We started the class with a 5 minute quizz about the chapters 2 and 3 of the book: Mark F. Bear, Barry W. Connors, Michael A. Paradiso – Neuroscience: Exploring the Brain (2015).
After that, some big questions were raised. Those were the questions that propel neuroscience to move forward and neuroscientists to still strive to learn more about how our brain works. The motivation to pursue such achievement is no small one: brain disorders have been becoming increasingly prevalent in our society and in order to cure them we must endeavour through uncharted territories, learning more about how the brain works, from a molecullar to a cognitive level. Our central nervous system has an incredibly complex structure, featuring hundreds of different kinds of transmitters and over 86 million neurons. This can be partly explained and tracked by the evolution of our species. Some questions about human brain enhancement were raised which also arose some interesting ethical questions by students in preseemo. Should we aim to enhance our brain at all? We can probably do so through magnetic stimulation. Genetic enhancement/editing is also an ever expanding field, which might show us some surprising aids to our life sooner than later. It was also pointed out that eating well and resting is a key factor in keeping the brain’s function’s at bay. After this interesting debate, we moved on to the topics on Chapter 3 of the book: The Neuronal Membrane at Rest. We went over the three main contributors to the resting membrane potential:
- Cytosol and Extracellular Fluids – Water is the main fluid both inside and outside of the neuron. Because of its highly polarity, it acts as a very good solvent for electrically charged atoms or molecules (ions). The molecules of water surround each ion in so called spheres of hydration, and effectively insulate the ions from one another. Ions are the major charge carriers in the conduction of electricity in biological systems, including the neuron. Water molecules attach themselves to different ions in different ways, which leads into a different transportation and membrane permeability to those ions . Some ions of particular relevance for this studies are the cations Na+ (sodium), K+ (potassium) and cation Ca2+ (calcium), and the anion Cl- (chloride).
- The Phospholipid Membrane – The neuronal membrane is made up of a sheet of phospholipids. They feature a stable arrangement, named phospholipid bilayer, that organizes itself in such a way that the hydrophilic heads face the outer side of the membrane and the hydrophobic tails face each other. This structure separates the neuron’s cytosol from the extracellular fluid.
- Proteins that span the membrane – After reviewing the basic structure of a protein, from its primary to quaternary structure, the protein’s many functions in the phospholipid bilayer were mentioned. Firstly, they provide structure, forming a cytoskeleton that provides the neuron’s its shape. Then, some proteins (enzymes) are catalizers to certain neuronal chemical reactions. The also make up the receptors that are sensitive to neurotransmitters. And last but definitely not least, they form channels and gates across the membrane which are routes for ions to cross it .
This mechanisms play an essential role in maintaining the resting potential stable. This movement of ions, however, does not happen in a random manner, but rather follows physical (diffusion) and electrical laws.
We then moved on to how the neuron membrane is at its resting potential: what are the concentrations of ions on each side of the membrane, what mechanisms help them keep it that way. The ion Potassium is more concentrated on the outside whereas Sodium, Calcium and Chloride are more concentrated on the inside. The sodium-potassium pump is mainly responsible for maintaining this large potassium concentration gradient across the membrane. At rest, the neuronal membrane is highly permeable to potassium, because of membrane potassium channels and it is negatively charged precisely due to the movement of potassium ions across the membrane.
This electrical potential difference work as one of the battery, kept stable through the work of ion pumps. We then moved on to focus on the Action Potential. We discussed the mechanisms that are responsible for the action potential and how it propagates down the axonal membrane.
We went over the different phases of an action potential, in the following order : resting potential > rising phase > overshoot > falling phase > undershoot, and its trigger, called the threshold, and how we can measure them through different experimental methods (voltage clamp). We went over each step of the depolarization (through Na+ Influx) and repolarization (through K+ exflux) of the cell membrane and were introduced to the structure and function of a voltage-gated-sodium channel (analyzed through the patch clamp technique). This channels works its way through some phases called activation, inactivation and deinactivation in order to regulate the passage of Na+ through it. Next came the propagation of the action potential through the neuron’s axon, aided by the myelin sheath that allows for a saltatory conduction of the impulse. After came the different classifications of synapses, according to mechanism and structure (electrical – gap junctions – VS chemical synapses), according to connection (axodentritic VS axosomatic VS axoaxonic VS dendrodendritic) and the neuromuscular junction complex. Finally we went through a brief overview of all the process of neurotransmitters transmissions: from synthesis and storage to release, reception, recovery and degradation. This are all complex processes mediated by different cell components in the axon, voltage-gated ion channels in its terminal and different enzymes and ion channels across the membrane. It was also discussed very briefly how the synaptic integration happens when multiple action potentials reach different dendrites of the same neuron.
Exercises Class 18.09
In the first exercise class we went over the different exercises on our first assignment. These regarded the structure of a neuron, the function of the glial cells, the different phases of the action potential, the action potential propagation velocity and finally a derivation for the equation that allows us to calculate the Equilibrium potential for a certain ion.