Cellular Communication: Introduction

The essence of cell signaling is that a regulatory molecule (ligand) binds to its specific receptor on a target cell, and this binding of ligand to receptor leads to changes in the activity of the target cell. The figure below schematizes different modes of cell signaling.

Different modes of cellular communication can be distinguished based on the distance that a regulatory molecule needs to travel to reach its target. Neurons are directly connected to their target cells via synapses. A neurotransmitter crossing the small space in the synapse only travels about 20 nanometers. Paracrines (commonly used in immune system signaling) are rapidly broken down, so they can only travel a few millimeters, and work on neighboring cells. By contrast, hormones (endocrine regulatory molecules) travel through blood vessels to reach their targets, which may include multiple tissues that are distant from each other. Thus hormones are much more likely to have systemic effects. Note that the same signaling molecule may work in more than one way. For instance, norepinephrine is a neurotransmitter, but it acts as a hormone when released from the adrenal medulla because then it gets to its targets via the circulation.

Another key difference between neuronal signaling and the two other types of signaling is in the timing of signaling.  Neuronal signaling is the most rapid type of signaling.  Fast synaptic transmission occurs when the neurotransmitter receptors at a synapse are ligand-gated ion channels. Neurotransmitter binding to the receptor opens or closes the channel, allowing ions to flow across the membrane and depolarize or hyperpolarize the cell, a process which takes less than a few milliseconds. There is also slow synaptic transmission, in which the neurotransmitter receptors are G-protein coupled receptors (see web page). There receptors require a few more steps to open ion channels, so that the the resulting signal takes longer (roughly a second).

Likewise, for paracrine and endocrine signaling, regulatory molecules may be working through G-protein coupled receptors or other receptors that have a more complicated process of signal transduction. In some cases, the result of signaling will ultimately involve changes in gene expression. Thus, depending on the type of receptor, endocrine or paracrine signaling may last some seconds, minutes, or even hours.