Final answer:
Graded potentials are primarily generated by ligand-gated channels, and also by leak channels, when neurotransmitters bind to receptors causing ionic flow across the neuron's membrane.
Step-by-step explanation:
The channels that contribute to graded potentials are primarily ligand-gated channels and to some extent leak channels. Voltage-gated channels are more involved in generating action potentials rather than graded potentials. When a neurotransmitter binds to a receptor, it often results in the opening of ligand-gated channels. These channels allow ions to flow into or out of the neuron, creating a graded potential such as depolarization or hyperpolarization depending on the type of ions that move across the membrane.
For example, when a channel opens on a postsynaptic membrane causing a negative ion to enter the cell, it can lead to a hyperpolarizing graded potential. Conversely, when a channel allows positive ions like Na+ to enter, it can cause depolarization. Graded potentials are localized changes in membrane potential and can summate to reach the threshold potential that initiates an action potential. They are critical in the process of neurotransmission, influencing whether or not a neuron will fire an action potential.