Synaptic Transmission
The human brain contains billions of individual cells called neurons. Each neuron communicates with other neurons forming trillions of connections. This lab activity shows you the process by which neurons communicate with each other by focusing on an example of just two nearby neurons. Before the invention of powerful microscopes, it was thought that the brain contained a single continuous neural net, but now it is clear that there are individual neurons separated by small gaps called synapses. This raises the obvious question that if neurons are not physically connected to each other, how do they communicate?
Neural Transmission Is Electrical and Chemical
The first movie in this lab activity shows two neurons next to each other. The neuron on the left is called the presynaptic neuron and the neuron on the right is called the postsynaptic neuron. Neurons transmit chemicals called neurotransmitters across gaps called synapses. The presynaptic and postsynaptic labels are used to indicate that the presynaptic neuron is in the position to release neurotransmitter, while the postsynaptic neuron is in a position to accept the neurotransmitter and generate an electrical signal. When these chemical neurotransmitters bind to the neuron on the other side of the synapse, an electrical signal is generated. In a simple case, the more neurotransmitter that makes it across the synapse, the greater the electrical signal. When the signal passes a threshold, an action potential consisting of a large voltage change is generated and transmitted down the length of the neuron’s axon to begin the process of chemical release all over again. Therefore, every neuron has the potential to play the role of a presynaptic and postsynaptic neuron.
Neurotransmitter Release and Binding
The second movie shows a close-up view of the synapse with the presynaptic neuron on the left and the postsynaptic neuron on the right. Inside the presynaptic neuron are shown little round sacks called vesicles that contain small amounts of neurotransmitter. These vesicles dock to terminals at the end of the presynaptic neuron and release the neurotransmitter into the gap between the neurons, which is called the synaptic cleft. Receptors are shown embedded into the dendrites of the postsynaptic neuron. The neurotransmitter makes its way across the synaptic cleft and docks to the receptors. This docking process is called binding. It is what causes changes in the postsynaptic neuron, one of which is an electrical voltage change. When enough neurotransmitter binds to the postsynaptic neuron, a large enough voltage change causes a nerve impulse called an action potential to travel from the postsynaptic dendrites down the axon to the axon terminal, beginning the whole process of neurotransmitter release and binding again. What causes the neurotransmitter to be released? When the action potential arrives and the axon terminal is depolarized, voltage gates open causing calcium to rush into the axon terminal. Calcium starts the process of causing the vesicles containing neurotransmitter to fuse with the neuron’s cell membrane. The effects of the voltage change from the arriving action potential and cause many of the fused vesicles to release neurotransmitter into the synaptic space. The neurotransmitter molecules then bind to specialized receptors in the postsynaptic membrane.
Excitation and Inhibition
The postsynaptic neuron can be either excited or inhibited depending on the particular neurotransmitter and receptor combination. This is depicted in the third video. For example, one common neurotransmitter is called acetylcholine. Its receptor is a sodium ion channel. When acetylcholine binds to the receptor, the receptor channel opens and positively charged sodium ions enter the postsynaptic neuron, which generates an excitatory postsynaptic response. Other neurotransmitters, like GABA, allow in negatively charged ions, which cause an inhibitory postsynaptic response. Mechanisms exist to remove or deactivate neurotransmitter from the synaptic space so that the effects of neurotransmission are brief and accurately follow the presynaptic signal. For acetylcholine, an enzyme in the synaptic space is called acetylcholinesterase. It acts to break down any acetylcholine in the synaptic space that has not bound to postsynaptic receptors. There are also special transporters located on the presynaptic neuron that serve to reuptake leftover neurotransmitter thereby clearing it from the synaptic space and allowing the presynaptic terminal to reuse the leftover neurotransmitter. In addition, there is a process called endocytosis in which the membranes used to form the vesicles inside the presynaptic terminal are recycled and refilled with neurotransmitter molecules, readying them for another round of synaptic transmission.