synapses and networks

Question 1

where are the typical locations of ion channels?

Answer 1

voltage-gate channels - axonal hillock (integration zone) and axon (conduction zone)

leak channels and ion pumps - entire neural membrane

voltage-gated ca2+ channels - axon terminals (output zone)

ligand-gated channels - dendrites and soma (input zone)

Question 2

what is contained in the neuron doctrine?

Answer 2

1. brains composed of separate neurons and other cells
2. cells are independent
3. neurons are polarised cells
4. info is transmitted from cell to cell across tiny gaps

Question 3

what is the basic description of what happens at a synapse?

Answer 3

signals are transmitted to another neuron

presynaptic to postsynaptic

Question 4

what is the synapse called when postsynaptic cell is muscle?

Answer 4

neuromuscular junction

Question 5

what characterises electric synapses?

Answer 5

gap junctions connect the cyctoplasms of 2 neurons

instantaneous current flow (need for fast responses e.g escaping)

synchronised activity e.g eye movement

Question 6

what occurs in chemical synapses?

4 steps

Answer 6

majority of synapses

1. resting potential
2. depolarisation -(excited + received signal + neurotransmitter bound to synaptic receptors in dendrites/soma)
3. action potentials (spikes) in integration zone if depolarisation reaches threshold
4. signal transmission to next neuron starts when AP reaches output zone (neurotransmitter released into synaptic cleft)

Question 7

what are the 3 types of frequently found synapses?

Answer 7

axo-dendritic
axo-somatic
axo-axonic

Question 8

what happens during synaptic transmission?

3 steps at presynatic neuron

3 steps at postsynaptic neuron

Answer 8

presynaptic neuron: depolarisation of axon terminal membrane opens Ca2+ channels so enter terminal due to lower concentration

increase is Ca2+ stimulates release of neurotransmitter from vesicles

when vesicles fuse with presynaptic membrane the neurotransmitter diffuses into synaptic cleft

postsynaptic neuron: neurotransmitter either crosses cleft and interacts with ionotropic receptors embedded in membrane of dendrite/soma

or crosses cleft and interacts with metabotropic receptors

reuptake - bind to autoreceptors in presynaptic cells

Question 9

what are ionotropic receptors?

Answer 9

ligand-gated ion channels which open when bound by neurotransmitter molecules

fast, signal transmission

Question 10

what are metabotropic receptors?

Answer 10

G protein-coupled receptors

interacts with ion channels or releases second messenger molecule in postsynaptic cell

slow, long-lasting, neuronal modulation

Question 11

what are teh 2 types of neurons?

Answer 11

spiking neurons:
action potentials

non-spiking neurons: constantly receiving and sending signals via graded potential (regardless of reaching threshold) so no on/off switch

Question 12

when does problem of signal loss occur?

and what are the solutions?

Answer 12

when transmission distances are longer as signals become weaker

solutions:

1. thicker axons
2. myelinisation of axons (Schwann cells which increase velocity of signal)
3. neuroglia cells - assist signal propagation & provide nutrients

Question 13

what does conduction velocity increase with?

Answer 13

increase in diameter and myelinisation of axons

Question 14

what are the Nodes of Ranvier?

what purpose do they serve?

Answer 14

small gaps in the myelin sheath where axon membrane is exposed

for ion conductance through voltage-gated channels (saltatory conduction as message jumps)

Question 15

what characteristics do most neurons in the mamalian system have?

and why?

Answer 15

short
unmyelinated (doesn't need to myelinated when short)

Question 16

what does the neuron picked up depend on?

Answer 16

type of neurotransmitter

duration of release

Question 17

what is a typical neurotransmitter at excitatory synapses?

and what does it do?

Answer 17

glutamate

causes depolarisation of postsynaptic neuron so likely to fire

Question 18

what is a typical neurotransmitter at inhibitory synapses?

and what does it do?

Answer 18

GABA

causes hyperpolarisation so postsynaptic neuron doesn’t fire

Question 19

what is spatial summation?

Answer 19

when postsynaptic potentials arrive together in the integration zone, they are summed up

Question 20

what is temporal summation?

Answer 20

if neurotransmitter released for longer time into synaptic cleft then postsynaptic potential is stronger

Question 21

what happens when more excitatory or inhibitory input arrives?

Answer 21

more excitatory - stronger output signal

more inhibitory - weaker output signal and neuron may not transmit output signal

Question 22

when will an action potential be generated?

Answer 22

if both excitatory potentials (from different neurons) are present and sum up to a depolarisation that surpasses threshold
then action potential will be generated (and signal transmitted)

Question 23

when will an action potential not be generated?

Answer 23

if inhibitory potentials (summing inhibitory input or subtracting smaller excitatory)
are present then action potential will not be generated and the signal will not be transmitted

Question 24

what determinesthe strength of the neuronal signal?

Answer 24

spatial and temporal summation

Question 25

what is divergence and convergence?

Answer 25

divergence - signal distributed to many neurons through divergence if their connections

convergence - collecting signals from many neurons