the communicating brain Flashcards
compare chemical and electrical synapses
CHEMICAL • channels open upon binding of chemical to receptor • channels on post-synaptic membrane • directionality • intermittent • variable reliability • highly regulated • plasticity of receptors
ELECTRICAL
• channels open depending on voltage difference
• channels on both pre- and post-synaptic membrane
• bidirectional
• continuous
• highly reliable
• channels provide pathway for flow of ions and small molecules which allows transmission of electrical signals
describe the structure of electrical synapses
electrical synapses are gap junctions between pre/post synaptic membranes
6 connexin subunits surround a central pore
hemichannel = one pore
hemichannels on pre and post synaptic membranes connect to allow rapid transmission of ions without passing through extracellular fluid
how does electrical coupling impact the entire network?
coupling increases average frequency of entire network but decreases excitation of average cell
by exciting others, cell is losing current and inhibiting itself
how do electrical synapses act like a circuit?
cell circuit can be simplified with two electrical properties
INPUT RESISTANCE:
• current flowing through resistor will give voltage change proportional to resistance
• resistance = size of channel
• many channels open = ↓ resistance
• adding synapses causes ↓ resistance and ∴ cells become less excitable
INPUT CAPITANCE:
• “ability to store electrical charge”
• lipid bilayer prevents charge passing through membrane
• somatodendritic compartment:
– very little time to charge lipid membrane (within ms)
– coupled cells
what happens as electrical signals are filtered through the synapse?
signal is filtered as it passes through the synapse, creating a lag between pre- and post-synaptic cell
this delay increases as frequency increases (post-synaptic lags behind)
active conductance changes EC coupling by blocking channels ∴ amplitude and latency of signal changes. this increases synaptic strength
how can electrical synapses be both inhibitory and excitatory?
electrical synapses can be both excitatory and inhibitory depending on the type of signal, type of cell or the state of cell
TYPE OF SIGNAL (AP shape):
– Shape of pre-synaptic action potential determines inhibitory or excitatory AP in post-synaptic potentials (also depends on cell type)
– Depolarising phase of AP is very fast (low synaptic strength → high frequency)
– This is very strongly filtered → not efficiently transmitted to postsynaptic neuron
– Hyperpolarisation is lower frequency → efficiently transmitted to postsynaptic neuron
– EPSP is much slower → low frequency → excitation is efficiently transmitted to post-synaptic cell
* draw graph
CELL TYPE:
– Two cells are electrically coupled; cell 1 and cell 2
– When cell 2 is excited at the same time or before cell 1, probability of AP is increased in cell 1 = increase firing
– Golgi cells are inhibited for a long period if they are excited after an EC-coupled cell = decrease firing
STATE OF CELL:
– If cell is in a depolarised state, can result in excitatory – inhibitory transmission
– If cell is in a hyperpolarised state, results in only excitatory transmission
does electrical coupling achieve complete synchronicity
electrical synapses tends to equalise both cells so that AP occurs at the same time
transmission is instantaneous due to ion movement
however, there is a very short lag (ms) between cells as it takes time to charge
how do cells become desynchronised?
excitation of cell 1 has strong inhibitory component to cell 2
therefore after stimulation the two cells are completely desynchronised
how is escaping current related to voltage difference?
escaping current is directly proportional to voltage difference
i.e. if two cells are excited at the same time, current is prevented from flowing because cells are depolarising at the same time
no ∆voltage = no current
what is coincidence detection?
coincidence detection = neural circuit can encode information by detecting the occurrence of temporally close but spatially distributed input signals
escaping current is modulated and decreased if both cells have similar potentials
coincidence detection used to decide if neuron will make output or not, based on if neighbour is receiving input
how are neuronal short-circuits created?
chemical synapses open due to chemical stimulation
some electrical current is lost through these open channels
this results in CURRENT SHUNTING = electrical transmission decreased
can synapses share electrical and chemical mechanisms?
yes, there are shared plastic mechanisms which effect both chemical and electrical synapses
T of F
shared electrical and chemical synapses increase responses to uncorrelated inputs and decrease correlated inputs
FALSE
shared electrical and chemical synapses:
– DECREASE responses to uncorrelated inputs
– ENHANCE correlated inputs
