Communication Between Neurons

Question 1

the synapse

Answer 1

synapse is the point of interaction between neurons. at the tip of the axon is the axon terminal (has alot of mitochondria). post synaptic has special proteins on the membrane called receptors to receive the released neurotransmitters. 3 types of synaptic connection: axodendritic, axosomatic, axoaxonic.

Question 2

synaptic neurotransmitter release

Answer 2

when action potential reaches the terminal it changes the membrane potential there as well (depolarized). specific voltage gated calcium channels open (more depolarized). each action potential releases several vesicles. neurotransmitter passively diffuses to post synaptic membrane (concentration gradient). neurons are relatively neurotransmitter selective.

Question 3

termination of synaptic transmission

Answer 3

3 ways = enzymatic metabolism, reuptake, uptake by glial cells

Question 4

enzymatic metabolism

Answer 4

inside the synaptic cleft, there are enzymes that are always present. when neurotransmitters are released into the synaptic cleft, the enzymes immediately metabolize them. very efficient and quick progress. most get metabolized before getting to the other terminal.

Question 5

reuptake

Answer 5

an efficient process. the membrane of the pre synaptic neuron has pumps/transporters. these pumps are specific to the neurotransmitter and they are taken up again and recycled. it gets metabolized inside the terminal.

Question 6

uptake by glial cells

Answer 6

all synapses in the brain have glial cells - maintenance and support cells. they supply the neuron with energy and metabolites and remove garbage. remove neurotransmitters from the synapse and stored where it can be reused. if they dont remove the neurotransmitter, signal would stay on longer

Question 7

synaptic potentials

Answer 7

post synaptic response can be excitatory (EPSP) or inhibitory (IPSP).

Question 8

EPSP

Answer 8

post synaptic membrane has depolarized - membrane is not as negatively charged as before. might make it easier to cross the threshold for an action potential.

Question 9

IPSP

Answer 9

post synaptic membrane becomes even more negative, reverses a depolarization that had already occurred. might make it harder to fire an action potential

Question 10

2 types of EPSPs

Answer 10

1. opening channels permeable to NA+ (enters cell)
2. closing leaky K+ channels (K+ cannot leave cell)

Question 11

2 types of IPSPs

Answer 11

1. opening channels permeable to Cl- (enters cell = hyperpolarization)
2. opening K+ channels (leaves the cell = hyperpolarization)

Question 12

spatial summation

Answer 12

several inputs act on a relatively limited area at the same time. the membrane potential in that area is summed up to a much stronger change. strong intensity of potential more resistant to interference.

Question 13

temporal summation

Answer 13

a single axon fires very rapidly multiple action potentials that are summed to produce a greater potential.

Question 14

neurotransmitter synthesis

Answer 14

• necessary enzymes are transported to the terminal where synthesis is taking place (ACh, DA). DA is synthesized from tyrosine converted into L-Dopa by tyrosine hydroxylase. L-Dopa is converted to DA by Dopa decarboxylase.
• neuropeptides precursors are synthesized in the soma and carried to the terminal via vesicles (replenishment is slower)

Question 15

pharmacology of the synapse

Answer 15

• agonist = imitates effects or increases effects of the neurotransmitter
• antagonist = reducing or inhibiting the effect of the neurotransmitter
• L-Dopa precursor to DA = agonist
• PCPA inhibits synthesis of serotonin = antagonist
• reserpine prevents storage of monoamines in vesicles = antagonist
• blackwidow spider venom stimulates release of ACh = agonist - (increased ACh not good)
• botulinum toxin inhibits release of ACh = antagonist
• nicotine, muscarine stimulates postsynaptic ACh receptors = agonist
• curare, atropine blocks postsynaptic ACh receptors = antagonist (complete paralysis)
• apomorphine stimulates DA autoreceptors = antagonist
• yohimbine blocks NE autoreceptors = agonist (increased NE causes anxiety)
• cocaine blocks DA, NE, serotonin reuptake = agonist
• physostigmine inactivates acetylcholinesterase (increases ACh) = agonist

Question 16

2 major types of receptors

Answer 16

ionotropic and metabotropic

Question 17

ionotropic receptors

Answer 17

ligand gated channel receptors. made of 4-5 subunits with a pore at the centre. fast acting (opens and closes fast) so the potential changes fast and is short lived. some are ion specific. usually the channel is blocked under normal conditions.
ex: two units of ACh needs to attach to 2 alpha subunits for the channel to open - not an ion specific channel (NA+ and K+).

Question 18

metabotropic

Answer 18

G protein coupled receptors. activates intra-cellular G proteins. takes longer to induce post synaptic response but the response is longer lasting. response is diverse. Receptor locks to a free floating G protein. using energy, the G protein breaks into 2 subunits which can move further to act on other proteins that are near the receptor and start a chain of events inside the cell (maybe 2nd mess cascade). G proteins can either inhibit or stimulate (Gi and Gs/Gq). Modulate effector enzymes that synthesize/breakdown 2nd mess cascade. cascade results in signal amplification. EX: norepinephrine binds to beta-adrenergic G protein coupled receptor. G protein breaks into 2 subunits, one of the subunits will act on adenyl cyclase which stimulates the conversion of ATP into cAMP (2nd messenger). cAMP activates PKA which acts on potassium channels to open them by phosphorylation.
EX: DA D2 receptors do the opposite of NE receptors - inhibit stimulation of adenyl cyclase and decrease protein phosphorylation.
EX: glutamate binds to mGluR G protein coupled receptor stimulating production of phospholipase C increasing production of DAG and IP3 (2nd messengers). DAG acts on PKC and IP3 binds Ca2+ channels on the smooth ER increasing Ca2+ concentration in cytoplasm. this increases protein phosphorylation.