musle physiology 1.1

Question 1

contraction

Answer 1

• activation of the force generating sites within muscle fibers - the cross bridges
• does not necessarily mean shortening
• can be isometric where muscle generating tension while its holding the joint in a steady position
• or even when lengthening (eccentric - down)

Question 2

alpha motor neurons

Answer 2

• innervate skeletal muscle fibers
• located in brain stem or spinal cord and carry out to skeletal muscles
• are myelinated and have large diamteter axons
• propagate fast, allowing signals from the CNS to travel to skeletal muscle fibres with minimal delay

Question 3

motor unit

Answer 3

motor neuron + muscle fibers it innervates

Question 4

how many muscle fibers does a single motor neuron innervated

Answer 4

a single motor neuron innervates many muscle fibers, but each muscle fiber is controlled by a branch from only one motor neuron

Question 5

what happens when AP occurs in motor neuron?

Answer 5

all the muscle fibers in its motor unit are stimulated to contract

Question 6

motor end plate

Answer 6

region of the muscle fiber plasma membrane that lies directly under the terminal portion of the axon

Question 7

neuromuscular junction

Answer 7

junction of axon terminal with the motor end plate

Question 8

events at neuromuscular juntion

Answer 8

AP arrives in a motor neuron to axon terminal

• Ca2+ diffuses into the axon terminal
• Ach released from vessicles to extracellular cleft to the motor end plate
• Ach binds to ionotropic reecptors (nicotinic)
• channel open, Na and K pass down diffusion gradient (Na in K out)
• depolarization of the motor end plate (end-plate potential) (EPP)
• Ap propagated over surface of muscle fiber into T-tubules *

Question 9

magnitude of EPP smaller or larger than that of EPSP

Answer 9

Magnitude of EPP much larger bc the neurotransmitter is released over a larger surface area, binding to many more receptors and opening many more ion channels

Question 10

how many EPP neccesary to depolarize

Answer 10

only one is normally more than sufficient to depolarize the plasma membrane to the end-plate membrane to its threshold potential

Question 11

muscular junction excitatory inhibitory both?

Answer 11

all muscular junctions are excitatory in skeletal muscle

Question 12

acetylcholinesterase

Answer 12

enzyme that breaks down Ach
-choline is then transported back into the axon terminals where it is reused in the synthesis of new Ach

-As we breakdown Ach, more Ach will come off the receptors and be broken down. No more Ach bound= ion chanels in the end plate close. Now it returns to its resting potential and can respond to the subsequent arrival of Ach released by another neuron AP

Question 13

neuromuscular junction

Answer 13

• neuron to motor end plate
• AP will propagate in both directions
• Cause an EPP
• magnitude of EPP is much larger than EPSP
• 1 EPP sufficient to depolarize
• All excitatory
• every AP in a motor neuron produces an AP in each muscle fiber in its motor unit

Question 14

inhibiting acetylcholinesterase

Answer 14

• organophosphates inhibit acetylcholinesterase
• in pesticides/nerve gases
• What happens? Ach is not destroyed, channels remain open-> cannot generate another AP
• receptors desensitized to Ach

Question 15

antidote for disruption of neuromuscular signaling

Answer 15

pralidoxime (reactivates acetylcholinesterase) and atropine (muscarinic receptor antagonist)

Question 16

excitation-contraction coupling

Answer 16

-the sequence of events by which an action potential in the plasma membrane activate the force-generating mechanisms (muscle contraction)

Question 17

ap skeletal muscle lasts

Answer 17

1-2msec

-is completed BEFORE any signs of mechanical activity begin (MUSCLE CONTRACTION)

Question 18

mechanical activity

Answer 18

follows an AP and last 100msec or more

AP’s are quick, mechanical activity longer

Question 19

electrical activity in plasma membrane

Answer 19

• *does not** directly act upon contractile proteins
• > increases cytosolic Ca2+ concentration

-when we continue to contract the muscle after the action potential is gone because of the calcium in the cytosol

Question 20

tropomyosin at rest

in ca2+ cross-bridge formation

Answer 20

• composed two intertwined polypeptides
• partially cover the myosin-binding site on each actin monomer, preventing cross bridges from making contact with actin

Question 21

troponin at rest

Answer 21

• holds tropomyosin in place and covers the myosin binding site on actin
• are all on the thin filament

Question 22

tropomyosin during contraction

what process allows cross bridges to form?

Answer 22

• moves away from myosin-binding site
• This process occurs when Ca2+ binds to troponin, resulting in the tropomyosin moving off the myosin binding site, allowing cross bridge formation
• if more Ca then greater contraction

Question 23

troponin 3 sites

Answer 23

I = Inhibitory

T= tropomyosin-binding

C=Ca2+

Question 24

mechanism of cytosolic increase in Ca2+

t-tubule

Answer 24

• extension of plasma membrane
• several ion channels (Ca2+)
• devoted to excitation-contraction coupling
• site for AP propagation

Question 25

mechanism of cytosolic increase in Ca2+ ## Footnote **DHP receptor (dihydropyridine)**

Answer 25

- **on t-tubule** - acts as a **_voltage sensor for calcium_**

Question 26

mechanism of cytosolic increase in Ca2+ ## Footnote **ryanodine receptor**

Answer 26

- on **sarcoplasmic reticulum** - has squigley line that connects to DHP receptor - _will let Ca2 out_

Question 27

mechanisms of cytosolic DECREASE Ca2+

Answer 27

- need to **remove Ca2+ from troponin** by **lowering Ca2+ concentration in the cytosol** to relax - using calcium ATPase

Question 28

calcium ATPase (pump)

Answer 28

- pumps calcium ions from **cytosol back into the lumen** of the **sarcoplasmic reticulum** - requires longer time - **Ca2+ is released from the reticulum** when an _AP begins in the t-tubule_ - Ca2+ pump **requires ATP** **-Ca2+ is exchanged with na+**

Question 29

sliding filament mechanism

Answer 29

**-tension is produced** by the sliding of **thin along the thick filaments** within the sarcomere - _no change in length_ of either the **thick or thin filaments** - movement of the cross bridges causes thick and thin filaments to **move past each other** _A band stays same size_, _I band gets smaller_

Question 30

power stroke

Answer 30

- pulls **thin filaments** towards _center of sarcomere,_ maximum contraction - sarcomere becomes shorter - occurs when myosin binds to actin, myosin will then detach and attach to next molecule, happens several times

Question 31

all sarcomeres contract/shorten simulataneously as well as relax simultaneously

Answer 31

true

Question 32

A band

Answer 32

thick filaments

Question 33

I band

Answer 33

thin filaments

Question 34

how many sarcomeres in a muscle

Answer 34

20000

Question 35

cross bridge cycle 4 steps - first step

Answer 35

1. attachment of the **cross bridge to a thin filament**, **actin binding**→ _cytosolic increase in calcium_ leads to attachment

Question 36

step 2

Answer 36

- movement of the crossbridge, producing tension in the thin filament - **power stroke occuring** **-release of ADP and organic phosphate which produces movement of cross bridge and powerstroke**

Question 37

step 3

Answer 37

- **detachment** of the _crossbridge from the thin filament_ - cross bridge **dissociation from actin** **-using ATP binding to myosin?**

Question 38

step 4

Answer 38

**atp bound to myosin** - ready for next cycle - _ATP hydrolysis which energizes cross-bridge_

Question 39

rigor mortis

Answer 39

**atp decreasing in cells,** **ca leaking out of SR**, _cant pump it back in and cross bridges are broken,_ _thick and thin cannot be pulled past each other_ -gradual **stiffening** of skeletal muscles that begins after death

Question 40

synaptic junction

Answer 40

neuron to neuron ap will propagate in one direction causes an EPSP or IPSP magnitude of EPSP is not as large as EPP usually need multiple EPSP to reach threshold and initiate an AP Excitatory or Inhibitory

Question 41

curare (poison blow darts)

Answer 41

binds strongly to nicotinic receptors so doesnt allow you to open those channels, ACh cant bind to them, when that happens you cant contract muscles, preventing you from breathing

Question 42

describe the whole process of the mechanism of cytosolic increase in Ca2+ \*\*review this

Answer 42

Ap comes through the motor neuron has vesicles with Ach, come to the motor end plates and attach to receptors for sodium and potassium and causes an action potential, calcium ? travels down the t tubules where voltage sensitive calcium DHP receptors activate the ryanodine receptors which let calcium out of the sarcoplasmic reticulum. This increases cytosolic concentration of calcium and leads to more calcium binding to troponin = more cross bridge formation

Question 43

what happens if there is an increase in calcium ?

Answer 43

more ca is binding to troponin and more tropomyosins are moving off the myosin binding sites on actin = greater contraction \*