lecture 5 notes

Lecture 5: 

Important parts of the neuron 

• Neuron cell bodies are located within the CNS, in peripheral ganglia and in the walls of the intestine 

• Dendrites increases the surface area of the cell body and are specialized for receiving signals 

• The base of the axon has a specialized region called the initial segment and axon hillock which is the site of action potential generation  

Integration  

• Most neurons are likely to be receiving both inhibitory and excitatory inputs and these inputs will summate. If the majority of the inputs open K channels, the cell membrane potential will be pushed towards Ek which then the cell is less likely to generate and output making it a inhibitory signal.  

• At each moment in time, neurons are integrating the sum of their excitatory and inhibitory inputs so their membrane potential reflects the timing and relative strength of their drive 

The initial segment  

• The base of the axon has a very high density of voltage-gated sodium channels which are opened by the depolarisation of the membrane  

• At each moment in time, if the summed inputs to the neuron cause a depolarisation, then voltage-gated sodium channel opening is made more likely and as the gated ion channels open, the cell becomes even more depolarised. If the depolarisation is significant it may cause enough voltage gated sodium channels to open that a major depolarising sodium influx results; this voltage is the threshold for initiation of an action potential  

Threshold  

• This is when EPSP becomes an action potential  

• Sudden large increase in sodium influx – threshold  

Action potentials 

• If the membrane is depolarised to threshold at the initial segment, an action potential will be generated  

• AP is typically drawn as a graph of voltage against time  

• The initial event occurs when the EPSP generates enough depolarisation to cause voltage-gated sodium channels to open, so the increased sodium conductance drives the membrane potential towards ENa (sodium equilibrium potential) - makes it more positive  

Voltage gated sodium channels  

• Channels are gated by the voltage across them  

• When cell is at RMP, the gate is closed and cell is relatively impermeable to sodium  

• As cell becomes depolarised (EPSP) the gate opens allowing sodium to enter cell  

• The increased sodium conductance allows sodium entry down its electrochemical gradient which drives the MP towards ENa 

• As the polarity of the membrane becomes more positive inside, a subunit of the channel protein swings into the pore and blocks the channel and this is called inactivation which stops the sodium movement into the cell  

Voltage gated potassium channels 

• Typically have a charged subunit that changes configuration when the voltage across the protein changes 

• When the voltage is inside positive the channel opens allowing increased K conductance 

• K flows out of the cell down its electrochemical gradient the membrane potential is driven back towards Ek meaning its making the cell more negative  

• Voltage gated K channel changes are slower than voltage gated sodium channels  

Undershoot 

• When voltage gated K channels open, the membrane potential is initially repolarised beyond the resting membrane potential and this is called the undershoot. This is caused by the slow kinetics of the voltage gated K channels. K conductance stays high for a relatively long time, pushing membrane potential towards Ek 

During absolute refractory period, it is impossible to generate another action potential  

During relative refractory period, it is possible to generate another action potential however it is harder to trigger since there are high concentration of K within the cell 

• These 2 points prevents action potentials from propagating backwards. I mean they still do but it does nothing since high concentration of K 

Action potential features 

• Once initiated, APs are normally the same size (don't get different sizes unlike local potentials) 

• Aps are said to be all or none meaning we either get a signal or we don't  

• Since APs don't vary in size, important information is encoded in their frequency of transmission --> intensity is the difference between strong and weak stimulus  

Action potential functions  

• Once initiated, action potentials are conducted along axons away from the site of their initiation  

• Once they reach the terminal of the axon, they trigger a release of signalling chemicals which can either be excitatory or inhibitory