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:
- Chemical synapses that continually release neurotransmitter-containing
vesicles depending on the membrane potential. Chemical synapses synapse
between photoreceptors and bipolar cells, for example.
- 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.