module 5 synaptic transmission

Synaptic Transmission Study Guide

1. Key Terms

1. Synapse

   • The junction between two neurons where information is transmitted.

2. Synaptic Cleft

   • The small gap between the presynaptic and postsynaptic membranes where neurotransmitters diffuse.

3. Synaptic Vesicle

   • Membrane-bound sac in the presynaptic terminal containing neurotransmitters.

4. Neurotransmitter

   • Chemical messenger released by neurons to communicate with other cells (e.g., ACh, dopamine).

5. (Neurotransmitter) Receptor

   • Protein on the postsynaptic membrane (or inside the cell) that binds neurotransmitters.

6. Small-Molecule Neurotransmitter

   • Usually rapid-acting; synthesized in the axon terminal (e.g., acetylcholine, glutamate).

7. Peptide Neurotransmitter

   • Larger, slow-acting; synthesized in the cell body and transported to the terminal (e.g., endorphins).

8. SNARE Proteins

   • Proteins (e.g., synaptobrevin, syntaxin, SNAP-25) that mediate vesicle fusion with the presynaptic membrane.

9. Horseradish Peroxidase (HRP)

   • An enzyme used experimentally to trace vesicle recycling and synaptic function.

10. Neurotransmitter Inactivation

    • Process of removing or breaking down neurotransmitters after they have performed their function.

11. Acetylcholine (ACh)

    • Key excitatory neurotransmitter at the neuromuscular junction (also in the CNS).

12. End-Plate Current

    • The ionic current through the ACh receptor channels at the neuromuscular junction end plate.

13. Excitatory Postsynaptic Potential (EPSP)

    • A depolarizing potential that makes the neuron more likely to fire an action potential.

14. Inhibitory Postsynaptic Potential (IPSP)

    • A hyperpolarizing potential that makes the neuron less likely to fire.

15. Temporal Summation

    • Summation of postsynaptic potentials over time when one presynaptic neuron fires rapidly.

16. Spatial Summation

    • Summation of postsynaptic potentials from multiple presynaptic neurons firing at the same time.

2. Mnemonics

A. Four Stages of Neurotransmitter Function

"S-S-R-I"

• S: Synthesis – Neurotransmitters are made (in the terminal or soma).

• S: Storage – Packaged into synaptic vesicles.

• R: Release – Ca²⁺ influx triggers vesicle fusion and neurotransmitter release.

• I: Inactivation – Neurotransmitters are removed or broken down.

B. Small vs. Peptide Neurotransmitters

"Small = Simple, Peptide = Pompous"

• Small: Synthesized at the terminal, stored in small vesicles, fast action.

• Peptide: Synthesized in the cell body, transported in large vesicles, slower action.

C. EPSP vs. IPSP

"Na⁺ for excites, K⁺ or Cl⁻ for inhibits"**

• If a channel lets Na⁺ in, the cell is more likely to reach threshold (EPSP).

• If a channel lets K⁺ out or Cl⁻ in, the cell hyperpolarizes (IPSP).

3. Tables & Diagrams

A. Small-Molecule vs. Peptide Neurotransmitters

4. Answers to the Questions

1. How did Otto Loewi show that nervous system communication must be chemical?

   • Otto Loewi conducted the frog heart experiment: He stimulated the vagus nerve of one frog heart, collected the fluid, and applied it to another heart, which also slowed. This demonstrated a chemical (later identified as acetylcholine) was responsible for the signal.

2. Name the 4 stages in neurotransmitter function.

   • Synthesis, Storage, Release, Inactivation (use the mnemonic S-S-R-I).

3. The presence of which ion inside the presynaptic cell is important for neurotransmitter release?

   • Calcium (Ca²⁺) influx triggers vesicle fusion with the membrane.

4. Name 2 differences in synthesis and storage of small-molecule neurotransmitters vs. peptide neurotransmitters.

   • Small-molecule are synthesized in the axon terminal and stored in small vesicles; peptides are synthesized in the cell body and stored in large dense-core vesicles.

5. What happens if acetylcholine is dumped on an acetylcholine-gated sodium channel? What if dopamine is dumped on that channel?

   • ACh on an ACh-gated Na⁺ channel → channel opens, Na⁺ flows in → EPSP.

   • Dopamine on the same channel → no effect, because the channel is specific to ACh.

6. Name 4 ways that a neurotransmitter’s action can be terminated.

   • Enzymatic breakdown (e.g., AChE for ACh)

   • Reuptake into presynaptic terminal (e.g., via transporters)

   • Diffusion away from the synaptic cleft

   • Uptake by glial cells (especially for glutamate, GABA)

7. How does the reversal potential of the ACh receptor at the neuromuscular junction show that these channels are permeable to more than one ion?

   • The reversal potential is between the equilibrium potentials for Na⁺ and K⁺, indicating both ions flow through the receptor.

8. Sodium’s reversal potential is higher than the threshold potential in most neurons. So a PSP caused by opening sodium channels is excitatory or inhibitory?

   • Excitatory (EPSP), because Na⁺ flows in, depolarizing the cell toward threshold.

9. Potassium’s reversal potential is lower than the threshold potential in most neurons. So a PSP caused by opening potassium channels is excitatory or inhibitory?

   • Inhibitory (IPSP), because K⁺ flows out, hyperpolarizing the cell away from threshold.