Chapter 10, Section 3
The Synapse And Membrane Potential
The Synapse
• Nerve impulses pass from neuron to neuron at synapses, moving from a pre-synaptic neuron to a post-synaptic neuron.
Synaptic Transmission Neurotransmitters are released when an impulse reaches the synaptic knob 1. Impulse (Action potential) reaches synaptic knob at the axon terminal 2. Calcium floods into the axon 3. Influx of Calcium causes the release of neurotransmitters into the synaptic cleft. 4. Neurotransmitter binds to post-synaptic cell
Some neurotransmitters are inhibitory – inhibit postsynaptic cell Others are excitatory – stimulate postsynaptic cell
Cell Membrane Potential Cell membrane is usually polarized (charged) • Inside the membrane is negatively charged relative to outside the membrane • Polarization is due to unequal distribution of ions across the membrane •Polarization is maintained by a series of ion pumps and channels
Na+/K+ Pump 1. Sodium/Potassium (Na+/K+) pump • Actively transport 3Na+ out of the cell, and 2K+ into the cell • Creates a high extracellular [Na+] and a high intracellular [K+] • Maintains polarization of cell membrane • Requires ATP
Non-gated Ion Channels 2. Non-gated ion channels “leak channels” • Always open • Allows for facilitated diffusion of specific ions • Cell membrane has many K+ leak channels, but only a few Na+ leak channels
Figure A. The sodium-potassium pumps transports sodium out of the cell, while transporting potassium into the cell. Figure B. Leak channels allow some of the potassium to leak out of the cell, contributing to the positively charged extracellular fluid.
Gated Ion Channels 3. Ligand-Gated Ion Channels • Open or close in response to a neurotransmitter or other molecule • Includes ACh receptors on motor endplates 4. Voltage-Gated Ion Channels • Open or close in response to small changes in membrane potential (millivolts=mV) • Voltage-gated Na+ channels open when membrane potential reaches -55mV.
Gated Ion Channels
Figure 10.15b. Ligand-gated Na+ channels (blue) open in response to neurotransmitters. Voltage-gated Na+ channels (pink) open in response to changes in membrane potential.
Distribution of Ions
• Potassium (K+) ions are the major intracellular positive ions (cations). • Sodium (Na+) ions are the major extracellular positive ions (cations). • This distribution is largely created by the Sodium/Potassium Pump (Na+/K+ pump). • This pump actively transports sodium ions out of the cell and potassium ions into the cell.
Resting Membrane Potential (RMP) •The Resting Membrane Potential (RMP) is the difference in charge between the outside and inside of a cell at rest. • For neurons the RMP is -70mV (inside the cell) • RMP is due to Na+/K+ distribution, along with negatively charged proteins within the cell
End of Section 3, Chapter 10
Figure 10.14c. The Na/K+ pump maintains the resting membrane potential of a neuron at -70mV.