Last week, we have learned about the structures of the synapse and the functions of neurotransmitters. Let’s dive deeper into the process of neurons carrying messages and how they communicate with other neurons.

There are three types of neurons: motor neuron, sensory neuron and inter neuron.

Motor Neuron

They are part of the CNS and they connect to the muscles, glands and organs throughout the body.

Sensory Neuron

They are activated by sensory input from the environment.

Interneuron

They are the neurons in between and connect spinal motor to sensory neurons.

 

Let’s begin with the neuron structure:

 

This diagram shows the different parts of the neuron. The first part is the cell body which is the cell’s life support center. The nucleus that is within the cell body is in charge of providing energy for the neurons to carry out its functions. The tree-like branches surrounding the cell body are called dendrites. Their purpose is to obtain information from the other neurons and sensory receptors. Some types of neurons do not have dendrites and some have multiple dendrites. They kind of look like arms that grab onto other information. The long chord looking thing that connects with the dendrites is called the axon. They make sure to carry the neuron’s message to the other parts of the body so that the body can function properly. The axon is then covered by layers of Myelin Sheath for some of the neurons; this helps speed neural impulses. Finally, the smaller branches are called terminal branches of axon and they form junctions with other cells.

Neuron Function

How an action potential moves along the neuron fibre

The first step is called Resting Potential. The neuron has more negatively charged ions in the fluid of the axon (sodium ions) with a “net” negative charge of around -70mV while the outside is positive (potassium ions).

The following step is called depolarization, which is when the sodium ions enter in the axon causing the fluid of the axon to become less negative. This happens with channels in the membrane that open to allow those Na+ ions to enter in the axon.

The third step is called Repolarization which is when the channels in the membrane open to allow K+ ions to leave the axon. The charge then returns back to -70mV but this causes the next segment of the axon to depolarize.

 

The final step is called Recovery Period which is when depolarization/repolarization causes action potential to move continuously down the axon.

 

Synapse Structure

There are many parts to the synapse but it is quite easy to understand once it’s been broken down into their sections.

The first part where the neurotransmitters are entering from the top is called the axon of sending neuron. Next is the axon terminal button which is in charge of sending a signal to the receiving neuron’s dendrite. The purple looking bubbles are called the synaptic vesicles and they are called this because they keep hold of the neurotransmitters. Next we have the presynaptic membrane which is found underneath right before the synaptic gap. Next we have neurotransmitters which are those dots that are inside the synaptic gap. The synapse consists of the tips of the terminal branches of axon, the tiny space between neurons (the gap) and the ends of the dendrites of receiving neuron. Then there is the receptor sites on receiving neuron which is where the neurotransmitters do a lock and key.

Synapse Function

How a signal is sent from axon of sending neuron to dendrite of receiving neuron.

When an action potential reaches the end of the neuron, a chemical signal (neurotransmitter) is sent to the next neuron. It starts off which the axon terminal button which produces neurotransmitter (NT) which stores NT in synaptic vesicles. They then recycle the neurotransmitters building blocks. After, the action potential then reaches the axon terminal, causing the synaptic vesicles to release neurotransmitters into the synaptic gap. The neurotransmitters then diffuses through the gap and does a lock in key with receptors on receiving neuron. The message of the neurotransmitters can either be received as excitatory, which stimulates action potential on receiving neuron or inhibitory, which represses AP on receiving neuron. To determine whether or not the receiving neuron sends its own action potential is by determining if it’s more excitatory than inhibitory which will be yes, or no if the inhibitory is less than the excitatory. This is determined by the action of the receptor: GABA causes an inhibitory reaction and glutamate causes an excitatory reaction.