5.2: Electrical vs. chemical synapses

Electrical synapses

Electrical synapses are the simpler of the two types of synapses. In an electrical synapse, the main driver of communication between two neurons is a change in potential, and the carrier of charge is almost always an ion. The electrical synapse simply means that the cells share their cytoplasm with an adjacent cell. Electrical synapses are what Camillo Golgi imagined when he proposed the reticular theory of nervous system organization. Oftentimes an entire network of many hundreds of neurons is connected by these synapses.

Imagine two neurons that are connected by an electrical synapse. First of all, both of them are complete cells on their own. Each one contains a complete plasma membrane surrounding the neuron, a nucleus, and all the individual organelles needed to carry out that cell’s basic life processes. Electrical synapses share the cytoplasm between the two connected cells, so ions, ATP, and larger signaling molecules and proteins are able to move between the two cells. For this to happen, there exists a specialized physical channel between the two that allows for the passage of cytoplasm called a connexon or hemichannel. Each hemichannel itself is made up of six transmembrane proteins called connexins (you can remember the difference between the channel and the individual protein because proteins often end with the letters -in). When two connexons contact each other, they interact closely with each other and form the gap junction, which is the structure that connects neurons electrically.

Neurons that are connected by electrical synapses are remarkably close to each other. The synaptic gap between the two electrically connected neurons is about 3.5 nanometers. Logically, the neurons must be close since the hemichannels are like a physical “bridge” between the two cells.

Electrical synapses are capable of passing information bidirectionally. This means that a signal does not always move sequentially from the presynaptic cell to the postsynaptic cell. Rather, ions and signaling molecules are free to move through the connexons in either direction. Also, each cell within an electricallycoupled network can receive inputs at any of the cells, making it able to detect several signals at once - the same way a huge satellite dish can detect more signals than a small dish.

Electrical synapses likely evolved because of evolutionary pressures that selected for speed. These synapses can pass signals as fast as electrical charges can move through an electrolyte-rich fluid like cytoplasm, which is almost instantaneous. Therefore, an escape reflex that is made up of communication across electrical synapses is advantageous for animals that need to escape predators. For example, crayfish exhibit a reflexive abdominal flexion response when exposed to threatening stimuli, causing the animal’s body to dart away from a threat within a fraction of a millisecond. Comparing across the phylogenetic tree, electrical synapses are often found in less complex organisms, including arthropods such as insects and crustaceans, where such reflexes are likely more critical for survival.

Another advantage of electrical synapses is that they can form a large network of interconnected neurons with synchronized Clinical connection Charcot-Marie-Tooth (CMT) disease Charcot-Marie-Tooth (CMT) disease is a rare genetic disorder that damages parts of the peripheral nervous system including the motor nerves, resulting in muscle weakness and difficulty with walking, and the sensory nerves, causing some to experience abnormal sensations such as tingling or pain in their extremities. These symptoms are characteristic of signal transduction failure resulting from deficits in myelin. One type of connexin protein, called Cx32, is heavily expressed in Schwann cells, the glia that produce myelin in the PNS. Mutations in the gene that codes for Cx32 are associated with the X-linked form of CMT disease, and knockingout the gene in experimental mice cause the mice to express similar symptoms as human CMT. Figure 5.3 People with CMT disease often have abnormally shaped feet. activity. For example, neuroendocrine cells in the hypothalamus are connected by electrical synapses. When the “go” signal arrives, all the cells depolarize at once, which can result in the massive release of hormones into the bloodstream. A network can also cause sudden, powerful inhibition. Like an angry mob of people chanting, a network of electrical synapses connecting inhibitory interneurons allows the network to send an immediate “shut-down” signal under specific circumstances.

Clinical connection

Charcot-Marie-Tooth (CMT) disease Charcot-Marie-Tooth (CMT) disease is a rare genetic disorder that damages parts of the peripheral nervous system including the motor nerves, resulting in muscle weakness and difficulty with walking, and the sensory nerves, causing some to experience abnormal sensations such as tingling or pain in their extremities. These symptoms are characteristic of signal transduction failure resulting from deficits in myelin. One type of connexin protein, called Cx32, is heavily expressed in Schwann cells, the glia that produce myelin in the PNS. Mutations in the gene that codes for Cx32 are associated with the X-linked form of CMT disease, and knockingout the gene in experimental mice cause the mice to express similar symptoms as human CMT.

Chemical synapses

At a chemical synapse, a signaling molecule is released by the presynaptic cell to influence the postsynaptic cell. These signaling molecules, generally called neurotransmitters, are synthesized or stored by neurons. After being released, these neurotransmitters diffuse randomly across the synapse, where they are able to affect nearby neurons once the chemical binds to its corresponding receptor.

Since chemical synapses do not rely on a direct physical protein “tunnel” to connect the two neurons, the distance between the two cells can be much larger. On average, a chemical synapse is a distance of about 20-40 nanometers, roughly a thousand times smaller than the diameter of a human hair.

A chemical synapse can pass a variety of signals, depending on the neurotransmitter and the receptor. For example, some signals are directly excitatory and allow positively charged cations to enter the neuron causing depolarization. Other signals are hyperpolarizing, and therefore inhibitory. And yet other signals are much more complex, inducing changes in protein expression that can modify cellular excitability over the course of minutes or hours.

Because of the complexity of the signals that chemical synapses can convey, evolutionary development through time has allowed for a tremendous variety of responses. Chemical synapses allow for fine-tuning of neural networks, giving these nervous systems a larger range of possibilities. The nervous systems of “higher” organisms like humans tend to have several chemical synapses since these signals are likely necessary for complex behaviors and cognition.

Many chemical synapses exist between the axon of one neuron and the dendrite of another neuron. One specific type of chemical synapse refers to the space between a motor neuron and muscle tissue, and this is called the neuromuscular junction, or NMJ. When the chemical signaling molecule acetylcholine (ACh) is released by the presynaptic motor neuron, it is detected by receptors that are expressed on the muscle. The release of ACh causes contraction of the muscle.