[SOUND] Welcome to this module. In this module, we'll discuss methods of communication in the brain, essentially how neurons communicate with each other in the brain. If you remember from one of our previous modules, we discussed that the neuron is the essential processing unit of the brain consisting of dendrites, a cell body, axons that transport the signal, and nerve endings that communicate the signal to the downstream neuron or the next neuron. There are two theories, at least historically, about how neurons communicate with each other. A theory supported by Golgi referred to as Reticular theory proposes that neurons form a continuous reticular net and are continuously connected with each other. A second theory proposed by Cajal, through his very meticulous neuron staining work that he did using a microscope to review the connections between neurons, suggested that neurons in fact are structurally independent, and that units interact by contiguity and not by continuity. This theory is referred to as the neuron doctrine, and has found a lot of support later, particularly in the 1950s through the development of the electron microscope. Neurons receive and process information. They transmit information both chemically and electrically. They pass information downstream to adjoining neurons. And they form neural networks that collectively support brain function. In this module, we'll go through a little bit of how they communicate with each other and how they support that collective brain function. There are essentially three types of neurons. There's the sensory neuron, which converts an external stimulus into an electrical signal that can be processed by the brain. There are interneurons, which process and relay information once it's been received from, for example, a sensory neuron. And finally, there are motor neurons, which convert electrical signal processed in the brain into muscle or gland movements, if you will, into physical action. On the membrane of each neuron are ion channels and ion pumps. They establish a difference in concentration of sodium, potassium, chloride, and calcium within the cell versus the outside of the cell. This establishes an electrical charge or what's called a resting state potential in the neuron. There's a voltage difference between the inside of the cell and the outside of the cell that is established by this ion channels and these ion pumps. At the synapse, at the communication point where external nerve endings form synapses with dendrites of adjoining neurons, this forms the site of the basic communication where electrical and chemical signals are transferred from one neuron to another. Now how exactly does that work? The flow of information goes from the presynaptic terminal as you see on the right, through the synaptic cleft to the what's called postsynaptic terminal or postsynaptic spine. Synapses from the primary site of interneuronal communication. Now, through the next few slides, we'll go through the individual steps that facilitate this synaptic transmission. So first, here's an example of a synapse. By opening one of the ion channels, an influx of calcium, for example, through this ion channel, causes available synaptic vesicles to fuse with the presynaptic membrane as you see in the image on the right-hand side. In the second step, this transmitter is released into the synaptic cleft from these synaptic vesicles. The transmitter, through the synaptic cleft, then binds with the post-synaptic member or receptors on the post-synaptic membrane. And these post-synaptic channels then either open or close based on this binding. The opening of ion channels in the postsynaptic membrane causes either an influx of ions into the postsynaptic neuron, or the ion channels close up and prevent an influx of ions into the postsynaptic neuron. The change in the balance of post-synaptic ions in the post-synaptic spine, causes the post-synaptic cell to either depolarize or further hyper polarize. If the post-synaptic voltage changes are large enough enough, an electrochemical pulse is generated which is referred to as an action potential. This action potential then travels rapidly along the axon where it can then in turn activate synaptic vesicle release downstream in synapses with other neurons. At the same time, the used synaptic vesicle is retrieved in the presynaptic terminal. And it's retrieved from the membrane and recycled. And new transmitter is synthesized by the cell's metabolic apparatus and stored in vesicles for future synaptic transmission. Post-synaptic stimulation changes the excitability of the post-synaptic cell. If this stimulation is excitatory, it depolarizes the membrane potential and makes it easier to reach an action potential threshold. And if the stimulation is inhibitory, it hyper polarizes the membrane potential, making it harder to reach the action potential threshold. Excitatory post-synaptic potentials are commonly referred to as EPSPs. Single EPSPs do not always cause a post-synaptic action potential. In some cases, neurons that form many connections with many synapses with adjoining cells, require a summation of EPSPs to generate an action potential. So here on the figure on the right, you will see that the accumulation of three EPSPs reaches the threshold to generate an action potential in the post-synaptic cell. We can record these post-synaptic potentials by the placement of an electrode. The electrode can be placed either inside the cell, as is indicated by number 1 in the right-hand figure, or on the outside of the neuron, as indicated by the number 2 in the right-hand figure. In the instance where the recording is done on the inside of the neuron, the influx of ions causes an increase in voltage inside the cell which are reflected in the voltage read up that will result from a reading like this. In case of recording from the outside of the neuron, positive ions float away from the extracellular electrode, causing a decrease in voltage that is recorded on the outside of the cell. So, there's a different voltage signature that can be obtained from recording on the inside of the cell, versus outside of the cell. Neurons are densely packaged and highly interconnected. The extracellular electrode cannot assess which neuron is causing the voltage change. Extracellular recordings thus measure activity of a local neuronal environment, and are therefore referred to as local field potentials. These local field potentials are incredibly important when we talk about the basis of the MRI signal in one of our future models about MRI image generation. So we've now talked about some basic communication principles that occur within the brain that allow cells to communicate with one another. In the next module, we'll discuss some of the functional differences between different types of neurons and brain circuits that support brain function that we are all familiar with. [SOUND]
