Biomedical Engineering Reference
In-Depth Information
E-cell
E-cell
c e
1
2
>3.2nS
1
2
<3.2nS
Figure 7.2: All-or-none synaptic transmission of bursting.
7.2.2 Suppression of Neuronal Firing
The bursting in the second neuron may be inhibited by an interneuron. Figure 7.3 shows the case where
Pyramidal cell 1 will tend to cause Pyramidal cell 2 to fire (e.g., c e > 3 . 2 nS ) in the absence of inhibition.
Interneuron cell 3 (closed circle), on the other hand, will tend to prevent Pyramidal cell 2 from firing.
Here it is the strength of the inhibitory connection ( c i , closed triangle) that determines whether cell 2
will fire or not. For example, if c i > 5 nS the strength of the inhibitory connection will prevent an action
potential from firing in cell 2. If c i < 5 nS , the inhibitory connection is not strong enough to prevent an
action potential in cell 2, but it may delay when cell 2 fires.
7.2.3 The Importance of Delays
The impact of an inhibitory cell is governed not only by the strength of the connection, but also the
timing. Figure 7.4 show a similar three cell network as Fig. 7.3, but c e and c i have been fixed so that cell
2 does not fire. The timing of the interneuron action potential, however, was delayed relative to the firing
of Pyramidal cell 1. As this delay becomes large, Pyramidal cell 1 has already committed to bursting and
the late inhibition has little effect.
7.2.4 Cyclic Activity in the Brain
The brain is known to generate a number of rhythms that vary depending upon the waking state and
mental tasks being performed. Although the function of these rhythms is not known, computational and
theoretical work have demonstrated how the brain may use the same neural circuits to generate different
rhythms. Typically, these circuits consist of tens to thousands, or even millions of cells. As a simple
demonstration, consider the left-hand panel of Fig. 7.5. The single pulse may activate Cell 1, which will
activate Cell 2 and on to Cell 3. If the delay for passing the electrical impulse through the network is long
 
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