Sciatic Nerve Bundle Case Study

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Question 3

Using any peripheral nerve bundle as a model, describe using figures (a) the general anatomical structure with key constituents identified and (b) the heterogeneity in the electrical conductivities of the different constituents.

For this question, I will be using the sciatic nerve as my model. The sciatic nerve consists of several main structures as can be seen in Figures 5 below.

Figure 5: Cross section of the sciatic nerve. The inset in the top image shows individual fibers wrapped in a myelin sheath. The image on the bottom shows the different component of the sciatic nerve1.

The three main structures of the sciatic nerve are the peroneal (Per) nerve, the sural (Sur) nerve, and the tibial nerve. The tibial nerve is further
…show more content…
How do these current flow patterns change with stimulation using microelectrode?

The current flow in the axon can be described by two main models, the Hodgkin-Huxley model and the Cable model. For myelinated fibers, we can use the Hodgkin-Huxley model for the nodes of Ranvier and the Cable model for the internode of the myelin sheath. For the unmyelinated axons, we can use just the Hodgkin-Huxley model by itself since there is no myelin sheath to interrupt the conductance described by the model. I will first discuss the current flow in the myelinated axon4.

The myelinated axon is composed of two parts, the unmyelinated nodes of Ranvier and the myelinated internodes. Current is conducted through the myelinated parts of the axon through a process called saltatory conduction in which sodium ions in the nodes of Ranvier interact with the sodium ions inside the myelinated internodes and subsequently with the sodium ions in the next node of Ranvier, vastly speeding up the process of propagating the action potential5. This process can be modeled by a partial differential equation obtained from the Cable theorem and describes the potential across the myelin layer, found in Equation 1

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