Learning Objectives After this miniclass, you should be able to: explain how a neuron maintains its resting membrane potential. explain the ionic movements that occur during an action potential. write scripts in Matlab. load data files in Matlab.

The mechanics of a neuron

Transmembrane voltage Extracellular Intracellular Equal +, - --ask, “if there is equal concentration of positive and negative ions on both side of the membrane, then why is there a potential difference across the membrane?” --then ask, “Why do positive charges collect on the external surface of the membrane?”

Sodium-Potassium Pump --the sodium-potassium pump establishes concentration gradients for sodium and potassium --three sodium ions are pumped out of the cell for every two potassium ions that are taken into the cell --due to concentration differences, the sodium ions want to diffuse back into the cell, while the potassium ions want to diffuse out of the cell --they are prevented from doing so, however, by the cell membrane http://hyperphysics.phy-astr.gsu.edu/hbase/biology/nakpump.html

A video explanation of the pump Sodium potassium pump

Ion channels Two primary kinds --ion channels restrict the free flow of ions across the cell membrane --ion channels only open under specific circumstances; some open depending on the voltage, others depending on the binding of other molecules Two primary kinds Voltage-gated: open or close depending upon the transmembrane voltage Ligand-gated: open or close depending upon the binding of a specific molecule

At rest, only K+ channels are significantly open --since only potassium channels are open, the potassium diffuses out of the cell and collects on the surface of the cell membrane, and attracts other negatively-charged ions within the cell to the interior cell surface --the cell membrane is therefore very well-approximated as a capacitor, and the resting membrane voltage tends to be -70 mV This results in the cell membrane functioning as a capacitor, establishing a transmembrane voltage typically around -70 mV (relative to the outside of the cell).

Perturbing the resting membrane potential --a neuron can be thought of as a nonlinear summation device, undergoing an all-or-nothing response to the total input signal that it receives --when the membrane voltage breaches a certain threshold, it undergoes an action potential

Generation of action potentials --the ion channels which allow sodium ions through are voltage-gated; when the membrane voltage breaches a certain threshold (often -55 mV or so), it causes these channels to open, and sodium ions flood into the cell --this causes the membrane voltage to increases, which causes the sodium ion channels to open even more. A positive-feedback process ensues, resulting in a steep increase in the membrane voltage --eventually the sodium ion channels close, but right around the same time, the potassium ion channels open, which causes potassium to flood out of the cell, which causes the membrane potential to decrease sharply --altogether, you get a rapid increase and decrease of the membrane potential—called a “spike”—which occurs in 1 to 2 milliseconds --all AP’s undergone by a given neuron are essentially the same

Propagation of Action Potential --action potentials propagate down the axon of a neuron

Myelin sheath oligodendrocyte --oligodendrocytes insulate neuronal axons with a fatty substance called myelin; this dramatically increases the propagation speed of action potentials --if myelinated, propagate at around 100 m/s --if unmyelinated, much slower (as slow as 1 m/s)

Synaptic transmission synapse --the ends of axons form connections with the dendrites of other neurons called synapses; synapses consist of a pre-synaptic axon terminal and a post-synaptic dendritic surface; in between these two cells lies a very small space called the synaptic cleft --when an action potential reaches a synapse, it causes molecules called neurotransmitters to be released into the synaptic cleft --these neurotransmitters bind to ion channels on the post-synaptic cell, which causes ions to flow into or out of the cell --different neurons transmit different kinds of signals: excitatory neurons excite other neurons and make them more likely to fire action potentials, while inhibitory neurons inhibit other neurons, making them less likely to fire action potentials Glutamate: primary excitatory neurotransmitter GABA: primary inhibitory neurotransmitter

Excitatory Neurons --this cartoon depicts results from a simple simulation of excitatory neurons --each of these three neurons is excitatory because when they fire, they cause the membrane potential of this neuron that they connect to to increase, which makes them more likely to fire action potentials --three PRE-SYNAPTIC NEURONS, one POST-SYNAPTIC NEURON

Inhibitory Neurons Have opposite effect of excitatory neurons

Writing scripts in Matlab x=-5:0.01:5; y=sin(x); plot(x,y) title('sin(x)','fontsize',18) set(gca,'fontsize',14) print('-djpeg','simple_plot')

Plotting a data file containing a neuronal voltage trace Start your script with the line: data=load('voltage_trace.txt') Then write code to do the following: Plot voltage versus time Label the x- and y-axes, and make their font size 18 Set the font size of the numbers on the axes to 14 Save a jpeg image of the plot