Author Archives: Kenzo Le Vaillant

The Serotonin system

Accessible Science

Introduction

This page is the first part of a study of the article

Serotonin and emotion, learning and memory by Alfredo Meneses and Gustavo Liy

Serotonin, also called 5-hydroxy tryptamine, is a famous neurotransmitter said to be involved in happiness.

However, that is only a part of its identity. Serotonin and the serotonin system is much more complex than that.

The serotonin system

Serotonin has 7 classes of receptors. 5HT1 to 5HT7. These are all involved in different functions and some work differently.

For example, the 5HT3 receptor class, has 3 different receptors. 5HT3A to 5HT3C. These are all ligand gated cation channels.

Cation

An ion with a positive charge.

In comparison, the 5HT1 (from A to E), 5HT2 (from A to C), 5HT4, 5HT6 and 5HT7 receptors are all GPCRs.

Even those of the same receptor type will have different responses.

When activated most of the 5HT1 receptors will cause hyperpolarisation while the 5HT2,3,4,6 and 7 will cause depolarisation.

Hyperpolarisation

The phase in which the membrane potential becomes lower than the resting potential.

It occurs after the repolarisation phase.

Repolarisation

The decrease in membrane potential that occurs after the depolarisation phase, the action potential.

A transporter

The receptors aren’t the only ones that uptake serotonin. Serotonin has its own transporter, the SERT.

SERT

A protein that binds to serotonin and takes it back to the presynaptic neuron to be reused.

However, when the rate of SERTs are too high compared to serotonin, this causes too much serotonin to be taken back before even being used. This blocking of the serotonergic transmission can be a cause of depression.

Projections

Serotonin projections and pathways are found in many areas of the brain including brain areas involved in memory, learning and emotions.

It is synthesised and stored in the raphe nuclei. The serotonin projections come from the raphe nuclei and extend to most of the forebrain areas and more.

Raphe nuclei

An area in the brainstem that produces and stores serotonin.

Forebrain

A.K.A. The prosencephalon.

It contains both cerebral hemispheres, the thalamic structures and more.

More simply, most of the brain except the brainstem and below, the cerebellum and regions around the 3rd ventricle.

One of the pathways is the dorsal raphe serotonin pathway that outputs to the amygdala and the frontal cortex. This pathway helps in the learning of defensive behaviours.

Dorsal raphe

The largest and back part of the raphe nuclei. It is where serotonin is sent from to the forebrain areas.

Another pathway is the dorsal raphe paraventricular serotonin pathway. This one outputs to the periaqueductal grey matter and inhibits the fight or flight reaction.

Fight or flight reaction

A stress response in which the SAM axis and adrenaline is strongly involved.

It activates the sympathetic nervous system and causes increased blood pressure, heart rate and blood sugar preparing for intense physical activity.

It also has effects through non serotonergic pathways as it has an influence on the transmission in cholinergic and glutamatergic pathways, the adrenergic system and the HPA axis.

Next Part : The funcion of serotonin in the brain

Definition : Hyperpolarisation

Accessible Science

Hyperpolarisation

The phase in which the membrane potential becomes lower than the resting. potential.

It occurs after the repolarisation phase and is caused by the prolonged opening of the potassium channels after the sodium channels are closed.

This causes potassium ions to continue leaving the cell causing the membrane potential to go down.

It prevents a neuron from being activated multiple times by the same signal.

Related

Depolarisation

Repolarisation

Membrane potential

Resting potential

Synapse

Neurotransmission