Neurosecretory cells spatially redistribute their pool of secretory vesicles upon stimulation. Recent observations suggest that in chromaffin cells vesicles move either freely or in a directed fashion by what appears to be a conveyor belt mechanism. We suggest that this observation reflects the transient active transport through molecular motors along cytoskeleton fibres and quantify this effect using a 1D mathematical model that couples a diffusion equation to advection equations. In agreement with recent observations the model predicts that random motion dominates towards the cell centre whereas directed motion prevails in the region abutting the cortical membrane. Furthermore the model explains the observed bias of directed transport towards the periphery upon stimulation. Our model suggests that even if vesicle transport is indifferent with respect to direction, stimulation creates a gradient of free vesicles at first and this triggers the bias of transport in forward direction. Using matched asymptotic expansion we derive an approximate drift-diffusion type model that is capable of quantifying this effect. Based on this model we compute the characteristic time for the system to adapt to stimulation and we identify a Michaelis–Menten-type law describing the flux of vesicles entering the pathway to exocytosis.

神经分泌细胞在刺激后在空间上重新分布其分泌小泡池。最近的观察表明，在嗜铬细胞中，囊泡通过似乎是传送带机制自由地或以定向方式运动。我们建议，该观察结果反映沿分子骨架纤维通过分子马达的瞬时主动传输，并使用将扩散方程与对流方程耦合的一维数学模型来量化此效应。与最近的观察结果一致，该模型预测随机运动在朝向细胞中心的方向上占优势，而定向运动在与皮质膜邻接的区域中占主导地位。此外，该模型解释了在刺激后观察到的定向运输向外围的偏向。我们的模型表明，即使囊泡的运输相对于方向无关紧要，刺激也会首先产生自由囊泡的梯度，这会触发向前运输的偏向。使用匹配的渐近展开，我们得出了能够量化这种影响的近似漂移扩散类型模型。基于该模型，我们计算了系统适应刺激的特征时间，并确定了描述囊泡进入胞吐途径的通量的米歇尔-门腾（Michaelis-Menten）型定律。