neurons and membrane potentials

1. ivyanatomy.com
section 3, chapter 10
The Synapse
And
Membrane Potential

2. Synaptic Transmission
Synaptic transmission is the mechanism that
transmits a signal from the pre-synaptic
neuron to the post-synaptic neuron.
An action potential causes the release of
neurotransmitters from the presynaptic cell
that diffuse across the synapse and bind to
the postsynaptic cell.

3. Steps involved in Synaptic Transmission
1. A nerve impulse (action potential) travels
down the axon to the axon terminal.
2. The action potential opens calcium channels
causing calcium to diffuse into the synaptic
knob.
3. The calcium influx triggers the release of
neurotransmitters from synaptic vesicles
into the synapse.
4. The neurotransmitters diffuse across the
synapse and bind to receptors on the postsynaptic cell
Some neurotransmitters are inhibitory
whereas others are excitatory, so the postsynaptic cell may be stimulated or it may be
inhibited depending on the neurotransmitter.

5. Cell Membrane Potential
The 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

6. Factors that maintain the cell membrane potential
1. Sodium/Potassium (Na+/K+) pump
The sodium/potassium pump actively transports
3Na+ out of the cell, and 2K+ into the cell.
• It creates a high extracellular [Na+] and a
high intracellular [K+]
• requires ATP
• The Na+/K+ pump only contributes a small
amount (-5mV) to the membrane potential.

7. Factors that maintain the cell membrane potential
2. Non-gated potassium channels “K+ leak channels”
• The cell membrane has many K+ leak channels, but
only a few Na+ leak channels
• K+ continually leaks out of the cell, making the inside
of the cell more negative.

8. Factors that maintain the cell membrane potential
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.

9. Factors that maintain the cell membrane potential
The distribution of ions across the membrane
creates a membrane potential (electrical gradient).
For a neuron at rest the membrane potential is -70mV inside the
cell. This is the Resting Membrane Potential
RMP = -70mV inside the cell.

10. RMP = -70 mV inside the cell.

11. Factors that change the cell membrane potential
Gated ion channels open and close in response
to a stimulus.

12. Gated Ion Channels
1. Mechanically-Gated Channels
• Open or close in response to physical stress.
• Touch, hearing, vibrations, ect.
2. Ligand-Gated Ion Channels
• Open or close in response to a ligand
(neurotransmitter, hormone, or other molecule)
• Includes ACh receptors on motor endplates
3. Voltage-Gated Ion Channels
• Open or close in response to small changes in the
membrane potential (millivolts = mV)
• Voltage-gated Na+ channels open when membrane
potential reaches -55mV.

13. 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.
End of section 3, chapter 10