Lecture 4 - Synapse Structure and Function

Question 1

how does a neuron hand off the electrical signal of an AP to cause an effect in another cell

Answer 1

it generates a chemical signal at a structure called a synapse

Question 2

chemical synapse - what happens at the axon terminus, primary NTs found in the PNS? CNS?

Answer 2

-the axon terminus synthesizes neurotransmitters and stores them in vesicles
-1 type of neuron usually makes 1 class or family of NT
-ACh and norepinephrine are the primary NTs found in PNS
-many different types of NTs in the CNS

Question 2

what is a synapse

Answer 2

its a structure in the nervous system through which a neuron passes a chemical or electrical signal to another cell

Question 3

what are the 4 major neurotransmitter families and examples

Answer 3

-amino acids (GABA, glycine, glutamate)
-acetylcholine (ACh)
-amines (seratonin, dopamine, epi, norepi)
-neuropeptides (oxytocin, vasopressin)

Question 4

how are neurotransmitters released

Answer 4

when AP reaches axon terminus, voltage gated Ca channels open –> Ca enters cell –> stimulates a series of biochemical events that triggers exocytosis of NT vescicles, NT released into cleft

role isnt to change the membrane potential but rather let Ca in

Question 5

after the neurotransmitter is released, how is the post synaptic cell stimulated? example

Answer 5

NT binds to different receptor proteins on post synaptic membrane

e.g. ACh binds to ACh receptors

Question 6

types of NT receptors

Answer 6

-lignd gated ion channels (nicotinic AChR)

-receptors acting through G proteins (muscarinic AChR)

-several others, most of which lead to changes (+ve/-ve) in the RMP of the post synaptic membrane

Question 7

what are the two important general signal transduction mechanisms for all cells that we need to know (think receptors)

Answer 7

-R is an ion channel
-R acts through G protein to activate enzyme/ion channel

Question 8

what is ionotropic and metabotropic

Answer 8

ionotropic = ligand gated ion channels. direct gating

metabotropic = G protein coupled. indirect gating

Question 9

do synapses always excite the post synaptic cell?

Answer 9

not always - it depends on the type of neuron and neurotransmitter

Question 10

how do EXCITATORY synapses work, what is it called, examples?

Answer 10

the stimulation of post synaptic membrane leads to the opening of the ligand gated Na or Ca channels which promotes excitation of membrane (e.g. depolarization). charge becomes less negative and favours the formation of an AP. This is called an excitatory post synaptic potential (EPSP).

example - nicotinic AChR at neuromuscular junction

Question 11

how do INHIBITORY synapses work, what is it called, examples

Answer 11

stimulation of post synaptic membrane opens K or Cl channels causing hyperpolarization (Charge becomes more negative) and may prevent the formation of an AP. This is called an inhibitory post-synaptic potential (IPSP)

example - GABA receptors in CNS - ligand-gated Cl channels

Question 12

synapses: summation concept

Answer 12

EPSPs and IPSPs “add up” (summate) over space (spatial summation) and time (temporal summation) and influence the membrane potential on the neuron surface, particularily the axon hillock, a structure to which these charges migrate

Question 13

what is the site where AP is produced

Answer 13

axon hillock

Question 14

what does the summation of EPSPs and IPSPs determine?

Answer 14

determine membrane action potential at axon hillock

Question 15

what limits the time over which temporal summation can occur?

Answer 15

hyperpolarization decreases as K+ leaks out slowly

Question 16

8 steps in the synapse (summary)

Answer 16

1. AP arrives at axon terminal
2. Ca2+ channels open
3. Ca2+ diffuses from ECF
4. neurotransmitter releases by exocyosis
5. Neurotransmitter diffuses across synpatic cleft
6. Neurotransmitter-receptor complex opens specific ions channels
7. Postsynaptic potential generated (Remember -this can be more positive OR more negative)
8. Local currents spread in all directions along cell membrane

Question 17

What happens to neurotransmitter molecules after synaptic transmission (e.g. AFTER they have generated a post synaptic response)

what do you have to consider for implications?

Answer 17

it depends on the NT and type of neuron - three most common outcomes are listed with examples below:
- ACh is broken down into acetic acid and choline by acetylcholinesterase. choline is taken up by presynaptic neuron and reused
- most amine neurotransmitters (NE, dopamine, seratonin) are taken up by the pre-synaptic neuron (re-uptake) and re-used, some is broken down by specific enzumes in axon terminus
- some diffuses away from synapse (GABA etc) to be metabolized by astrocytes

implications for toxins and drugs that interfere with this action (esterase inhibitors, re-uptake inhibitors, etc)

Question 18

pharmacology: ion channels and synapses - examples of drugs and toxins act at synapses, mechanisms

Answer 18

many drugs and toxins act at synapses
-other examples: organophosphates, sarin, curare, atropine, strychnine, tensilon, neostygmine, etc

mechanisms:
-block release of NT (botulism, tetanus toxins)
-block re-uptake of NT (SSRI, SBARI)
-block receptor (atropine)
-block mechanism (sarin, organophosphate, tensilon)

Question 19

electrical synapse - what are specialized, what happens when in contact, how does depolarization work, cardiac muscle and conduction pathways

Answer 19

-gap junction proteins in neighbouring cells are specialized
-when in contact, open to form water filled pore directly between 2 cells
-depolarizations sweep from cell to cell as if it were a single cell
-in cardiac muscle and conduction pathways, numerous cells connected by electrical synapses