1. Chapter 12- 2
Neural tissue

2. Signal transduction Neurons use gated channels to transmit signals.
Activation of these channels cause local changes in the membrane potential.

3. Types of potential changes
Graded (local) potentials
all cell membranes can generate
act locally
strength is related to the strength of the signal
get weaker the further they get from the original signal
Can be excitatory or inhibitory
Action potentials
only on excitable membranes
are self propagating
exhibit “all or none” behavior

4. Graded potential
A change in potential that decreases with distance and varies in strength based on the strength of the stimulus
Result in a localized can be depolarization or hyperpolarization
Depolarization - a shift in the transmembrane potential toward 0 (becomes less negative)
Hyperpolarization - a shift in the transmembrane potential away from 0 (becomes more negative)
Repolarization - after a cell depolarizes a shift back towards its typical resting potential

5. Depolarization and Hyperpolarization
opening Na+ channels would depolarize the cell
opening of K+ channels would repolarize or hyperpolarize the cell
opening of Cl- channels would hyperpolarize the cell
Figure 11.9

6. Depolarization waves
Figure 11.10

7. Local Potentials Graded
Magnitude varies from small to large depending on stimulus strength or frequency
Can summate or add onto each other
Spread (are conducted) over the plasma membrane in a decremental fashion: rapidly decrease in magnitude as they spread over the surface of the plasma membrane.
Can cause generation of action potentials
Seeley, Stephens and Tate

8. Action Potential
Appears when region of excitable membrane depolarizes to threshold. In neurons, it is a property of the axon and normally begins at the axon hillock.
Steps involved
1. Sodium channel activation and depolarization of the membrane
2. Sodium channel inactivation
3. Potassium channel activation and membrane repolarization
4. Return to normal permeability

9. Generation of an Action Potential
Figure

10. Steps of an action potential

11. Generation of an Action Potential

12. Characteristics of action potentials
Generation of action potential follows all-or-none principle
Refractory period lasts from time action potential begins until normal resting potential returns.
Absolute refractory period - the neuron cannot re-fire. Na+ Channels are either open or inactivated
Relative refractory period - when Na+ channels closed and can become reactivated. K+ channels are still open so a new stimulus must cause more Na+ entry than K+ exit.

13. Refractory periods

14. Characteristics of action potentials
Continuous propagation
spread of action potential across entire membrane in series of small steps
Saltatory propagation
action potential spreads from node to node, skipping internodal membrane

15. Continuous Propagation
Propagation along an Unmyelinated Axon

16. Saltatory Propagation
Propagation along a Myelinated Axon

17. Synapse Site of intercellular communication
Neurotransmitters released from synaptic knob of presynaptic neuron to a postsynaptic neuron or other effector.
Neuromuscular junction
Neuroglandular junction
Presynaptic membrane
Postsynaptic membrane
Synaptic cleft

18. Electrical synapses Rare
Pre- and postsynaptic cells are bound by interlocking membrane proteins that allow ions to pass between them (Gap junctions and connexons)

19. Chemical synapses Common
Release a chemical neurotransmitter between the pre- synaptic and post-synaptic membranes
Excitatory neurotransmitters cause depolarization and promote action potential generation
Inhibitory neurotransmitters cause hyperpolarization and suppress action potentials.
Depends on the receptors and ion channels present on the post-synaptic membrane

20. Structure of a Typical Synapse

21. Neurotransmitter Functions
Direct effects (ion channels)
Figure 11.22a

22. Neurotransmitter Functions
Indirect effects via G proteins

23. Some neurotransmitters
Cholinergic synapses
very common
release Acetylcholine (ACh)
Are always excitatory,
work by opening Na+ channels
Adrenergic synapses
release norepinephrine (NE),
depolarizing effects,
released in the sympathetic NS
Work via G proteins and cAMP

24. Some neurotransmitters
Other important neurotransmitters
Dopamine - released in the CNS and in some peripheral nerves. Can be excitatory or inhibitory, causes artery vasodialation, alters mood
Serotonin - released in the CNS, low levels lead to depression
GABA (gamma aminobutyric acid) (an amino acid) in CNS, is generally in inhibitory neurons, opens Cl- channels

25. Cholinergic transmission

26. Cholinergic synapses
Synaptic delay occurs as calcium influx and neurotransmitter release take appreciable amounts of time
ACh broken down
Choline reabsorbed by presynaptic neurons and recycled
Synaptic fatigue occurs when stores of ACh are exhausted

27. Removal of neurotransmitter
Seeley, Stephens and Tate

28. Neuromodulators
Are neurotransmitters or other substances that generally influence pre-synaptic or post-synaptic cells response to other neurotransmitter
They can be peptides (opiods), gases (CO2, NO), lipids (arachadonic acid), purines (eg ATP, adenosine)
They can alter membrane potential or cell function directly (alter ion channels) or indirect effect (activate second messengers)
Can exert influence via lipid-soluble gases