section 3, chapter 10 nervous system I

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

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

Non-gated Ion 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 • 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 cellFigure 10.14c. The Na/K+ pump

maintains the resting membrane

potential of a neuron at -70mV.potential of a neuron at -70mV.

End of Section 3, Chapter 10