1. A junction that mediates information transfer from one neuron:
The Synapse
A junction that mediates information transfer from one neuron:
To another neuron, or
To an effector cell (muscle, secretory…)
Presynaptic neuron—conducts impulses toward the synapse (sending side)
Postsynaptic neuron—transmits impulses away from the synapse (receiving side)

2. Axodendritic—axon of one to dendrite of another
Types of Synapses
Axodendritic—axon of one to dendrite of another
Axosomatic—axon of one to soma of another
Less common types:
Axoaxonic (axon to axon)
Dendrodendritic (dendrite to dendrite)
Dendrosomatic (dendrite to soma)

3. Axodendritic synapses Dendrites Axosomatic synapses Cell body
Axoaxonic synapses
(a)
Axon
Axon
Axosomatic
synapses
Cell body (soma) of
postsynaptic neuron
(b)
Figure 11.16

4. Less common than chemical synapses
Electrical Synapses
Less common than chemical synapses
Neurons are electrically coupled (joined by gap junctions)
Communication is very rapid, and may be unidirectional or bidirectional
Are important in:
Embryonic nervous tissue
Some brain regions

5. Specialized for the release and reception of neurotransmitters
Chemical Synapses
Specialized for the release and reception of neurotransmitters
Typically composed of two parts
Axon terminal of the presynaptic neuron, which contains synaptic vesicles
Receptor region on the postsynaptic neuron

6. Synaptic Cleft
Fluid-filled space between presynaptic and postsynaptic neurons, found at chemical synapses
Transmission across the synaptic cleft
Is a chemical event (not electrical)
Involves release, diffusion, and binding of neurotransmitters
Provides unidirectional communication between neurons

7. Synaptic Transmission Summary
Action potential arrives at presynaptic neuron’s axon terminal and opens voltage-gated calcium channels
Calcium enters neuron terminal and causes synaptic vesicle fusion with cell membrane
Neurotransmitter exocytosis occurs
Neurotransmitter diffuses across cleft and binds to receptors on postsynaptic neuron
Ion channels in membrane of post-synaptic cell open, causing excitation or inhibition (graded potential)
Neurotransmitter diffuses away from receptors as it is broken down in the cleft and/or taken back up by pre- synaptic neuron

8. Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic neuron
Presynaptic neuron
Postsynaptic neuron 1
Action potential arrives at axon terminal.
Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. 2
Mitochondrion
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+ entry causes neurotransmitter- containing synaptic vesicles to release their contents by exocytosis. 3
Synaptic cleft
Axon terminal
Synaptic vesicles 4
Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane.
Postsynaptic neuron
Figure 11.17, step 4

9. 5 Ion movement Graded potential
Binding of neurotransmitter opens ion channels, resulting in graded potentials.
Figure 11.17, step 5

10. 6 Enzymatic degradation Reuptake Diffusion away from synapse
Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse.
Figure 11.17, step 6

11. Termination of Neurotransmitter Effects
Neurotransmitter effect is terminated in a few milliseconds by
Degradation by enzymes
Reuptake by astrocytes or axon terminal
Diffusion away from synaptic cleft

12. Synaptic Delay
Neurotransmitter must be released, diffuse across synapse, and bind to receptors
Synaptic delay = time needed to do this ( ms)
Synaptic delay is the rate-limiting step of neural transmission (in short neurons at least)

13. Postsynaptic Potentials
Graded potentials
Strength determined by:
Amount of neurotransmitter released
Time the neurotransmitter is in the area
Types of postsynaptic potentials
EPSP—excitatory postsynaptic potentials
IPSP—inhibitory postsynaptic potentials

14. Table 11.2 (1 of 4)

15. Table 11.2 (2 of 4)

16. Table 11.2 (3 of 4)

17. Table 11.2 (4 of 4)

18. Excitatory Synapses and EPSPs
Neurotransmitter binds to and opens chemically gated channels that allow simultaneous flow of Na+ and K+ in opposite directions
Na+ influx is greater than K+ efflux, causing a net depolarization
EPSP helps trigger AP at axon hillock if EPSP is of threshold strength and opens the voltage- gated channels

19. postsynaptic membrane that brings the neuron closer to AP threshold.
An EPSP is a local
depolarization of the
postsynaptic membrane
that brings the neuron
closer to AP threshold.
Neurotransmitter binding
opens chemically gated
ion channels, allowing
the simultaneous pas-
sage of Na+ and K+.
Membrane potential (mV)
Threshold
Stimulus
Time (ms)
(a) Excitatory postsynaptic potential (EPSP)
Figure 11.18a

20. Inhibitory Synapses and IPSPs
Neurotransmitter binds to and opens channels for K+ or Cl–
Causes hyperpolarization (inside of cell becomes more negative)
Reduces the postsynaptic neuron’s ability to produce an action potential

21. hyperpolarization of the postsynaptic membrane and drives the neuron
An IPSP is a local
hyperpolarization of the
postsynaptic membrane
and drives the neuron
away from AP threshold.
Neurotransmitter binding
opens K+ or Cl– channels.
Membrane potential (mV)
Threshold
Stimulus
Time (ms)
(b) Inhibitory postsynaptic potential (IPSP)
Figure 11.18b

22. Integration: Summation
One EPSP cannot induce an action potential
EPSPs can sum to reach threshold
IPSPs and EPSPs can cancel each other out

23. Integration: Summation
Temporal summation
One presynaptic neuron sends several or many impulses in a short time to the postsynaptic neuron
Spatial summation
Multiple presynaptic neurons stimulate the post-synaptic neuron simultaneously

24. Figure 11.19a, b E1 E1 Threshold of axon of postsynaptic neuron
Resting potential E1 E1 E1 E1
Time
Time
(a)
No summation:
2 stimuli separated in time
cause EPSPs that do not
add together.
(b)
Temporal summation:
2 excitatory stimuli close
in time cause EPSPs
that add together.
Excitatory synapse 1 (E1)
Excitatory synapse 2 (E2)
Inhibitory synapse (I1)
Figure 11.19a, b

25. Figure 11.19c, d E1 E1 E2 I1 E1 + E2 I1 E1 + I1 Time Time (c)
Spatial summation:
2 simultaneous stimuli at
different locations cause
EPSPs that add together.
(d)
Spatial summation of
EPSPs and IPSPs:
Changes in membane
potential can cancel each
other out.
Figure 11.19c, d

26. Neurotransmitters
Most neurons make two or more neurotransmitters, which are released at different stimulation frequencies
50 or more neurotransmitters have been identified
Classified by chemical structure and by function

27. Chemical Classes of Neurotransmitters
Acetylcholine (Ach)
Released at neuromuscular junctions and some autonomic neurons
Synthesized in the pre-synaptic neuron
Degraded by acetylcholinesterase (AChE)

28. Chemical Classes of Neurotransmitters
Biogenic amines include:
Norepinephrine (NE)
Epinephrine
Serotonin
Many others
Broadly distributed in the brain
Play roles in emotional behaviors and the biological clock

29. Chemical Classes of Neurotransmitters
Amino acids include:
GABA—Gamma ()-aminobutyric acid
Glutamate
Many others

30. Functional Classification of Neurotransmitters
Excitatory (depolarizing) and/or inhibitory (hyperpol.)
Determined by receptor type on postsynaptic neuron
GABA usually inhibitory
Glutamate, epinephrine usually excitatory
Acetylcholine
Excitatory at neuromuscular junctions in skeletal muscle
Inhibitory in cardiac muscle

31. Neurotransmitter Actions
Direct action
Neurotransmitter binds to channel-linked receptor and opens ion channels
Promotes rapid responses
Examples: ACh; glutamate and GABA at some of their synapses

32. Neurotransmitter Actions
Indirect action
Neurotransmitter binds to a G protein-linked receptor and acts through an intracellular second messenger
Promotes long-lasting effects
Examples: Norepinephrine, epinephrine, serotonin; glutamate and GABA at some of their synapses