MODEL OF WHOLE NEURON

This section brings together the entire neuron, combing the dendrite, soma, axon, and presynaptic terminal

MODEL OF WHOLE NEURON Neurons are responsible for the transmission and analysis of all electrochemical communication within the brain and other parts of the nervous system.

MODEL OF WHOLE NEURON Dendrites and axons can be modeled as a series of cylindrical compartments, each connected together with an axial resistance Note: Both dendrite and axon are connected to the soma.

Each neuron is composed of a cell body called a soma, a major fiber called an axon, and a system of branches called dendrites. Axons, also called nerve fibers, convey electrical signals away from the soma and can be up to 1 m (3.3 ft) in length. Most axons are covered with a protective sheath of myelin, a substance made of fats and protein, which insulates the axon. Myelinated axons conduct neuronal signals faster than do unmyelinated axons. Dendrites convey electrical signals toward the soma, are shorter than axons, and are usually multiple and branching.

MODEL OF WHOLE NEURON Note : the dendrite and axon do not have to have constant- diameter cylinders, but may narrow toward the periphery

Figure illustrates the axon compartment with active channels at the axon hillock and the node of Ranvier

MODEL OF WHOLE NEURON To model the myelinated portion of the axon, a set of passive compartments, like the dendrite compartment, can be used with capacitance, passive ion channels, and axial resistance

Figure is a portion of the axon with myelin sheath, with three passive channels, and an active component for the node of Ranvier. The structure in Figure can be modified for any number of compartments as appropriate. The soma can be modeled as an active or passive compartment depending on the type of neuron.

To model the neuron in Figure 11.33, Kirchhoff ’s current law is applied, giving

MODEL OF WHOLE NEURON Because neurons usually have other channels in addition to the three of the squid giant axon, a model of the neuron should have the capability of including other channels, such as a fast sodium channel, delayed potassium conductance, or high-threshold calcium conductance

Additional ion channels can be added for each compartment in Equation 11.49, by adding for each compartment for channels i ¼ 1, n. The values of Cm, RTH, Ra, and Gi are dependent on the size of the compartment and the type of neuron modeled.

MODEL OF WHOLE NEURON A complete model of the neuron can be constructed by including as many dendritic branches as needed, each described using Figure 11.17, each modeled by:

a soma with passive or active properties using either:

MODEL OF WHOLE NEURON an axon using Equation 11.49

Except for the terminal compartment, two inputs are needed for the dendrite compartment; the input defined by the previous compartment’s membrane potential and the next compartment’s membrane potential. Additional neurons can be added using the same basic neuron, interacting with each other using the current from the adjacent neuron (presynaptic terminal) to stimulate the next neuron.

MODEL OF WHOLE NEURON Many of the principles discussed in this chapter also apply to these other cells but the action potential defining equations are different. For example, the cardiac action potential can be defined with a DiFranceso–Noble, Luo–Rudy, or other models rather than a Hodgkin–Huxley model of the neuron.