Basic neurophysiology

Basic neurophysiology

1. types of neurones

   • unipolar sensory neurons-one long axon which connects receptors to the spinal cord or brain
   • multipolar inter- and motorneurons-have many dendrites and one axon
   • bipolar neurons- one dendrites and one axon

2. resting potential

   • neurons process info in the form of electrical signals (Ap's). Proteins act as ion channels & ion pumps in plasma men & move ions across men
     • inside the cell, [K+] high, balanced by a high conc of -vely charged proteins & other anions (A-).In ECF (outside the cell) [Na+] high,balanced by Cl-
       • cell mems of neurone have high selective permeability for K+- diffuses out of cell, impermeable anions left behind creating a electrical potential.
         • Inside of cell now has net negative charge, electrical force starts to pull K+ back in.
           • When certain potential difference reached, electrical force pulling K+ in exactly counterbalances conc grad=equilibrium
             • the electrical potential diff that exactly balances an ionic conc grad is called EQUILIBRIUM POTENTIAL (measured using microelectrode technique)
               • can calc equilibrium constant using nerst equation: v=(RT/zF)ln (CR/Ci). For K+, [K+]out=5mM & [K+]in=150mM. EK+=61.5 x log10 (5/150)=-91mV
                 • neurons are also permeable to Na+- large electrochemical gradient for Na+ drives Na+ into the cell. Makes resting mem potential more positive (-70mV)
                   • Goldman equation
                   • At -70mV, K+ leaks out neuron, Na+ leaks in. The Na-K pump actively transports ions to compensate for Na+ & K+ leaks using ATP (3 Na+ out, 2 K+ in)
                 • nonexcitable cells-mem potential depends only on K+.
             • electrical & chemical forces combine to become electrochemical gradient causing movement of ion

3. ACTION POTENTIAL

   • in inter- and motor neurone, info flow is directional-incoming signals are intergrated & if signal large enough=AP generated & conducted along axon
   • Depolarisation phase
     • some voltage gated Na+ channels open=Depolarisation of mem to threshold (-55mV)=open more voltage gated Na+ channels-rapid depolarisation.
       • peak depolarisation,Na+ channels inactivated-because of +ve charge inside cell & conc grad-K+ voltage gated channels open-K+ moves out=REPOLARISATION
         • resting mem reached but some K+ channels still open, more K+ move out=HYPERPOLARISATION (below -70mV)
           • Na/K+ pump restores K+ to 150mV and Na+ to 15mV=RESTORATION PHASE. All voltage gated channels closed
       • K+ leak channels always open but effect overcome by Na+
   • Na channels
     • At resting mem potentials= closed/ as neuron depolarisaed=open/ at fixed time after channel opens=inactivates
   • absolute refractory period
     • after the neurone has generated an AP it cannot generate another one (From the moment Na+ channels open at threshold until inactivation)
   • relative refractory period
     • immediately after absolute refractory period, neuron can develop AP but only if it is depolarised to a value MORE POSITIVE than threshold
   • Hodgkin-Huxley model of excitability (from experiments with giant squid axons)
     • 1.AP caused by inward Na+ & outward K+
     • currents flow through voltage-gated conductance channels which undergo a conformational change upon changes in mem potential-alters their conductance
     • AP is ALL or NONE
     • only INA & IK are needed to produce an AP.All other gated channel currents can indirectly modify INA & IK, altering mem potential & therefore conduct
   • Conduction of APs in Axons
     • myelinated-APs can jump between Nodes of Ranvier-faster propogation (saltatory conduction)=quick response. Unmyelinated-moves slower in current loops
     • Speed much faster when diameter of axon bigger
     • if temp higher=AP moves quicker/if cold can block AP
     • impulses travel along neurons at speed from 1 to 120 m/s
     • fibres
       • A fibres-large myelinated axons that conduct AP at the speed 15-120m/s
       • B fibres-medium diameter myelinated axon that conducts Ap at speed of 3-15m/s
       • C fibres: unmyelinated axons that conduct AP at speed <3m/s
     • chemicals can affect electrical signals in neurons
       • Tetrodotoxin-puffer fish/Saxitoxin-from marine organisms that cause red tide/Procaine-local anasthetic=all block VOLTAGE GATED NA+ CHANNELS
         • local anasthetics block pain by blocking transmission to the brain
       • Tetraethylammonium chloride TEA-BLOCKS VOLTAGE GATED K+ CHANNELS
       • Quabain-from ripe seeds of African plants-BLOCK SODIUM-POTASSIUM PUMP