Electrical pulses fired by a postsynaptic neuron are called action potentials or spikes. These action potentials move away from the cell, travelling along the cell’s axon until it reaches the axon’s branches. At this point, it travels to the axon terminals. The axon terminal Is where presynaptic neurons release the neurotransmitter message to other neurons.
The synapse is one of the most important concepts in neuroscience. It is the space between the post-synaptic dendrite receiving the neurotransmitter and the presynaptic axon producing the neurotransmitter to be received. The space between the postsynaptic neuron and the presynaptic neuron is known as the synaptic cleft. It is this cleft with the neurotransmitter travels across when going from the presynaptic to the postsynaptic cleft.
Other neurons receive input from environmental energy rather than from neurotransmitter input. For example, photoreceptors, existing in the eye, convert photos into neurotransmitters. Likewise, auditory hair cells in the ear bound when sound pressure is received in the inner ear. Somatosnsory receptors serve a similar purpose in response to impact on the skin. The nose and tongue have similar sensory neurons serving the specific purpose of their senses.
In addition to these inputs, the nervous system possesses outputs. One example is the motor system, which possesses motor neurons. These neurons send neurotransmitters to muscle cells which then contract when received. On the outside of the membrane is a binding site for neurotransmitters. Neurotransmitters become bound to it, causing receptor protein to refold. This refolding opens up a pore in the receptor complex so that this or that ion can enter the cell membrane.
This is important because these cell membranes are otherwise impermeable. Such receptors are known as “neurotransmitter-gated” receptors. Not all receptors are like this, however. When the receptor structure is in the same binding site as the ion pore, it is known as an “ionotropic” receptor. It is typically excitatory and inhibitory receptors for which this holds true.
Many neuromodulators are different. The ion channel pore is in a different place for the neurotransmitter’s binding site. On the outside of the membrane, there is a neurotransmitter changes the shape of part of the channel protein on the membrane’s interior so that it produces neural modulator that is intracellular and internal. Such modulators are internal messengers in the cell’s interior whose purpose is to amplify the actions of biochemical cascades. When neurotransmitters bind to such receptors, they can alter DNA expression in the nucleus of the cell or open up nearby channel pores. Such receptor types are called metabotropic receptors, and their effects are longer lasting but slower than ionotropic receptors.
Amthor, Frank (2011-11-09). Neuroscience For Dummies (Kindle Locations 1434-1436). Wiley. Kindle Edition.