An excitatory post synaptic potential has a reversal potential more positive than the action potential threshold, whereas an inhibitory post synaptic potential has a reversal potential more negative than threshold.
Excitatory postsynaptic potentials (EPSP) carry the neuron's potential closer to its volatile threshold. on the other hand Inhibitory postsynaptic potentials (IPSP) change the charge across the membrane to be further from the volatile threshold.
These potentials are also known as graded potential because their size or amplitude is directly proportional to the strength of the triggering event. Example include when large stimulus leads to the generation of a strong graded response and a small stimulus leads to the generation of a weak graded response.
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If a non native species got introduced then it would hurt the animals already there because there is no animals that are hunting it down. Also it would cause a imbalance in the food chain which can cause so animals to become extinct. If the animal is left unchecked then that animal can become the best in the food chain because nobody can get rid of it.
External fertilization cannot happen on land since it needs a medium for the fertilization to be able to take place, but land cannot provide the medium.