muscle physiology

Question 1

how is the resting membrane potential calculated

Answer 1

ICF voltage minus the ECF voltage

Question 2

How is the resting membrane potential maintained

Answer 2

via Na and K passive leak channels and also the Na/K ATPase (more +ve leaving cell than entering therefore -ve ICF)

Question 3

why is the ECF at 0mV

Answer 3

there is no membrane keeping ions there so they diffuse away leaving behind a voltage of 0

Question 4

action potential

Answer 4

rapid change to the membrane potential caused by a rapid activation and fluxes of ion currents

Question 5

describe how calcium is released in skeletal and cardiac muscle contraction

Answer 5

1 - peak of AP sees opening of voltage gated Ca channels in t tubules of sarcolemma
2 - this allows small influx of Ca into the cell, some of which binds to the ryanodine receptor on the sarcoplasmic reticulum
3 - this binding causes SR to release Ca stores
4 - Ca goes on to stimulate contraction

Question 6

describe how calcium is released in smooth muscle contraction

Answer 6

1 - Ca enters the cell via both voltage gated and chemically gated channels (e.g binding of ACh)
2 - some of this calcium binds to RYR and also IP3R on the SR and calcium stores are released
3 - calcium goes on to stimulate contraction

Question 7

during relaxation, how is calcium removed

Answer 7

• some returns back to the SR via SERCA

- some leaves cell to accommodate future influx via NCX and PMCA

Question 8

in a relaxed state, in what form is actin in with regards to tropomysosin

Answer 8

myosin head binding sites on actin are hidden by tropomyosin and the troponin complex

Question 9

How does calcium stimulate muscle contraction in skeletal and cardiac muscle

Answer 9

• calcium targets troponin complex which reconfigures it and pulls troponin away from the myosin head binding sites

Question 10

how does calcium stimulate muscle contraction in smooth muscle

Answer 10

• Ca binds to calmodulin
• Ca-calmodulin complex activates myosin kinase
• myosin kinase phosphorylates myosin heads so that they can then bind to actin

Question 11

does ATP provide energy for muscle contraction or relaxation

Answer 11

relaxation

Question 12

describe the cross bridge cycle

Answer 12

1 - Binding of Ca exposes actin binding sites and cycle begins
2 - binding of myosin to actin sees release of phosphate
3 - ADP released during power stroke
4 - calcium moves out to re energise SR
5 - ATP required to break bonds between myosin and actin which relaxes the muscle
6 - ATP provides energy to cock myosin head for next cycle

Question 13

cramp

Answer 13

occurs when muscle is depleted of ATP therefore stays in the contracted state and cant relax

Question 14

rigor mortis

Answer 14

stiffening of limbs after death as no ATP to relax them

Question 15

what might abnormalities to the RYR cause

Answer 15

calcium sparks - small amounts of Ca being released and binding to myofilaments when muscles are meant to be relaxed

Question 16

calcium sparks

Answer 16

Ca released and binding to myofilaments when muscles are meant to be relaxed. May also lead to insufficient stores in SR
significant impact in cardiac muscle