A brief Introduction to Molecular Neurobiology

(…as told by a UC Berkeley MCB Graduate)

Rita BarakatCenter for Aphasia and Related Disorders (CARD)

University of California Berkeley Wednesday, November 18th 2015

The material covered in this presentation is a summary of approximately one year’s worth of an undergraduate Molecular Neurobiology curriculum.

If you have specific questions regarding the details referenced in certain slides, I will be more than happy to provide more information!

A brief disclaimer…

There are hundreds of different types of neurons, and it is estimated that each neuron can form 1000 or more unique connections with other neurons, creating the complex neural networks that we are able to visualize today.

The way these connections relay information is binary, in that the electrical/ electrochemical signal transmitted is either excitatory (typically causing a depolarization of the post-synaptic cell), or inhibitory (typically causing a hyperpolarization of the post-synaptic cell).

Below are some of the more “general” classifications of these neuron cell types

(organized by morphology/ function):◦ Horizontal Cells (A) ◦ Bipolar Cells (B)◦ Multipolar Cells ( C) ◦ Stellate (Golgi Type II) Cells (D) ◦ Pyramidal Cells (E) ◦ Purkinje Cells (F) ◦ …AND MANY MORE!!

Neuron Classification and Morphology

Many visualization techniques have been developed for imaging neurons at the single-cell resolution; below is a brief summary of some of these methods: ◦ Nissl Stain (ideal for visualizing the soma and distinguishing between cortical

layers in situ) ◦ Golgi Stain (ideal for visualizing single neuron morphology, a Potassium

Dichromate-based staining technique that is still relatively mysterious in terms of its staining selectivity)

◦ Immunohistochemistry (ideal for staining neurons that express a specific receptor/ protein, utilizes the endogenous mechanism of antibody-antigen binding)

◦ Retrograde/ Anterograde/ Viral Tracing (involves the injection of a tracer molecule, either a fluorophore or a dye, that will travel in the retrograde or anterograde direction and stain the neurons that are connected within a particular circuit. The viral transfection-version of this technique involves infecting a circuit with a virus, such as the Rabies virus, that will allow for more precise visualization of the cells connected in series)

◦ Two-photon Microscopy (a high-resolution imaging technique which involves the ejection of two photons of equal magnitude but opposite sign wavelengths, such as +/- 45 nm, which allow for a very narrow range of the receptive field to be visualized)

◦ “Brainbow” (utilizes selective expression of fluorescent proteins that distinguishes the soma, as well as some limited visualization of the axon extensions, of individual neurons)

The way we see Neurons

Examples of Neuron Visualization Techniques

For every neuron, there are on average ten glial cells assisting with various physiological processes (acquiring nutrients, initiating an immune response, myelination, etc.)

There are five major Glial Cell types (the sixth type, known as the Radial Glial Cell, is present primarily during development). Below is a summary of these five types, their morphology, location and function: 1. Astrocytes (CNS): The most abundant glial cell, they are named for their star-

shape. They have been to shown to assist in providing neurons with nutrients/ oxygen.

2. Oligodendrocytes (CNS): The “myelinating” glial cell of the central-nervous system, these cells are able to detect/ “decide” which axons should be myelinated, and wrap around the axon to form a “process” that then signals for myelination.

3. Schwann Cells (PNS): Perform a similar/ same function to the Oligodendrocytes, but on nerve fibers in the periphery. Unlike oligodendrocytes, these cells do not form a process on the axon fiber, and need additional cell assistance to influence myelination.

4. Ependymal Cells (CNS): These glial cells line the choroid plexus membrane in the ventricles and produce CSF.

5. Microglia (CNS): The smallest and least abundant glial cell, they function as the immune cells of the nervous system (initiate the immune/ inflammation response).

The Glial Cell Types

While the majority of connections between neurons are formed between axons and dendrites, the Axon fiber itself is essential in initiating and propagating the electrochemical signal (the Action Potential) that ultimately conveys information to downstream, “higher” brain regions.

In Molecular Neurobiology and Biophysics, we think of the axon as being analogous to a wire that would transmit electrical information in the form of electron flow. Based on the principles of Ohm’s Law (V = IR), we can then assign qualitative and quantitative values to different features of the axon (such as the diameter, length, and degree of myelination).

The Axon

Particular synapses can be strengthened or weakened depending on the frequency of firing from a pre-synaptic cell onto a post-synaptic cell. A high firing frequency is associated with Short-Term Facilitation (STF), a phenomenon that can last up to a few hours. A low firing frequency is associated with Short-Term Depression (STD), which can decrease the strength of a monosynaptic connection for a brief period of time.

The primary difference between “short” vs. “long” term changes in synaptic strength is the recruiting of proteins for transcription, which only occurs in the cases of extreme high or extreme low frequency firing periods that induce Long-Term Potentiation (LTP) or Long-Term Depression (LTD).

A significant increase in the synaptic Calcium Ion concentration has been linked to LTP, whereas a moderate increase has been linked to LTD.

Facilitation/ Potentiation (STF/LTP) and Depression (S/LTD)

“The cells that fire together wire together”!

“Hebb’s Postulate” provides a pre-synaptic explanation for neural plasticity, which applies primarily to small (mono- or di-synaptic) connections, that explains the associative phenomenon between neural firing (see above quote)

A new classification for a form of Hebbian plasticity that relies on a close temporal relationship between the pre-synaptic and post-synaptic firing is referred to as “Spike-Timing Dependent Plasticity” (STDP).

Hebbian Plasticity

On a larger scale, synapses can encode relative strengths that ultimately translate into meaningful information in the “higher” cortical areas. This pre- and post-synaptic phenomenon is referred to as Homeostatic Plasticity, and essentially involves the encoding of a “homeostatic” baseline strength of a particular synapse that allows it to be distinguishable relative to neighboring connections. ◦ Pre-Synaptic “Scaling”: A pre-synaptic neuron can modify its firing

magnitude/ frequency in order to control the strength of its relationship to the post-synaptic neuron.

◦ Post-Synaptic “Modification”: The post-synaptic neuron cannot control the magnitude/ frequency of the signal it will receive from the pre-synaptic neuron, however, it can still maintain a relative synaptic strength based on the degree of transmitter receptors are expressed on the cell membrane.

Homeostatic Plasticity

Pre-Synaptic “Scaling” and Post-Synaptic “Modification”

In thinking about Neuron/ Glial cell morphology and function in a broader context:• “How can we take advantage of the physiological properties of a unique Neuron or

Glial cell type in a potential therapeutic approach?” e.g.) The potential role of the oligodendrocytes in remyelinating spared neurons post-lesion,

and how exactly to trigger this process based on what is known about oligodendrocyte activity in vivo.

In thinking about Plasticity (Hebbian vs. Homeostatic) in a broader context: • “How can we stimulate synaptic strengthening (LTP) in order to influence the

activity of a particular circuit?” • e.g.) Providing electrical/ magnetic stimulation to a tract that has been damaged in order to

revive and rebuild the synaptic strength of the overall circuit

Combining the powers of Molecular Neurobiology and Cognitive Neuropsychology!

Potential Applications to Language Research