Chemical Messengers

Question 1

what is a synapse ?

Answer 1

anatomically specialised junction between two neurons, at which the electrical activity in one neuron, the presynaptic neuron, influences the electrical (or metabolic) activity in the second, postsynaptic neuron

Question 2

where are neurotrasnmitters stored ?

Answer 2

membrane-bound synaptic vesicles in the pre synaptic neuron.

Question 3

explain how neurotransmitters are released ?

Answer 3

When an action potential in the presynaptic axon reaches the end of the axon and depolarizes the axon terminal. The voltage-gated calcium channels in the membrane open, and calcium diffuses from the extracellular fluid into the axon terminal near the docked vesicles. The calcium ions induce a series of reactions that allow some of the docked vesicles to fuse with the presynaptic plasma membrane and liberate their contents into the synaptic cleft by the process of exocytosis. Once released from the presynaptic axon terminal, neurotransmitter diffuse across the cleft. A fraction of these molecules bind to receptors on the plasma membrane of the postsynaptic cell.

Question 4

non peptide neurotransmitters - monoamines ?

Answer 4

include noradrenaline , dopamine , 5 – Hydroxytryptamine ( 5-HT) . All of these have a low molecular weight.

Question 5

non peptide amino acids ?

Answer 5

GABA , glutamate and glycine

Question 6

non peptide other category ?

Answer 6

acetylcholine , ATP and NO

Question 7

criteria for neurotransmiter ?

Answer 7

1. Synthesised within neurone
2. Stored in nerve terminal
3. Released by nerve stimulation in Ca2+ dependent manner
4. Exogenous drug must mimic endogenous neurotransmitter
5. Antagonist must inhibit neurotransmitter & exogenous drug
6. Specific mechanisms exist for inactivation

Question 8

for syntheisis within the neurone , provide an example ?

Answer 8

dopamine production , tyrosine comes from dietary amines. These are then converted to L – Dopa by tyrosine hydroxylase which is the rate limiting step. This is then converted to dopamine by DOPA decarboxylase

Question 9

for the dopaminergic pathways , where does the substantia nigra send impulses to ?

Answer 9

striatum

Question 10

ventral tegmentum ?

Answer 10

send impulses to the nucleus accumbens and forebrain

Question 11

pituitary gland ?

Answer 11

impulses to the hypothalamus

Question 12

for the release of dopamine , what needs to occur ?

Answer 12

nerve stimulation

Question 13

describe mimicry ?

Answer 13

cAMP is measured in the corpus striatum after nerve stimulation or adding dopamine. It decreases the cAMP. You must see the same effect , if not then it is not a neurotransmitter.

Question 14

name antagonist that can inhibit the neurotransmitter ?

Answer 14

D2-R antagonist

Question 15

for inactivation , how does this occur for dopamine and noradrenaline and why ?

Answer 15

neuronal re-uptake this prevents continuous stimulation and activation of the receptors at the post synaptic neurone.

Question 16

how does prosac work ?

Answer 16

antidepressant the prevents re-uptake of 5-HT

Question 17

what are enkephalins used for ?

Answer 17

pain management and they are neuropeptides.

Question 18

similaritites between classical and neuopeptide neurotransmitters ?

Answer 18

• Storage in vesicles which are specific regions
• Ca2+-dependent release
• 2nd messengers /  ion channel activity

Question 19

differences ?

Answer 19

Synthesised in cell body as part of large precursor protein before Transported to nerve terminal where they are processed to the active peptide. They are released at lower concentrations
The mechanism of inactivation are unclear however , they are removed slowly from synaptic cleft and have a longer action

Question 20

describe slow transmission ?

Answer 20

This takes seconds – minutes and they involve G proteins and cytoplasmic messengers.

Question 21

dopamine is involved , explain ?

Answer 21

they act on D1 to increase cAMP – PKA , while the action on D2 is a decrease in the cAMP.

Question 22

5-HT is involved explain ?

Answer 22

act on 5HT1 receptors they cause an increase in cAMP , while the 5HT2 receptors cause an increase in the IP3 and DAG

Question 23

describe fast excitatory neurotransmission ?

Answer 23

This occurs in 10’s of milliseconds and they have a direct effect on the receptor operates ion channels. This causes ions to flow and changes the membrane potential due to depolarisation. When a messenger binds to the receptor ion channel , this causes the channels to open so that Na+ and K+ ions can flow through , this leads to depolarisation of the membrane potential. This leads to an Excitatory Post Synaptic Potential ( EPSP).

Question 24

is a single EPSP enough for an A.P ?

Answer 24

nope , there needs to be several EPSP for an action potential and this is called summation.

Question 25

describe temporal summation ?

Answer 25

when one presynaptic neurone releases neurotransmitter many times over a period of time. The total amount of neurotransmitter released may exceed the threshold value of the postsynaptic neurone.

Question 26

describe spatial summation ?

Answer 26

when multiple presynaptic neurones together release enough neurotransmitter (e.g. acetylcholine) to exceed the threshold of the postsynaptic neurone. For example, neurone A and neurone B may individually release insufficient neurotransmitter but when these quantities are combined, threshold may be exceeded and an action potential generated.

Question 27

describe fast inhibitory neurotransmission ?

Answer 27

This is when the channel that was closed becomes open ( same as excitatory). However it differs due to the influx of Cl- into the cell which causes repolarisation of the cell membrane and this is called inhibitory Post Synaptic Potential.

An example of a neurotransmitter that causes this is GABA.