1. Neuroscience Fundamentals 112C Ian Parker
Biophysics of intracellular neuronal signaling
Ca2+ signaling

2. Ca2+ signaling
Resting [Ca 2+ ] in cytosol maintained at ~ 50 nM by actions of pumps & exchangers. So, only a little Ca entering the cytosol will give big (a few mM) increase in concentration.
Cytosolic Ca can……
Activate membrane ion channels
Regulate enzyme activity
Modulate protein function
Regulate gene expression
Kill cells! (necrotic & apoptotic death)
It has functions in virtually all cells of the body.
Christened a ‘life and death’ messenger

3. Calcium is the ONLY link between electrical activity of a neuron (or any other cell) and the end response of the cell: e.g. Ca2+ influx through voltage gated channels in presynaptic terminal evokes transmitter release Ca2+ liberation from SR triggers muscle contraction (skeletal or cardiac muscle) Ca2+ influx through ligand-gated channels involved in LTP Unlike other ions (Na+, K+, Cl-) it is the CHEMICAL signal carried by Ca2+ that is important, not the electrical charge of the ion.

4. Sources of Ca2+; and Ca2+ permeable channels
Extracellular fluid (~ 2 mM)
Plasma membrane channels Voltage-gated channels (N, P, Q, L-type)
Ligand-gated channels: e.g. several neurotransmitter- activated channels have appreciable Ca2+ permeability (nicotinic ACh, NMDA)
Store-operated channels: open in response to depletion of e.r calcium stores
Endoplasmic/sarcoplasmic reticulum (~ 1mM)
SR membrane (skeletal muscle)- Ryanodine receptors; opening coupled to voltage sensors in plasma membrane
SR membrane (cardiac muscle)- Ryanodine receptors; opening triggered by Ca2+ entering through plasma membrane voltage-gated channels
ER membrane Ryanodine receptors; opening triggered by cytosolic Ca2+
IP3 receptors; opening requires both IP3 and cytosolic Ca2+

5. Diffusion The only way calcium ions can transmit information from one place in a cell to another.
A particle (ion, molecule, organelle, whatever…) undergoing diffusion follows a ‘random walk’ [run web movie]
The mean distance L it will have moved (in 3-dimensions) from its starting point after time t is given by L = sqrt (6Dt) ; where D = diffusion coefficient (um2 sec-1)
So, mean distance increases as square root of time
E.g. a molecule with D = 17 um2 sec-1 will travel 100 nm in 0.1 ms; 1 mm in 10 ms; 10 mm in 1 s; 100 mm in ~2 min.
Compare ‘chemical’ and electrical signaling in neurons…

6. Only a few % of the Ca2+ ions in the cytosol are ‘free’
Most bind to stationary Ca buffers (proteins), which slow their diffusion
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Diffusion coefficient for free Ca2+ in water is ~ 200 mm2 s-1,
in cytosol it is about 10 mm2 s-1

7. Spatial and temporal aspects of Ca2+ signaling
Because Ca2+ diffusion in the cell is hindered by immobile buffers, transient Ca 2+ elevations can be highly localized, and specifically regulate only nearby targets – e.g. neurotransmitter release.
Or, Ca2+ ions can propagate as a wave throughout a cell: e.g. to communicate signals to nucleus
Sustained Ca2+ elevations kill cells (e.g. glutamate neurotoxicity). So Ca waves are generated periodically. Frequency of repetitive waves encodes stimulus strength.

8. ‘Digital’ vs. ‘Analog’ encoding of information
Weak stimulus
Strong stimulus
Strength encoded by frequency or pattern of all-or none signals
Strength encoded in a continuously-graded manner by signal amplitude

9. IP3/Ca2+ signaling pathway
Cytosol
Ca2+ Channel
[Ca]Local>10mM
Pump
IP3R ER
Cell
Membrane
Pump

10. Global Ca2+ signaling Ca2+ + IP3 [Ca2+]cyt IP3 receptor Ca2+ waves
cytoplasm
Ca2+
IP3 +
IP3 receptor
Ca2+
waves
in a
whole
cell
[Ca2+]cyt

11. - - + Global Ca2+ signaling Ca2+ + IP3 [Ca2+]cyt IP3 receptor Ca2+
cytoplasm
Ca2+
IP3 +
IP3 receptor -
Ca2+
waves
in a
whole
cell
[Ca2+]cyt - +

12. Ca2+-induced Ca2+ release propagates Ca2+ wavesy x
Cytosol z
Cytosol space
ER membrane
ER store
Cytosol
Membrane ER

13. Local & global Ca2+ signals
10 mM

14. Model of local & global IP3 - evoked Ca2+ signals
Switched to other one.

15. IP3-dependent Ca2+ signals are ordered hierarchically
local
cellular
molecular
stochastic
periodic