Discovery of neurotransmitter: neurotransmitters was first discovered in 1921 by Otto Loewi, an Austrian scientist
Neurotransmitter ,along with electrical signals are the languages of the nervous system- the means by which each neuron communicates with others to process information and send messages to the rest of the body. In another means neurotransmitter are an exogenous chemicals that implement neurotransmission, this chemical molecule is a type of messenger which transmits signals between nerve from: nerve cell to nerve cell , nerve cell to muscle cell or from nerve cell to gland cell.
These signals which are transmitted between two nerves occurs in the synaptic cleft ( a gap between neurons at the nerve synapse) and here is where the electrical signals have travelled along the axon are briefly converted into chemical ones through the release of neurotransmitters, causing a specific response in the receiving neuron. Therefore neurotransmitters are classified into Excitatory and Inhibitory according to their functions.
Excitatory and Inhibitory
Excitatory transmitters, these neurotransmitters increase the rate of neuron firing which is the opening of the ion channels and allow the Na+ inside which causes depolarization. Whereas Inhibitory these transmitters decrease the rate of firing level of a neuron in which causes hyperpolarization which make more potassium K+ ions outside then the sodium Na+ ions inside. There are also effects , depending on the specific receptor types which they interact. For example amino acids GABA and glycine are usually inhibitory, while glutamate is typically excitatory. On the other hand , Ach and NE each bind to at least two receptors types that causes opposite effects. For example , acetylcholine is excitatory at neuromuscular junctions with skeletal muscle and inhibitory when released on cardiac muscle.
Mechanism of action: Direct and Indirect:
Neurotransmitter that open ion channels are said to act directly. These neurotransmitters provoke rapid responses in the postsynaptic cell by promoting the changes in the membrane potential. Ach and the amino neurotransmitters are direct acting. Whereas the indirect acting neurotransmitters promotes a more longer-lasting effects by acting through intracellular second-messenger molecules. In this way their mechanism of action is similar to that of many hormones. The biogenic amines and the peptides are indirect neurotransmitters. Neuromodulator is the term used to describe a chemical messenger released by a neuron that does not directly cause EPSP’s or IPSP’s but instead affects the strength of synaptic transmission. Neuromodulators are the neurotransmitters , neuropeptides hormones that have spatially distributed temporally extend effects on the recipient neurons and circuits. Neuromodulators secreted by a small group of neurons diffuse through large areas of the nervous system , affecting multiple neurons. A neuromodulator may act presynaptically to influence the synthesis, release, degradation, or reuptake of neurotransmitter. Alternatively, a neuromodulator may also act as postsynaptically by alternating the sensitivity of the postsynaptic membrane to neurotransmitter. Receptors for neuromodulators are not necessarily found at synapse. Instead , a neuromodulators may be released from one cell to act at many cells in its vicinity , similar to paracrines chemical messengers that act locally and are quickly destroyed. The distinction between neurotransmitters and neuromodulators is fuzzy , but chemical messengers such as NO, adenosine , and a number of neuropeptides are often referred to as neuromodulators. In some neurons, AP’s generated at the axon hillock propagate back up into the dendrites. This current flow may alter the effectiveness of synapses by opening voltage-gated Ca2+ into the dendrites and promoting synaptic potentiation. Synaptic potentiation can be viewed as learning process that increases the efficiency of neurotransmission along a particular pathway. Indeed , the hippocampus of the brain , which plays a special role in memory and learning exhibits an important type of synaptic plasticity called long-term potentiation (LTP). Presynaptic inhibition; events at the presynaptic membrane can also influence postsynaptic activity , presynaptic inhibition occurs when the release of excitatory neurotransmitter by one neuron is inhibited by the activity of another neuron via an axonal synapse. More than one mechanism is involved , but the end result is the less neurotransmitter is released and bound forming smaller EPSP’s.
They are membrane receptor proteins that exist in the neuronal and glialcells , they are present in both postsynaptic and presynaptic neurons. Neurotransmitter receptors can bind to neurotransmitters and therefore change the membrane potential of a neuron. The neurotransmitter receptors are capable of receiving signals that trigger an electrical signal by regulating the activity of the ion channels.
Types of neurotransmitters:
There are many times of neurotransmitters present in humans brain but there are major types
1. Dopamine : it can play many different roles in human’s C.N.S/ brain depending on the location where dopamine is present in brain. In other words in the frontal cortex , dopamine it plays a role in which it controles the flow of information to other areas of the brain and not only that but it also has a role in attention , problem-solving , and also in memory.
2. Serotonin: it is inhibitory neurotransmitter. It is inlovled with moods, sleep cycle , pain control and digestion. Fact: most of the serotonin found in the gastrointestinal tract in the body and there is only about 10% of serotonin is found in brain. In addition serotonin can somehow help in forming blood clots and also increase sex hormons.
3. Acetylcholine: acetylcholine comes from a chemical word called choline ,which is found in some of the food like nuts , eggs and seafood . ACh has a very important role in the formation of memories, verbal and logical reasoning and also concentration. It also has an important role in helping with synaptogenesis which is the production of healthy new synapse in the brain. In case of lack of ACh in the neuromuscular junction it can cause no action and relaxation in the muscle.
4. GABA: is also an inhibitory neurotransmitter which helps in the balancing of neuron that might be over the firing level. GABA here is useful when it comes to anxiety or fear , because when GABA is released it will help to relax you and keep you in a calm . it also has a role in vision and motor control.
5. Noradrenalin: noradrenalin can be also known as (epinephrine) it is a hormone which is secreted by adrenal gland ( a gland that is present on top of the kidneys). Noradrenalin helps is activating the sympathetic nervous system it helps to fight stress, and it is an excitatory neurotransmitter. Noradrenalin also has a role in emotions, attention, learning, and in sleeping and dreaming. When released into blood stream it increases heart rate, blood flow in muscles and glucose energy level.
Drug and its effects:
Drugs targeting the neurotransmitter of such systems affect the whole system , this explains the complexity of action of some drugs. In addition one of the most important drugs that affects the autonomic nerves system is nicotine, which is present is cigarettes/smoking which plays a very important role in health. Autonomic system in sympathetic and parasympathetic are activated by ACh release .so the ACh receptors present on the neurons are of nicotinic type which means that they are ligand-gated ion channel (LICs ,LIGIC). As for when neurotransmitter released to bind to a receptor protein the channel will open and will allow positive ions cross the cell membrane and this will cause depolarization. The nicotine acting as ACh land of these synapse so when a drug is taken, it binds to these ACh receptors and lead to activation of ganglionic neurons and depolarization. Another effect is the norepinephrine and epinephrine influence target effectors by binding to the ?-adrenergic or ?-adrenergic receptors. Drugs that affect the sympathetic system affect these chemical systems. drug that enhances adrenergic function is known as a sympathomimetic drug.
Diseases and metabolic disorder:
When neurotransmitters are balanced it leads to a healthy life . but sometimes there can happen that there is an imbalance which can cause illnesses and diseases such as depression and Parkinson’s disease.
Depression: is a word we are all familiar with, because of today’s world the amount of people suffering from depression, it became a normal word and not a diseases. Not only humans who suffer from depression but also animals can suffer from such a diseases. there are many causes of depression like decrees in serotonin in brain and decres norepinephrine level.
it is a disorder that affects the neurons which are responsible for movement of the body. These movement neuron are transmitted by dopamine , the death of these neurons can cause some symptoms like balance problems .
Parkinson’s disease PD treatment : L-dopa is a predecessor to dopamine that crosses the blood brain barrier and starts solving movement problems in the body.