Navigating the Wonders of Neurons and Neural Networks 

Like cells are the building blocks of life, neurons are the building blocks of the nervous system

Neurons

We have 100 million neurons (nerve cells) in our human nervous system. With 80% located in the brain!

It plays a crucial role in communication by transmitting signals electrically and chemically, to the nervous system. 

We have 3 types of neurons: 

• Sensory neurons
• Carry messages from the peripheral nervous system to the central nervous system
• Short axons and long dendrites
• These are activated by sensory input from the environment (like touching a hot surface with your fingertips)
• Relay neurons
• Connect sensory neurons to motor or other relay neurons
• Short dendrites and short axons
• Allows sensory neurons and motor neurons to communicate
• Motor neurons
• Connects central nervous system to effectors like muscles and glands
• Short dendrites and long axons
• Transmits spinal cord to skeletal and smooth muscles and can control our muscle movements

Speaking of dendrites and axons, what is the difference?

Axons are long and unbranched, whereas dendrites are short and highly branched.

Structure of neurons

From a millimetre to a metre long, neurons vary in size.

Their features: 

• Cell body (a.k.a soma)
• Contains nucleus (so it contains the genetic material of the cell)
• Dendrite
• Branch-like structures which receive nerve impulses from other neurons and carry them to the cell body
• Axons
• Carries impulse away from the cell body down the length of the neuron
• Transmits electrical impulses so they can be received by other neurons
• Myelin Sheath
• Covers axons and protects it as it is a fatty layer
• Speeds up the electrical transmission of impulses 
• Nodes of Ranvier
• Segmented gaps on myelin sheath
• Speeds up transmission by forcing it to "jump" across gaps on axon
• Terminal buttons
• End of axons, and communicate with next neuron in a chain across a gap, known as a synapse

Why does myelin sheath need nodes of Ranvier (gaps)? if it was continuous, it would have a reverse effect and slow down electrical impulses.  

Electrical transmission- firing of neurons 

A neuron in a resting state is negatively charged compared to the outside

Activated by a stimulus, the inside of the cell becomes positively charged for a split second causing an action potential (rapid sequence of changes in voltage along a membrane because information is transmitted down an axon) to occur. 

The electrical impulse then travels down the axon towards the end of a neuron. 

Synaptic transmission 

Neural networks = are how neurons communicate with each other within groups

In the photo above, you can see a synapse. Each neuron is separated from the next neuron by a synapse. Synapses are essential in allowing neurons to communicate with each other and allowing transmission of nerve impulses between each other. 

As you can see, between the synapses, there is a space. This space is called the synaptic clefts. 

Between the synaptic clefts, there is a presynaptic terminal and postsynaptic receptor site.

The presynaptic terminal is the nerve cell that fires the neurotransmitter and releases it in the synaptic cleft. The postsynaptic receptor site takes up the neurotransmitters once it enters the synaptic gap. 

Synaptic transmission is the process of communication between neurons occurring. Signals within neurons are transmitted electrically. Signals between neurons are transmitted chemically. 

Once an electrical impulse reaches the end of a neuron (presynaptic terminal) the release of a neurotransmitter is triggered from synaptic vesicles (tiny sacs).

Using the second picture above:

Presynaptic terminal = top right one. 

    Synaptic vesicles = small round things in it

    Neurotransmitter = seen in both pictures are the tiny balls in the gaps

    Postsynaptic receptor sites = bottom left of second picture

Neurotransmitters

Having seen these photos, I have never felt so interested in this section. I hope you feel the same :p

As we know, the chemicals diffusing through the synapse to the next neuron in the chain is the neurotransmitter. 

Here comes the next step:

The neurotransmitter crosses the gap and is taken up by the postsynaptic receptor site, which is the dendrites of the next neuron. 

The chemical message is converted back into an electrical impulse and the process of transmission begins again in the other neuron. 

Several dozen neurotransmitters have been identified in the brain, spinal cord and glands. Each neurotransmitter has its specific structure allowing it to fit perfectly into the post-synaptic receptor site. 

Neurotransmitters have specialist functions. 

Excitation and inhibition

Neurotransmitters can either have an excitatory effect or an inhibitory effect on the neighbouring neuron. 

What are excitation and inhibition?

Excitation =  When a neurotransmitter increases the positive charge on a postsynaptic neuron. This then increases the likelihood of a neuron firing and passing on an electrical impulse. 

    e.g. adrenaline

Inhabitation = When a neurotransmitter makes a postsynaptic charge more negative. This decreases the likelihood of a neuron firing and passing on the electrical impulse.   

     e.g. serotonin 

Summation 

Whether a postsynaptic neuron fires is decided by the process of summation. 

The net effect of a postsynaptic neuron is inhibitory, then the postsynaptic neuron is less likely to fire

The net effect of a postsynaptic neuron is excitatory, then the postsynaptic neuron is more likely to fire, causing the inside of the neuron to be positive. 

Once an electrical impulse is created, it travels down the neuron. 

The action potential of a postsynaptic neuron is triggered once the sum of excitatory and inhibitory signals reaches the threshold (enough to transmit between 2 states).