Neurophysiology

Question 1

Resting membrane potential

Answer 1

• All cells have an electrical potential (Vm) that exists across a membrane when the cell is in resting condition (Vrest) =
  
  • -65mV
• Vrest originates from the ability of the cell to selectively allow different ions across its membrane
  
  • This is determined by the permeability of the ions, which is determined by the amount of open channels.
• The Goldman-Hodgkin-Katz constant field equation combines the Nernst equations for all ions and takes permeabilities into consideration

Question 2

Membrane potential

Answer 2

• The membrane potential is dictated by ion permeability. Most ions cross through the membrane via pores or channels, therefore no ion flow and membrane potential can be generated unless there are specific channels present in the membrane allowing ions to get through
• The ions that are responsible for the resting membrane potential are: K+, Na+ and Cl-
  
  • During resting potential, all the channels are closed. However K+ leaks out the membrane. P_K determines E_K which is -90mV
  • However, Vrest is not identical to E_K because there is also some leakage through Na+ and Cl- channels, where Na+ and Cl- ions are brought into the cell raising the membrane potential to -70mV
  • In a typical axon at rest the P_K : P_Na : P_Cl= 1 : 0.04 : 0.45

Question 3

Signal propagation in nerve cells

Answer 3

• There are 2 types of signal propagation
  
  • Passive propagation: due to static membrane properties and is determined by measuring the voltage change resulting from a current pulse passed through the axonal membrane that may not be large enough to generate action potential - here there is diminishing of signal as distance increase. Axon diameter also affects signal propagation where the large the diameter, the further it can travel. Therefore passive propagation usually only happens over short distances. (myelin also influences signal propagation - behaves like insulation)
    
    • Passive propagation are graded = bigger stimulus leads to bigger response and summated = multiple stimuli = summated response
  • Active propagation (action potential): Electrical properties, triggered by changes in Vm. These properties enable the conduction of electrical signals without decrement over large distances
    
    • Action potentials are all or non-signals therefore a threshold (approximately -55mv) has to be achieved an action potential is generated.
    • It always stays the same size and does not get larger for a stronger stimuli.

Question 4

Properties of the voltage-gated Na+ and K+ channels in active propagation

Answer 4

• There are 3 states of the Na+ channels
  
  • Resting state: M gates are closed and h gates are open. This is -70mv (inside of the cell is negative while the outside is positive). K+ n gates are closed (however, leaky K+ gates still allows for K+ ion flow)
  • When the voltage across the membrane decreases to threshold -55mv, the m gates open. This allows Na+ to start entering. This causes a +ve loop as more Na+ enters the membrane potential goes up and this causes an increase in opening of Na+ gates. This causes the rise to peak AP which is +40mv. However before Ena is reached, due to slow kinetics, h gate 1msec after depolarisation, the h gates inactivate which is the closing, making the membrane no longer permeable to Na. Repolarisation now begins and a new action potential cannot be made until the internal gates h are open. This is the absolute refractory period
  • Voltage gated potassium channels slowly open – these are called delayed rectifiers as they take 2msec from threshold depolarisation to open. This allows potassium flow out of cell, speeding up repolarisation, leading to hyperpolarisation which closes the n gates and opens the h gates while the m gates close – Inactive Na+ channel state
    
    • During this hyperpolarised state, you can generate another action potential but a stimulus has to be strong enough to overcome the hyperpolarisation and the threshold potential. However there is reduced amplitude because fewer Na+ channels are available to open
    • Behind the AP, Na+ channels are in an inactive state and cannot open. Therefore the spreading current has no effect on these channels and action potentials cannot be triggered

Question 5

myelin sheath affects on active propagation

Answer 5

• Axons are wrapped in myelin sheath which consist of several layers of a specialised membrane (70-80% lipids and 20-30% proteins)Myelin is uniform and impermeable to movement of ions
  
  • Short gaps - nodes of Ranvier - exist between the myelin sheaths, exposing the axon
• Membrane areas covered by myelin do not become depolarised and therefore cannot generate action potentials. This forces the current to travel down the axon to the node of Ranvier where there is no myelin and the concentration of V-gated Na+ channels is high. Thus AP jump from one node to another.
• Between the nodes, there is passive spread of potential
• This whole process is called Saltatory conduction

Question 6

Types of synapse

Answer 6

• Electrical synapses: No chemical transduction involved and is common in
  
  • Escape reflex neurones
  • Cardiac muscle cells
  • Epithelial cells E.g. Gap junction - contains protein pores (Connexon - made of 6 subunits) that bridge the gap between two cells, it is large enough to allow substance with a molecular weight of up to 1000 to pass through
  • Properties of gap junctions: allows substances with a molecular weight of up to 1000 to pass through, is continuous, without transmission delays and can be bi-directional
• Chemical synapse: Chemical substance used as intermediate to convert electrical signals in the pre-synaptic cell intro electrical signals in the post-synaptic cells
  
  • Found in neurones
  • Properties: there is no cytoplasmic continuity, requires a chemical transmitter with a transmission delay of 1-5msec and is unidirectional
  • Otto Loewi 1921, I a double frog experiment, he discovered that there was chemical transmission of nerve impulses
• Receptors to neurotransmitters are either ionotropic or metabotropic. Ionotropic: Directly linked to an ion-channels, also called ligand-gated ion channels or Metabotropic: G-protein coupled receptors, not directly linked to an ion channel, use a second messenger. Ionotropic responds faster

Question 7

Criteria for identifying a neurotransmitter

Answer 7

• When applied experimentally to postsynaptic membrane -mimics normal transmission
• Manufactured in the presynaptic cell and stored in its terminals
• Released when presynaptic neurones is excited
• Should be a mechanism for removing substance from the synaptic cleft (enzyme or uptake) e.g. - removal of Ach in the NMJ

Question 8

events in synaptic transmission

Answer 8

1. Action potential propagation in pre-synaptic neurone
2. Electrical depolarisation causes opening of Ca2+ ions
3. Ca2+ flow through the pre-synaptic membrane and therefore increasing the [Ca2+] inside the presynaptic neurone
4. High [Ca2+] activates a set of Ca2+ sensitive proteins synaptotagmin attached to vesicles that contains a neurotransmitter
5. Synaptobrevin found on vesicles change shape and dock vesicles to membrane of pre-synaptic cell- V-SNARE (attached to vesicle) binds to syntaxin on T-SNARE (found on target membrane protein in pre-synaptic neurone)
   
   1. This all occurs on active zone - dense collection of SNARE proteins
6. Vesicles and protein are recycled through endocytosis
   
   1. Slow clathrin dependent process: where the clathrin cage sits on the inside of membrane and a pore held open by dynamin which acts as an entry point to clathrin – causing clathrin dependent endocytosis.
      
      1. Dynamin twists off the vesicle and this slow clathrin vesicle formation is followed by fast-acidification to transport neurotransmitter into the vesicle
   2. Bulky endocytosis: Which is fast and only relies on dynamin and larger vesicles are formed. This is followed by slow-acidification

Question 9

Post-synaptic events

Answer 9

• EPSP and IPSP are both post-synaptic potentials
• An excitatory postsynaptic potential (EPSP) is a postsynaptic potential that makes the postsynaptic neuron more likely to fire an action potential.
  
  • This temporary DEPOLARISATION of postsynaptic membrane potential, caused by the flow of positively charged ions into the postsynaptic cell, is a result of opening ligand-gated ion channels.
• These are the opposite of inhibitory postsynaptic potentials (IPSPs)
  
  • HYPER-POLARISATION of post-synaptic neurone which usually results from the flow of negative ions into the cell or positive ions out of the cell. This forms a synaptic potential that makes a postsynaptic neuron less likely to generate an action potential.
  • E.g. is to inhibit motor neurones of antagonistic muscles
• The postsynaptic potential representing the sum of all the postsynaptic potentials arrive at the active synaptic boutons: composite postsynaptic potential – this determines if an action potential should fire or not.
  
  • When the composite PSP rises above the threshold, action potentials are generated in the initial segment (axon hillock) – where it has high density of voltage gated Na+ channels

Action potentials are non-decremental - therefore do not decrease in size, where synaptic potentials are decremental.

• Temporal summation of EPSP: Adding together of EPSP’s generated by firing of the same pre-synaptic terminal at high frequency to generate an action potential in the post-synaptic neurone
• Spatial summation: Adding together of EPSP’s generated by firing of two or more pre-synaptic neurones simultaneously to generate an action potential in the post-synaptic neurone