An electrode arrangement for sensing electrical activity in target tissue is described. A support electrode has an elongate electrode body with a base end and a penetrating end for insertion into the target tissue. A fixation electrode has an elongate electrode body with a base end and a penetrating end at an angle to the electrode body. The electrodes are joined together with their electrode bodies in parallel so that the penetrating end of the fixation electrode penetrates a fixed distance into the target tissue so that at least one of the electrodes senses electrical activity in the target tissue.