Neuron

Neuron

Neuronal Cell Structure

Resting Membrane Potential

Graded Potentials

Action Potential

Ionic Basis of the Action Potential

The effect of Myelination

Axon Terminal

Neuronal Cell Structure

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The neuron is a directional cell, receiving input from other neurons on its dendrites, resulting in the inflow of cations. Since these are usually positive ions crossing the membrane, their movements point in the direction of the current. These currents create graded potentials that sum in the cell body (soma). The first part of the cell membrane that is capable of being voltage-sensitive is called the axon hillock, which is the junction of the soma and the axon. If the graded potential is greater than (i.e.: more positive or less negative) than the threshold, the neuron will fire an Action Potential. The axon is a tube to conduct the signal to the target cell or cells. The axon splits into many axon terminals, allowing the signal to be delivered to many targets simultaneously. Each axon terminal ends in a rounded widening of the cell called a synaptic bouton (French, for “button”) that is the part that releases neurotransmitters onto the target.

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Resting Membrane Potential

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The resting membrane potential is maintained by the Na+/K+ pump. Each cycle of the pump is driven by the lysis of one ATP into ADP + Pi (inorganic phosphate). Each cycle pushes out 3 Na for 2 K. So, the pump pushes both ions against their concentration gradient, and results in the net transfer of 1 positive charge outside of the cell.

There are fixed large polyanions inside the cell (proteins). Thus, Chloride anions move out of the cells. For each ion, both electric potential and concentration gradients exist. If ion pores are opened, that specific ion will move until the electrochemical gradient is zero. Every cell in the body has a negative resting potential. Excitable cells (nerves, muscle) generally have a lower, more negative resting membrane potential.

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Graded Potentials

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When the incoming signal touches the dendrite (at the dendritic spine), it results in the opening of an ion pore. If the receptor is an excitatory one, it allows cations in to form a positive current (Excitatory Post-Synaptic Potential, EPSP). If the receptor is inhibitory, it leads to the creation of an IPSP.

When the incoming signal touches the dendrite (at the dendritic spine), it results in the opening of an ion pore. If the receptor is an excitatory one, it allows cations in to form a positive current (Excitatory Post-Synaptic Potential, EPSP). If the receptor is inhibitory, it leads to the creation of an IPSP.

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Positive potential bring the membrane potential closer to zero (i.e.: the cell becomes less polarized or depolarized). Inhibitory inputs lead to hyperpolarization. The amount of time the ion pore remains open depends on how much neurotransmitter was secreted into the incoming synapse, resulting in a weak or strong depolarization. Given that pumps are constantly working to restore the membrane potential back to the resting one, the bigger the disturbance, the longer it lasts. If subsequent inputs are close enough together in time, a temporal summation results where the postsynaptic potential gets bigger. That high concentration of cations diffuses down the pipes of the dendrites, the currents adding together like tributaries of a river that are constantly evaporating (due to the pumps). If two different inputs come in from two different cells, then two tributaries from two different areas in space meet up, causing a spatial summation.  These depolarizations and hyperpolarizations can have any size, which is why they are called graded potentials. If a threshold depolarization reaches the axon hillock, an action potential is generated. (see next section)

The following image summarizes these processes at a decent level of detail.

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Action Potential

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The resting membrane potential is negative, so what happens when sodium pores (Voltage-gated Na+ channels) open is that sodium flows in to the cell. After 1 msec or so, the membrane repolarizes, letting K+ ions out passively, while actively pumping Na+ out.

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If you were to graph the cytoplasmic voltage versus time on an oscilloscope, you would see the standard shape of the action potential as the depolarization wave passes the electrode.

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Ionic Basis of the Action Potential

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For the MCAT, you are expected to know the individual ion channels that are responsible for the action potential. There are two major players here: the Voltage-gated Na+ channels, and Voltage-gated K+ channels. Since K+’s electrochemical gradient tends to push it out of the cell, these K+ channels can be called “rectifiers,” because they cause a net movement of positive charge outside of the cell that opposes the effect of the Na+ channels. The Voltage-gated Na+ channels have 3 states: Closed and Ready, Open, and Closed and Inactive. So, let’s follow it step-by-step:

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  1. An above-threshold potential causes the closed-but-ready Voltage-gated Na+ channels to open. Sodium follows its electrochemical gradient, entering the cell and depolarizing it. Even though the equilibrium point of the gradient is greater than 0 mV, the change in the potential initially depolarized the cell, so it is still called a depolarization.
  2. The Voltage-gated K+ channels are triggered by a higher threshold than the Na+ channels, so they begin to open after the Na+ channels do. The K+ outflow is a negative current that opposes the positive Na+ current.
  3. The Na+ channels close and become inactive. Since the only current is now the negative current of the K+, the cell becomes more negative. Since the K+’s equilibrium is at a lower (more negative) potential than the resting potential, this corrective (rectifying) current actually overshoots the resting potential. This hyperpolarization is what is responsible for the relative refractory period, during which it is harder to cause the same segment of membrane to start a new action potential. The absolute refractory period is when no action potential can be transmitted under any circumstances, caused by the Na+ channels becoming inactive.
  4. When the K+ channels begin to close, the only active transfer of ions is due to our old friend the Na+ /K+  ATPase. This leads to an asymptotic approach of the membrane potential back to resting levels.

The following diagram summarizes this nicely, so be able to draw it, or describe it aloud. The “story” of the Action Potential is a story of ion channels.

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The effect of Myelination

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Myelin is a cellular insulating material, that works very much like the rubber that we wrap wires in. In the PNS, myelin is laid down by Schwann Cells, which each wrap a single section of a single axon. In the CNS, a single oligodendrocyte can myelinate multiple axons, even multiple sections on the same axon. The gaps between the myelinated segments are called Nodes of Ranvier.

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Saltatory Conduction is the “jumping” of the Action Potential from Node to Node. In the myelinated sections, the axon is just like a lead pipe for water. In the nodes,  because you have exposed membrane, the action potential is refreshed to the standard strength (because there is some reduction in the membrane potential difference due to diffusion (and thus dilution of the cationic current).

Useful Analogy: Think of two different pipes. Your standard garden hose is the myelinated section. Now, imagine that you are trying to run water through a pipe made of uncoated cotton. Water will go through the pipe, but you will always be losing water current from the lumen because the wall is leaky. As you got farther from the spigot, the current would decrease and eventually stop. This cotton hose is the unmyelinated section. Unless you spent energy at certain intervals to repressurize the hose with water, it couldn’t transmit very far. Thus, the unmyelinated sections are packed full of Na/K pumps and the voltage-gated channels necessary to replenish the amplitude (repressurize) the Action Potential.

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Axon Terminal

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Finally, as the Action Potential arrives at the bouton,  it depolarizes the membrane in the bouton, where there are Voltage-gated Calcium channels. This calcium  influx leads to exocytosis of the vesicles containing neurotransmitters. The extra membrane will be reabsorbed through endocytosis, and transported back up the microtubules in the axon to the cell body where they were generated and filled with neurotransmitters (both small molecules as well as peptides that can only be made in the ribosomes in the soma).

Video of how kinesin “walks” vesicles down the microtubules:

https://www.youtube.com/watch?v=4TGDPotbJV4

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