Final answer:
The cell in the pathway that experiences spontaneous graded potentials and triggers the release of excitatory neurotransmitters is the sensory neuron, due to the generator potentials that can directly initiate action potentials.
Step-by-step explanation:
The next cell in the pathway that displays spontaneous, graded potentials leading to the release of excitatory neurotransmitters is a sensory neuron.
Graded potentials are variations in membrane potential that can initiate an action potential if they are strong enough. In the context of sensory neurons, these are often referred to as generator potentials. Generator potentials occur in the dendrites of unipolar sensory neurons and can directly generate action potentials in the axon of the cell. When a sensory stimulus, such as temperature, is strong enough, it can cause the membrane potential to reach a threshold that triggers an action potential at the initial segment of the axon, which is immediately adjacent to the sensory endings. This action potential then travels along the sensory pathway, ultimately leading to a response. An excitatory postsynaptic potential (EPSP) is a depolarizing graded potential at the synapse that can contribute to reaching the threshold for action potential initiation, and is facilitated by the high density of voltage-gated Na+ channels located at the initial segment of the axon or the axon hillock.