1. Synapses
Figure 11.17

2. Fluid-filled space separating the presynaptic and postsynaptic neurons
Synaptic Cleft
Fluid-filled space separating the presynaptic and postsynaptic neurons
Prevents nerve impulses from directly passing from one neuron to the next
Transmission across the synaptic cleft:
Is a chemical event (as opposed to an electrical one)
Ensures unidirectional communication between neurons

3. Synaptic Cleft: Information Transfer
Neurotransmitter
Ca2+
Na+
Axon terminal of
presynaptic neuron
Action potential
Receptor 1
Postsynaptic
membrane
Mitochondrion
Postsynaptic
membrane
Axon of
presynaptic
neuron
Ion channel open
Synaptic vesicles
containing
neurotransmitter
molecules 5
Degraded
neurotransmitter 2
Synaptic
cleft 3 4
Ion channel closed
Ion channel
(closed)
Ion channel (open)
Figure 11.18

4. Synaptic Cleft: Information Transfer
Ca2+
Axon terminal of
presynaptic neuron
Action potential 1
Axon of
presynaptic
neuron
Figure 11.18

5. Synaptic Cleft: Information Transfer
Ca2+
Axon terminal of
presynaptic neuron
Action potential 1
Mitochondrion
Axon of
presynaptic
neuron
Synaptic vesicles
containing
neurotransmitter
molecules 2
Figure 11.18

6. Synaptic Cleft: Information Transfer
Ca2+
Axon terminal of
presynaptic neuron
Action potential 1
Postsynaptic
membrane
Mitochondrion
Axon of
presynaptic
neuron
Synaptic vesicles
containing
neurotransmitter
molecules 2
Synaptic
cleft 3
Ion channel
(closed)
Ion channel (open)
Figure 11.18

7. Synaptic Cleft: Information Transfer
Neurotransmitter
Ca2+
Na+
Axon terminal of
presynaptic neuron
Action potential
Receptor 1
Postsynaptic
membrane
Mitochondrion
Postsynaptic
membrane
Axon of
presynaptic
neuron
Ion channel open
Synaptic vesicles
containing
neurotransmitter
molecules 2
Synaptic
cleft 3 4
Ion channel
(closed)
Ion channel (open)
Figure 11.18

8. Synaptic Cleft: Information Transfer
Neurotransmitter
Ca2+
Na+
Axon terminal of
presynaptic neuron
Action potential
Receptor 1
Postsynaptic
membrane
Mitochondrion
Postsynaptic
membrane
Axon of
presynaptic
neuron
Ion channel open
Synaptic vesicles
containing
neurotransmitter
molecules 5
Degraded
neurotransmitter 2
Synaptic
cleft 3 4
Ion channel closed
Ion channel
(closed)
Ion channel (open)
Figure 11.18

9. Synaptic Delay
Neurotransmitter must be released, diffuse across the synapse, and bind to receptors
Synaptic delay – time needed to do this ( ms)
Synaptic delay is the rate-limiting step of neural transmission

10. Postsynaptic Potentials
The two types of postsynaptic potentials are:
EPSP – excitatory postsynaptic potentials
IPSP – inhibitory postsynaptic potentials

11. Excitatory Postsynaptic Potential (EPSP)
Open Na+ channels
Figure 11.19a

12. Inhibitory Postsynaptic (IPSP)
Open K+ and Cl- channels
Figure 11.19b

13. Summation A single EPSP cannot induce an action potential
Temporal summation
Spatial summation
Figure 11.20

14. Chemical Neurotransmitters 50 different neurotransmitters
Acetylcholine (ACh)
Biogenic amines
Amino acids
Peptides
Novel messengers: ATP and dissolved gases NO and CO

15. Neurotransmitters: Acetylcholine
Degraded by the enzyme acetylcholinesterase (AChE)
Released by:
All neurons that stimulate skeletal muscle
Some neurons in the autonomic nervous system
Tetanus – bacterial toxin

16. Neurotransmitters: Biogenic Amines
Catecholamines – dopamine, norepinephrine (NE), and epinephrine
Indolamines – serotonin and histamine
emotional behaviors

17. Neurotransmitters: Amino Acids
Include:
GABA – Gamma ()-aminobutyric acid
Glycine
Aspartate
Glutamate
Found only in the CNS

18. Neurotransmitters: Peptides
Include:
Substance P – mediator of pain signals
Beta endorphin, dynorphin, and enkephalins
Act as natural opiates; reduce pain perception
Bind to the same receptors as opiates and morphine
Gut-brain peptides – somatostatin, and cholecystokinin

19. Neurotransmitters: Novel Messengers
ATP
Is found in both the CNS and PNS
Produces excitatory or inhibitory responses depending on receptor type
Induces Ca2+ wave propagation in astrocytes
Provokes pain sensation

20. Neurotransmitters: Novel Messengers
Nitric oxide (NO)
Activates the intracellular receptor guanylyl cyclase
Is involved in learning and memory
Carbon monoxide (CO) is a main regulator of cGMP in the brain

21. Functional Classification of Neurotransmitters
Two classifications: excitatory and inhibitory
Excitatory neurotransmitters cause depolarizations (e.g., glutamate)
Inhibitory neurotransmitters cause hyperpolarizations (e.g., GABA and glycine)

22. Functional Classification of Neurotransmitters
Some neurotransmitters have both excitatory and inhibitory effects
Determined by the receptor type of the postsynaptic neuron
Example: acetylcholine
Excitatory at neuromuscular junctions with skeletal muscle
Inhibitory in cardiac muscle

23. Neurotransmitter Receptor Mechanisms
Direct: neurotransmitters that open ion channels
Promote rapid responses
Examples: ACh