Biomedical Engineering Reference
In-Depth Information
the K
1
eux. Membrane repolarization closes the Ca
21
channel. The K
1
current gradually decays as Ca
21
is pumped out of the cytoplasm. Provided
that the neuron is receiving tonic excitatory input from some glutamatergic
source, the NMDA receptor will re-open after the hyperpolarization has
dissipated. The NMDA receptor transforms a steady excitatory drive into an
alternating rhythm of bursts of spikes separated by silent periods of hyper-
polarization, a very useful feature of pattern generators. The rhythm of NMDA
oscillators is in the range of 0.1-3 Hz.
28,29
Simple pacemaker neurons cannot produce complex rhythmic patterns of
activity. Synchronized oscillations can be created either by neurons sharing or
distributing the timing functions among themselves through mutually exciting
or inhibiting one another via synaptic connections. When large numbers of
neurons are involved and phase relationship requisites are met, the cyclic
behavior is best produced by an interactive network of neurons, the network
oscillators. Breathing, locomotion and other rhythms are generated by reticular
networks that allow for changes in relative timing and in phase magnitude with
respect to the other constituent neurons of that particular network. The basic
design of an oscillatory network involves a diffuse excitatory input from an
extraneous source so that the discharge is automatic when neurons are not
being inhibited. Such inhibitory connections among the network of neurons
determine the order and timing of the activity patterns.
Oscillating networks generally follow a ring structure with directed exci-
tatory and inhibitory interconnections. Each node or group of neurons
functions as a burst generator that drives a particular phase of the movement
cycle and tends to inhibit its immediate predecessors. It is the retrograde
inhibitions that are the most important and constant feature of the ring. A ring
cycles freely in the direction counter to the inhibitory connections. Because the
entire network is subjected to diffuse excitation, firing in any node is mainly due
to the spontaneous depolarization following the removal of the inhibition.
The rate of cycling is therefore dependent on the strength and duration of the
inhibitory effects. The stronger and longer lasting the inhibition, the slower is the
cycle frequency. The coupling of individual oscillators can occur in several ways.
Phase coupling entrains the start of a cycle in one oscillator to a specific phase of
another so that a travelling wave of activity will pass rhythmically. Relaxation
coupling is the manner in which a group of oscillators become abruptly
synchronized. Their individual cycles are concurrently reset by the simultaneous
inhibition of all the individual oscillators. When the inhibition subsides, all of
the oscillators resume their synchronous discharge. This is the mechanism
responsible for synchronizing populations of thalamic neurons that project to the
cerebral cortex. Synaptic connections between excitatory and inhibitory thalamic
neurons force each individual neuron to conform to the rhythm of the group.
These coordinated rhythms are then passed to the cortex by the thalamocortical
axons, which excite cortical neurons. This is how a small group of centralized
thalamic cells can impose synchronicity to a much larger group of cortical cells.
Membrane potential oscillations are always entrained throughout popu-
lations that are joined by gap junctions. Such oscillations occur in many
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