Lecture 17 - The role of Ca2+ in the nervous system Roles of calcium in the nervous system - Soluble Ca2+ can act as a: o Charge carrier (e.g. cardiac action potential) > o Signalling molecule - Most of intracellular Ca2+ is buffered -> bound to another molecule and occasionally released - Insoluble form -> structural constituent of bones and teeth o Concentration of free calcium is very low (large electrochemical gradient across membrane for Ca2+) Calcium and action potentials - At the NMJ, Ca2+ facilitates action potential - triggered neurotransmitter release from the presynaptic cell - Low extracellular Ca2+ conc. reduces the likelihood of EPP generation in the postsynaptic cell - Ca2+ serves as a second messenger during transmitter release Ca2+ entry into the presynaptic terminal Experiment: - Na+ and k+ channels inactivated - Voltage clamped cell (constant voltage level) o -70 mV Cell 1: - Moderate depolarisation (-25mV) results in a presynaptic Ca2+ current followed by the postsynaptic EPP ( end plate potential) Cell 2: - Stronger depolarisation (+50 mV) does not trigger a presynaptic Ca2+ current / postsynaptic EPP - Why? Reversal potential for Ca2+ was nearly reached > not enough driving force for calcium to enter the cell -> no influx Mechanism of neurotransmitter release Cell 1 Cell 2 +50 mV presynaptic potential 50 0 (mV) -70 -25 mV 0 presynaptic a" current (??) -3 -6 A 'tail' Ca2+ current -25 postsynaptic current (HA) -50 -75 time - 5 ms - The synaptic vesicle membrane and postsynaptic membrane interact - Important proteins on the postsynaptic membrane: Syntaxin and SNAP- 25 - Synaptotagmin is an important synaptic vesicle membrane that binds to Ca2+ molecules SNARE COMPLEX o Primary role of SNARE proteins is to mediate vesicle fusion to allow exocytosis of Ca2+
Exocytosis- mediating SNARE complex: 1. Free SNARES on vesicle and plasma membranes o Synaptobrevin and synapotagmin attached to vesicular membrane are not yet in contact with the proteins on the postsynaptic membrane 2. SNARE complexes form as vesicle docks o Synaptic vesicles eventually bump into other membrane > Dock o Synaptobrevin interacts with SNAP-25 and syntaxin 3. Synaptotagmin binds to SNARE complex 4. Entering Ca2+ binds to synaptotagmin o Calcium enters the cell and binds to synaptotagmin (calcium sensor) o The calcium sensor undergoes conformational change o This change pushes the presynaptic vesicle closer to the plasma membrane o plasma membrane curves -> brings the two membranes together 5. Fusion of the membranes leads to exocytotic release of neurotransmitter Synaptic vesicle membrane Syntaxin Synaptotagmin -SNAP-25 Presynaptic plasma membrane Exocytosis-mediating SNARE complex Neuroscience, Purves (ch.5) (1) Free SNARES on vesicle and plasma membranes O Synaptotagmin Synaptobrevin Vesicle Ca2+ channel Syntaxin SNAP-25 (2) SNARE complexes form as vesicle docks (3) Synaptotagmin binds to SNARE complex (4) Entering Ca2+ binds to synaptotagmin, leading to curvature of plasma membrane, which brings membranes together (5) Fusion of membranes leads to exocytotic release of neurotransmitter
Pathologies associated with impaired presynaptic vesicle exocytosis and Ca2+ cycling LEMS (Lambert- Eaton MS): - Attacks