Question

Na⁺ channels are opened by excitatory neurotransmitters, while it is the inhibitory neurotransmitters that will open either chloride or potassium ( Cl^– or K⁺ ) channels.

Provide a rationalization for why this is what is observed in neurons. Discuss it with regards to the effects that these ions have on an action potential firing in a neuron.

Answer 1

A resting neuron has more Na+ ions outside and more K+ ions inside. This distribution of Na+ and potassium is maintained by the Na+-K+ ATPase. An action potential or a wave of depolarisation is generated when Na+ ions move in due to the opening of voltage gated sodium channels. Influx of positive charge can only generate an action potential if threshold potential is achieved. A resting neuron has -70mV resting membrane potential and if a nerve impulse is to be generated this membrane potential should cross the threshold potential of -55mV. Threshold can be achieved if there is influx of Na+ ions. Hence, the excitatory neurotransmitters open the Na+ channels to achieve the threshold value. If neurotransmitter is opening a K+ channels then this would cause the efflux of K+ions and will make the neuron hyperpolarized. Similarly if a neurotransmitter has opened  Cl- channels then this would also makes the neuron hyperpolarized.

Na+ influx would cause a neuron to fire an action potential whereas K+ efflux and Cl- influx would inhibit not generate any action potential.