The answer requires an understanding of Na, K, the Na/K pump, and what’s called saltatory conduction in nerve axons. This will advance your thinking in the context of neuroscience, systems biology, ions, membranes, neural networks, and real life applications.
Many nerve axons are insulated with a fatty, lipid layer called myelin. While this layering is protective, its spatial integrity is not continuous along the length of an axon. There are unmyelinated bare spots, called nodes of Ranvier. Briefly, when your arm or leg ‘falls asleep’, there is compression of nerves, axons, extracellular space, membranes, etc. This disturbs the myelin and quite literally obliterates the nodes of Ranvier. These nodes normally allow action potentials to jump and propagate much faster along myelinated axons vs unmyelinated axons. At each node, there are Na/K pump channels that normally pump Na out of the axon and K into it, thus keeping a resting ion potential across the membrane. When a neural impulse or stimulus occurs, a depolarization at the node reaches threshold, resulting in an action potential where Na channels open allowing Na into the axon and K channels close. This change in the electrochemical gradient across the membrane jumps from one node to the other and is the basis of saltatory conduction, sending the action potential very quickly down the axon. This excitability is essentially how neurons send their efferent messages to synapses and through networks so that a specific outcome or behavior can be elicited.
When compression occurs however, these action potentials and efferent signals are blocked and the result is paresthesia numbness. When the compression is relieved, the myelin must structurally reorganize its insulating function. The nodes during this process have also become spatially disorganized, resulting in a highly charged electrochemical gradient along the axon that is also highly depolarizing. The result is the pins and needles sensation we feel.
The key in understanding what relieves this sensation of pins and needles is ATP. The Na/K pump requires ATP to properly function and ATP requires O2. When compression is relieved, blood flow is restored to the area. This in turn allows blood cell hemoglobin to bring an O2 supply to the ATP requirement of the Na/K pump. The result is a working Na/K pump and return to a resting membrane potential across the axon, effectively ending our pins and needles….
