1. Neurons, Signals, Synapses
Chapter 37

2. Dendrites receive signals from other neurons
I. Neuron Structure
Dendrites receive signals from other neurons
Cell body contains nucleus and organelles
Axon transmits signals to other neurons or muscles
Dendrites
Cell body
Figure 48.5 Structural diversity of neurons.
Axon
Fig. 37.5
Motor neuron

3. Figure 37.2 Presynaptic neuron Stimulus Cell body Axon
Synaptic terminals
Synaptic terminals
Synapse
Figure 48.4 Neuron structure and organization.
Postsynaptic neuron
Neurotransmitter

4. Synapse – junction of a nerve cell with another cell
Relative terms depending on cell location in network:
Pre-synaptic cell – cell sending signal across synapse
Post-synaptic cell – cell receiving signal

5. Cyan = neuron cell bodies Red = support cells (glia)
Figure 37.3
Real neurons
Green = dendrites
Cyan = neuron cell bodies
Red = support cells (glia)
80 m
Cell bodies of neurons
Figure 48.6 Glia in the mammalian brain.

6. II. Resting potential Bioflix: How Neurons Work
Intro + Resting Potential

7. II. Resting potential
Resting potential = charge difference across a cell membrane of ALL Body Cells
Develops because of Na+ & K+ movement
Ion movement through 3 membrane proteins:
Na+-K+ pump
Passive K+ channels
Passive Na+ channels

8. Figure 37.6 Key Na K Sodium- potassium pump OUTSIDE OF CELL
Potassium channel
Sodium channel
Figure 48.7 The basis of the membrane potential.
INSIDE OF CELL

10. Description of ion movement
Na+-K+ pump: pumps 3 Na+ out & 2 K+ in to the cell
Result:
[Na+] = 15 mM inside 150 mM outside
[K+] = 140 mM inside 5 mM outside

11. Passive K+ channel: always open
K+ leaves cell
Cl- and Protein- are stuck inside cell
Result:
(-) charge now greater inside cell
(-) charge attracts (+) charge on outside, electrical gradient limits K+ efflux (outflow)

12. III. Action potential
Bioflix: Action Potential

13. III. Action Potential Depends on voltage-gated Na+ & K+ channels
Passive channels that are closed at rest
Open in response to a change in voltage across cell membrane
Like an electrically operated gate

14. Description of ion movement
Stimulus changes membrane potential (voltage)
If change is large enough that voltage exceeds a threshold, voltage-gated Na+ channels open
Na+ flows into cell
Change in potential CLOSES voltage-gated Na+ channels and OPENS voltage-gated K+ channels
K+ leaves cell

15. Na+-K+ pump re-establishes gradients of Na+ and K+
Action potential spreads because Na+ diffuses
along the inside of the cell membrane, changing
voltage & opening the next batch of voltage-
gated Na+ channels

16. Figure 37.11 Key Na K 4 Falling phase of the action potential 3
Rising phase of the action potential
50
Action potential 3
Membrane potential (mV)
Threshold 4 2
50 1 1 5 2
Depolarization
Resting potential
100
Figure The role of voltage-gated ion channels in the generation of an action potential.
Time
OUTSIDE OF CELL
Sodium channel
Potassium channel
INSIDE OF CELL
Inactivation loop 1
Resting state 5
Undershoot

17. IV. Synapse
Bioflix: How Synapses Work

18. A real synapse Postsynaptic neuron
Synaptic terminals of pre-synaptic neurons
5 m
Figure Synaptic terminals on the cell body of a postsynaptic neuron (colorized SEM).
Figure 37.16

19. Presynaptic cell Synaptic cleft
Figure 37.15
Presynaptic cell
Postsynaptic cell
Axon
Synaptic vesicle containing neurotransmitter 1
Postsynaptic membrane
Synaptic cleft
Presynaptic membrane 3
Figure A chemical synapse. K
Ca2 2
Voltage-gated Ca2 channel
Ligand-gated ion channels 4
Na

20. At chemical synapse
Electrical signal crosses the synaptic cleft = gap between neurons
How?
A.p. reaches end of neuron
Voltage changes in the neuron membrane
Voltage-gated Ca2+ channels open
Ca2+ binds to vesicles containing neurotransmitter
Vesicles fuse with neuron membrane

21. (continued)
Neurotransmitter is released by exocytosis into the synaptic cleft
Neurotransmitter binds to receptors on next neuron
Receptors are ligand-gated Na+ channels = Na+ channels that open when the right molecule (ligand) attaches
Neurotransmitter detaches from receptors and is degraded by enzymes to stop signal

22. What happens next?
The postsynaptic cell responds if the postsymatic potential reaches threshold
Postsynaptic cells could be :
Nerve cells
Muscle cells
Glandular cells