<prompt> In epidural anaesthesia, local drug deposited inIntrathecal spaceExtraduralParaspinalIntervertebral space  <response>: Extradural 494- Lees 13th 361- KDT6th 128-A. yadav. Anaesthesia 3rdEpidural Anaesthesia - mainly used for controlling post operative pains Main site of action is Anterior and posterior nerve root Drugs diffuses through dura and arachnoid and inhibits descending pathways in spinal cord Epidural extradural anaesthesia Spinal Intradural anaesthesia Extradural - The extent to which drugs reach their site of action is termed bioavailability Intradural - although the dura is a relatively rough, avascular fibrous membrane, the arachnoid with its tight overlapping cells represents 90 of resistance in drug permeability and keeps CSF confined to the subarachnoid space Epidural anaesthesia - LA injected in the this space acts primarily on nerve roots in the epidural as well as subarachnoid spaces to which it diffuses and small amount permeates through intervertebral foramena to produce multiple paravertebral blocks 