In continuation of the theme of my previous article, today we will discuss two things: action potential and neurotransmitters. From comprehending these micro concepts, we will be able to appreciate the macro science of neurobiology.
Before we can jump into the world of action potential, let’s have a quick review of the anatomy of a neuron: the dendrites, finger-like structures that receive incoming neural signaling, the soma, or cell body, which contains the nucleus, the axon, the appendage of the neuron that transmits impulses away from the body (1), the axon hillock, the bridge between the soma and axon which has the gates that either initiate or prevent an action potential, myelin, the axon’s insulation that confers speedy neural communication, the nodes of Ranvier, gaps between the myelin-sheath coverings of the axon, and the axon terminals, which house neurotransmitters that will or will not be released into the synapse, the gap between two neurons in which neurotransmitters are or are not released. Synapses “talk,” while dendrites “listen.”
Now that we have refreshed our memory of neuroanatomy, we can examine action potential. An action potential is the electrical impulses that send signals around a person’s body, a temporary shift (from negative to positive) in the neuron’s membrane potential caused by ions suddenly flowing in and out of the neuron (2). Neurons have the all-or-nothing principle, where a neuron either reaches an action potential and fires or it does not fire. There is no in between.
The action potential occurs like this: when a neuron is resting, the ion channel gates that are located on the neuron’s membrane are closed. On the positive, extracellular side of the neuron, there is a high concentration of sodium ions, and inside of the nerve cell, potassium ions are abound in a great amount. A resting neuron is negatively charged, and its resting potential is -70mV. Depending on the electrochemical message sent from the presynaptic neuron, if a neuron decides to fire, the ion channel gates will allow more positive sodium ions to flow into the cell. This process of the domino effect of ion channel gates opening is known as depolarization.
A neuron typically fires at around -55mV, which is its threshold. At around +40mV, the ion channel gates will reopen and usher out the potassium ions to the extracellular space, recreating the original positive external and the negative internal environments of the neuron. This is called repolarization. At the time of repolarization, the neuron’s potential is approaching -70mV again. Subsequently, the ion channel gates are left open for too long, and too many potassium ions are released. This initiates a period of hyperpolarization, where the neuron’s potential dips below -70mV. Eventually, the neuron returns to homeostasis and stabilizes at the -70mV resting potential. Now that we understand how the economy of neural communication is achieved, in my next article we will learn about the currency of neural signaling: neurotransmitters.
Featured Image is from Washington.Edu