Lecture 4 Flashcards
neurotransmission
intracellular = electrical transmission WITHIN the cell
extracellular = electrical transmission BETWEEN cells
if only K+ channels are open, K+ leave the cell until the membrane potential reaches -80mV
if only Na+ channels are open, Na+ enters the cell, making the membrane potential more positive until it is roughly +62mV
at rest there are lots more K+ channels than Na+ channels open, therefore at rest the membrane potential is -70mV which is near the potassium equilibrium potential
electrical signals
- can be measured with a voltmeter
- resting membrane potential is negative
- action potential is a wave of transient depolarisation that travels down the axon
- sodium channels initiate transient depolarisation
key concepts of ion channels
- holes in the membrane that allow ions to enter and leave the cell
- they are selective for different ions
- can be open all the time e.g. K+ leak channels that set resting membrane potential
- others are opened by different stimuli
ion flow down electrical and chemical conc gradients
the action potential
- conveys a fast signal from one place to another in the body
- generated by changes in membrane permeability due to opening and closing of bolted gated ion channels
- a self-regenerating electrical wave
- is a transient change in membrane potential
only occurs if a threshold membrane potential is achieved in the axon initial segment
- this is called the axon hillock which transiently opens voltage gated sodium channels
the action potential events
- threshold potential is reached
- depolarisation due to opening of Na+ channels
- depolarisation due to inactivation of sodium channels and opening of voltage=gated K+ channels
- hyper-polarisation as voltage-gated potassium channels are still open
three phases of action potential
- rising phase (depolarisation)
- Na+ channel opens at -70mV
- K+ channel shut - falling phase (repolarisation)
- Na+ channel shuts so it is inactive
- K+ channel opens - undershoot (hyper-polarisation)
- goes back to resting state
- relatively few ions move during an action potential
- barely changes the conc gradient as long as keep pumping ions back
action potentials are all or nothing
for action potentials to be generated, the local depolarisation must reach a threshold point, which is the voltage at which Na+ channels start to open
this results in positive feedback
action potential propagation
- action potentials propagate along axons, they are same size all along
- as action potential moves, it depolarises next bit of membrane and opens Na+ channels
- if enough Na+ channels are opened, the membrane potential reaches threshold potential and action potential propagates along
- the area that has just generated an action potential cannot fire another as Na+ channels now inactivated
absolute and refractory periods
- all Na+ channels inactivated = absolute refractory period
this enforces one way transmission - some Na+ channels inactivated = relative refractory period
only very strong stimuli can re-open the Na+ channel and generate action potentials
saltatory conduction and myelin sheath
MYELINATION =
- myelin insulates membrane - less charge loss
- action potentials travel from on node of ranvier to next and this is saltatory conduction
- faster/ more efficient as fewer ions flow so less ATP needed to pump them back
summary of action potentials
- membrane reaches a threshold voltage, voltage-gated Na+ channels briefly open which depolarises the cell
- voltage-gated K+ channels open and depolarise the cell
- depolarisation spreads along membrane activating nearby Na+ channels
- inactivation of Na+ channels means action potential propagates in one direction and sets limit on firing frequency
- action potentials are all or nothing and only occur once threshold is reached
- myelin speeds action potentials and makes them more energy efficient
postsynaptic membrane - excitation (glutamate)
- depolarisation of dendrites by ion flow through glutamate receptors generates an excitatory post-synaptic potential (EPSP)
- drives membrane potential towards the threshold for action potential firing
summation
- usually need summation to get action potential generated
- single EPSP is sub-threshold
- lots of EPSPs summate over time to reach threshold = temporal summation
lots of synapses active at same time - EPSPs summate to reach threshold = spatial summation
postsynaptic membrane - inhibition
GABA = main inhibitory transmitter in brain
opens chloride channels (GABAa receptors), allowing negative charge into cell, generating inhibitory post-synaptic potential (IPSP)
HYPERPOLARISES MEMBRANE
summation of excitatory and inhibitory inputs = synaptic integration
summary of synapses
- info passed between neurones at synapses
- depolarisation of axon terminal = transmitter release
- NTs activate receptors on post-synaptic neurone dendrites
- receptors excite or inhibit post-synaptic cell
- soma integrates excitatory and inhibitory synapses to ‘decide’ whether to fire a potential
- info coded by changing frequency and timing of action potential firing
