Session 3-4 [CAP in Sciatic]

Session 3-4 [CAP in Sciatic] - BIO335 Labs 3 & 4 LAB...

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BIO335 Labs 3 & 4 1 LAB INSTRUCTIONS Sessions 3 & 4 COMPOUND ACTION POTENTIALS in SCIATIC NERVE In this and next week’s sessions, we will continue to study how action potentials travel along nerve fibers – this time, the frog sciatic nerve. We will use essentially the same method that was used to record from earthworm giant axons: extracellular metal electrodes and differential AC amplification will be used to detect changes in extracellular field potentials as action potentials pass down the fiber. But this vertebrate preparation introduces some significant changes. - The nerve must be dissected out of the animal for recording. The quality of your data depends on your isolating a long nerve segment with minimal damage. To help with this VIEW THE DISSECTION VIDEOS (link on Blackboard) before coming to lab. - The sciatic nerve includes many thousands of axons, which fall into different classes with conduction properties that you will study. Unlike the earthworm nerve cord, there are no giant axons to give large signals from single action potentials. Instead, the extracellular voltage signals that you will record are compound action potentials, arising from action potentials induced simultaneously in populations of axons. - Like the earthworm giant fiber, many axon types in this vertebrate nerve have an insulating myelin sheath. In vertebrates, myelin consists of spiral wrappings of a glial cell around the axonal membrane. This increases the effective membrane resistance , and reduces membrane capacitance , enabling these axons to conduct action potentials faster and farther than would be possible in an unmyelinated axon of the same diameter. - You will also use NIA2 to model a single myelinated axon , and observe the effects of changing the number of myelin layers and other axon properties. A partial demyelination tutorial mimics the effects of demyelinating diseases such as Multiple Sclerosis (MS). I. BACKGROUND The Frog Sciatic Nerve The sciatic nerve, the major nerve innervating the hindlimb, contains thousands of axons which carry information in the form of action potentials to and from the spinal cord. These include sensory axons, which conduct action potentials from peripheral receptors into the spinal cord and somatomotor axons, which extend from motor neurons in the spinal cord and terminate on muscles. In addition to a functional classification, the axons can also be grouped by conduction velocity (CV), which correlates with their diameter and extent of myelination. Axons in the sciatic nerve can be classified by CV into three groups: A, B and C. In the frog, the "A" group consists of myelinated afferent (sensory) and efferent (motor) axons of diameters ranging from 18 µ m ( α subgroup) to 14 µ m (ß subgroup) to 11 µ m ( δ subgroup), with conduction velocities of 42, 25 and 17 m/sec. respectively. The "B" group consists of preganglionic myelinated nerve
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Session 3-4 [CAP in Sciatic] - BIO335 Labs 3 & 4 LAB...

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