Nervous System: Classification and Neurohumoral transmission


The nervous system can be divided into two (depending on the location of the system associated organs in the body)

 


 

 

Central Nervous System (CNS) is present on the central longitudinal axis of the body and consists of Brain and Spinal cord.

 

 

 Peripheral Nervous System (PNS) is present outside the central longitudinal axis and consists of Cranial nerves and Spinal nerves.

The main function of PNS is to act as communication mechanism between the CNS and the body.

The messages between the CNS and the body are received and sent via nerves.

A nerve is a group of neurons which conveys the message from one site to another site.

PNS can be further subdivided into two broad categories on the basis of functioning of the nerves i.e, Voluntary or Involuntary 

Somatic Nervous System (SNS) is voluntary in nature, meaning it will work as per the will of the human. E.g, if you want to raise your hand you can simply do it.

Whereas, Autonomic Nervous System (ANS) is involuntary in nature. It will work on its own pace. E.g., one can’t increase or decrease his/her heart rate as per will. It will increase or decrease as a homeostatic mechanism which depends on the surrounding of the body.

Whether PNS or ANS, the common feature is the way by which the message is received and sent via the nerves. i.e, Neurohumoral Transmission.

 

Neurohumoral Transmission: The mechanism by which the message from one neuron to other neuron or the effector organ is sent through a chemical i.e., Neurotransmitter

Neurotransmitter: It is synthesized and stored in the neuron. After completion of the action, the neurotransmitter is either metabolized by enzyme(s) or is reuptake by the neuron itself.

Fig: Neurohumoral Transmission





In the fig above various sites have been given which are important in Neurohumoral Transmission and could be the target of various drugs.

A: Neurotransmitter synthase could be inhibited to block the synthesis of a neurotransmitter

B: Certain drugs can promote or inhibit the release of neurotransmitter in the synaptic cleft. Even the neurotransmitter can hinder its own release.

C: Drugs can bind with receptor to produce effects similar to the neurotransmitter or to block the effects of neurotransmitter. e.g., agonist and antagonist.

D: Drugs can also target the presynaptic receptors.

E: Drugs can block transporter which reuptake the neurotransmitter by the presynaptic neurone to increase the duration of effect of neurotransmitter.

F: Drugs can block the activity of enzyme which is responsible for the metabolism of the neurotransmitter which results in the increased effect of the neurotransmitter. 

G: Transporter responsible for the uptake of the metabolites of the neurotransmitter which could be utilized for the synthesis of fresh neurotransmitter.

 

ANS is involuntary in nature and it consists of nerves, ganglia and plexuses which communicate with heart blood vessels, visceral organs, glands, smooth muscles, etc. throughout the body.

Ganglia are complex structure that contain axondendritic synapses between preganglionic and postganglionic neurons.

Most visceral reflexes of ANS are mediated through the CNS. The information on the status of visceral organs is transmitted to the CNS through two main sensory systems:

a. The cranial nerve (Parasympathetic) visceral sensory system

b. The spinal nerve (Sympathetic) visceral afferent system

 

On the efferent side, ANS can be subdivided into two

1. Sympathetic or thoracolumbar division of ANS

2. Parasympathetic or craniosacral division of ANS

 

The neurotransmitter of all preganglionic autonomic neuron, most postganglionic parasympathetic neuron and few postganglionic sympathetic neurone is Acetylcholine (Ach)

 

Majority of postganglionic sympathetic neuron have Noradrenaline or Norepinephrine (NE) as neurotransmitter.

 

The terms cholinergic and adrenergic is loosely used for the neurons having Ach and NE as neurotransmitter, respectively.

 

Acknowledgement: All the figures above were created in biorender.com .


 

 

 

 

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