.png)
Walking, thinking, eating, speaking are all functions that are guided by neurons. Even as you are reading this sentence right now millions of signals are firing through your brain processing what you are reading, the smell of your surroundings, and even the feel of your clothes. These signals are fired through brain cells known as neurons. However, unlike electricity these neurons don't touch each other to transfer these signals. Instead, the space in between neurons is called the synapse.
What Are Neurons?
Neurons are nerve cells that send messages all over the body to and from the central nervous system (which consists of the spinal cord and the brain) and eventually to the brain. As the brain develops and learns some new neurons may grow and build more pathways, called neural circuits, that transfer signals between different areas of the brain. However, some neuroscientists also believe that the addition of too many neurons can disrupt the brain's communication system (see problems with synapse below for more information).
There are three types of neurons:
Sensory neurons - carry information from the sensory organs (eyes, ears, mouth, etc.) to the brain
Motor neurons - control voluntary muscle activity (ex. walking and talking) and carry messages from the nerve cells in the brain to the muscle
Interneurons - act as the capillaries of the nervous system by transferring signals between sensory and motor neurons directly without going to the brain
The Signal Through the Neuron
The process of the neuron involves the entire neuron starting from the dendrites at the top of the neuron all the way down to the axons at the end of the neuron. The signal through the neuron is quite a process but it happens quite fast taking about 0.5-100 meters/second. Below are the steps that a signal takes to go across the neuron:
The neurotransmitter binds to the specific receptors in the dendrites which releases an electrical signal down the dendrites towards the soma or the cell body (the organelle that carries out the cell function).
This signal passes through the cell body into the axon which is wrapped with myelin that speeds up the impulses conducted through the axon. The act of signals traveling down the axon is called the action potential. Between each myelin is the Node of Ranvier.
Once the signal reaches the end of the axon, they stimulate neurotransmitters in vesicles which leave through the axon terminals and enter the synaptic cleft
Finally they bind to the respective receptor (like a key fitting a lock) to stimulate another set of signals in the next neuron
In order for the action potential to occur or the impulse to travel down the axon, a certain level of energy is needed to start this. Similar to how one manually starts a lawn mower, it must pass through a threshold in order to start the action potential down the axon. This process is called the all-or-nothing principle.
When it does pass this threshold the depolarization occurs. This is when channels, called sodium-ion channels with the help of ATP (aka. energy), open up allowing the positive ions to enter making the inside of the cell more positive.
In repolarization, this positive charge exits as the energy is removed from the sodium ion pump which leads to a much more negative charge inside the neuron. However, in what is known as the refractory period (aka. hyperpolarization) the neuron returns to its original state. Once it returns to its original state it can once again take signals.
The Process of Synapse
Between two neurons there is a very small gap where the process of synapse occurs. This gap is called the synaptic cleft and it is where neurotransmitters or electrical signals are exchanged to stimulate the action potential of the next neuron. In order to understand the process of synapse there are three main parts you should know:
Presynaptic endings - these are the ending or the axon terminals of the neuron that contains the neurotransmitters that are being sent to the next neuron
Synaptic cleft - as mentioned before it is the gap between two neurons where neurotransmitters and electric signals pass
Postsynaptic endings - this is the beginning or the dendrites of the neuron which the neurotransmitters and electrical signals are being sent to
While having the potential to send on messages to "connecting" neurons, it can also change these messages. Postsynaptic neurons can also send back neurotransmitters to the presynaptic cell telling them to change how often a particular neurotransmitter should be released.
Types of Synapse
There are two main types of synapse: the chemical synapse and the electrical synapse. The process explained above is the chemical synapse but the electrical synapse also has similar functions.
Chemical Synapse
The neuron that is sending the neurotransmitter to the next neuron (the presynaptic neuron) triggers a release of chemical messengers, called neurotransmitters. The neurotransmitters bind to the specialized receptors of the postsynaptic cell (receiving neuron) This neurotransmitter either excites or inhibits an action potential in the next neuron.
Electrical Synapse
Electrical synapse allows electrical signals to travel through the gap junction of two neurons, which rapidly speeds up the transfer of signals. The protein channels in the neurons make it possible for the positively charged current to flow from the presynaptic neuron to the postsynaptic neuron.
Here is a quick diagram of the differences between a chemical and electrical synapse:
Problems with Synapse
Many problems arise from the mishap of synapses. Dysregulated synapse can cause many neurological problems including autism, down syndrome, startle disease, and epilepsy and sometimes even neurodegenerative disease including Alzheimer's and Parkinson's disease.
Epilepsy
Epilepsy is a disorder of the brain characterized by repeated seizures (an alteration of behavior due to a temporary change in the electrical functioning of the brain). Because epilepsy does in fact involve changes in the electrical conduction of the brain, it can affect the synapse greatly. Namely, in epileptic people there is often the disappearance of the presynaptic terminals, a modification of the neurotransmitter content in the vesicles, and/or an alteration in the control of the release machinery.
Down Syndrome
Down syndrome includes neurological complications including dystonia, epilepsy, psychiatric problems, and cardiac, auditory, and visual defects. Similar to epilepsy there can be significant changes in the axons or the neurotransmitter itself. Another study by the University of Michigan, suggested that there is increased axon growth and synapse locations that put a break on the neuron's other activities. With this change in neuronal activities there is significant inhibition of the sensation, cognition, and behavior parts of the brain (and sometimes other parts of the cerebral cortex).
Startle Disease
Those with startle disease can have an excessive startle reaction such as eye blinking or muscle spasms to sudden unexpected noise, movement, or touch. In glycinergic synapse, often with subunits of glycine (an amino acid that makes a protein) receptors, if there is a loss-of-function mutation it can cause startle disease.
Autism
Finally, autism spectrum disorder also is caused by malfunctions in the synapse. Autism causes developmental challenges caused by differences in the brain. For more information visit our blog post on Exploring Autism. Those with autism have a surplus of synaptic connections (aka. too many connections between neurons). Though the brain does get rid of excess synaptic connections, due to a slowdown in this process Autism can occur.
Sources