Electrophysiology Lecture Notes PDF

Scientific basis of neurophysiology Maram Samy Neuronal axons are covered in an insulating substance called myelin, which is a component of Schwann cells (in the peripheral nervous system) and oligodendrocytes (in the central nervous system). The effect of the myelin sheat...

Scientific basis of neurophysiology Maram Samy Neuronal axons are covered in an insulating substance called myelin, which is a component of Schwann cells (in the peripheral nervous system) and oligodendrocytes (in the central nervous system). The effect of the myelin sheath is to provide insulation and allow faster conduction along the axon, as well as providing metabolic support for the cell. Along the course of the axon there are tiny gaps between myelinating cells called nodes of Ranvier (which are up to 2 mm apart). The action potential jumps between these nodes by a process called saltatory conduction, which speeds up the process of conduction. Reflex Arc Nerve action potential The principals of nerve conduction study NCS involve the application of a depolarizing square wave electrical pulses to the skin over a peripheral nerve producing: (1) a propagated nerve action potential (NAP) recorded at a distant point over the same nerve: (2) a compound muscle action potential (CMAP) arising from the activation of muscle fibers in a target muscle supplied by the nerve. In both cases these may be recorded with surface or needle electrodes The principals of nerve conduction study Surface electrodes are designed to give information about the whole of a muscle stimulated, giving data for the time taken for the fastest axons to conduct an impulse to the muscle and the size of the response. The principals of nerve conduction study The aim of nerve conduction studies NCS Our minimum knowledge set above has shown us that peripheral nerves contain many nerve fibers of different diameters, degrees of myelination, and afferent or efferent connections. The NCS studies the fastest 20% of these fibres and the aim of the investigation is to document focal or continuous abnormalities in the length of the mixed, motor or sensory nerve. Particular attention is paid to the following questions as the test progresses: Is the fastest conduction velocity normal? Is the velocity gradient normal. Normally nerves closer to the neuraxis and more cephalad conduct faster than more distal and caudal nerves. Is the CMAP normal in size and shape? Does the CMAP alter in size, shape or duration between stimulation points? Waves of nerve conduction study The time it takes for electrical impulses to travel from the stimulation to the recording site is measured. This times is called and is measured in milliseconds. The size of the response is called and measured in millivolts. By stimulating two or more different locations along the same nerve, nerve conduction velocity across different can be measured. Nerves studied Motor Nerves Sensory Nerves Compound motor action potential CMAP CMAP CMAP - Axonal loss (most common) - Conduction block that is caused by demyelination in the segment between the stimulant and the recorded muscle - NMJ disorder - Myopathy CMAP conduction velocity CV (m/sec) Sensory nerve conduction study SNAP It is a compound potential that represents the summation of all the individual sensory fiber action potential. Usually they are biphasic or triphasic action potentials. The sensory nerve action potential (SNAP) is obtained by electrically stimulating sensory fibers and recording the nerve action potential at a point further along that nerve. The stimulus must be supramaximal.  Mixed nerve conduction studies (MNAP) Not pure sensory so can’t assess the integrity of sensory nerve nor the DRG In routine NCS the largest and fastest fibers are recorded. The sensory muscle afferents (Ia fibers) are recorded only during mixed nerve studies where the entire mixed nerve is stimulated. Since the Ia fibers have the largest diameter and the maximum myelin they are often the earliest fibers affected by demyelinating lesions. Te recorded action potential is the summation of all the individual sensory and motor fiber action potentials. Temporal dispersion and phase cancellation Who has the larger temporal dispersion and phase cancelation SNAP or CMAP ?  Late response waves H reflex F wave A wave H reflex Submaximal stimulation of he afferent sensory fiber which will causing orthrodromic conduction to the spinal cord causing synaptic stimulation of the alpha motor neuron causing an evoked H response in muscle A rudimentary M response is produced when a few motor axons ae directly stimulated. H reflex Latency : from 28 – 20 milliseconds (side to side difference 0.5 to 1 ms) Above 60 years old add 1.8 ms Location for testing - Soleus muscle : tibial nerve S1 pathway - Flexor carpi radialis : median nerve C7 pathway - Vastus medialis : femoral nerve : L4 pathway F wave Supramaximal stimulus initiates two responses One antidromic motor response to the spinal cord One orthodromic motor respons to the recording electrode. F wave is 5% of the CMAP height Configuration and latency change with each stimulation (why?) Due to the Renshaw cells that inhibit impulses from raveling in the same path every time Significance radiculopathy and polyneuropathy A wave Not a true wave Seen between f response and M wave The A reflex (Axon reflex) is a small motor potential that is identical in latency and configuration with each successive stimulation and seen in re innervated nerves when submaximal stimulus is given - It represents collateral sprouting following a damaged nerve - also shows that stimulus is submaximal THANK YOU