Which cells are in the nervous system?

Neurons and glia

The nervous system consists of two kind of specialized cells. Neurons process and transmit information by electrical and chemical signalling; and glia maintain homeostasis, form myelin, provide support and protection for neurons. An adult has approximately 100 billion neurons. Neuroscience is a relatively new branch of science. Charles Sherrington and Santiago Ramón y Cajal are considered to be the main founders of neuroscience. Neurons are fed with glucose. For this, a large amount of oxygen is needed. Glia cells mainly need glycogen. Glucose is so important, because this is one of few things that can pass the blood-brain barrier.

The structures of an animal cell

The structure of a neuron is very much similar to the other animal cells. Most of the animal cells share the following structures:

• The nucleus serves as the control centre of a cell and contains the cell's chromosomal DNA.
• Mitochondrion perform metabolic activities and is akin to a cellular ‘power plant’.
• Ribosome creates proteins.

The structure of a neuron

Usually a neuron has three very important parts:

• Axon, a long branch of a neuron that conducts electrical impulses away from the soma.
• Soma (also called the cell body), contains the nucleus and other basic structures.
• Dendrites, branched projections that conduct electrical impulses received from other neurons to the soma.

The myelin sheath is an insulating layer around the axon. It has intervals called the nodes of Ranvier. The myelin sheath accelerates the action potential. An afferent axon is an axon that imports information into a structure. An efferent axon is an axon that exports information from a structure. An interneuron is a neuron that connects afferent neurons and efferent neurons in neural pathways.

Differences between neurons

Neurons can vary in shape, size and function. The shape determines the connection with other neurons and thus also the contribution of a neuron to the nervous system. The function of a neuron is related to its shape.

Glial cells

Glial cells, sometimes called neuroglia, are non-neuronal cells with several functions:

• Astrocytes support endothelial cells that form the blood-brain barrier, synchronise the activity of the axons, control the blood flow and remove waste material.
• Microglia work as an immune defence in the nervous system by removing waste material and viruses.
• Oligodendrocytes insulate the axons by building the myelin sheath.
• Schwann cells work as oligodendrocytes.
• Radial glia control and guide the migration of neurons.

The blood-brain barrier

The vertebrate brain does not replace damaged neural cells, as damaged cells in other parts of the body are being replaced. This is why we need a blood-brain barrier. The blood-brain barrier blocks out most viruses, bacteria and harmful chemicals. However, it also blocks out most nutrients and – for consequently – many forms of medication. The barrier lets different things through in different parts of the brain. The blood-brain barrier mainly exists to make the chance of brain damage as small as possible.

How the blood-brain barrier works

Some chemicals can cross the blood-brain barrier:

• Small uncharged molecules (e.g. oxygen, carbon dioxide).
• Water (via special protein channels).
• Molecules that dissolve in the fats of the membrane (vitamin A & D and certain types of drugs that have an influence on the brain, like antidepressants.

Some other essential chemicals are actively transported into the brain. These chemicals include glucose (energy source), amino acids, choline, some vitamins, purines, iron and some hormones. A virus that manages to enter the nervous systems probably stays there (e.g. rabies, herpes).

How does a nerve impulse work?

The characteristics of impulses in an axon are very well adjusted to the needs people have for a certain kind of information processing. This is related to different mechanisms.

The resting potential of the neuron

The membrane of a neuron is electrically loaded. This means that there is a difference in charge between within and outside of the membrane. In case of polarisation, there is a difference in electrical charge of the two locations. The resting potential is the difference in voltage in a resting neuron.

Sodium and potassium

The membrane is selectively permeable. That means that some molecules, like the oxygen molecule, can pass through the membrane, while other molecules may never pass or rarely pass through the border of the membrane. In the membrane, there are special portals for sodium and potassium. In the resting potential, potassium can enter those portals in an average speed. The sodium portal is closed in the resting potential. With the help of a sodium-potassium pump, which is an enzyme, three sodium ions are transferred out of the membrane, while two potassium ions are transferred inside the membrane. The sodium ions will be more than ten times concentrated outside the membrane than inside the membrane and potassium ions will be more concentrated inside the membrane than outside the membrane. This will result in a difference in charge. This sodium-potassium pump is an example of active transport (energy is needed). The pump is only active because of the selective permeability of the membrane, otherwise the sodium ions that had been moved out of the membrane, would enter again. Some potassium ions that had been pumped into the membrane, will leak out again. This will increase the electrical gradient. When a neuron is in rest, two forces are trying to get sodium into the cell: the electrical gradient and the concentration gradient. Sodium has a positive charge and the inside of the cell has a negative charge. The electrical gradient wants to pull sodium into the cell (because positive and negative charges pull each other). Sodium is more concentrated outside the cell than in the cell, which will result in the sodium ‘wanting’ to enter the cell more eagerly than leaving the cell. These two gradients will result in sodium wanting to move quickly. However, when the membrane is in rest, the sodium channels are closed and no sodium will escape to the outside (except for the sodium that is being pushed outside by the sodium-potassium pump). Potassium also has a positive charge and the inside of the cell has a negative charge. The electrical gradient wants to pull potassium inside. But, potassium is more concentrated inside the cell than outside the cell, which will result in the concentration gradient wanting to push potassium to the outside. If the potassium channels would be open, only a small percentage of the potassium would flow to the outside. The two potassium gradients are almost equally balanced. They can’t be completely balanced because of the sodium-potassium pump.

Why is there a resting potential?

The main functions of the sodium-potassium pump are to maintain the resting potential, transport of membrane transporter proteins and to regulate cellular volume. The resting potential’s function is to help the neuron react quickly on a stimulus.

The action potential

Nerve impulse is an action potential. It is an event in which the electrical membrane potential of a cell rapidly rises and falls. Electrical gradient refers to the difference in electrical charge between the inside and outside of the cell. The inside of a cell in the phase of the resting potential is negatively charged (approximately 70mV). Concentration gradient is the difference in distribution of ions across the membrane. Molecules tend to move from areas of greater concentration to areas of lesser concentration.  The voltage-gated channel is a membrane protein controlling sodium entry. At the time of depolarization those channels open. Still even during the peak of the action potential the difference between concentrations remain. After the action potential voltage-gated potassium channels open, sodium carries a positive charge out of the axon and the membrane returns to the phase of resting potential. Local anaesthetic drugs change the sodium channels of the membrane and prevent the flow of sodium ions (thus blocking action potential).

The ‘all-or-none law’

The all-or-none law states that the strength by which a nerve fibre responds to a stimulus is not dependent on the strength of the stimulus. If the stimulus is any strength above threshold, the nerve or muscle fibre will give a complete response or otherwise no response at all. However, the greater the frequency of action potentials, the greater the intensity of the stimulus.

The myelin sheath and saltatory charge

The speed of the process is influenced by the thickness of the axon and whether or not there is myelinisation. Most axons have myelinisation with small gaps on regular distances (the Ranvier buttons). If action potential jumps from one gap to the other, this is called saltatory conduction.

The refractory period

The refractory period is the time after action potential when the cell resists further action potentials. This can be divided into the absolute refractory period (approximately 1 ms, when the action potential is impossible in any case) and the relative refractory period (extra 2-4 ms, when the new action potential requires greater stimulus).

Local neurons

The story explained above, does not go for all neurons. Smaller, local neurons don’t produce action potentials – they make gradual potentials that can vary in size. These neurons are very hard to study and don’t follow the all-or-none law.