How is your spinal cord working? Wait what! Weird question huh! Actually I found this question quiet funny. But it also put me to think that we always ask how is your brain working? Actually, we don’t ask this too.
I mean generally, our functioning majorly depends on our nervous system. From coordination to emotion to growth to our five senses. If any unit of this system fails, it can cause mild to severe problems. But the question is, what are its units i.e. forming components?
That unit is neuron along with non-neuronal cells or glial cells. Glial cells contain the same importance as that of neuron cells if not more. These cells are supporting cells for neurons that cover almost double the volume of nervous tissue more than the neuron itself. Let’s understand these.
This time, I will be stating them and structures related to it. Therefore, this time we will learn about:
- Glial cells
- Schwann cell
- Ependymal cells
So, let’s get started.
These cells that are smaller but more in number and, like I already said, provide support to neurons. What kind of support and cells we are talking here? Well…
- They provide protection, form myelin sheath (more about it later), clearing the surrounding environment of neurons i.e. debris or dead cells and maintaining homeostasis for proper signalling function of neurons.
- Some of the glial cells are oligodendrocytes, astrocytes, microglia, ependymal cells and radial glial cells in CNS. In PNS (peripheral nervous system) there are Schwann cells, satellite and enteric glial cells. Let’s spend some time here to explain briefly some of these. For the rest of the cells mentioned here, I will detailing them later on.
- Satellite glial cell- These cells cover neuron cell bodies in PNS and provide nourishment and cushioning to them.
- Enteric glial cell- These cells are found in nervous system of gut lining (the ganglia of myenteric and submucosal plexus) and maintain homeostasis there.
Radial glial cell
These cells are the progenitor of neurons.
- Also gives rise to some other glial cells like oligodendrocytes and astrocytes.
- These cells are named as such due to presence of long radial processes.
- There’s another significant function, it is to mentor the movement and axonal out-growth of newly formed neurons radially from the ventricular zone (zone containing neuronal stem cells including radial glia).
These are the functional and structural unit of our nervous system. Well, this is the simplest definition that I have learned from my childhood book.
- These cells are the actual cells through which impulses/signals are transmitted through from CNS to target cell/organ and vice-versa.
- Like I said couple of times, these are permanent cells i.e. do not divide.
- Structure consist of
- Dendrites– Tree-like projections which are the signal (input) receiving unit (from other neurons).
- Cell body or soma- Think of it like an abdomen that carries vital organs and spaces. Similar to that, it carries the vital structures for a neuron i.e. nucleus, ER and golgi complex etc. We can say that, dendrites and axon arise from here only.
- Axon– The longitudinal part which is signal passing unit of neuron from cell body to axonal terminals i.e. output of cell body. Also known as by nerve fibre.
Q#1. Can you visualise a neuron?
A#1. Hmm… we will know about that later. But 1st we need to know about what is Nissle body!
Nissl body or granules are ER with ribosomes which are found in nerve cell body and dendrites. And what does the ER with ribosomes (rought endoplasmic reticulum) do? They synthesize proteins. These proteins flow towards the dendrites and axon for structural maintenance.
To visualise neurons, Nissl staining is done during their histological study. For this, basic dyes are used as nissl body is basophilic (base-loving). These basic dyes are positively charged and tag the negatively charged DNA and RNA.
It is an insulating sheath (Q#2) that wraps around neuron.
- This insulation increases the conduction velocity of signals across the axon.
- Minimises signal loss.
- This is also favoured by some gaps present at regular intervals where myelin is absent, called Nodes of Ranvier.
- This conduction is known as by saltatory conduction (saltatory means jumping as conduction of impulse jump from one node to another).
- These nodes have high number of voltage gated sodium channels (A#1).
- Made up of protein and fat.
Q#2. How does insulation helps for increment of conduction velocity?
A#2. Actually, I answered the question already. Due to the presence of nodes at regular intervals, the impulse jumps on them regularly thereby speeding up the conduction speed. This is very significant especially during dangerous event or life-threatening situation when we need quick response.
You can read more about it here.
Or by reading About#1, you will get more clear idea.
A#1. Voltage gated channels: These are proteins present in membrane that are specific for specific location. For example here, we are talking about sodium channels (actually calcium channels are also present).
The channel on activation causes rapid flux of ions like sodium, calcium or potassium. And the activation occurs via any appropriate stimulus.
Voltage gated sodium channel are densely presented at nodes and start of the axon just after the cell body, therefore activating them cause a huge influx of positive sodium ions in the axon which in turn will cause a rapid depolarisation of axon membrane. This will happen as soon as the impulse will travel to the next region of polarisation (other node of ranvier).
These are a type of glial cell that wraps around axon and create the myelin sheath. (P#1)
- These cells are derived from neuroectoderm.
- Actually, one oligodendrocyte can produce myelination on more than one axon.
- Present in CNS. Later we will learn about Schwann cell and their similarity/difference with oligodendrocyte.
P#1. Multiple sclerosis: In this, the oligodendrocytes are damaged causing demyelination of neurons.
Causes: Autoimmune (AI) disease.
Symptoms: Varies on various factors like the location of nerve fiber, person and course.
- Weakness of limbs (naturally is one-sided).
- Barber chair phenomenon or Lhermitte’s sign– Electric-shock sensation (that radiates toward spine or limbs) during flexion of neck. You can check yourself by bending your neck towards your chest.
- Dizziness and Loss of balance
- Slurred speech
- Loss or blurriness of vision or diplopia (double vision where two images appear of a single object).
Treatment: Only progression can be slowed down and symptoms could be managed. But can’t be cured permanently.
- Using disease-modifying drugs like glatiramer, cladribine, dimethyl fumarate that reduces the flare-up events and progression of disease. If you are interested about more drugs, you can go here.
- Steroids for the same.
- Therapies to adapt yourself for your daily routine inspite of the condition, to reduce pain or fatigue.
These are a type of glial cell that wraps around axon and create the myelin sheath. (P#2)
- Originate from neural crest cells (themselves formed from embryonic germ layer called ectoderm).
- Found in PNS unlike oligodendrocyte.
P#2. Guillain-Barré syndrome: In this, during nerve fibre damage, shcwann cells are unable to remyelinate the nerve fibres leading to loss of proper conduction.
Causes: Damage is caused by our own immune cells i.e. also an AI disease.
- Bacterial infection like Campylobacter jejuni.
- This bacteria contains ganglioside-like epitopes (simply, part of antigen which is LPS i.e. lipopolysaccharide that is a virulence factor of gram-negative bacteria) that activates autoantibody. This mechanism is called molecular mimicry.
- Weakness in muscles
- Tingling in limbs
- Gait problem
- Speech impediment
- Breathing and swallowing problems
- Diplopia or blurred vision
Treatment: People can be treated, thankfully.
- One treatment is to prevent the attack of auto-antibodies by providing normal healthy antibodies in the circulation. This is done by intravenous immunoglobulin. Here, a healthy compatible blood is donated intravenously inside the patient.
- Or simply filtering the blood or more appropriately plasma from such antibodies where it is replaced by albumin (A#2) containing solution. This is called plasma exchange.
- Assisting by giving pain killers, ventilation facility or clearing pathways for egestion by laxatives.
A#2. Albumin: Protein produced by liver, present in plasma maintaining osmotic balance, carrying bilirubin, enzymes, ions etc.
These are star shaped (Greek Astron- star) glial cells.
- Derived from neuroectoderm.
- Most abundant glial cells in CNS.
- Roles are
- Support and repair provision to neurons.
- Protecting neurons from oxidative damage by releasing antioxidants.
- Essential component of BBB (A#3).
- Buffer for extraneuronal K+ presence.
- Buffer for glycogen fuel reserve i.e. preventing hypoglycaemic condition for neurons.
- Removal of neurotransmitter (Q#3).
Q#3. Removal of neurotransmitter, why is it necessary?
A#3. Actually, more suitable word here is uptake of neurotransmitter. This uptake is needed if we want a continuous and efficient conduction without any interruption.
A#3. Blood brain barrier: It is a protecting highly selective semi permeable barrier formed by endothelial cells.
- It is responsible for protection against many pathogens or immune cells, hence is protective against multiple sclerosis, tumor etc.
- It blocks hydrophilic molecules and blood components from entering into CSF.
- It allows hydrophobic molecules like oxygen and hormones.
These are tiny glial cells that are also present in CNS.
- Derived from mesoderm.
- Role is mainly as to perform phagocytosis during tissue damage.
These are simple columnar ciliated glial cells found in CNS.
- Derived from neuroectoderm like astrocytes.
- These cells are found in cavities like ventricles (Q#4) and spinal cord’s central canal.
- These cells are covered with cilia and microvilli that has independent function.
- Cilia provide flow to CSF (cerebrospinal fluid) thereby facilitating its circulation.
- Microvilli is needed for CSF absorption. It happens by arachnoid granulations that has villi for this purpose.
Q#4. How many ventricles are there in our body?
A#4. If you are saying 4, then congrates. You are also wrong just like many others. This question is a trick question, as it is an incomplete one.
There are many ventricles present in our body like in brain here or in heart.
Though the answer is 4. There are 4 ventricles present in brain. In heart, there are 2.
A brief description about ventricles of brain- These are cavities in which CSF fluids fills the inside of it and circulates. These ventricles are continuous.
There are 2 lateral, 1 third ventricle and 1 fourth ventricle. By the way these ventricles has some openings. Do you know them?
So, these were all about our nervous system… Kidding. How can our nervous system be this small? Actually, it is one of the most complex system in our body and competitive field for doctors.
You may know already that neurosurgeons are one of the most stressed doctors. They are satisfied but are totally mentally burned out from their duties. I mean, no matter how small disease or disorder you have associated with your brain or nervous system, it IS going to affect you somehow. However, no field or role is small just like a disease. I am not saying others role is less significant but this field affect you the most.
Though, I myself want to test these things out as at the moment I far away from such a luxury to change people’s lives. Frankly, at the moment, I don’t even know what doctor I will be become. May be a neurosurgeon or an orthopedic. Let’s see.
But whatever that will be, there is a long road ahead. Till that, I have to keep working, keep learning. And keep writing and posting new articles also, so that you can also learn and grow with me.
I hope you have liked it. Leave comment and your feedback.