ELECTRICAL EXCITABILITY CONDUCTION OF THE NERVE IMPULSE (PDF)

‫وزارة التعليم العالي و البحث العلمي‬ ‫جامعة بابل‬ ‫كلية طب حمورابي‬ ELECTRICAL EXCITABILITY CONDUCTION OF THE NERVE IMPULSE Session 4.2 Presented By MSc. Hayder S. Alhussainy  Aim of the lecture Outline how resting...

‫وزارة التعليم العالي و البحث العلمي‬ ‫جامعة بابل‬ ‫كلية طب حمورابي‬ ELECTRICAL EXCITABILITY CONDUCTION OF THE NERVE IMPULSE Session 4.2 Presented By MSc. Hayder S. Alhussainy  Aim of the lecture Outline how resting nerve fiber membrane is raised to By the end of this lecture the student should be able to threshold Describe the results of extracellular recording Demonstrate the local circuit theories of propagation of the nervous impulse Apply the effect of nerve fiber diameter and myelin on nervous conduction Distinguish some clinical application as demyelination  Electrical stimulation occurs under a cathode (negatively charged); excitability will be reduced under an anode (positively charged). This can be used to stimulate an axon or group of axons to threshold, thus initiating an action potential.  Conduction velocity is calculated by measuring the distance between the stimulating electrode and the recording electrode and the time gap between the stimulus and the action potential being registered by the recording electrode: conduction velocity = distance / time How is the action potential conducted along an axon? A change in membrane potential in one part can spread to adjacent areas of the axon This occurs because of local current spread shown diagrammatically below Conduction velocity is determined by how far along the axon these local currents can spread90 When local current spread causes depolarization of part of the axon to threshold then an action potential is initiated in that location  The further the local current spreads down the axon the faster the conduction velocity of the axon will be. Properties of the axon that lead to a high conduction velocity include: A high membrane resistance A low membrane capacitance A large axon diameter (this leads to a low cytoplasmic resistance) Capacitance, C, is the ability to store charge. This is a property of the lipid bilayer. A high capacitance takes more current to charge (or a longer time for a given current) and can cause a decrease in spread of the local current, especially with brief current pulses. The membrane resistance depends on the number of ion channels open. The lower the resistance the more ion channels are open and the more loss of the local current occurs across the membrane, thus limiting the spread of the local current effect.  These local currents cause the action potential to propagate down the axon. Note that the action potential will not begin to go backward because an area of axon that has just fired an action potential is refractory, i.e. it cannot fire another action potential until it has recovered from being refractory.  The myelin sheath Conduction velocity is increased considerably by myelination of axons. Large diameter axons such as motoneurones are myelinated, smaller ones such as C-fibres (sensory neurones) are not. The effect of myelin is to reduce the capacitance and increase the resistance of the axon. Myelin is formed by special cells: Schwann cells - these myelinate peripheral axons Oligodendrocytes - these myelinate axons in the CNS  Demyelination There are certain diseases where areas of some axons can lose their myelin sheath. The most well known condition is multiple sclerosis. This is a disease of the immune system where myelin is destroyed in certain areas of the CNS. This can have dramatic effects on the ability of previously myelinated axons to conduct action potentials properly. This can lead to decreased conduction velocity, complete block or cases where only some action potentials are transmitted.