Announcements Tutorial next Thursday, Oct 9 –Submit questions to me Mid-term schedule Go vote!

Today –Action Potential Conduction Next Lecture –Synaptic Transmission

Action Potential Conduction Axon hillock Region of neuron where AP usually starts

Action Potential Conduction Why are Action Potentials needed? 1.First look at current flow without APs. 2.Second look at current with APs

Passive Current Flow Record voltage Inject current axon

voltage distance 0.63V 0 0 Length constant V= Vo – 0.63Vo Inject current Record voltage

Passive Current 1.Current decays very rapidly along the length of an axon 2.The length constant is the distance over which the potential drops to 63% of the highest value 3.Typical length constants range only from 1-5 mm

Length Constant Depends on: 1.Resistance across the membrane (‘leakiness’) 2.Longitudinal resistance to current flow (varies with axon diameter)

Passive Current Flow Inject current axon Membrane Resistance Longitudinal Resistance

Action Potential Conduction Record voltage Stimulate Action Potential axon

Action Potential Conduction 1.APs constant amplitude at all points along the axon

Na+ Inject current

Na+ Inject current

Sequence of Events leading to AP propagation 1.Stimulus opens Na+ channels & cause AP 2.Depolarizing current flows down the axon 3.Local depolarization opens Na+ channels downstream & initiate a new AP 4.Na+ channels close (inactivate) & K+ channels open 5.Local depolarization opens Na+ channels downstream and initiate a new AP

Na+ Inject current

Conduction Velocity Record voltage Inject current axon Measure distance between recording sites Measure time between APs

1.Axon diameter 2.Myelination Small unmyelinated  0.5 m/s Large myelinated  120 m/s

Myelinated nerve Myelin Formed by: Schwann cells (periphery) Oligodendrocytes (central) Node of Ranvier

Myelin

Na+ Saltatory conduction

Myelin Myelin increases speed of conduction by: 1.Increasing membrane resistance Reduces ‘leakiness’   length constant 2.Voltage-gated channels only at Node of Ranvier APs generated only at the Node

Mulitple Sclerosis Demyelination of axons –Impaired AP conduction –Symptom depends on nerves affected Optic nerve  blindness Motor nerves  weakness or paralysis

Summary & Key Concepts 1.Currents flow passively down axon decay described by length constant 2.Action potentials propagation due to sequential opening of Na+ channels in response to local depolarization 3.Conduction velocity determined by axon diameter and myelination - length constant 4.Myelin  trans-membrane resistance and Na+ channels only at Nodes Saltatory conduction

Sample Question AP conduction velocity will increase following an increase in the electrical resistance of the: A.Axon membrane B.Node of Ranvier C.Schwann cell