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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The Marriage of Art and Science: Scanning Electron Microscopy

By Joseph


As both an aspiring artist and scientist, I find special appreciation for technologies like the scanning electron microscope, or SEM.  In addition to fueling scientific discovery, the SEM illuminates the tiniest of nature’s beauties.  Although it is difficult to avoid a SEM image every once in awhile in our modern lives, I’d like to give you what I think is a top hits list.

Image courtesy Iowa State University

But first, a little background on the SEM. The SEM is a type of microscope that creates an image by bouncing electrons off an object instead of light.The microscope produces an electron beam that travels vertically through a vacuum inside of the microscope, focused along the way by electromagnetic fields and lenses.  When the electron beam hits the specimen on the stage, it causes electrons and X-rays to be emitted from the sample.  Detectors then catch these “backscattered” electrons and X-rays and convert them into an image.  As the electron beam is interacting with individual atoms in the sample, the SEM enables extremely high-resolution images.  The narrow beam of electrons is moved across the sample in successive rows, thereby “scanning” the sample.  The detector reading the “backscattered” electrons is a combination of a scintillator and photomultiplier in which the accelerated electrons cause the scintillator to emit flashes of light which are captured, amplified, and displayed by the photomultiplier.  In addition, the narrow width of the electron beam causes SEM images to have a very large depth of field, enabling images that convey the full 3-dimensional character of the sample.

Now that you have a general concept of how the images are produced, let’s take a look at what I think are the TOP 5 SEM images (note:  SEM images are black and white, so any color is “pseudo-color” applied by a computer):

Ant.  So svelte.   Image courtesy Dartmouth Electron Microscopy Facility.

Human sperm.  As the story goes, when the microscope was first invented in the 17^th century, one of the first things that was examined was human sperm (most likely of the inventor).  I’d like to think the same thing happened with the SEM.Image courtesy Science Photo Library, Barcroft Media L.

Spiny-headed worm.  Yeah this could be in your gut.  Right now.  Image copyright Museum of Science, Boston, 1996.

Mammalian lung epithelium.  Red blood cells are sitting on top of ciliated and non-ciliated bronchiolar cells.  Image courtesy Dartmouth Electron Microscope Facility.

Human eyelashes, mostly awesome because they look gross.  Image courtesy Science Photo Library, Barcroft Media L.


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