Biomedical Engineering Reference
In-Depth Information
Appendix A
Bioelectromagnetic Forward Modeling
A.1
Neuronal Basis of MEG and EEG Signals
Neurons in the brain function electrically, as well as chemically, and have associated
electric and magnetic fields which can be detected outside the head. We first discuss
the anatomical and physiological properties of neurons in the mammalian brain, that
contribute to the electric and magnetic fields detected by MEG and EEG. Neuronal
currents are considered at a level sufficient for understanding the primary source of
these fields.
A.1.1
Neurons and Synapses
Neurons are cells that are highly specialized for signal processing and conduction
via electrochemical and electrical processes. The morphological structure of a neu-
ron includes a cell body, called the soma, and elaborate branching structures called
dendrites and axons that enable communication with sensory receptors, distant neu-
rons, etc. Inputs to a neuron are collected in a continuous fashion by the dendrites,
and represented as a spatially and temporally continuous variation of the transmem-
brane voltage. Multiple inputs are summed in the dendritic tree, and the net input is
often represented as transmembrane voltage at the soma. When the somatic voltage
reaches some threshold, a discrete voltage pulse is generated, called an action poten-
tial, which propagates down the axon as a discrete output. The end of the axon has
elaborate branching to enable communication with target neurons.
Neuronal communication occurs for the most part via chemical transmission
through synapses, although a small minority of neurons also communicate elec-
trically through gap junctions. When an action potential reaches the presynaptic
terminal of a synapse with another neuron, it causes the release of neurotransmitter
into the synaptic cleft between two cells. The released neurotransmitter binds with
some probability to sites of the postsynaptic terminal, stimulating a flow of current
