Regulation Of Neurotransmission

The level of control exerted by either division of the ANS at any instant depends largely on the concentration of transmitter available for binding to the receptors located on the effector. Axons of sympathetic postgan-glionic neurons release norepinephrine from varicosities located at their terminals. These terminal regions resemble strings of pearls in that the varicosities are pearl-like expansions distributed along the axon. A single varicosity is represented in Fig. 4. Several factors

Synthesis

Tyrosine

Synthesis

Tyrosine

FIGURE 4 Synthesis, release, and reuptake of norepinephrine at sympathetic (adrenergic) nerve varicosities. AAD, aromatic L-amino acid decarboxylase; DbH, dopamine ^-hydroxylase; NE, norepinephrine; Tyr Hyd, tyrosine hydroxylase. (Modified from Vanhouette PM, Fed. Proc. 1978, 37:181-186.)

affect the temporal release of norepinephrine from varicosities. The frequency of action potentials arriving at a varicosity is a primary determinant of the flux of calcium ions into that varicosity. This influx of calcium in turn controls the vesicular release of norepinephrine from the varicosity. The amount of norepinephrine available for release depends on its synthesis and on its reuptake by the varicosity. The steps in the synthesis pathway are illustrated in Fig. 4. It should be noted that dopamine is converted into norepinephrine by the enzyme dopamine ^-hydroxylase. This enzyme is also packaged into the norepinephrine-containing vesicles of the varicosities and is released along with norepi-nephrine into the extracellular space. Because dopamine ^-hydroxylase is not degraded as rapidly as norepinephrine, it is often used as an indicator of the level of overall sympathetic activity.

The amount of norepinephrine released from a varicosity in response to a specific pattern of axonal action potentials is modulated by receptors located on that varicosity. A varicosity having presynaptic receptors located on its surface is illustrated in Fig. 5. A variety of compounds, such as histamine, 5-hydroxy-tryptamine, and acetylcholine, may act to decrease the amount of norepinephrine released by a train of action potentials, whereas other substances such as angiotensin II may augment its release. The concentration of norepinephrine within the junctional cleft may itself modulate release of norepinephrine. Varicosities with autoreceptors (a2-adrenergic receptors) inhibit the release of norepinephrine when these receptors bind with norepinephrine. This negative feedback mechanism tends to conserve norepinephrine and stabilize its concentration within the cleft.

In addition to the rate of norepinephrine release, the concentration of norepinephrine available for binding to receptors is modulated by several other factors, which are illustrated in Fig. 6. These include neuronal uptake, extraneuronal uptake and subsequent degradation by some effector cells, and diffusion into capillaries.

Smooth Muscle Cells

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