Final answer:
A nerve impulse is transmitted across a synapse via neurotransmitters released from the presynaptic cell, which then bind to receptors on the postsynaptic cell.
Step-by-step explanation:
A nerve impulse is transmitted across a synapse from a presynaptic cell to a postsynaptic cell primarily via neurotransmitters. In this process, the presynaptic cell releases neurotransmitters into the synaptic cleft upon the arrival of an action potential. These molecules then travel across the cleft and bind to receptors on the postsynaptic cell, enabling the transmission of the impulse. In essence, the binding of neurotransmitters to the postsynaptic membrane allows the nerve impulse to propagate through the receiving neuron.
This process begins with the depolarization of the presynaptic membrane, triggering voltage-gated Ca2+ channels to open. Calcium ions flowing into the cell then initiate a cascade that leads to the fusion of synaptic vesicles with the presynaptic membrane and the subsequent release of neurotransmitters. It is through the action of neurotransmitters, rather than a direct connection, a spark, or solely via Na+ or Ca2+ ions, that the signal is transferred to the postsynaptic cell.