1. Calcium Release Channels
Recommended reading
Signal Transduction ed. by Heldin and Purton Chapter 14 “Calcium Signaling”
Additional extra recommended reading for those who want to learn more:
Ionic Channels of Excitable membranes Bertil Hille
Molecular Cell Biology pp pp pp pp
Kandel, Schwartz and Jessell Principles of Neural Science
3rd Edition
Chapter 5 pp 66 to 79
Chapter 8 pp 104 to 118
Appendix A pp 1033 to 1039

3. NCE mM [Ca] NMDA, AMPA rec nAChR CNG VR/temp rec VDAC Stretch receptor
TRP
mM [Ca]
SOC
IP3R
SERCA
100 nM [Ca]
ROC
VGCC
mM [Ca]
Channels and transporters involved in maintaining ion homeostasis in cells.
PMCA, plasmamembrane calcium ATPase; VGCC, voltage-gated calcium channel; ROC, receptor operatedd channel; SOC, store operated channel; TRP, transient receptor potential - channels thought to be involved in store operated calcium influx; stretch receptors, VR/temp rec, vanilloid receptors, pain and temperature sensors; CNG, cyclic nucleotide gated channels; nACHR, acetylcholine receptors; NMDA, AMPA receptors; NCE, sodium/calcium exchanger; VDAC, mitochondrial channel; IP3R, inositol 1,4,5-trisphosphate receptor; RyR, ryanodine receptor; SERCA, sarcoplasmic/endoplasmic reticular calcium ATPase.
PMCA
RyR
SERCA

4. Some of the roles of calcium signaling in cells

5. How calcium signals may be propagated in cells

6. Sarcoplasmic Reticulum (SR) / T Tubule System
specialized form of endoplasmic reticulum, whose membrane is rich with active transport systems (Ca-ATPase pumps) that pump intracellular calcium into the SR. Thus SR is the storage site for calcium used to activate contractile proteins.
b) sac-like structure that surrounds each myofibril.
c) enlarged sac at A-I band junction, lateral sacs.
d) actin potential is carried to the fiber interior through the transverse tubules (i.e. t tubules). The tubule fluid is identical in composition to the extracellular fluid. The t tubules are juxtaposed to the lateral sacs. When an action potential is propagated along the t tubule, the lateral sacs are signaled by an undefined mechanism to release their calcium into the sarcoplasm, activating the contractile proteins.
Summary of Excitation-Contraction Coupling
Action potential at surface membrane
Transmission into interior along transverse tubules
Release of calcium from lateral sacs of SR
Calcium diffuses to filaments and initiates contraction
Calcium taken up by SR causes relaxation

7. Excitation-contraction coupling

10. Twitch
Response to single action potential

11. Summation
Increase in contractile response to a second action potential that occurs during the contractile response produced by the previous action potential.

12. Tetanus
Response to multiple stimuli delivered at a rate sufficient to produce a fused contraction.
Basis for increased force with multiple stimuli - Not due to increased release of calcium from SR, single action potential releases sufficient calcium to elicit a maximum response.
Several factors act together to prevent the maximum possible force from being developed by the muscle. These factors are:
a) The rapid removal of free calcium from the cytoplasm back into the SR;
b) the finite time required for the troponin-tropomyosin complex to undergo conformational change before crossbridges can attach to actin
c) the finite time required for crossbridges to attach to actin and then undergo the conformational change to the force generating state.
Therefore, successive stimuli allow calcium to be elevated for a sufficient time so that significant crossbridge attachment and cycling can occur.

13. Role of Ca++ in contraction
Role of Ca++ in contraction- if we deprive a muscle cell of Ca it will not contract. However, if remove the outer membrane of muscle cell, and bath "skinned fiber" in a [Ca] of 10-6 M, muscle contracts.
note that muscle tension increases as [Ca] increases, and that contraction requires Mg++, and ATP.