Neurotransmitters - Anatomy & Physiology

Nerve - © fig.cox.miami.edu 2008

Introduction

Neurotransmitters are chemicals that are used to relay, amplify and modulate signals between neurones and cells. They can be classified into two main groups; small-molecule transmitters (SMT) and neuropeptides. SMTs are synthesised within nerve terminals via enzymes that are produced within the cell body. Within most nerve junctions, the terminal membrane of the nerve contains numerous specific transport proteins that facilitate the transport of the majority of the SMTs back into the nerve terminal, effectively recycling the neurotransmitter. Neuropeptides are constructed of around 3 - 40 amio acid molecules that are synthesised within the cell body and are then transported to along the axon to the nerve terminal within vesicles running along microtubules. Neuropeptides are broken down by extracellular enzymes called peptidases once the neurotransmitter has been released. A small proportion of neuropeptides can bind to postsynaptic receptors in the nerve terminal membrane and can be taken up by endocytosis, although the degree of recycling of neuropeptides is lower than that of SMTs.

Types of neurotransmitter

Small-molecule Neurotransmitters

There are two major sub-groups of SMTs; amino acids and biogenic amines. Amino acid These include:

• Acetylcholine (Ach)

• Monoamines - epinephrine (E), norepinephrine (NE), dopamine (DA), serotonin (5-HT) and melatonin.

• Three or four amino acids, depending on exact definition used: primarily glutamic acid, gamma aminobutyric acid (GABA), aspartic acid & glycine.

• Purines -(Adenosine, adenosine triphosphate (ATP), Guanosine triphosphate (GTP) and their derivatives.

• Peptides - vasopressin, somatostatin etc.

• Histamine (HA)

• Single ions - such as synaptically released zinc.

• Gaseous - nitric oxide (NO) and carbon monoxide (CO).

Function

Release of excitatory neurotransmitters from the presynaptic membrane cause channels in the postsynaptic membrane to open and cause an increase in sodium ion concentration within the postsynaptic cell and a decrease in potassium ion concentration. This leads to a depolarisation of the postsynaptic cell, which is propagated further along the axon by an action potential (AP).

Inhibitory neurotransmitters cause hyperpolarization of the postsynaptic cell making it unable to generate an action potential.

Receptors determine the reaction of the neurotransmitter meaning that the same neurotransmitter may cause an excitatory effect on some membranes whilst an inhibitory effect on others e.g ACH can be either excitatory to skeletal muscle cells or inhibitory to both smooth muscle and cardiac muscle.

Actions

Neurotransmitter	Action
Acetylcholine (Ach)	Skeletal muscle movement (sympathetic) Visceral movement (parasympathetic)
Epinephrine	As norepinephrine, produced by adrenal glands.
Norepinephrine	Conciousness (sympathetic).
Dopamine	Addiction, love, motivation.
Seretonin	Emoticon, conciousness, sleep and thermoregulation.
Melatonin	Regulates the onset of sleep and seasonal changes in the body e.g winter weight gain.
ATP	Timing mechanism of G proteins by displacement of ADP.
GTP	Timing mechanism of G proteins by displacement of GDP.
Vasopressin	Metabolic rate, metabolism.
Somatostatin	Inhibits production of other hormones.
Histamine	Gastric secretions increased, dilation of capillaries, constriction of bronchial smooth muscle and decreased blood pressure.
Zinc	Release of neuropeptide Y.
NO	Enlargement of genital organs leading to erection.
CO	Sleep cycle.
Glutamate	Exitatory, found in CNS.
GABA	Inhibitory, found in CNS.
Glycine	Spinal reflexes, motor.

Neurotransmitter systems

See somatic and autonomic nervous systems.

Degradation and elimination

• Acetylcholine is derived from Choline.
• Serotonin [5-Hydroxytryptamine (5-HT)] is derived from Tryptophan.
• GABA is derived from Glutamate.
• Histamine is derived from Histidine.
• Epinephrine, norepinephrine and dopamine are derived from Tyrosine.
• Adenosine is derived from ATP.
• Nitric oxide is derived from Arginine.

Neurotransmitters must be broken down once they reach the post-synaptic cell to end the excitatory or inhibitory signal and prevent over stimulation/perminent inhibition.

• e.g, ACH an excitatory neurotransmitter, is broken down by acetylcholinesterase (AchE).

• Choline is taken up and recycled by the pre-synaptic neuron to synthesize more ACH.

• Other neurotransmitters such as dopamine are able to diffuse away and are eliminated from the body via the kidneys, or destroyed in the liver.