section 3, chapter 10 nervous system i
DESCRIPTION
nervous system lecture for anatomy & physiologyTRANSCRIPT
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