Explain the structure and function of a synapse. Discuss the synthesis, function and elimination of noradrenalin at a noradrenergic synapse.
The correct answer and explanation is:
Correct Answer:
Structure and Function of a Synapse:
A synapse is the junction between two neurons where communication occurs. It consists of three main parts:
- Presynaptic neuron terminal – contains synaptic vesicles filled with neurotransmitters.
- Synaptic cleft – a small gap (about 20-40 nm) between neurons.
- Postsynaptic membrane – contains receptor proteins that bind neurotransmitters.
When an electrical signal (action potential) reaches the presynaptic terminal, it triggers the release of neurotransmitters into the synaptic cleft. These neurotransmitters then bind to receptors on the postsynaptic membrane, altering its permeability and generating a new electrical signal.
Noradrenergic Synapse (Norepinephrine Synapse):
1. Synthesis of Noradrenaline (Norepinephrine):
Noradrenaline is synthesized in the presynaptic terminal from the amino acid tyrosine:
- Tyrosine → DOPA (via tyrosine hydroxylase)
- DOPA → Dopamine (via DOPA decarboxylase)
- Dopamine → Noradrenaline (via dopamine β-hydroxylase, inside vesicles)
2. Function of Noradrenaline:
Noradrenaline is stored in synaptic vesicles. When an action potential reaches the terminal, Ca²⁺ ions enter the cell, causing vesicles to fuse with the membrane and release noradrenaline into the synaptic cleft. It binds to adrenergic receptors (α or β) on the postsynaptic neuron or target tissue, triggering effects such as increased heart rate, vasoconstriction, or alertness (depending on the receptor subtype and tissue involved).
3. Elimination of Noradrenaline:
Noradrenaline is mainly removed by reuptake into the presynaptic neuron through norepinephrine transporters (NET). Once reabsorbed, it can either be repackaged or broken down by enzymes:
- Monoamine oxidase (MAO) – in the mitochondria of the presynaptic neuron.
- Catechol-O-methyltransferase (COMT) – in surrounding tissues.
In summary, a noradrenergic synapse enables fast, regulated transmission using noradrenaline, with synthesis from tyrosine, action via adrenergic receptors, and elimination via reuptake and enzymatic degradation.