What are the major neurotransmitters in the CNS?
Written by Elijah Dave M. Cordova
Neurotransmitters allow brain cells to communicate with each other. They enable the transfer of information across gaps among neurons. Among the major neurotransmitters in the CNS are acetylcholine and catecholamines. Serotonin and GABA (γ-aminobutyric acid) complete the list.
Before we go into each of them, let us first classify neurotransmitters.
- Excitatory. They cause neuron firing of an action potential to a receiving neuron.
- Inhibitory. It is the opposite of excitatory action. It inhibits the firing of action potentials across synapses.
- Neurohormones. Neurons synthesize and secrete them into the bloodstream. Oxytocin and vasopressin are examples.
- Neuromodulators. They influence other neurotransmitters and affect several neurons at once.
Acetylcholine (Ach) is a neuromodulator that affects your attention and memory. It also impacts how you learn things through integration. Ach cells originate in your brainstem and midbrain. They then travel to every area of the CNS through synapses.
In the Peripheral Nervous System (PNS), Ach is an excitatory neurotransmitter. Neuromuscular junctions often use Ach to send signals between your nerves and muscles. For example, acetylcholine signals parasympathetic smooth muscle movement.
Choline acetyltransferase catalyzes the production of acetylcholine. It separates the acetyl part of acetyl coenzyme A (acetyl-CoA). It then joins this acetyl part to choline to form the product.
Catecholamines are neurohormones crucial to maintaining homeostasis in your body. Your adrenal glands on top of your kidneys produce them. Dopamine is a catecholamine. So, too, are norepinephrine (NE) (i.e., noradrenaline (NA)) and epinephrine (adrenaline) catecholamines.
Dopamine is a neuromodulator across many brain regions. One of its main functions is in the prediction and learning of rewards. Axons of your midbrain house dopamine.
L-amino acid decarboxylase (i.e., DOPA decarboxylase) synthesizes dopamine from L-Dopa. It is a precursor chemical. This same enzyme also facilitates the synthesis of histamine and serotonin, another neurotransmitter.
Dopamine β-hydroxylase synthesizes noradrenaline from dopamine. Its cells originate from your brainstem. Its main function is regulating the sympathetic nervous system. It helps regulate your body systems based on specific situations.
You can think of it as what regulates your arousal to specific situations. In times of stress, norepinephrine increases your alertness or wakefulness. It is so that you can respond in a proper manner and ensure your survival.
Epinephrine is the hormonal equal to norepinephrine. They work together to form your “fight or flight” response which you need to survive.
During stress, norepinephrine constricts your blood vessels to raise your blood pressure. Meanwhile, epinephrine forces your heart to contract with much greater strength. Its actions increase blood pressure and cardiac output.
Your body releases noradrenaline to your blood circulation at a low dose. Your body releases adrenaline in stressful moments alone.
Serotonin uses the amino acid L-tryptophan in its synthesis. Hence, it is also known as 5- hydroxytryptamine (5-HT). Most of it is in the raphe nucleus of your brain stem. Serotonin also cannot pass through your blood-brain barrier. It performs its functions in your brain alone.
Serotonin, like dopamine, is a neuromodulator. Unlike adrenaline and noradrenaline, it does not have a specific function. Instead, it affects many brain regions and body systems in consequence. Serotonin affects your mood, sleep, circadian rhythms, and body temperature, among others.
GABA is a derivative of glutamate (glutamic acid), a non-essential amino acid. It is the main inhibitory neurotransmitter of your brain. This means it inhibits the transmission of messages along neural pathways. It makes sure that your brain does not send random messages.
The enzyme glutamic acid decarboxylase (GAD) synthesizes GABA from glutamic acid. It demands cofactor pyridoxal phosphate (derived from Vitamin B6) in its synthesis. As GABA increases in your brain, it inhibits the action of GAD. With this, it manages its own synthesis.
GABA-A and GABA-B are its two receptors. They act on both receiving and transmitting targets. They do this to fine-tune the responses of your CNS. GABA cells work through lateral inhibition. This mechanism ensures the highlighting of important information in your brain.
In essence, GABA blocks noise or irrelevant message transmission. GABA also promotes sleep but inhibits brain regions that promote awakening. Abnormalities in GABA contribute to anxiety disorders which unnecessary brain activity often causes.
What triggers neurotransmitter release?
You have now come to appreciate the role of neurotransmitters in your body. Now, we ask how they travel across synapses. The opening of voltage-gated calcium channels triggers the release of neurotransmitters. These neurotransmitters travel to your synapses.
Your brain cells have dendrites that receive information and axons that conduct information. A myelin sheath can protect your axons and speed up its message transmission. Degradation of this myelin sheath leads to multiple sclerosis. It leads to a lack of muscle control.
Nerve cells also have the soma or cell body. It contains all things your nerve cell needs. The information your dendrites receive may belong from the environment or other neurons. Most neurons have one axon and many dendrites. These are multipolar neurons.
Each axon terminates on the next neuron at a synapse. These synapses are less than a millionth of an inch apart. A synapse is a specialized structure for transferring information.
The tip of each axon is the axon terminal. This part of your neuron contains synaptic vesicles that house neurotransmitters. Meanwhile, the postsynaptic membrane of the target cell has receptors for those neurotransmitters.
Your axon terminal also contains voltage-gated calcium channels. An action potential travels from your dendrites to your axons. When an action potential reaches your axon terminal, it changes its membrane potential. This action potential also opens these voltage-gated calcium channels.
Now, calcium flows into your axon terminal through diffusion. This event increases the concentration of calcium in your axon terminal. This calcium causes the fusing of vesicle and axon terminal membrane proteins.
This fusion allows your neurotransmitters to communicate outside the neuron through the synapse. These neurotransmitters diffuse into the synapse and bind to target cell receptors. They fit the receptors like a key to a lock.
Three things affect this process.
The frequency of action potentials fired down your axon opens more calcium channels. More calcium would flow in the axon terminal. More synaptic vesicles would fuse with the membrane of your axons. Hence, more neurotransmitters would travel into the synapse.
The duration of action potentials fired down your axon means longer neurotransmitter release.
Third, the blockage of target cell receptors causes failure of message transmission. It can also show the presence of a disease. Myasthenia gravis is muscle weakness due to circulating antibodies that block acetylcholine receptors.
When the train of action potentials stops firing, your calcium channels will close. Calcium stops flowing into your axon. The fusion between your vesicles and axon membrane stops. Hence, there are no more neurotransmitters that travel down your synapses.
Where can you find vesicles of neurotransmitters?
Neurotransmitter vesicles are also known as synaptic vesicles. These vessels store neurotransmitters for transport across synapses. They are in the neuron region known as the axon hillock or axon terminal. This region is the release zone.
Synaptic vesicles contribute a great deal toward the transmission of neurotransmitters across neurons. They aid in facilitating the communication between the CNS and the rest of your body.
When an action potential travels down your axon terminal, it generates a set of reactions. This results in the release of neurotransmitters kept by your synaptic vesicles. These neurotransmitters travel through synapses to communicate with target cells.
Removal of neurotransmitters from the synapse entails either of the following:
Here, uptake pumps take your neurotransmitters from the synapse. Then, your axon terminal membrane would close off. An example of this would be serotonin uptake pumps. Selective serotonin reuptake inhibitors (SSRIs) induce serotonin buildup in your body.
- Deactivating Enzymes.
These enzymes break down your neurotransmitters. An example of this would be acetylcholinesterase (AchE). It breaks down acetylcholine in your synapses. Inhibition of its action or production builds up acetylcholine in your synapse.
How does neurotransmission affect human behavior?
Everything psychological is biological. Our biological condition affects our ideas, impulses, and moods. Our neurons communicate with neurotransmitters. This communication leads to motion and emotion. They make us move and feel.
Endorphins make you feel good. They are like opium in that they associate with the control of pain and pleasure. Your systems would flood with endorphins after exercising or eating delicious food. Falling in love can make you feel good too.
Norepinephrine controls alertness and arousal as discussed in a previous section.
Glutamate, from which we derive GABA, is also a neurotransmitter. It helps manage memory. Having too much glutamate in your CNS could cause migraines or seizures. Hence, some people sensitive to glutamate avoid monosodium glutamate (MSG). It is an ingredient you can find in ramen.
Serotonin affects your feelings of hunger, your mood, and your sleep. We link depression to low amounts of serotonin. Some antidepressants like SSRIs treat depression by increasing serotonin levels in the brain.
Acetylcholine affects learning, memory, and muscle action. Deterioration of acetylcholine-producing neurons causes Alzheimer’s. It is a progressive neurological degradation due to brain degeneration.
Dopamine affects attention, emotion, and learning. It also impacts movement and pleasure. An excess of dopamine causes schizophrenia. It is a chronic psychiatric illness where you disassociate behavior, emotion, and thought. Excess dopamine also causes other addictive or impulsive behaviors.
Acetylcholine and dopamine neurotransmitters can be excitatory or inhibitory. It depends on the type of receptors they encounter.
Like neurotransmitters, hormones act on the brain. Some hormones are even identical to certain neurotransmitters. Hormones also affect our arousal, mood, and circadian rhythms. Besides this, they can impact physical growth and aid in sexual reproduction. They can also control your metabolism.
Neurotransmission occurs at high speeds. Hormones take their time and deliver slow communications through glands. Hence, hormones linger. This explains why it takes time to calm down after having a bout of severe stress or trauma.
Adrenaline works with noradrenaline for the fight or flight response.
Your endocrine and nervous systems work together to affect your behavior.
What neurotransmitter causes happiness?
Does happiness make you healthy? Does being healthy make you happy? We may go different ways here but we can agree that both are important. Hence, we now concern ourselves with what neurotransmitter causes happiness.
Seven neurotransmitters provide a general feeling of well-being. Adrenaline, dopamine and endocannabinoids are among them. So, too, do endorphin, GABA, oxytocin, and serotonin belong.
Adrenaline elicits an exhilarating feeling. It also creates a surge in energy. It makes you feel alive.
Dopamine drives behaviors driven by rewards and seeking pleasure. Several addictive drugs act on the dopamine system. These drugs block dopamine reuptake, leaving it in your synapses longer.
Endocannabinoids that include anandamide have similar effects to marijuana. They prevent anxiety and burnout. They contribute to the runner’s high you would get after sustained running.
Endorphins resemble opiates. They are painkillers in essence.
GABA creates a sense of calmness by inhibiting the firing of neurons. Benzodiazepines are anti-anxiety medications that work by increasing GABA.
Oxytocin is a neurohormone linked to feelings of intimacy and affection. Couples distant from each other for a long time have decreased oxytocin levels.
Serotonin affects your appetite, mood, social interaction, and performance. High serotonin levels mean increased self-esteem and feelings of worthiness.
Where is GABA neurotransmitter produced?
GABA is the brain’s main inhibitory neurotransmitter. It is also a major inhibitory neurotransmitter in the spinal cord. Glutamate decarboxylase uses Vitamin B6 (pyridoxine) to synthesize GABA from glutamate. GABA production happens in the brain and β-cells of the pancreas.
Several diseases exhibit GABA deficiencies:
- Dystonia and spasticity.
Dystonia is involuntary muscle contraction. It causes repetitive and twisting motions. Spasticity refers to stiff muscles. A deficiency in GABA signaling can cause these two diseases.
- Hepatic encephalopathy.
It refers to impaired brain function due to advanced liver disease. Elevated ammonia levels can bind to a GABA complex and cause this disease.
- Huntington’s disease.
It is an inherited disease the causes the progressive degeneration of brain neurons. A lack of GABA exhibits this disease.
- Pyridoxine deficiency.
It is when your body does not have Vitamin B6 to produce GABA. Frequent seizures during infancy are common.
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