Neurohumoral Transmission in the Autonomic Nervous System

Neurohumoral transmission is the transmission of a nerve impulse from a presynaptic to a postsynaptic neuron that passes through a humoral substance like a biogenic amine, an amino acid, or a peptide. This neuromodulator affects the release of a transmitter by acting on presynaptic transmitter neurons. 

The process includes several functions such as production, storage, release, receptor contact, and deactivation of the transmitters. Non-synaptic neuronal terminals may also emit biogenic amines, which may modulate the activity of other neurons. This article will talk about  Neurohumoral transmission in the autonomic nervous system. 

The autonomic nervous system is involuntary and has all the impulse reactions. As the autonomic nervous system is a visceral efferent system, it can send motor impulses to the visceral organs. This nervous system consists of innervated smooth muscles, cardiac muscles, and glands that work automatically and constantly without conscious effort. 

Neurohumoral Transmission (NHT)

When the nerves transfer signals, there is a release of humoral (chemical) substances during the transmission of an impulse. These transmissions take place through a synapse and neuro-effector junction. All these transmission areas are categorised under neurohumoral transmission.  All the treatments used in autonomic medications exert their pharmacological effects by modifying critical phases in the neurohumoral transmission mechanism.

The primary neurotransmitters are Noradrenaline (NA or norepinephrine, NE) and Acetylcholine (ACh). These neurons are released from postganglionic sympathetic and parasympathetic nerve ends. But, Acetylcholine is transmitted via the ganglia from preganglionic nerve endings of both systems. Cholinergic (NANC) transmitters are released through specific nerve terminals. These include nitric oxide, dopamine, serotonin, ATP, and GA.

Chemical Signalling in the Autonomic Nervous System is described below: 

In this, a synapse is a point where all the autonomic neurons link to a target. Synapses are an essential part of the autonomic system. They are classified as cholinergic or adrenergic. The classification depends on synapse  (ACh) or norepinephrine (NE). 

Another feature of the adrenergic system is the presence of epinephrine which is also a signalling chemical. The incorporation of a methyl group (CH3) in epinephrine is the chemical difference between norepinephrine and epinephrine.

Acetylcholine

Acetylcholine (ACh) is a type of chemical substance that is used in many neurotransmission processes. It is a transmitter that helps neurons present in the central nervous system (CNS). Also, these transmitters help in connecting the peripheral nervous system (PNS). 

Acetylcholine is used in various activities of the nerves, such as preventing neurons from various diseases or medicines that affect the neurotransmitter’s function. Due to the structure of the transmitters, it is called Acetylcholine. These transmitters are composed of acetic acid and choline-based chemical compounds.

There are various roles of Acetylcholine. It helps in various processes of the neuron system. All these transmitters are used in motor neurons and help in inducing muscular contractions. The functions of this essential neurotransmitter are involved in every movement of the body. These neurons are involved in many such processes, from the stomach and heart to breathing. One such example is the blink of an eye. Acetylcholine also helps in mental processes, including memory and cognition.

Function of Norepinephrine

Norepinephrine is often called noradrenaline. It is a type of hormone plus a neurotransmitter. It is also a crucial component of your body’s immune system that improves fight-or-flight response. Sometimes, these hormones are used in drugs to raise and maintain blood pressure and help cure difficult medical situations. 

It is a chemical messenger that helps the nervous system carry the nerve signals between the nerve ends to another nerve cell, muscle cell, or gland cell as a neurotransmitter. Your kidneys also release a hormone from their adrenal glands. Norepinephrine is a neurotransmitter that produces dopamine. 

Epinephrine Hormone 

Epinephrine is also a neurotransmitter and hormone used on many medications to treat allergic reactions, restore heart rhythm, and manage mucosal congestion, glaucoma, and asthma. They are also used as injections for treating insect bites or stings, foods, drugs, latex, and other sources of life-threatening allergic responses. 

This hormone belongs to a group of drugs known as alpha- and beta-adrenergic agonists (sympathomimetic agents). And help in relaxing the airway muscles and constricting the blood vessels.

Physiological steps involved in the neurohumoral transmission

• Axonal Conduction: Sensory nerves receive the information from the organs and then transfer it to CNS in the form of impulse through efferent autonomic nerves

• Transmission through Ganglia and Neuro-effector Junctions: After the information comes into the axonal terminal, there are a number of events that take place here 

• Receptor events on the post-junctional membrane: The transmitters bind together with receptors on the post effector cell membrane

• The fate of Neurotransmitters: The adrenergic neurotransmitter norepinephrine re enters the presynaptic nerve terminal (uptake I) or diffuses away from the receptor sites to a maximal extent (uptake II)

Role of Acetylcholine in the Transmission of an Impulse

Acetylcholine is a part of the somatic nervous system in the PNS. These neurotransmitters play an important role in helping the muscles to work actively. Acetylcholine also helps regulate a variety of processes in the autonomic nervous system by acting on neurons in the sympathetic and parasympathetic nervous systems. 

These transmitters help in smooth muscle contraction and blood vessel dilatation and enhance bodily secretions with a slower heart rate. For example, if the brain transmits some signals in your left brain to pick some object. The nerve fibres will carry the signal to neuromuscular junctions to help complete the task. The cholinergic neurotransmitter, in turn, will transmit the signal across this junction, producing the intended reaction in that specific muscle.

Conclusion

In this article, we talked about how Neurohumoral transmission works with the help of Acetylcholine and various other neurotransmitters. In the nervous system, two cells cannot pass signals directly to each other, but they require some kind of network for transmission. As a small electrical signal, these neurons help transmit signals or impulses from one end to the other end. Chemical messengers such as acetylcholine help in bridging the gaps known as synapses.

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