Signals in nerve cells
Previously we got some information about neurons, the cells that are involved in carrying a signal (or information) to and from the brain. In this week’s blog we will briefly describe a part of this transmission process known as the action potential. We had some previous information about the process as a group but for some members the topic itself was completely unknown, so we made certain that everyone understood the topic correctly. The action potential takes place when a neuron is fired. During the potential, the neural membrane opens and allows ions to move through it causing a depolarization. This depolarization moves along the axon and carries the signal forward.
Before the Action Potential: Neuronal membrane at restThere are many charged ions inside and outside the neural membrane. A positively charged ion is known as cation and negatively charged ion is called anion. The cations calcium (Ca2+) and sodium (Na+) have positive charges are mainly present outside the nuclear membrane. A calcium ion contains 2 positive charges while a sodium and potassium ion have one positive charge. Chloride (Cl–) is an anion and it has only one negative charge. The ions move to and from a cell through ion channels as most ions cannot pass the cell membrane with diffusion.
When a neuron is at rest, i.e. not sending signals, it is slightly negatively charged has negative charged relative in comparison to the outside of the cell creating a potential difference. This electric potential difference is called the resting potential which is typically -70 mV. There are a high concentration of sodium ions outside the cell whereas inside the cell there are potassium ions and a high concentration of chloride ions. The ion channels are mainly closed, but some potassium ions move outside. This ion concentration is maintained by an ion pump that moves three sodium ions out and two potassium ions in by active transportation. This expends energy but allows for quick reaction to stimulus.
Information Transmission: During the Action PotentialWhen an impulse is sent out from a cell body, the sodium channels open and the sodium cations surge into the cell. Inside of the cell starts to become less negative, i.e. depolarize and once the cell reaches -55mV the neurons will fire an action potential sending the electrical signal down the axon. During the action potential, the depolarization is so large that the potential difference briefly reverses polarity, which means inside of the cell becoming positively charged. The action potential is always of same size i.e. there is no big or small action potential (this principle is also known as “All or None” law). The action potential is renewed in every Ranvier node. This ensures that full intensity of the signal is carried down the nerve fiber and hence the signal persists when it travels further from the source.
After the Action PotentialAs more sodium ions enter the cell, the potassium ions start to move slowly outside the cell. This process continues until the cell reaches its peak action potential value which is about 40 mV. At peak action potential, the sodium channels close but potassium ions keep moving out of the cell and the membrane gets closer to the resting potential.
Eventually, the membrane potential starts to turn negative again and briefly turns more negative than the resting potential before finally reaching it. This is called hyperpolarization. The ion pump starts working and restores the membrane to resting potential. After the action potential there is a small refractory period, during which the cell resists new action potential. During the absolute refractory period, the membrane cannot create a new action potential. During the relative refractory period, the cell would need a bigger stimulus to cause another action potential.Sources:
- Images https://courses.lumenlearning.com/wm–biology2/chapter/resting-membrane-potential/
- Image http://faculty.washington.edu/chudler/ap.html#:~:text=The%20action%20potential%20is%20an,This%20is%20the%20threshold.
- Text has been partially referenced from the book and from the sources mentioned above