Question

Describe how an action potential is propagated through an axon, and how neurotransmitters are released in response to the action potential reaching the synaptic terminal

Answer 1

Propagation of action potential through axon: The axon of neurons are basically two types , myelinated and non-myelinated. Myelinated axon are covered with myelin sheath and non-myelinated fibres are not covered with myelin sheath.

Propagation through myelinated axon: The propagation of action potential through myelinated axon is called saltatory conduction because the conduction of action potential is leaping around the axon fibre to reach the axon terminal. In resting condition the inner surface of the axon is -ve (negative) charge and outside of the axonal membrane is +ve (positive). This is called resting membrane potential (-70 mV). When the action potential is arrived in a portion of axon, the Na+ channels are opened and lots of sodium ions are entered into the axon from the extracellular fluid. This makes the inner surface of the axon membrane become +ve (positive) and outer surface become -ve(negative). The myelin sheath doesn't have any sodium channels so this part of the axon is impermeable to any ion influx. So the action potential is travelled from +ve charge to -ve charge of the neighbouring part of axon by leaping the myelin sheath. When the action potential travelled to the next segment of axon, the previous segment is again returned to its resting potential by opening K+ (potassium) channels, and causes K+ efflux. This way the action potential are travelled through the entire axon by leaping the myelin sheath.

Propagation through non-myelinated axon: The propagation of action potential in non-myelinated axon is not like myelinated axon because, this axon doesn't have myelin sheath. When a action potential arrives a segment of non-myelinated axon it causes the reversal of the membrane charge. The innerside of the membrane become +ve charge and extracellular part become -ve. Thats why the membrane potential become +35 mV and causes depolarization of the membrane. The action potential then travels to the next segment from +ve to -ve charge and the previous segment become repolarized and regain its resting potential. This way the action potential is travelled through the non-myelinated axon, segment by segment not by leaping around.

The propagation of action potential through the myelinated axon is faster than the non-myelinated axon because, the myelin sheath in myelinated axon increases the diameter of the axon. The increasement of the thickness also increases the velocity of the propagation of action potential through the axon.

Release of neurotransmitter from the axon terminal:

When the action potential is arrived the axon terminal it causes the opening of voltage gated calcium channels. This entry of calcium ions through this channels causes the synaptic vesicles to dock in the docking region of presynaptic membrane. The Neurotransmitters are filled with in the synaptic vesicles. The calcium entry is the sensor for the exocytosis process of this neurotransmitters. Calcium binds with synaptotagmin protein in the synaptic vesicles and guide the vesicle to the docking site of presynaptic neuron. There are others proteins like SNAP-25 , Syntaxin1 present on the docking site of the membrane . These docking protein binds with the vesicles protein for the fusion of synaptic vesicles in the presynaptic membrane. After fusion the Neurotransmitters are released from the synaptic vesicles to the synaptic cleft region by exocytosis.

This way neurotransmitters are released from the axon terminal when the action potential is arrived in the synaptic terminal. After exocytosis this neurotransmitters are then binds with the postsynaptic membranal receptor for physiological responses.