Prolongation of a Signal by a Neuronal Pool—“Afterdischarge”
Thus far, we have considered signals that are merely relayed through neuronal pools. However, in many instances, a signal entering a pool causes a prolonged output discharge, called afterdischarge, lasting a few milliseconds to as long as many minutes after the incoming signal is over. The most important mechanisms by which afterdischarge occurs are the following.
Synaptic Afterdischarge. When excitatory synapses dis-charge on the surfaces of dendrites or soma of a neuron, a postsynaptic electrical potential develops in the neuron and lasts for many milliseconds, especially when some of the long-acting synaptic transmitter sub-stances are involved. As long as this potential lasts, it can continue to excite the neuron, causing it to trans-mit a continuous train of output impulses. Thus, as a result of this synap-tic “afterdischarge” mechanism alone, it is possible for a single instantaneous input signal to cause a sustained signal output (a series of repetitive discharges) lasting for many milliseconds.
Reverberatory (Oscillatory) Circuit as a Cause of Signal Prolon- gation. One of the most important of all circuits in theentire nervous system is thereverberatory, or oscilla-tory, circuit. Such circuits are caused by positive feed-back within the neuronal circuit that feeds back to re-excite the input of the same circuit. Consequently, once stimulated, the circuit may discharge repetitively for a long time.
Several possible varieties of reverberatory circuits are shown in Figure 46–14. The simplest, shown in Figure 46–14A, involves only a single neuron. In this case, the output neuron simply sends a collateral nerve fiber back to its own dendrites or soma to restimulate itself. Although this type of circuit probably is not an important one, theoretically, once the neuron dis-charges, the feedback stimuli could keep the neuron discharging for a protracted time thereafter.
Figure 46–14B shows a few additional neurons in the feedback circuit, which causes a longer delay between initial discharge and the feedback signal. Figure 46–14C shows a still more complex system in which both facilitatory and inhibitory fibers impinge on the reverberating circuit.A facilitatory signal enhances the intensity and frequency of reverberation, whereas an inhibitory signal depresses or stops the reverberation.
Figure 46–14D shows that most reverberating path-ways are constituted of many parallel fibers. At each cell station, the terminal fibrils spread widely. In such a system, the total reverberating signal can be either weak or strong, depending on how many parallel nerve fibers are momentarily involved in the reverberation.
Characteristics of Signal Prolongation from a Rever- beratory Circuit. Figure 46–15 shows output signalsfrom a typical reverberatory circuit. The input stimu-lus may last only 1 millisecond or so, and yet the output can last for many milliseconds or even minutes. The figure demonstrates that the intensity of the output signal usually increases to a high value early in rever-beration and then decreases to a critical point, at which it suddenly ceases entirely. The cause of this sudden cessation of reverberation is fatigue of synaptic junc-tions in the circuit. Fatigue beyond a certain critical level lowers the stimulation of the next neuron in the circuit below threshold level so that the circuit feed-back is suddenly broken.
The duration of the total signal before cessation can also be controlled by signals from other parts of the brain that inhibit or facilitate the circuit. Almost these exact patterns of output signals are recorded from the motor nerves exciting a muscle involved in a flexor reflex after pain stimulation of the foot (as shown later in Figure 46–18).
Continuous Signal Output from Some Neuronal Circuits Some neuronal circuits emit output signals continu-ously, even without excitatory input signals. At least two mechanisms can cause this effect: (1) continuous intrinsic neuronal discharge and (2) continuous rever-beratory signals.
Continuous Discharge Caused by Intrinsic Neuronal Excitability. Neurons, like other excitable tissues, dischargerepetitively if their level of excitatory membrane potential rises above a certain threshold level. The membrane potentials of many neurons even normally are high enough to cause them to emit impulses con-tinually. This occurs especially in many of the neurons of the cerebellum, as well as in most of the interneu-rons of the spinal cord. The rates at which these cells emit impulses can be increased by excitatory signals or decreased by inhibitory signals; inhibitory signals often can decrease the rate of firing to zero.
Continuous Signals Emitted from Reverberating Circuits as a Means for Transmitting Information. A reverberatingcircuit that does not fatigue enough to stop reverber-ation is a source of continuous impulses. And excita-tory impulses entering the reverberating pool can increase the output signal, whereas inhibition can decrease or even extinguish the signal.
Figure 46–16 shows a continuous output signal from a pool of neurons. The pool may be emitting impulses because of intrinsic neuronal excitability or as a result
of reverberation. Note that an excitatory input signal greatly increases the output signal, whereas an inhibitory input signal greatly decreases the output. Those students who are familiar with radio trans-mitters will recognize this to be a carrier wave type of information transmission. That is, the excitatory and inhibitory control signals are not the cause of the output signal, but they docontrol its changing level of intensity. Note that this carrier wave system allows a decrease in signal intensity as well as an increase,whereas up to this point, the types of information transmission we have discussed have been mainly pos-itive information rather than negative information. This type of information transmission is used by the autonomic nervous system to control such functions as vascular tone, gut tone, degree of constriction of the iris in the eye, and heart rate. That is, the nerve exci-tatory signal to each of these can be either increased or decreased by accessory input signals into the rever-berating neuronal pathway.
Rhythmical Signal Output
Many neuronal circuits emit rhythmical output signals—for instance, a rhythmical respiratory signal originates in the respiratory centers of the medulla and pons. This respiratory rhythmical signal continues throughout life. Other rhythmical signals, such as those that cause scratching movements by the hind leg of a dog or the walking movements of any animal, require input stimuli into the respective circuits to initiate the rhythmical signals.
All or almost all rhythmical signals that have been studied experimentally have been found to result from reverberating circuits or a succession of sequential reverberating circuits that feed excitatory or inhibitory signals in a circular pathway from one neuronal pool to the next.
Excitatory or inhibitory signals can also increase or decrease the amplitude of the rhythmical signal output. Figure 46–17, for instance, shows changes in the respiratory signal output in the phrenic nerve. When the carotid body is stimulated by arterial oxygen deficiency, both the frequency and the amplitude of the respiratory rhythmical output signal increase progressively.
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