Answer 10

voltage gated calcium channels in t tubules sense depolarization

4 of these channels tug on the “foot” of a different calcium channel, the ryanodine receptor, which is in the SR membrane –> RYR opens, calcium enters sarcoplasm

otherwise; the voltage sensoryin one calcium channel opens a different calcium channel (RYR)

Question 11

actin and calcium process

Answer 11

-when calcium is released from SR it binds troponin
-causes tropomyosin strand to move
-reveals myosin binding site on actin
-cross bridge forms between actin and myosin
-myosin head group tilts
contraction

Question 12

ratchet theory of contraction

Answer 12

ATPase in myosin head group cleaves ATP
-extends headgroup, which binds to actin
-binding is followed by tilting of headgroup, which slides actin about 5-10nm
-new ATP molecule binds myosin head group which releases actin
-ATP then cleaved to tilt head group again
-cycle repeated as long as calcium levels are high

Question 13

energy generation for muscle contraction

Answer 13

-additional substrate that muscle uses
-creatine metabolized by CK to get creatine phosphate, pretty much only happens in muscle

Question 14

sources of ATP over time

Answer 14

phospho-creatinine is the primary source of energy for the first 30 seconds

glycolytic = anaerobic metabolism. fast but not super efficient. used if exposive contraction happens

aerobic = used in long time scale

order: phospho-creatinine –> glycolytic –> aerobic

Question 15

heterogeneity of skeletal muscle - white/fast twich vs red/slow twitch

Answer 15

white/fast twitch = short contraction times, larger in size and more extensive SR, adapted for anaerobic metabolism (predatory animals have more of this)

red/slow twitch = long contraction time, smaller in size with less extensive SR, adapted for aerobic metabolism (long distanced running)

Question 16

removal of calcium

Answer 16

-calcium ATPases or pumps reset the intracellular calcium level by moving calcium out of the cell or back into the SR
-cell surface pump is called the PM calcium ATPase (PMCA)
-SR pump is called the sarcoplasmic/endoplasmic reticulum ATPase (SERCA)

Question 17

smooth (involuntary muscle) - visceral vs multiunit –> what controls it, how AP works, how are cells connected in visceral, components of multiunit

Answer 17

visceral = controlled mainly by local stimuli with some nervous regulation, cells connected by gap junctions, may have spontaneous slow waves that trigger APs

multiunit = controlled by nervous stimulation (ACh or NE) ; [ciliary muscles, piloerector muscle, large blood vessel tone]
APs or decremental depols may contraction

Question 18

skeletal muscle vs smooth muscle

Answer 18

-APs in visceral smooth mucles are slower than in skeletal muscle and persist longer
-graded depolarizations can cause contraction, APs cause more forceful contraction