Brain–Computer Interfaces - Index of

206 D.M. Taylor and M.E. Stetner
electrical phenomenon specific to neurons and muscle cells. These electrically active
cells naturally have a voltage difference across their cell membrane due to different
concentrations of ions inside and outside the cell. Special channels within the cell
membrane can be triggered to briefly allow certain ions to pass through the membrane
resulting in a transient change in the voltage across the membrane – an action
potential. These action potentials are often called “spikes” as they can be detected
as a spike in the measured voltage lasting only about a millisecond.
Although the action potential itself is the fundamental unit of neural activity,
our perceptions, thoughts, and actions emerge from how action potentials form
and travel through the complex network of neurons that make up our nervous
systems. Action potentials travel within and between neurons in a very specific unidirectional
manner. Each neuron is made up of four parts (depicted in Fig. 1) –the
dendrites, the cell body, the axon, and the synaptic terminal. Neural signals flow
through each neuron in that order.
1) The dendrites are thin branching structures that can receive signals or synaptic
inputs from many other neurons or from sensory receptors. Some synaptic
inputs will work to excite or increase the probability that an action potential will
be generated in the receiving neuron. Synaptic inputs from other neurons will
inhibit or reduce the likelihood of an action potential taking place.
2) The large cell body or soma contains the structures common to all cells, such
as a nucleus as well as the machinery needed to make proteins and to process
other molecules essential for the cell’s survival. Most importantly, the cell
Fig. 1 The four main parts of
a neuron. The dendrites
receive synaptic inputs (“yes”
or “no” votes) from upstream
neurons. The large cell body
or soma combines the
synaptic inputs and initiates
an action potential if enough
“yes” votes are received. The
action potential travels down
the axon (sometimes long
distances) to the synaptic
terminal. The action potential
triggers neurotransmitters to
be releases from the synaptic
terminal. These
neurotransmitters become the
“yes” or “no” votes to the
dendrites of the next neurons
in the network. Arrows
indicate the direction in
which the electrical signals
travel in a neuron