Neuro transmitters

Chemicals used for the transmission of messages in nervous system are called as the neurotransmitters. According to the molecular size they can be divided into small molecular rapidly acting transmitters and large molecular slowly acting transmitters. The  former  include the Acetylcholine, Epinephrine, serotonin etc. and the later includes the hormones such as Oxytocin, Vasopressin etc.

According to the mode of action  neuro transmitters can be classified as excitatory neurotransmitters and the inhibitory neurotransmitters.

The excitatory neurotransmitters genrally excite the target organ by causing  the opening of sodium channels and  increased  inflow of sodium ions into the target cell and thereby the establishment of positive charge inside the membrane of the target cell.Eg.Acetylcholine

The inhibitory neurotransmitters generally inhibit the target organ by promoting the opening of chloride ion channels and  the  inflow of chloride ions and inhibiting the inflow of sodium ions into the target cell  membrane.This results in establishment of negative charge inside the cell membrane. Eg.Dopamine.

Important  Neurotransmitters

A. Small molecular Neurotransmitters

a. Acetylcholine: Is screted as neurotransmitter in many areas of the brain by the preganglionic fibres of the sympathetic nervous system,post ganglionic fibres of the parasympathetic nervous system and also the post  ganglionic  fibres of some sympathetic nerve.In most instances  it shows excitatory effect but is inhibitory in some parasympathetic nerve endings for eg.The Vagus nerve endings terminates into the heart.

 b.Nor-epinephrine: Is secreted by many neurons located in the brain stem,pons  and the hypothalamus. It is also found in the post ganglionic nerve endings of the sympathetic nervous system.In brain it control mood by promoting wakefulness. Nor-epinephrine is excitatory on some organ and inhibitory on some others.

 c. Dopamine:Is secreted by neurons that originate in the substantia nigra.The effect of Dopamine is generally inhibitory in action.

  d.Glycine :Is secreted mainly the synapses in the spinalcord.It generally act as an inhibitory transmitter.

 e.GABA (Gamma Amino Butyric Acid) :Is mainly secreted by the nerve terminals in the spinal cord, cerebellum and many areas of the cerebral cortex. GABA is an inhibitory transmitter.

 f.Glutamate: Is secreted by the pre synaptic terminals of many sensory nerve and also the nerves in cerebral cortex. It is generally excitatory in action.

 g.Serotonin: Is secreted at the nerve endings of the brain stem and also the dorsal horn of  the  spinal cord  and hypothalamus. Serotonin  act as an inhibitor of pain pathways in the spinal cord and in brain it helps to control the mood of the person and may cause the sleep.

 h.Nitric oxide:Is a small molecular transmitter substance. It occurs in those areas of the brain which control the long term behavior such as memory. Compared to that of other neurotransmitters it is not pre synthesized at the pre synaptic nerve   endings  and is very rapid in action. Instead it is always synthesized instantly when it is needed and then immediately diffused out of the pre synaptic terminal

B.Large molecular Neurotransmitters                                                                      This includes the various releasing hormones secreted from the  hypothalamus and  trophic  hormones from pituitary (Neuropeptides) ,  gut hormones,  pancreatic hormones  and other tissue hormones.

a. Rleasing - hormones from hypothalamus

This includes the TTRH (Thyro Tropin  Releasing  Hormone), Somatostatin and Luteinizing hormone- releasing hormone.

b.Pituitary peptides

This includes the prolactin, Growth hormone, Melanocyte Stimulating Hormone (MSH), ACTH, Oxytocin, Vasopressin,Beta endorphin etc.

c.Gut hormones

This includes the Gastrin, Cholecystokinin, Insulin, Glucagon etc.

d.Other tissue hormones

such as Angiotensin II, Bradykinin, Carnosine,  Calcitonin etc.