Question

An action potential is stopped by a chemical traveling down a motor neuron. Give 3 hypotheses for how this chemical might halt the propagation of an AP. For each hypothesis, state the physiological mechanisms impacted by the chemical.

Answer 1

Key facts: action potential and synapses

• Neurons communicate with each other via electrical events called ‘action potentials’ and chemical neurotransmitters.
• At the junction between two neurons (synapse), an action potential causes neuron A to release a chemical neurotransmitter.
• The neurotransmitter can either help (excite) or hinder (inhibit) neuron B from firing its own action potential.
• In an intact brain, the balance of hundreds of excitatory and inhibitory inputs to a neuron determines whether an action potential will result.

Neurons are essentially electrical devices. There are many channels sitting in the cell membrane (the boundary between a cell’s inside and outside) that allow positive or negative ions to flow into and out of the cell.

synapses are the junctions where neurons pass signals to other neurons, muscle cells, or gland cells. Most nerve-to-nerve signaling and all known nerve-to-muscle and nerve-to-gland signaling rely on chemical synapses at which the pre synaptic neuron releases a chemical neurotransmitter that acts on the postsynaptic target cell

There are three main events that take place during an action potential:

1. A triggering event occurs that depolarizes the cell body. This signal comes from other cells connecting to the neuron, and it causes positively charged ions to flow into the cell body. Positive ions still flow into the cell to depolarize it, but these ions pass through channels that open when a specific chemical, known as a neurotransmitter, binds to the channel and tells it to open. Neurotransmitters are released by cells near the dendrites, often as the end result of their own action potential! These incoming ions bring the membrane potential closer to 0, which is known as depolarization. An object is polar if there is some difference between more negative and more positive areas. As positive ions flow into the negative cell, that difference, and thus the cell’s polarity, decrease. If the cell body gets positive enough that it can trigger the voltage-gated sodium channels found in the axon, then the action potential will be sent.
2. Depolarization - makes the cell less polar (membrane potential gets smaller as ions quickly begin to equalize the concentration gradients) . Voltage-gated sodium channels at the part of the axon closest to the cell body activate, thanks to the recently depolarized cell body. This lets positively charged sodium ions flow into the negatively charged axon, and depolarize the surrounding axon. We can think of the channels opening like dominoes falling down - once one channel opens and lets positive ions in, it sets the stage for the channels down the axon to do the same thing. Though this stage is known as depolarization, the neuron actually swings past equilibrium and becomes positively charged as the action potential passes through!
3. Repolarization - brings the cell back to resting potential. The inactivation gates of the sodium channels close, stopping the inward rush of positive ions. At the same time, the potassium channels open. There is much more potassium inside the cell than out, so when these channels open, more potassium exits than comes in. This means the cell loses positively charged ions, and returns back toward its resting state.
4. Hyper polarization - makes the cell more negative than its typical resting membrane potential. As the action potential passes through, potassium channels stay open a little bit longer, and continue to let positive ions exit the neuron. This means that the cell temporarily hyper polarizes, or gets even more negative than its resting state. As the potassium channels close, the sodium-potassium pump works to reestablish the resting state.