2.4 Neural Firing

Vocabulary

Neural Firing

Parts of the Neuron

Neurons are our body’s nerve cells which make up the nervous system. For a neuron to fire, or communicate with another neuron, information must first be gathered in by the dendrites of the receiving neuron. From there, the information passes through the cell body to the axon.

Image Courtesy of Open Source Textbook

Action Potential

Action potential must occur for the message to continue to travel down the axon. This only occurs if the neuron’s threshold has been met - meaning it has received enough stimulation🔋 from the original sending neuron. If this threshold is met, the action potential occurs and the message travels down the axon via a process of depolarization. If the threshold is not met, nothing happens. Neurons have an all-or-none response - they either fire or they don’t. 

This action potential occurs through the relocation of ions. At resting state, the neuron has a negative (-70mv) ➖charge. The negative ions are inside the neuron, while positive ions are outside the neuron. Then, specific neuron receives neurotransmitters (more of this explained later: think of it as a signal). When enough of this signal is received, meeting the threshold, the membrane of the neuron becomes permeable, allowing the positive ions to rush into the neuron, bringing the charge of the cell to about +40mv ➕. When the charge changes, a series of signaling occurs within the neuron like a bullet 💣💨(the speed of this electric message firing is actually 120 m/s, which is about 270 mph!! 😵). 

Image Courtesy of Teach Me Physiology

Though you are not required to memorize the complicated steps of action potential (that's more AP Biology material!), try to know what each step of the graph means, because it can come up as a MCQ! Just understand what each section of the graph means (its signficance) and you'll be good to go!

• Basically, the action potential is associated with depolarization. Depolarization is the process that carries the neural impulse through the axon, action potential is what must happen for the process to occur.

There are two types of signals / neurotransmitters:

• Excitatory -- Pushes neuron's "accelerator"🚦; makes a neuron more likely to reach action potential and fire
• Inhibitory -- Pushes a neuron's "brake"🛑; makes it less likely for a neuron to reach action potential

Terminal Branches

Remember those neurotransmittors? Well once the message has passed through the axon, it reaches the terminal branches. The terminal branches of a neuron contain neurotransmitters which are then released. These neurotransmitters cross the synaptic gaps between neurons and are gathered in by dendrites of a new neuron, continuing the communication process📩

Image Courtesy to Wikipedia

The synapse is where two neurons meet and neurotransmitters are released into it. There is both an electrical synapse, which relays quick🐆 messages to another cell, and a chemical synapse, which sends messages slowly🦥 to another cell.

Neurotransmitters are stored in vesicles in the axon terminal. When there is a neural impulse, the vesicle binds with the edge of the axon terminal and the neurotransmitters are released into the synaptic cleft.

• Neurotransmitters that do not find a home on the dendrites of a neuron are absorbed back into the original neuron through a process called reuptake.

Neurotransmitters

Neurotransmitters often act as agonists or antagonists in our body. An antagonist neurotransmitter binds to the dendrites of a neuron and prevents or blocks🙅 its response. An example of this is the poison, Botulin. Botulin causes paralysis because it blocks the release of acetylcholine, an important neurotransmitter in muscle action. 

Agonists, on the other hand, bind to receptor sites and mimic the effects of a specific neurotransmitter. Opiates are an example of an agonist as they mimic the effects of endorphins in our body (which is why they produce a morphine-like effect).

• Drugs trick our brains into thinking that they are neurotransmitters. As mentioned above, drugs mimic the effects of endorphins so much to the point that our brain stops producing natural endorphins. This is why there is such a withdrawal when people stop trying to take drugs💊. The brain of a long-term drug user just cannot produce something significant for your health anymore because of the way drugs trick our brain. Here is a chart of the functions of some key neurotransmitters. It is good to memorize this since it is sometimes tested on.

Table and Content Courtesy of Myers' Psychology for AP - 2nd edition

After Firing

After a neuron fires and reaches action potential, it goes into its refractory period, where it cannot fire. This period of rest😴 prevents one signal from combining with another. The neuron becomes slightly more negative than -70mv during this period. After its refractory period, the neuron comes back to -70mv, and the neuron reaches the resting potential, where the cell is polarized and ready to fire again once it reaches threshold.

🎥Watch: AP Psychology - Neurons and Neurotransmitters