Channel blockers

Channels are not receptors. 

If we use the analogy of a receptor being a door with the ligand as a key, a channel is more like a tunnel with a gate.   

Channels are found in all cell membranes.  They are tiny.  The archetypal channel pore is just one or two atoms wide at its narrowest point and is selective, accepting only one type of ion, such as sodium, calcium or potassium. These ions move through the channel pore single file nearly as quickly as the ions move through free fluid.

In some ion channels, passage through the pore is governed by a "gate," which may be opened or closed by chemical or electrical signals, temperature, or mechanical force, depending on the type of channel.  In effect, therefore the overall action is not dissimilar to the receptor, channels have doors or ‘gates’ and can only be opened by the right key – the right ligand.  So in principle the action of ligands and functions still holds good.  The door or gate regulates how much goes in and out.  Gates and doors to perception!

“Voltage-activated" channels underlie the nerve impulse and are especially prominent components of the nervous system.

Most of the offensive and defensive toxins that organisms have evolved for shutting down the nervous systems of predators and prey, for example, the venoms produced by spiders, scorpions, snakes, fish, bees, sea snails and others,   work by modulating ion channel conductance and/or kinetics.

In addition, ion channels are key components in a wide variety of biological processes that involve rapid changes in cells, such as cardiac, skeletal, and smooth muscle contraction, epithelial nutrient transport {epithelial cells are tissue cells that perform functions like  secretion, selective absorption, protection, transcellular transport and detection of sensation), T-cell activation (our system of immunity)  and pancreatic beta-cell insulin release.

In the search for new drugs, ion channels are a frequent target.