1210 Week 16

1210 Week 16

1. Lecture 1

   • An Action Potential
     • there are 4 phases - rising, overshoot, falling, undershoot
     • 1 - resting state - all voltage gates are closed
     • 2 - rising - depolarisation, Na+ gates open and Na+ floods the cell
     • 3 - overshoot - the threshold has been reached and so an action potential occurs as more Na+ floods in
     • 4 - falling - repolarising, Na+ gates are inactivated and K+ gates are opened and K+ leaves the cell
     • 5 - undershoot - Na+ gates remain closed and K+ open
   • Changes in Resting Potential
     • stimuli cause a patch of the nerve membrane to become depolarised
     • positive ions flow to the negative areas and vice versa
     • local currents are created that depolarise the adjacent membrane areas and allow for a wave of depolarisation
     • hyperpolarising currents produce passive changes in potential
     • depolarising current produce passive changes until the threshold is reached
     • at the threshold an action potential occurs
     • once the threshold has been reached it is all-or-nothing
     • an action potential is a constant amplitude
     • the size of a stimulus will change the frequency of firing not the size of the action potential
   • Nerve Inhibition/Excitation
     • tetrodotoxin - blocks voltage gated Na+ channels and paralyses people
     • absolute refractory periods are the time when no matter the size of the stimulus it isn't possible to generate a new action potential
     • relative refractory periods are the time when a larger than normal stimulus would be needed to cause a new action potential
     • refractory periods make sure that action potentials are unidirectional
     • mylinated axons have nodes of ranvier that are the only places where action potentials can occur so they jump along the axon speeding up conduction - saltatory conduction

2. Lecture 2

   • Preparations
     • single cell - isolated neurones, no synaptic connections, cell lines or oocytes
     • brain/spinal cord slices - neurones in vitro, local synaptic connections, intact
     • larger in vitro - brain stem/spinal cord kept intact to allow maintenance of longer connections
     • intact nerve - recording from an axon
     • invertebrate - snails brain
     • in vivo - whole animal recordings, most realistic recording environment
     • brain slice - brain is immediately placed in synthetic CSF and a vibrotome used to slice, they can then be used for many different neural measurements
   • Clamps
     • current clamp - a current is injected into the cell so the membrane potential can be measured
     • voltage clamp - the membrane potential is controlled so the current can be measured flowing through ion channels
       • adding tetraethyl-ammonium blocks K+ channels and stops the outward current
       • adding tetrodotoxin blocks Na+ channels and stops the inward current
     • patch clamp - glass electrode is applied to the membrane and gentle suction applied forming a seal, various configurations can be applied to record electrical activity
     • whole cell patch clamps - measure the entire cell, the membrane is split and the electrode placed in the opening - both current and voltage clamps can then be used
     • 2 electrode voltage clamp - used in large cells, 1 electrode records voltage and the other passes current
   • Recordings
     • intracellular
       • electrode is placed within the cell
       • can be used to measure - resting potential, action potentials, firing rate, synaptic potentials
       • dyes can be used to work out the morphology of the neurone which allows for correlation of electrophysical and neuroanatomical properties
     • extracellular
       • records voltage, firing rate, synaptic potentials
       • multibarrelled electrodes allow multiple recordings and application of a number of different drugs - microiontophoresis
     • single channel - the current of a single ion channel, microscopic currents
   • Real life Applications
     • extracellular recordings can be used in absence epilepsy to measure firing rate and looking at the effects of drugs on the increased firing rate
     • oocytes can be injected with exogenous mRNA sequences to synthesise the foreign protein - this can be done with mRNA for ion channels to make functional channels
     • families with a history of epilepsy have a gene mutation that causes problems with the K+ channel and occytes can be used to investigate how they are affected and possible solutions