The neurone Flashcards

Question

What are the differences in distrubution of excitatory/inhibitory synapses along a post-synaptic neurone?

Answer 1

- Excitatory synapses are mostly axo-dendritic. - Inhibitory synapses are mostly axo-somatic. - Having inhibitory synapses closer to soma than excitatory ones allow for more effective shunting inhibition, which is more powerful.

Answer 2

1. Axo-axonic synapses cause pre-synaptic depolarisation, leading to inactivation of Na_vs and decreased magnitude of APs. 2. Inhibition due to IPSP decreases magnitude of APs. 3. Secondary messengers via metabotrophic receptors (e.g. α₂-adrenoreceptors) decrease influx of Ca²⁺.

Answer 3

- Timing is crucial and the inhibitory signal needs to arrive at virtually the same time as AP. - Arriving too early means inhibitory effects become attenuated when AP arrives. - Arriving too late obviously results in no effect.

Answer 4

More selective inhibition of certain selective excitatory inputs instead of general inhibition of all excitatory inputs.

Answer 5

- I_A is active when the neurone is polarised and acts as an outward K⁺ current clamping the membrane at the RP. When a stimulus causes NaVs to open and inward depolarising Na⁺ currents to occur, they are reduced by I_A, reducing rate of depolarisation. As neurone becomes depolarised, I_A inactivates, allowing further depolarisation to threshold and thus AP to be produced. - I_A effectively spaces out APs along spike trains. - Stimulus of greater magnitude causes faster depolarisation and faster inactivation of I_A.

Answer 6

1. Neurone is depolarised by initial stimulus and RP. 2. Depolarisation causes T-type Ca²⁺ channels to activate and an inward Ca²⁺ current that depolarises neurone much more quickly. 3. Once threshold is reached, AP is generated. 4. Several APs fire in rapid succession, significantly increasing intracellular [Ca²⁺]. 5. Ca²⁺-activated K⁺ channels are activated, producing outward K⁺ current that terminates bursting. 6. Ca²⁺ is extruded from neurone by NCX and PMCAs, inactivating K⁺ current and allowing burst cycle to repeat.

Answer 7

- Reduces firing frequency of bursts, allowing time for T-type Ca²⁺ channels to reactivate following AP in burst. - Becomes greater after each AP in burst due to higher [Ca²⁺]_i, steadily hyperpolarising neurone and eventually terminating burst. - Allows for spike frequency adaptation and the steady decrease in firing frequency over time.

Answer 8

Allows for neurones that need to be tonically activsted to 'break' during prolonged stimulation, preventing complete inactivation of APs.

Answer 9

1. Convergence 2. Divergence

Answer 10

Group of motor neurones that receive EPSPs from group of 1A affterents, but is insufficient to depolarise neurone to threshold and generate AP.

Answer 11

1. Linear summation: When stimulus units are non-overlapping and so stimulation of 2 units produce response which is sum of 2 individual unit responses. 2. Supralinear summation: When the stimulus units are overlapping and so stimulation of 2 units result in stimulation of some fibres in suliminal fringe, resulting in the overall response being greater than sum of unit responses. 3. Occlusion: If there is too much overlap, then 1A fibres from 2 units may in turn stimulate the same motor neurones and so have 'wasted' effect, resulting in overall response being less than the sum of unit responses.

Answer 12

1. Feedforward inhibition 2. Feedback inhibition

Answer 13

To allow for inhibition of antagonistic muscle groups during muscle reflexes.

Answer 14

1. Prevents over-firing of motor neurone and fatigue. 2. Stalilises neuronal firing rate so that firing rate is proportional to magnitude of stimulating current. 3. To allow for spike-frequency adaptation in motor neurones.

Answer 15

After tetanus, Ca²⁺ stored intracellularly increases as a result of significant increase in [Ca²⁺]_i during tetanus. This results in greater facilitation of vesicle priming, reulting in more NT being available for release upon post-tetanic stimulation.

Answer 16

An input is strengthened if it has a role in stimulating a firing cell, even if it alone cannot stimulate the same cell.

Answer 17

1. On post-synaptic membrane, NMDA (Glu) receptors (selective to Ca2+ ions) are present, along with AMPA recepors (non-selective cation channel). 2. When the synapse is stimulated, Glu is released from the pre-synaptic terminal and binds to both NMDA and AMPA receptors. 3. When Glu binds to NMDA, the channel opens, but is blocked by Mg2+ ions. 4. However, when AMPA opens in conjunction with NMDA, influx of cations (mainly Na+) causes depolarisation, which makes NMDA Ca2+ permeable. 5. Influx of Ca2+ causes a number of changes including: - Increased AMPA receptor density - Increased NO and arachidonic acid synthesis (both retrograde messenger that modulate NT release) 6. These changes increase the sensitivity of the synapse. 7. They only occur if both NT present and depolarisation of post-synaptic neurone occurs.

Answer 18

- In some pre-synaptic terminals, there may be multiple NTs and/or neuromodulators (e.g. neuropeptides). - Neuromodulators may be held in bigger vesicles and so need sustained pre-synaptic APs to ne released, while bursts of activity favours NT release alone. - Neuromodulators don't cause post-synaptic potential themselves. Instead, they change the post-synaptic effects of the NT.

Answer 19

1. To inhibit antagonistic groups of effectors in a reflex. 2. To control an excitatory signal and prevent explosive chain reaction of excitation throughout CNS, restricting excitation to only the desired pathway.

Answer 20

There is greater percision, so that the actual sources of the excitatory inputs can be targeted individually, not just the post-synaptic neurone receiving inputs from a number of different sources.

Answer 21

- Neuropeptide Y (NPY) - Substanc P - Vasoactive intestinal peptide (VIP) - Cholecystokinin (CCK) - Calcitonin gene-related peptide (CGRP)

Answer 22

- Function: Chemical/electrical isolation of excitatory inputs so that they are not affected by changes in rest of neurone. - Disadvantage: High resistance attenuates excitatory input.