Final answer:
A neurotransmitter is a chemical released by the axon terminal of a neuron when a nerve impulse arrives. It travels across the synapse, binding to receptors on the next neuron to propagate the impulse. Afterwards, it is degraded or reabsorbed by the presynaptic neuron.
Step-by-step explanation:
The chemical substance you are referring to is called a neurotransmitter. It is a type of chemical that carries nerve impulses from one neuron to another across a synapse, the small gap between neurons. When a nerve impulse reaches the end of an axon, the axon terminal releases neurotransmitters. These chemicals travel across the synapse and bind to receptors on the dendrite of the receiving neuron.
This binding alters the electrochemical potential of the postsynaptic cell, thereby launching the next electrical impulse. Neurotransmitters can either be excitatory, helping to trigger the next impulse, or inhibitory, reducing the likelihood of an impulse being triggered. Once the neurotransmitters have fulfilled their function, they are quickly degraded or taken back up by the presynaptic cell to ensure the system is ready for the next nerve impulse.