Graded cells and Spiking cells

Some cells, particularly in the retina, are graded cells. When you monitor a graded cell, you will not hear anything because the cells do not spike.

Graded cells communicate only over short distances and do not require a spike mechanism. They synapse to other cells either by a number of mechanisms:

  1. Chemical synapses that continually release neurotransmitter-containing vesicles depending on the membrane potential. Chemical synapses synapse between photoreceptors and bipolar cells, for example.
  2. Gap junctions. These are ohmic connections that connect the cells' membrane potentials directly. Horizontal cells are connected to each other by gap junctions, so they form a continuous resistive sheet.