# Neuron

Neurons are the individual cellular unit in the nervous systems.

• Axon terminals connect to dendrites.
• Cell body = Soma

There are different ions that play highly important roles in the neurons, such as $\ce{Cl-, Na+, K+, Ca^2+}$. They can be moved between the intercellular and extracellular space through different mechanisms:

• Diffusion: particles move to make concentrations uniform on both sides.
• Electrostatic: same charge repel, different charge attract.

Besides these two mechanisms, there are sodium and potassium pumps that require ATP to work (energy).

Neurons have different types of ion channels:

• Passive, that allow for steady changes to happen.
• Active, that open, close and inactivate depending on the membrane potential. The membrane potential changes the probability of something to happen to the channel, but it does not determine it fully. Kinetics also play a role.
• $\ce{Na+}$ open and close fast.
• $\ce{Na+}$ inactivation is slow.
• $\ce{K+}$ open and close slow.
• The kinetics is related to the probabilistic temporal dynamics of moving between different states.

The membrane acts as a Capacitor and thus causes voltage to change gradually. If a membrane has a higher capacitance, it has a greater ability to build up charges on its two sides and so it will take longer for the membrane voltage to change. Similarly, if the capacitance is lower, the membrane voltage will change more quickly. We will return to this point with the idea of a ‘time constant’ later in this lesson.

• The voltage across the membrane is measured relatively to the extracellular space.
• Membrane potential: Voltage across the membrane at any point in time; can vary from -90 mV to +60mV.
• Membrane resistance: Resistance to the flow of ions across membrane; generally represented in Ohms per square mm.
• Axial resistance: resistance to the flow of ions down the axon.
• Resting potential: membrane potential of a neuron that is “at rest”, i.e., not sending or receiving signals; usually between -60 mV and -70 mV.

A cell can be either depolarized or hyperpolarized. These terms are compared to the resting potential, thus:

• A cell is depolarized when there is a positive change in the membrane potential, i.e., it’s less negative.
• A cell is hyperpolarized when there is a negative change in the membrane potential, i.e., more negative.

A neuron’s axon is covered by myelin sheaths, having regular gaps between them, called nodes of Ranvier. These sheaths cause a condition called saltatory conduction increasing the conduction velocity of action potentials.