A Model of the Median Sensory Nerve Compound Action Potential Leading to a Method for Nerve Fibre Diameter Distribution Determination

1 INTRODUCTION Nerve conduction studies (NCS) are one of the basic tools of the electrodiagnostic clinician and are performed in order to evaluate the integrity of peripheral nerve function. Using such techniques diagnosis of various diseases and disorders of nerve are possible. During NCS a peripheral nerve is stimulated using an electrical pulse of sufficient intensity to recruit as many nerve fibres as possible. This elicits a volley of action potentials (APs) in the individual nerve fibres which is then recorded at a distal point usually using surface electrodes. Usually only two components of the recorded response (called the compound action potential or CAP), the distal amplitude and the distal/peak latency are routinely recorded though the proximal latency is also used to construct an average measure of conduction velocity (CV). In this study a model of the sensory CAP of the median nerve as measured using bipolar electrodes is proposed and using this model an additional measure is extracted from the CAP. This measure represents the distribution of conduction velocities within the nerve trunk and from this a measure of nerve fibre diameter distribution can be ascertained. 2 GENERAL INFORMATION The median nerve is derived through the spinal nerve roots C6-T1 and travels the whole length of the arm along through the carpal tunnel at the wrist to the palm of the hand. The median nerve innervates both the pronator teres muscle in the forearm and the abductor pollicis brevis muscle (often abbreviated to APB, it is this muscle that is responsible for thumb abduction). In humans the median nerve is made up from around 6500 nerve fibres of which over three quarters are sensory with external diameters ranging from 6-12m as found by dissection [1]. The relationship between the external fibre diameters and conduction velocity has been found to be a linear one in which CV increases with fibre diameter. this relationship is often expressed thus conduction velocity (m/s)=k.fibre diameter(m) (1) where k6 [2]. There are currently several methods for decomposing the CAP into its constituent nerve fibres according to diameter such as the techniques by Tu et al.[3], Pollak et al.[4] and Hirose et al.[5] yet none of these methods have gained widespread acceptance as each method suffers from its own particular shortcomings e.g. Tu's method takes no account of the bipolar configuration of recording electrodes used in such measurements and its influence on the CAP morphology.