Synaptic Transmission

Transcript

0:00 – 0:30  [Basic Definition]  Synaptic transmission is the process of transmitting information from one cell to another across a synapse.  There are two broad categories of synaptic transmission: 1) chemical and 2) electrical.  During chemical synaptic transmission, the presynaptic neuron releases neurotransmitter that crosses a synaptic cleft and binds receptors on the postsynaptic target – this is a unidirectional process.  During electrical synaptic transmission, an electrical signal passes directly from one cell to another through a gap junction – this is a bidirectional process.

0:30-2:30  

Spiking activity is shared by neurons through two major modes of synaptic transmission, chemical and electrical. In both cases there is a pre-synaptic side formed by the axon terminal of one neuron and a post-synaptic side formed by the dendrite of another neuron. However, there are key differences in how each mode operates.

Chemical neurotransmission relies on the SNARE complex, which helps dock ‘readily releasable pools’ of synaptic vesicles in the active zone and initiates vesicle exocytosis, releasing neurotransmitters into the synaptic cleft. Voltage-gated calcium channels are located in the active zone and provide the source of calcium necessary to trigger this process. Vesicles are then recycled and refilled after use.

The coupling of voltage-gated calcium channels to the SNARE complex elegantly links activity levels to neurotransmitter release. As presynaptic activity levels increase, more spikes invade the axon terminal activating more voltage-gated calcium channels. This allows more calcium to flow in, which more effectively activates the SNARE complex. This in turn leads to more vesicles fusing and more neurotransmitter release, which translates into stronger activation of post-synaptic receptors and thus more effective communication.

Neurons can either excite one another using acetylcholine or glutamate to increase spiking activity or inhibit one another using glycine or GABA to decrease spiking activity. There are two major types of receptors that so-called excitatory and inhibitory neurotransmitters activate.

Ionotropic receptors contain ion channels that allow direct current flow and so provide a fast means to alter the post-synaptic membrane potential in response to neurotransmitter release. Metabotropic receptors are coupled to G-proteins that lead to secondary effects on neuronal excitability and thus are slower in time course.

Electrical neurotransmission relies on electrical synapses formed by gap junctions that directly connect the pre- and post-synaptic neuron. Thus spikes arriving in the axon terminal can move directly to the dendrite, with the amount of current flow limited by the number of gap junctions. In contrast to chemical neurotransmission, both subthreshold and suprathreshold signals can pass through gap junctions. Thus, electrical synapses are faster, more reliable, and bi-directional forms of communication between neurons.

2:30-3:00  [Parallel Vocabulary] In introductory classes you learned about synaptic transmission.  But Neuroscience is an interdisciplinary field, so there are many words for this term – For example, it might also be called synaptic communication.  Ultimately the effect of synaptic transmission depends on how the postsynaptic target is affected.  Synaptic transmission is excitatory if the postsynaptic cell becomes more likely to spike and it is inhibitory if the opposite occurs. So…synaptic transmission is an umbrella term that can include both excitation and inhibition.

3:00-4:00  [Here’s a real world example]  Synaptic transmission is important in almost every aspect of neural communication including sensation, motor control, and cognition.  For example, did you know that many poisonous substances produce toxic effects in the body because they disrupt the process of synaptic transmission.  A tetanus infection is caused when a puncture wound becomes infected by Clostridium bacteria that are commonly found in dirt and rust – these bacteria produce toxins that target inhibitory spinal cord neurons and destroy the synaptobrevin protein, which is part of the SNARE complex.  Recall that the SNARE proteins are required for docking and exocytosis of synaptic vesicles. When the SNARE proteins are selectively destroyed in inhibitory neurons – this leads to an overall increase in excitation that  produces extreme muscle contractions called tetanus.  In extreme cases, a tetanus infection produces muscle spasms that impair breathing and can even break bones.  Treatments include mechanical ventilation to assist breathing, and muscle relaxants to treat the muscle contractions.

4:00-6:00 [Follow along with this example]

6:00-6:30 [Here are a few readings to help you review]
1) Neuroscience Exploring the Brain (Bear)

• Chapter 5: “Synaptic Transmission”.

2) Principles of Human Physiology (Stanfield)

• Chapter 8: “Synaptic Transmission and Neural Integration”

Media attributions

Neuronal synapse by Edk006 is licensed under a Creative Commons Attribution Sharealike 3.0 Unported license (CC-by-SA 3.0).

Synapse illustration by Nrets is licensed under a Creative Commons Attribution Sharealike 3.0 Unported license (CC-by-SA 3.0).

Image from “Foundations of Neuroscience” by Casey Henley is licensed under a Creative Commons Attribution NonCommercial Sharealike license (CC-by-NC-SA 4.0).