Synaptic Function and Plasticity: The Microscopic Machines That Make Us Tick

By Joseph

           Chemical synapses of the nervous system- whether from worm or from human- rely on similar structural and chemical principles.  Similarly, in the nervous system, neurons are called upon to use generally similar structures and capabilities to perform a multitude of different tasks.  For example, neurons that innervate the mammalian retina are tasked with encoding information about changes in the intensity of light, requiring synapses of these neurons to rapidly alter the amount of neurotransmitter released.  In addition, synapses throughout the body must be plastic—able to alter the strength of both the bond and message between the cells sending and receiving the signal.  As such, after an intriguing lecture, a student leaves class with a fundamentally different brain than when he entered, for the processes of learning and memory are encoded in the ability of the brain to remodel itself in response to input.  As we will see, important microscopic components play a critical role in enabling both the plasticity and diversity of powerful functions in neurons.

General representation of a chemical synapse.  An axon of the presynaptic neuron forms a synapse with a dendrite of  the postsynaptic neuron.  Synaptic vesicles (purple) are trafficked to the synapse itself, where they fuse with the plasma membrane to release neurotransmitter into the synaptic cleft.  Image courtesy scienceblogs.com.

          The primary function of chemical synapses of neurons is to send messages to their postsynaptic partner via the coordinated release of neurotransmitter.  Neurotransmitters are small proteins that are trafficked to the synapse within special compartments called synaptic vesicles, which mediate their release into the space between the neuron and its partner, called the synaptic cleft.  Here, neurotransmitters are received by a receptor on the opposite cell to propagate an incoming signal.  As a neuron’s primary means of intercellular communication, neurotransmitter release can also be the event to begin a cascade of synaptic remodeling.  Therefore, it is critical to understand how the proteins or protein structures that mediate neurotransmitter release function.

            One such structure is the dense projection, called the “t-bar” in invertebrates and the “synaptic ribbon” in vertebrates (for now I’ll stick with the term t-bar).  The t-bar is so named because in transmission electron microscopy images of invertebrate synapses, it appears as an electron-dense body projecting from the synapse.  Although few protein components of the t-bar are known, (and those that are known are poorly understood) the t-bar is thought to function in coordinating different pools of synaptic vesicles and mediating neurotransmitter release.  In addition, more extreme expressions of t-bar structures, such as the synaptic ribbon, are thought to act like vesicle transport and fusion dynamos that enable the tasks of powerful synapses like those of the retina.  Without synaptic ribbons or t-bars, we likely would lack the rich visual senses or hair-trigger reflexes that make us such premium dancers.

Transmission electron micrographs and schematic illustrations of dense projections in a variety of organisms.  Note that the dense projection clusters synaptic vesicles at the synapse, where they fuse with the membrane to release neurotransmitter.  Dense projections can appear as everything from T-bars in Drosophila (E-G) to synaptic ribbons in the vertebrate retina (J).  Other organisms shown are the worm C. elegans (A, B), crayfish (C, D), skate (H, I), frog (K, L), lizard (M, N), and human (O, P).  Image adapted from Zhai and Bellen, 2004.

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