All about Nervous System Cells

Our body is made out of many organs, (as organs are made of a number of cells) but these organs cannot function alone, they need to be part of a system (i.e the stomach needs a mouth and an oesophagus so that the food can arrive and be digested in it) but a system can also be a regroupement of many parts of the body that have a similar function.

That's why almost all organs are part of a fairly precise system with a fairly precise function.

 In the human body we can make out 11 systems:

Integumentary System, Nervous System, Skeletal System, Endocrine System, Muscular System, Cardiovascular System, Lymphatic System, Respiratory System, Urinary System, Reproductive System and finally the Digestive System.

Today we are focusing on the Nervous System. We would like to start citing its components and then go on to explain their function in the system and the body as a whole.

Two types of cells make up the majority of the nervous system: Neurons and Glial cells.

Neurons: 

Or what can be called "Nerve Cells", are the basic element of communication, they are the cells that can send a message from any place in the body whether its picking up information from an exterior environment, like touch, smell, taste, sight and hearing (depending on the type of receptor cell its picking up from) or from the internal environment, like blood pressure, body temperature, etc…

A typical Neuron has a Cell Body (that contains most of cellular organelles including nucleus and mitochondria), Dendrites which are tree like extensions that receive information either from receptor cells, or from a previous neuron. An Axon, which is the main extension that sends the information coming from the cell body and dendrites to one or many  neurons, by the help of Axon Terminals. Axon Terminals are extensions from a single Axon that connect to dendrites with Synapses. A synapse is the place of "contact" between the axon terminal and the dendrite.

We divide the neurons into types depending on their function, either sending information to the Central Nervous System (CNS) or receiving information from the CNS and sending it to the effectors needed, like muscles. So three basic classes exist:

1-Afferent neurons: they transmit the information coming from the receptor cells to the CNS. they are the neurons that help you feel pain, pressure, temperature or vibration depending on the receptor to which they are connected to. That's why they are called Sensory Neurons.

2-Efferent neurons: they transmit the information coming from the CNS to muscles essentially (or effectors). They are the neurons that help you do something about the pain you felt (ex: hot water gets spilled over your hand, you feel the pain, you put your hand in cold water). That's why they are called Motor Neurons.

3-Interneurons: they are the ones to receive the "data" from the Sensory Neurons (afferents) in the Spinal cord and delivering the response to the Motor Neurons (efferents). They play the role of a middle-man, usually small (always inside the CNS), they can also have connections between each other.

A question that might come to your mind, is when does the differenciation between axon and dendrites occur? since originally they all start as small protrusions coming from the Soma or Cell body.

There is basically 5 stages in which the polarization is achieved: 
Stage1: lamellipodial and filopodial protrusions.
Stage2: Multiple immature neurite extensions.
Stage3: Breaking of symmetry: Axon Specification.
Stage4: Axon and dendrite outgrowth, branching.
Stage5: Spine morphogenesis, synapse formation.

Glial Cells: 

Also called "Neuroglia", the term Glia comes from a Greek word meaning "Glue", since in the 19th century, it was presumed that they held the nervous system together in some way. But now it is known that they are of a main importance in keeping the nervous system well and functioning, they do not interfere directly in synaptic functions as Nerve Cells do. Their roles include supplying the neurons with nutrients and oxygen, cleaning remains of dead neurons and insulating the neurons from one another. They are also known as "Supporting cells".

4 types of supporting cells exist, 3 in the Central Nervous System (CNS): Astrocytes, Oligodendrocytes and Microglia, 1 in the Peripheral Nervous System (PNS): Schwann Cells.

Before we begin describing each type of the supporting cells and its function, we will define what the Central and Peripheral Nervous Systems consist of:

Central NS: Is protected by the skull, bones of the spine and a blood-brain barrier. It includes the brain and the spinal cord, along with the second cranial nerve, optic nerve and retina.

Peripheral NS:  Is not protected by any bones, it is thus exposed to all mechanical injuries and toxins. It includes all the nervous tissue outside the Central NS, which are nerves and ganglia, 12 cranial nerves, and 31 spinal nerves.

Now that we have a notion of what are the Central and Peripheral NS, we can begin explaining each type of supporting cell:

Astrocytes: The most abundant of cells in the NS and found in the CNS, they are star-like shaped cells, unlike a neuron they don't have an axon. They are known to have many roles and are essential and necessary for a working NS. Their main roles include:

-providing neurons with the right nutrients from the blood stream like glucose, providing the right ionic environment and extracellular ionic balance, healing and repairing of the brain and spinal cord after traumatic injuries.

Fig1: Representation of myelinated axons in the CNS, showing how Oligo

dendrocytes wrap themselves around many axons forming myelin sheaths.

Oligodendrocytes: found in the CNS, their main function is to speed up nerve conduction, and this is done by a very simple way, supposedly the electrical message would be very slow if it had to travel all the axon, so by putting a barrier of myelin (white fatty substance) the electrical message would have to jump over it, this makes it a lot faster (see image of Oligodendrocyte for more details), the region between each myelin sheath and the other is called Ranvier Node. So the signal jumps from node to node, until it reaches the axon terminals (see image of Shwann Cell for more details).

Microglia: found in the brain and spinal cord they represent the first and main immune system in the CNS. They may be called "resting" macrophages, but they are the fastest moving cells in the central nervous system and are activated when there is an injury in the brain or spinal cord. They also have processes that send and retract protrusions (something like extensions) to scan their environment and domain. Considering the speed of these processes, it appears that they can scan the whole brain in several hours for any harm or injury. When activated they are called to the place of injury or harm and eliminate damaged cells by phagocytosis.

Shwann Cells: are the only supporting cells of the PNS (peripheral nervous system). Just like oligodendrocytes, their main objective is to speed up the electrical signals (or action potentials) travelling through the axon. The only difference between them and oligodendrocytes, is that each Schwann Cell wraps itself around a single axon making only one block of myelin sheath. Oligodendrocytes on the other hand, are bigger cells that send extensions and wrap themselves around many nerve axons at once (see Fig2 for more details). But these cells (Schwann) have many other important functions in the PNS, like guiding regrowth of nerve axons and cleaning residues. 

Fig3: Representation of a Shwann myelinated axon, showing  the saltatory conduction of 

the action potential due to a series of blocks of myelin sheaths.  

Fig2: Myelinated axons dependending

on  the type of the supporting cell

It has been proven by many studies that when a PNS axon is cut due to an injury, the part linked to the effectors (muscles and other) withered away leaving behind it, the Schwann cells that used to myelinate it , staying in place, those Schwann cells helped the cut axon find its way back when growing again.

Knowing the two types of cells that make up the majority of the nervous system (i.e Neurons and Glial Cells), we can start describing the essential parts (organs) that make up the CNS (Central Nervous System) and the PNS (Peripheral Nervous System).