Cycle 4 Methods Electrical Signals Dendritic membrane potentials (post-synaptic, backpropogation) Smaller membrane potential than action potentials, EPSPs longer than IPSPs (why?) Axon hillock, Axonal membrane potentials (action potentials) Larger membrane potential than post-synaptic potentials Location and conductance of specific ion channels determines current flow Sinks suck + charge into cell; Sources push + charge out of cell Current flow generates extracellular fields (e.g. dipoles along a dendrite) Electric fields decay exponentially with distance. Draw current flow (sources and sinks) for Pyramidal cell getting feedforward inhibition. Assume input excitatory synapses include apical dendrites, inhibitory synapses are on cell body.

Cycle 4 Methods Electrical Signals Dendritic membrane potentials (post-synaptic, backpropogation) Axon hillock, Axonal membrane potentials (action potentials) Measuring across the fields (across the gradient) results in a ‘field’ potential (units = Volts). Requires signal and reference probes. Theoretically the two probes are equal opposites, but in practice the reference is placed in a ‘neutral’ spot. Signal probes close to cell bodies may record both dendritic (EPSP/IPSPs) and axonal potentials (spikes).

How can a field potential reflecting dendritic activity be larger than action potentials?

Cycle 4 Methods Signal and reference probes: EEG measures across skin, muscle, skull, meninges and brain Large distance between signal and reference means many signals are detected…source localization issues, bias to superficial layers ECoG measures across meninges and brain Smaller distance between signal and reference means fewer signals are detected…but signal probe is still remote to actual signal source. Bias to superficial layers

Cycle 4 Methods Depth (intracranial) electrodes record brain* *but don’t forget your reference location! Smaller distance between signal and reference means fewer signals are detected, and unit activity can be detected. “LFP” signal falls with distance so typically, measures a few mm^2 volume. MEG measures resultant electromagnetic field that exists across tissue such as skull and skin. But very small magnitude, and detects many signal sources, thus localization problem,