Negative transmittance

FIG. 19 Light transmittance ( = 650 mm) of alternating mnltUayer of particles lb and 2 vs. number of deposited particle layers. (+) or (—) indicate that positively or negatively charged particles, respectively, were deposited in the last adsorption step (substrate glass modified with 3-AMDS and subsequently acidified). (From Ref. 156.)... 

In quadrature detection, the transmitter offset frequency is posidoned at the center of the F domain (i.e., at F2 = 0 in single-channel detection it is positioned at the left edge). Frequencies to the left (or downfield) of the transmitter offset frequency are positive those to the right (or upheld) of it are negative. 

The exact process(es) by which a2-adrenoceptors blunt release of transmitter from the terminals is still controversial but a reduction in the synthesis of the second messenger, cAMP, contributes to this process. a2-Adrenoceptors are negatively coupled to adenylyl cyclase, through a Pertussis toxin-sensitive Gi-like protein, and so their activation will reduce the cAMP production which is vital for several stages of the chemical cascade that culminates in vesicular exocytosis (see Chapter 4). The reduction in cAMP also indirectly reduces Ca + influx into the terminal and increases K+ conductance, thereby reducing neuronal excitability (reviewed by Starke 1987). Whichever of these releasecontrolling processes predominates is uncertain but it is likely that their relative importance depends on the type (or location) of the neuron. 

Unlike other transmitter systems, there are no obvious meehanisms for dampening glutamate release. Presynaptic autoreceptors for glutamate are mostly of the kainate type (see below) and appear to act as positive rather than negative influenees on further release of the amino acid. Although poorly characterised at present, inhibitory autoreceptors of the metabotropic type of receptors may act to inhibit release of glutamate. 

Figure 9.2 illustrates a typical example of normalized transmittance, T(z), of CdTe QDs against the sample position z from the focusing point vdth and vithout aperture [17]. Since the peak ofthe normalized transmittance for the closed aperture precedes the valley, the sign ofthe nonlinear refractive index of CdTe QDs is negative. 

To consider pH as a controlled variable, we use a pH electrode to measure its value and, with a transmitter, send the signal to a controller, which can be a little black box or a computer. The controller takes in the pH value and compares it with the desired pH, what we call the set point or reference. If the values are not the same, there is an error, and the controller makes proper adjustments by manipulating the acid or the base pump—the actuator.2 The adjustment is based on calculations using a control algorithm, also called the control law. The error is calculated at the summing point where we take the desired pH minus the measured pH. Because of how we calculate the error, this is a negative feedback mechanism. 

It is even possible for an individual noise pulse to exceed — ET so that a negative reading of Ex will be obtained. This happens in the real world and therefore must be taken into account in the mathematical description. This is a good place to also note that since the transmittance of a physical sample must be between zero and unity, Es must be no greater than If, and therefore when If is small an individual reading of Es can also be negative. Therefore it is entirely possible for an individual computed value of T to be negative. 

The postsynaptic receptors on any given neuron receive information from transmitters released from another neuron. Typically, postsynaptic receptors are located on dendrites or cell bodies of neurons, but may also occur on axons or nerve terminals in the latter case, an axoaxonic synaptic relationship may cause increases or decreases in transmitter release. In contrast, autoreceptors are found on certain neurons and respond to transmitter molecules released from the same neuron. Autoreceptors may be widely distributed on the surface of the neuron. At the nerve terminal, they respond to transmitter molecules released into the synaptic cleft on the cell body, they may respond to transmitter molecules released by dendrites. Functionally, most autoreceptors appear to decrease further transmitter release in a kind of negative feedback loop. Autoreceptors have been identified for all the catecholamines, as well as for several other neurotransmitters. a2-adrenergic receptors are often found on noradrenergic nerve terminals of postganglionic sympathetic nerves, as well as on noradrenergic neurons in the CNS [36], and activation of these receptors decreases further norepinephrine release. Dopamine autoreceptors,... 

The transmittance of the structure A depending on the input power P was evaluated via calculation of Tfz) (Fig.l3) and T2 z) (Fig.l4). It is seen that self-focusing of the light beam in the core of nonlinear waveguide increases with input power, but rate of the increase diminishes so that for the powers P > 7 (a = 1.8 pm) and P > 4 (a = 3.0 pm) the dependence is weak. Negative slope of the curve in this range results from the mentioned above soliton-like... 

Ionotropic receptors are ligand-gated ion channels (left half of the table). The receptors for stimulatory transmitters (indicated in the table by a ) mediate the inflow of cations (mainly Na""). When these open after binding of the transmitter, local depolarization of the postsynaptic membrane occurs. By contrast, inhibitory neurotransmitters (GABA and glycine) allow cr to flow in. This increases the membrane s negative resting potential and hinders the action of stimulatory transmitters hyperpolarization, 0). 

Symbolized by D, the transmission density is the negative decadic logarithm of the transmittance thus, D = log... 

Presynaptic or prejunctional receptors are located on the presynaptic nerve endings and function to control the amount of transmitter released per nerve impulse and in some instances to affect the rate of transmitter synthesis through some as yet undetermined feedback mechanism. For instance, during repetitive nerve stimulation, when the concentration of transmitter released into the synaptic or junctional cleft is relatively high, the released transmitter may activate presynaptic receptors and thereby reduce the further release of transmitter. Such an action may prevent excessive and prolonged stimulation of the postsynaptic cell. In this case, the activation of the presynaptic receptor would be part of a negative feedback mechanism... 

The nervous system has several properties in common with the endocrine system, which is the other major system for control of body function. These include high-level integration in the brain, the ability to influence processes in distant regions of the body, and extensive use of negative feedback. Both systems use chemicals for the transmission of information. In the nervous system, chemical transmission occurs between nerve cells and between nerve cells and their effector cells. Chemical transmission takes place through the release of small amounts of transmitter substances from the nerve terminals into the synaptic cleft. The transmitter crosses the cleft by diffusion and activates or inhibits the postsynaptic cell by binding to a specialized receptor molecule. In a few cases, retrograde transmission may occur from the postsynaptic cell to the presynaptic neuron terminal. 

The principle of negative feedback control is also found at the presynaptic level of autonomic function. Important presynaptic feedback inhibitory control mechanisms have been shown to exist at most nerve endings. A well-documented mechanism involves an 2 receptor located on noradrenergic nerve terminals. This receptor is activated by norepinephrine and similar molecules activation diminishes further release of norepinephrine from these nerve endings (Table 6-4). Conversely, a presynaptic Breceptor appears to facilitate the release of norepinephrine. Presynaptic receptors that respond to the transmitter substances released by the nerve ending are called autoreceptors. Autoreceptors are usually inhibitory, but many cholinergic fibers, especially somatic motor fibers, have excitatory nicotinic autoreceptors. 

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