Final answer:
The key ion initiating electrical signals in neurons is the sodium ion (Na+), which causes depolarization and triggers an action potential when the neuron receives sufficient stimulus. Subsequently, calcium ions (Ca++) play a critical role at the synapse in neurotransmitter release, and potassium ions (K+) restore the resting potential after signal transmission.
Step-by-step explanation:
The ion that plays a key role in initiating electrical signals in neurons is the sodium ion (Na+). When a neuron receives a strong enough signal, the sodium ion channels on its membrane open, allowing the influx of Na+ into the cell. This influx causes the cell to depolarize, reducing the voltage difference across the cell membrane. This change in electrical potential (depolarization to around -60mV) can bring the neuron to a threshold that triggers an action potential, a self-propagating reversal of the resting membrane potential. The action potential then travels down the neuron's axon towards the synapse, where neurotransmitters are released to continue signal transmission to adjacent neurons.
At the synapse, the arrival of the action potential stimulates Ca++ voltage-gated channels to open, allowing calcium ions (Ca++) to enter the presynaptic cell. The entry of Ca++ triggers the release of neurotransmitters into the synaptic cleft, enabling chemical synaptic communication with the postsynaptic neuron. Following the release of neurotransmitters and action potential, potassium ion (K+) pumps help to restore the resting potential by moving K+ out of the neuron, thereby terminating the action potential and resetting the neuron for another signal.