section 3, chapter 10

The Synapse And

Membrane Potential

ivyanatomy.com

Synaptic transmission is the mechanism that transmits a signal from the pre-synaptic neuron to the post-synaptic neuron.

Synaptic Transmission

An action potential causes the release of neurotransmitters from the presynaptic cell that diffuse across the synapse and bind to the postsynaptic cell.

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 post-synaptic cell

Some neurotransmitters are inhibitory whereas others are excitatory, so the post-synaptic cell may be stimulated or it may be inhibited depending on the neurotransmitter.

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

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.

Factors that maintain the cell membrane potential

1. Sodium/Potassium (Na+/K+) pump

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.

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.

Factors that maintain the cell membrane potential

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.

RMP = -70 mV inside the cell.

Factors that change the cell membrane potential

Gated ion channels open and close in response to a stimulus.

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.

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.

Gated Ion Channels

End of section 3, chapter 10