Biomedical Engineering Reference
In-Depth Information
amplitudes. This introduces the possibility that perception and temporal
representation might be the result of such synchronized network activity that
spans over five ranges of magnitude in frequency. Oscillations and oscillatory
synchronicity are the most energy-ecient mechanisms through which
information is conveyed. The non-dissipating solitary wave is an example of
such a mechanism. Acoustic-electromagnetic (optical, conformational,
rotational, oscillating) solutions are the fundamental components of quantum
wave matrices in living molecules. How the brain stores patterns and generates
creativity using so little energy might be explained by such an energy-effective
mechanism.
Brain oscillations appear to be the result of a finely tuned and optimized
interplay between the intense intracellular activity and the dynamic
properties of neural networks and circuits. Neurons interconnected in neuronal
networks and circuits behave surprisingly like miniature electrical oscillating
circuits.
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Resonance, oscillation and the intrinsic frequency preferences of
neurons are due to their intrinsic resistivity, membrane capacity and the
conductance of ionic channels which all decide the resonant behavior.
Figure 5.14 shows an analogy between classical electronic components forming
an electric resonator and the neuron. An impedance amplitude profile using
input complex signals containing all the frequencies of interest shows, by fast
Fourier transform (FFT), that the neuron, like a miniature electronic circuit,
can discriminate between input frequencies and intrinsically select preferred
frequencies.
Rhythmic oscillations are a basic feature of the membrane potentials found
in spontaneously active neuronal cells or neuron networks. These cells or
networks generate the patterns responsible for walking, breathing, chewing and
other rhythmic movements. There are several mechanisms through which
oscillatory activity can be produced. These include interactions among ion
channels, inhibitory interactions among neurons in cyclic networks, cascades of
metabolic reactions and/or cyclic transcriptions of genes. These mechanisms
each operate with different time periods. For example, one such mechanism is
established by the action of the hyperpolarization-activated current, a cationic
current critical for the neuronal pacemaker activity. It is a slowly developing
inward current (depolarizing) activated by hyperpolarization of the membrane
beyond the resting potential and produced by a mixed Na
1
/K
1
conductance.
The time constant of current activation varies from 1-2 s at close to rest
potential to 100-400 ms at maximum hyperpolarization. An increase in intra-
cellular Ca
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regulates the current such that it operates at more depolarized
membrane potentials.
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Another example of the molecular underpinnings of oscillatory behavior is
related to the action of a specific glutamatergic receptor. In pacemaker neurons,
the NMDA glutamate receptor generates the pacemaker rhythms and provides
a mechanism for synaptic plasticity. NMDA is an amino acid derivative that
acts as a specific agonist to the NMDA receptor and therefore mimics the
action of glutamate on that receptor. In contrast to glutamate, NMDA binds to
and regulates the above receptor only and does not bind to any other glutamate
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