Neurons are the only cells in the body that have the ability to carry information all over the system. They have unique cellular components called dendrites and axons that transmit data through synapses between these cells, where they are also broken down.
A synapse is defined as a gap between neurons, a space between cells. It is made up of a presynaptic end and a postsynaptic end, as well as a synaptic cleft. The presynaptic cell is the one sending the message across the synapse to the receptor sites of the postsynaptic cell.
There are also two types of synapses, chemical and electrical. In an electrical synapse, the presynaptic cell releases neurotransmitters once it reaches a certain charge. The electrical synapse then sends this charge to the terminal axon of the presynaptic cell, which can transmit into neighboring dendrites, the bloodstream, another axon and even into the extracellular space. This gives the nervous system direct access to multiple parts of the body.
For an interaction between neurons to happen, an electrical impulse should go down the axon towards the synaptic terminal. This will cause vesicles of neurotransmitters to move to the presynaptic membrane, discharging them into the synaptic cleft.
According to Dale’s law, a principle in neuroscience, it was first believed that a neuron generated and distributed only one kind of neurotransmitter. However, amongst many other medical breakthroughs of today, evidence has emerged indicating that neurons may be able to produce a variety of these chemical messengers.
Diffusion of Neurotransmitters Across the Synaptic Cleft
In this process, neurotransmitter compounds disperse all over the synaptic cleft where they are able to reach their specific receptor site at the postsynaptic end, triggering an electrochemical reaction in the neuron as a result. As neurotransmitters stick to a receptor, the postsynaptic end causes a shift in the cell’s electrical excitability, making it more likely to start an action potential. When the amount of excitatory neurotransmitters reaches a certain density the message will be transmitted to the next end point. Many medications and neurotoxins affect the transmitting or receiving sites of the synapse.
Release of Neurotransmitters
Discharge of these chemical messengers happen unusually fast through cellular secretion, brought on by incoming nerve impulses or action potentials. Inside the presynaptic nerve terminal, vesicles that contain neurotransmitters are situated in close proximity to the synaptic membrane.
In many cases, an incoming action potential causes an increase of calcium ions that which enables neurotransmitter vesicles to stick with the presynaptic membrane. These are generated by means of the voltage-dependent, calcium-selective ion streams located at the tail current of the synapse. Once released from the vesicles, the neurotransmitters can cross the cleft to the receptor site.
The fusion of a vesicle is known as a stochastic system. This is often a random process which usually results in a usual malfunction of synaptic transmission at the tiny synapses. This is a typical scenario in the central nervous system. Huge chemical synapses like the neuromuscular junction possess a synaptic release probability of 1.
The esicle fusion is powered by the action of a couple of proteins in the presynaptic station called SNAREs. Overall, the protein complex that mediates the arrival and mixture of presynaptic vesicles is known as the active zone. Then, the membrane applied by the fusion process is eventually recovered by endocytosis and reused for the creation of new neurotransmitter-filled vesicles.
Receptors on the other side of the synaptic cleft join neurotransmitter molecules. These receptors can react in two common ways:
First, the receptors have the ability to immediately unlock ligand-gated ion paths in the postsynaptic cell membrane. This helps ions to enter or exit the cell which alter the nearby trans-membrane potential.
The consequential alteration in voltage is known as a postsynaptic potential. When it comes to depolarizing currents, the outcome is excitatory and inhibitory when it comes to hyperpolarizing currents.
Regardless of whether a synapse is excitatory or inhibitory will depend on what types of ion channel conduct the postsynaptic currents, which scientists claim is a process of the sort of receptors and neurotransmitter used at the synapse.
The other way a receptor can have an impact to the membrane potential is by assisting the generation of chemical messengers within the postsynaptic neuron. The excitatory and inhibitory response to neurotransmitters can then be enhanced by these types second messengers.
Role in Memory
It is generally accepted that the synapse helps build memories. Since neurotransmitters stimulate receptors across the synaptic cleft, the link between the two neurons is strengthened when both of them are working simultaneously. The intensity of two attached neural pathways is said to lead to the storage of information, leading to memory formation. This method of synaptic conditioning is called long-term potentiation.
By chaging the discharge of chemical messengers, plasticity of synapses could be regulated in the presynaptic cell. Altering the function and quantity of postsynaptic cell receptors can also be controlled. This is one of the reasons supraphysiological loads of neurotransmitters available from illicit drugs alter brain structure and function.
Adjustments in postsynaptic signaling is usually related to N-methyl-d-aspartic acid, receptor-dependent long-term potentiation and long-term depression, that happen to be the most examined kinds of plasticity at excitatory synapses.
The Cerebrospinal Fluid (CSF) is a watery liquid consistently generated and assimilated. Present in the ventricles, spinal cord, and around the surface of the brain, it is soaked up in the venous system, and is created in the choroid plexus – a framework of enfolded arteries in the lateral, third, and fourth ventricles.
CSFstreams from the lateral ventricle to the third with the help of the inter-ventricular foramen, also known as the Foramen of Monro. The cerebral aqueduct, or the Aqueduct of Sylvius, holds the third and fourth ventricles together, while the Magendie and Luschka foramens are where they pass through to get to the subarachnoid space.
Its absorption into the bloodstream happens in the superior sagittal sinus through structures known as the arachnoid villi. These are one-way valves that permit the cerebrospinal fluid into the bloodstream as needed, or once its pressure is higher than the venous pressure, but do not allow blood to enter. In case production surpasses release, CSF stress increases, resulting to conditions such as hydrocephalus to happen. The acquired fluid in the course of a lumbar puncture would help lower the pressure, but this is a risky procedure and must only be undertaken with medical supervision.
Cerebrospinal Fluid in the Brain
Maybe you have heard athletes talk about concussion, or maybe you’ve seen it in the news. But for those of you who do not know, it is basically trauma to the brain taken from a physical impact.
What most people do not know, however, is that our brain uses a framework wherein holes present in our skull help reduce the severity of a concussion. You read that right: we have holes in our skull that actually shield us from severe trauma. Learn more about this and the cerebrospinal fluid’s role in the ventricles, choroid plexus, and the blood-CSF barrier, among many others, as we move on in this chapter.
If we were to cut the brain in half, you would see the huge indentations inside it – these cavities are known as the ventricles of the human brain. It also has the same name as the sections in our hearts, but for the sake of this book, it should not be confused with them.
There are four ventricles present in the brain. The first and second ventricles are generally known as the right and left ventricles, as they are located on each side of the brain, while the third and fourth ventricles, however, are not coupled, and are situated along the midline of the brain, right smack in the middle of the two.
The Choroid Plexus and CSF
The ventricles are a somewhat boring topic by itself, as they are simply just the gaps in your brain. What is more interesting and vital lies within these holes or cavities.
If we were to take a look inside the brain without pulling it out of our skulls, these holes are stuffed with fluid made from a framework known as the choroid plexus. Again, this structure is situated in the ventricles, and is also responsible for generating and producing the cerebrospinal fluid.
The CSF performs many essential roles in safeguarding the mind. It covers the brain and spinal cord in a defensive material that supports them during any physical injury to the head. Though as protective as it may be, it is not strong enough to prevent all traumas, especially those severe in nature.
Consider it like a pillow filled with water. You put one on top of a wall then decided to punch it. As a result, it will contort and reform itself to deal with the impact. Since the water pillow can endure the mild hit your fist made, you would not be able to harm the wall behind it. However, strike the pillow hard enough, and your fist will go through to the other side and will be able to hit the wall. The same concept applies to the brain, the skull, and the cerebrospinal fluid. In case an excessive force impacts your head, your brain will greatly hit your skull, which may then lead to a concussion.
Besides serving as a cushion and a physical barrier to the brain, the CSF also acts as a chemical buffer. It helps dilute toxic substances that may enter the body and the central nervous system to give other systems more time to counteract them.
Cerebrospinal fluid (CSF) analysis is a method of searching for environmental elements that can influence both the brain and the spine. This series of medical tests performed on a sample of the fluid, is far better in analyzing and recognizing signs of illnesses or diseases concerning the central nervous system, than that of any blood tests.
However, it is harder to acquire spinal fluid compared to a blood sample. Getting into the spinal canal with a needle takes practice and should only be done by specialists with expertise in spinal anatomy. Knowledge about any existing brain or spinal conditions should always be noted before attempting such a procedure.