What are neurotransmitters?

Neurotransmitters are chemicals in the body that transmit signals from a nerve cell (neuron) to a target cell across a synapse

-   A neurotransmitter is released from a nerve cell by the arrival of an electrical impulse.   The impulse is transferred across the synapse to a target cell when the neurotransmitter binds to a receptor in the membrane on the postsynaptic side of the synapse.

-   The receiving target cell can be:

•  An adjacent nerve cell

•  A muscle fiber  (to stimulate movement)

•  A body organ cell

•  Other tissue cell

Myelin sheath.    Consists of Schwann cells that encircle axon like a jelly roll, act as insulators and are separated by gaps of unsheathed axon called Nodes of Ranvier. Instead of a continuous traveling down the axon, the action potential jumps from node to node (called saltatory conduction), thereby speeding up propagation of impulse.

Release of neurotransmitters

Neurotransmitters are usually released following arrival of an action potential at the synapse.    This causes the voltage across the cell membrane (membrane potential) of a cell (in this case a presynaptic neuron) to rapidly rise and fall.

•  Follows a graded electrical potential.

•  Occurs without electrical stimulation as a low level “baseline” release.

Neurotransmitter synthesis

Neurotransmitters are made via just a few biosynthetic steps, from simple precursors, such as amino acids readily available from diet.

Have excitory or inhibitory effect (or both), depending only on the type of receptors they activate .    An excitory effect increases the probability that the target cell will fire an action potential.

Types of Neurotransmitters (packaged in neurotransmitter vesicles, which are membrane enclosed sacks that store and release neurotransmitters into synapse at presynaptic neuron terminal so that they can be detected by receptors on the postsynaptic neuron)

•  Amino acids.    E.g. GLUTAMATE (excitatory at >90% of the brain's synapses), ASPARTATE, GABA (inhibitory at >90% of synapses that do not use GLUTAMATE), GLYCINE.

•  Monoamines.     DOPAMINE, NOREPINEPHRINE

•  Peptides.    E.g. ß-endorphin (engages with opioid receptors in CNS). Also considered neurotransmitters (shown to be released by presynaptic terminals to produce an action, but are not packaged in vesicles)

•  Single ions.    E.g. zinc

•  Gas molecules.    E.g. carbon monoxide, NITRIC OXIDE

Re-uptake of neurotransmitters

For nerve cells to communicate, neurotransmitters are secreted by one neuron and picked up by receptor proteins on the surface of another neuron.  Once the message has been delivered, a neurotransmitter is either destroyed or reabsorbed into the cell that made it and its activity is over. This process is known as re-uptake.

Neurotransmitter effects

Most psychoactive drugs exert their effects by altering the actions of some neurotransmitter systems.   E.g. Addictive drugs, such as cocaine and amphetamines, primarily affect the DOPAMINE system. Addictive opiate drugs primarily indirectly regulate DOPAMINE levels.

-   Cocaine.   Blocks reuptake of DOPAMINE back into the presynaptic neuron leaving DOPAMINE in the the synapse for a longer time to bind to receptors on postsynaptic neuron. This elicits a pleasurable emotion, until prolonged exposure causes down-regulation of receptors.

-   Prozac.     A selective SEROTONIN reuptake inhibitor (SSRI), which blocks reuptake of SEROTONIN by the presynaptic cell and potentiates longer effect of SEROTONIN;

-   Alpha-methyl-p-tyrosine (AMPT).    Prevents conversion of tyrosine to the DOPAMINE precursor (L-Dopa); can cause depressive symptoms.