Give scientific reasons:
$(1)$ In an excited nerve fibre region,a positive charge is created inside the plasma membrane.
$(2)$ In some animals,the conduction of nerve impulses is saltatory.

(N/A) $(1)$ In the excited region,$Na^+$ ion channels open. At this time,the interior of the neurilemma contains negatively charged proteins and a lower concentration of $Na^+$. Consequently,a large amount of $Na^+$ enters the cell through ion channels,resulting in the formation of a positive charge inside the neurilemma.
$(2)$ In myelinated nerve fibres,the medullary sheath is not continuous. At regular intervals,it forms nodes of Ranvier. The action potential (conduction of nerve impulse) jumps from one node to the next,which is why it is called saltatory conduction.