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Threshold Potential

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In neuroscience, the threshold potential is the critical level to which
a membrane potential must be depolarized to initiate an action potential.


Tony had just managed to fall, well, asleep would’ve been a bit exaggerated, but at least into a semi-conscious haze on the sleeping pad that barely seemed to make a difference to the hard ground underneath, when Steve bellowing at the top of his voice roused him.

“Avengers! Dinosaurs! Assemble!”

Half awake, Tony crawled out of the tent and summoned his armor. Steve might well be overreacting to an unexpected appearance of a herd of peaceful herbivores—he really wasn’t up to par with his dinosaur taxonomy—but better safe than sorry.

In hindsight, he should’ve paid more attention to the sudden flare of pain through his calf when the greaves closed around his left leg, but with the adrenaline of the sudden wake-up call and an impending fight with prehistoric creatures at hand, he brushed it off as a worse than usual muscle cramp. Not like those were unfamiliar to him.

Sam and Natasha had also gotten out of their tents, and the three of them rushed to Steve's aid. He hadn’t been exaggerating with the full alert: he was badly outnumbered trying to hold back a pack of six—what were they, anyway? They looked like Velociraptors, only they were at least twice as big.

“Jarvis? Species ID?” Tony asked, firing up the boot jets to launch himself into the fray.

“My database suggests the genus Utahraptor at 94 percent probability,” the AI replied. “And the family Dromaeosauridae at 99 percent.”

“Your everyday raptor’s bigger cousins. Great. At least it’s something I haven’t seen before,” Tony said.

“Adding it to your life list, sir,” Jarvis noted. And why yes, of course Tony was keeping a list of his dinosaur sightings. Much more exciting than regular birding.

As menacing as these things were, he wasn’t the least bit worried. He had his suit; he could take down a couple of overgrown angry proto-birds without breaking a sweat. The main challenge was, he didn’t want to seriously harm the beasts, and he knew the others wouldn’t, either. These were unique, ancient animals that had only survived in the Savage Land, after all.

Sam had joined him in the air, and they swooped down, firing non-lethal blasts at the creatures, managing to draw four of them away from Steve and Natasha. With one foe for each of them, the two hand-to-hand combatants shouldn’t have any trouble, either. He glanced at them, and saw Steve easily block a clawed foot aimed his way, returning a kick of his own at the dino’s head that sent it staggering back. Natasha unleashed her bite on the one she was fighting, and it backed away, growling, tossing its head.

Tony blinked to clear his vision. The HUD looked wrong, almost as if he were seeing double. Was that a glitch in the tech, or something in his eyes? No time to worry about that. He'd look into it later.

One of the four dinosaurs Tony and Sam were herding off was bigger than the others, and seemed to be the leader of the pack. He focused a blast on it, managing to make it lose its footing. As soon as it got up, it roared what was clearly a command at its buddies. Like most of the dromaeosaurids Tony had met, these seemed to be smart. Make them realize that you’re not a quick, tasty snack, and they’d see their mistake and retreat. The battle was over almost as suddenly as it had begun.

With the dinosaurs falling back and jogging away, Tony landed on the jungle clearing close to Steve and Natasha. The witty remark he’d had ready died on his lips when he realized his entire mouth felt oddly lax.

“Jarvis,” he tried, but it came out as a slurred “Jah”, his tongue suddenly a useless, thick, heavy weight.

Something was really, really wrong. His calf was still hurting, on top of everything else. He’d easily been able to ignore it during the fight. A regular cramp left a lingering ache often enough. They never hurt like this, though, burning like there was a red-hot branding iron pressed against in his skin.

The strange, warm heaviness was spreading, already at his neck; it didn't hurt, but it was deeply disconcerting. His second attempt at alerting his AI never made it past at an inarticulate, distressed grunt.

“Sir? I’m afraid I didn’t catch that,” Jarvis said, in a tone that was as close to puzzled as the AI ever got.

Sam, Natasha and Steve were chatting away right next to him, with Sam going on excitedly about the dinosaurs they’d been fighting. They obviously had no idea he was in trouble.

Tony tried to step forwards and gesture at the others. He very nearly stumbled and fell over: his feet had become entirely immobilized, as had his hands, each finger perfectly relaxed and utterly impossible to move. Fortunately, he did manage to raise his arms in what he hoped could only be interpreted as a distressed wave. By the time he brought them down by his sides, he could no longer bend them at the elbows.

The three other Avengers stopped talking and turned to stare at him.

“Hey, Stark? Everything all right?” Steve asked, looking like he couldn’t decide whether he should be amused or worried.

“He’s certainly being more quiet than usual,” Natasha commented. “Not a single quip so far.”

Saliva was building up in his mouth, and he couldn’t even bring himself to swallow it down. What the hell was going on with him? He felt like he wasn’t getting enough air. The readout on the HUD said his O2 levels were fine. Maybe it was all in his head—was he just freaking out? He was definitely breathing faster than normal, but he couldn’t help it. Not with this odd paralysis that was almost complete, every voluntary muscle in his body beyond his control.

“Sir? You need to calm down,” Jarvis urged. Yeah, obviously the AI would read this as some kind of a bizarre panic attack—which Tony obviously never got, that was ridiculous—since this wasn't exactly a scenario he'd made contingencies for, and Jarvis couldn’t tell what Tony was thinking and had no way to measure, whatever it would take to measure this, nerve impulses, maybe?

Now that he thought about it, the only thing that could possibly explain what was happening to him would be some kind of poison, what with that sting in his leg. A fast-acting neurotoxin from a venomous critter. No doubt the Savage Land had plenty of those. Problem was, figuring that out did absolutely nothing to help him.

Steve was right in his face, peering into his unmoving eyes, nose almost pressed against his helmet. “Tony? Say something!”

Oh how he wanted to, but as much as Tony tried, he couldn’t get out as much as a squeak. He couldn't breathe. He wasn't imagining that. This was a level of nightmarish that even his nightmares rarely managed. He wasn't just trapped in the armor, he was also trapped in his body. Without the armor keeping him upright, he would’ve already been a heap on the ground. He could see his oxygen saturation dropping, dipping below 90%. He was screaming at his diaphragm to move and his lungs to expand, but it wasn’t happening, and the involuntary reflexes weren’t kicking in, either, though he was getting light-headed, his vision starting to fray at the edges. A few minutes, and he was going to pass out.

A hand appeared in front of his face and tore off the faceplate, taking away the HUD. Good riddance. The numbers hadn't been helping at all. Without the metal between his and Steve’s faces, he could feel Steve’s breath against his skin, which was an ironic sort of torment, since he couldn’t draw air in himself.

“Oh no, he looks—is he dead?” Steve said, horrified.

“No, no, he’s alive, but something’s wrong,” Sam said, close behind Steve. “His vitals are all over the place, and he's not breathing.”

Took you long enough to put those scanners to use, Falcon, Tony thought, as Steve grabbed hold of Tony’s armor to tip him over and place him on the ground.

“What is it?” Steve asked, his gaze skimming Tony’s armor-clad body, and fixating on his chest. “Something wrong with the arc reactor?”

Use your eyes, Steve! Nothing was wrong with that, for once, though the way his heart was hammering against it felt like it was going to dislodge the device. He needed air, he wasn't going to last much longer, come on—

“Nothing that I can pick up. I think this is something else,” Sam said, his face hovering next to Steve’s, both getting blurry and sliding out of focus.

“Did he get hit in the fight?” Steve asked.

“No! He didn’t even go close to the dinosaurs. There’s absolutely nothing wrong with his armor. No damage anywhere,” Sam said.

His vision was narrowing and Steve was just staring at him all anxious and totally useless damn it Cap he was dying here what was taking so long—

“Guys!” Natasha's sharp voice cut in. “Whatever it is, he’s not going to last much longer if we don’t do something about the ‘not breathing’ situation.”

—ohthankgod for Natasha being there and not getting overwhelmed with worry because he was that close to passing out barely able to see and an ominous ringing in his ears—

—Steve's fingers pinched his nose, Steve’s lips were on his, and there was oxygen. Finally.

Since there was actually nothing wrong with his lungs aside from the total loss of muscle control, the air Steve was breathing into him was as good as an actual inhalation would’ve been. Another breath followed, seconds later, and the looming darkness was rapidly dispelled from his vision, his head clearing again.

Tony still couldn’t move, though.

All he could do was lie there helplessly, entirely dependent on Steve. At least it was Steve. There could’ve been worse fates. It was all sorts of wrong that every five or so seconds, Steve’s lips pressed against his, and he couldn’t feel happy about that. Under any other circumstances he would’ve been over the moon, but this ranked pretty high on the list of the scariest things he’d ever gone through.

“We still need to figure out what’s wrong,” Steve said between breaths. “I don’t get it. Why didn’t he say anything?”

Steve taking the time to talk meant it took him that much longer to return to Tony, which wasn't good at all, he needed that air, he did not want to go through the whole looming suffocation and death thing again please—

Ah, there, Steve was back, with another lungful of blessed air.

Going by the complete lack of any words addressed to Tony, Steve didn't even realize he was conscious. Sure, he'd managed to keep his eyes open, but he couldn't move them any more than any other part of his body. There was no way he could let the others know he saw and heard and felt what was happening. He would seem completely unresponsive to them.

“We should get him out of the armor,” Natasha said. “Make sure we haven’t missed any injuries.”

“Right, that should be doable, I think this is one of the lighter armors that come off easily,” Sam said. “Jarvis? Can you break him out of the suit?”

“I’m afraid not, Mr. Wilson. You don’t have the authorization,” Jarvis answered, his voice loud in Tony’s ears. Thanks for nothing, J, and damn the security protocols. Of course, there was someone who could get around that. Steve! Let's hear it!

“Jarvis, I’m sure you’re aware he’s in trouble,” Steve said, right on cue. Tony didn’t even mind that he was talking instead of breathing. “Come on, there’s got to be an override.”

“Voice recognition: Captain America. Override authorization accepted,” Jarvis announced, and the suit split at the seams around Tony. Yess.

After the next breath, Steve pulled off Tony’s helmet and felt around his head and neck for injuries—of which there obviously were none—before setting his head gently onto the soggy, grassy ground. Natasha and Sam stepped in to remove the rest of the armor, working between breaths, lifting him to get the back plate off, hands touching his shoulders and sides. Eventually, they reached his legs.

“Whoa! What’s that?” Sam exclaimed. He’d been the one at the left leg, then. “Widow! Are you crazy? Don’t touch it!”

“Afraid of spiders, Falcon?” Natasha said, sounding amused. “Can’t let it crawl away if we want to find out what the venom is.”

“But there’s no way to do that here! We're hundreds of miles from the nearest lab, and it'll take the others several hours to get here!” Sam said.

Sam was right, of course. They were on their own for now, and they didn’t have the equipment to run a complex chemical analysis, let alone come up with an antivenin, which they also would need to get from somewhere, if it even existed, and if it didn’t, they’d have to synthesize it, which would take ages. Shit.

Tony was so screwed and this was getting really old really fast.

“A spider did this?” Steve asked. Tony could see him frown at the two others before he crouched in for another breath and blocked Tony’s field of vision entirely.

By the feel of it, Sam was poking at Tony’s calf, tearing off the undersuit. “That definitely looks like a bite mark, so, yeah. Spider. A seriously huge and creepy one that got caught between the armor and his leg, and somehow survived without getting squished to death.”

“Okay. We know what's wrong. What can we do?” Steve asked.

“Keep doing what you’re doing,” Natasha replied. Tony heard her soft footsteps moving closer, and then she was by his head, her face next to Steve’s. Her hand found its way to Tony’s shoulder, giving it a squeeze. “I should’ve guessed earlier that it might be something like this. I’m sorry, Stark.”

“He’s—“ Steve began. Tony could see the horrified realization dawn on his face as he grasped what Natasha was implying.

“Yes, I think he’s conscious, he just can’t reply. I know a thing or two about poisons, and everything seems to point to some kind of a powerful paralyzing agent,” Natasha said.

Tony wanted to hug her. Scold her for not figuring it out earlier because she totally should have, but also hug her and thank her because they’d finally got it. Unfortunately, Steve seemed so stricken with shock that he was again taking longer than usual between breaths. Which should be fine. Tony should be fine without air for a few minutes. He’d rather have that air though. He really didn't like this. Steve. Air. Now would be good.

“I had no idea,” Steve said, his eyes wide. He put a hand on Tony’s cheek. “If you can hear me—I’m sorry, I honestly didn’t realize—it’s going to be all right. We’re going to fix this.”

Okay that was all sweet and nice of him but Tony didn’t need apologies, he needed oxygen—

Which he got, finally, though the next breath Steve gave him was different. It was as if something had shifted because Steve knew Tony was aware of what was going on. His touch was ever so slightly softer and more cautious. Maybe a tiny bit closer to a really weird kiss than a dispassionate rescue breath. No, that had to be just Tony’s imagination running wild. Maybe he was getting delirious. Likely to happen with oxygen deprivation.

“What can we do?” Steve asked again, his voice even more urgent.

“We don’t know what the venom is, and anyway we wouldn't have antivenin to it even if we did,” Sam answered, his voice quite close but his face not visible. “The good news is that he doesn’t seem to be getting any worse.”

If Tony could’ve, he would’ve made a comment about how it would be difficult to get much worse than this except by actually dying, though he knew that wasn’t strictly true. At least he still had a pulse and was still thinking—which wasn’t exactly a blessing at this point and he might’ve preferred passing out—but yeah, okay, things could get worse. Better not jinx it.

“With poisons like this, it’s usually the respiratory paralysis that kills you. This could be the worst it’s going to get,” Natasha said. “All we need to do is to wait it out.”

Tony had feared that would be the conclusion they’d come to. Thanks a bunch, guys. Wait it out. Yeah, easy for them to say, they weren’t the ones trapped inside their own skin. Not that he had any better suggestions off the top of his head, but could they at least try to toss around a few ideas?

“We have no way to tell how long that’ll take, though,” Sam said. “Unknown venom, unknown dose. I’ll call the others. If this turns out to be a long-lasting thing…”

“I agree, calling for help is a good idea,” Steve said. “While Falcon’s at it, Tony and I will be moving to a tent. Widow, you keep watch in case more dinos show up.”

Ah, things were looking up a little. If he had to go through another ten minutes, or half an hour, or—shudder to think—several hours of this, he’d appreciate not lying on the mushy ground.

Steve gave him another breath, said, “All right. I’m going to pick you up and carry you to your tent,” and went on to do just that.

Forget about the tent and the warmer and drier surface beneath his back, what made all the difference was the change in the way Steve treated him now that he knew Tony was aware of his surroundings. Suddenly, things weren’t half as scary.

Between the next few breaths, Steve carefully placed a pillow under Tony’s head, and even went as far as to run a soothing hand through his hair. “Hang in there. It’ll pass, sooner or later. I'll be right here with you, no matter how long it takes.”

That was annoying pep talk and approaching cloying levels of sweet, but Tony wasn’t about to complain. He could use all the cheering up and support he could get. Getting it from Cap was a nice bonus, too. He did always secretly enjoy that, even if he often complained about it, mainly for appearances' sake.

There was a rustling sound of tent canvas: someone at the door. “Cap?” Sam said. “I got hold of the others and they’re heading our way. Shouldn’t take more than three hours.”

“Okay, that’s good,” Steve said.

“How’s he doing?” Sam asked.

“You’re the one with all the scanners,” Steve said. “But at least not worse, as far as I can tell.”

“Scanners say you’re right,” Sam said. “Okay, I’ll be out there keeping Widow company. If there’s anything we can do, let us know.”

Then it was just Tony and Steve again, in the small tent brightly lit by a led lantern. Steve pulled off his gloves and cowl. Without the mask covering his features, his deeply concerned expression was even more evident. Tony wished he could say that it was better now. So many things he wanted to say and couldn’t. He also wanted to tell Steve how stunning he looked, and how out of all the people in the world Steve was the one he would’ve picked to help him if he had to go through this. But nope, all he could do was wait and accept the next tenderly delivered breath.

Natasha showed up a little later with an ice pack, which she proceeded to place against the bite in Tony’s calf, wrapping a bandage around it. It helped a little, numbing that constant, gnawing burn.

Things got kind of monotonous after that. Nothing going on except for the constant breaths from Steve, and the occasional soothing words and touches. A few times, he felt Steve’s fingers at his neck, checking his pulse, but most of the time, it was just a warm hand on his shoulder, or his arm, or his cheek.

He couldn't help thinking that those touches went a bit beyond what he'd expect from a friend, even a friend as close as Steve. If it had been only today and entirely limited to this poisoning and paralysis scenario, he might've brushed it off, but he'd kind of gotten this feeling before, often enough, with the little touches here and there. A tad more hugging and carrying and pats on the shoulder than would've been strictly necessary. He considered himself something of an expert on interpreting this sort of thing. He was Tony Stark, after all. Flirting was his forte. He always returned the flirting in kind, of course, but so far, no luck getting anywhere with that, and he hadn't wanted to push it. If he was wrong about it, things could get very awkward.

If he made it through this, he promised himself, he was going to give it a go. So, he really had to make it through this.

Tony had no idea how long this had been going on. He started counting breaths at some point, and found it surprisingly soothing. That was what numbers tended to do to him. Numbers and Steve’s lips put together, that was even better.

There were quite a few breaths. He got up to fifty.

“Don’t worry, Tony. I’m not going to go anywhere until you’re okay,” Steve was saying, his thumb brushing Tony’s cheek.

Sam showed up to check on them again, then Natasha. They didn’t even have any news. Tony was genuinely touched that they wanted to keep tabs on how he was doing.

He hit 240. With one breath every five seconds, that added up to twenty minutes. Twenty minutes, plus however long he'd been in this tent before he started counting. It came to him that he was extremely lucky to have a tireless super soldier caring for him. He’d done rescue breathing himself, a few times, and he knew it took effort. Steve wasn’t complaining, he kept going, steady as a metronome, and heck, Tony was almost starting to feel like he could handle this, like he could let his mind relax and be all zen about the whole thing. For the short term, anyway—and he wasn’t going to start thinking about the long term and the unlikely but terrifying possibility that this wasn’t a poison that would eventually be metabolized and fade away but something more sinister that had somehow permanently damaged his nerves leaving him stuck like this forever, oh, great, he was thinking about that, oh god, what if it was?

Steve’s fingers were pinching his nose again, lips locking with Tony’s for another breath. 241. What’s one more. This wasn’t going to be permanent. A little longer. He could do this.

He began trying to distract himself with the numbers, to come up with any interesting features he could think of. Prime numbers. Good primes, happy primes, lucky primes.

He’d just reached 396, which happened to be an Osiris number—over half an hour passed, he was pretty damn proud he was still sane at this point—when he got the nagging feeling that something was different.

He couldn’t point a finger—haha—at what had changed, and that made him anxious. Was the poisoning getting worse? It wasn't supposed to! He’d made it through more than half an hour of this, surely that had been enough suffering for one night! His mind was racing, his heart was pounding, and he—

He breathed.


He tried moving his fingers: nothing. Tried saying something: not a peep. But Steve’s lips were not currently attached to his and there was air. It felt like it wasn’t quite enough, but yeah, he was definitely breathing, his lungs inflating on their own accord. He was going to be okay. It would be over soon.

Steve hadn’t noticed the change yet, but went on for another three breaths, before he pulled back and frowned, staring very closely at Tony’s face. He rested a hand over Tony’s mouth and nose, and his expression lit up in the most radiant, beautiful way.


Tony’s facial muscles were still stubbornly refusing to move, but he did manage an undignified squeaky noise that had the vowel “e” in there.

“Tony! It’s okay, give it time, I’m sure the rest of it will pass soon. Welcome back!” Steve said. In a perfect world, he would’ve proceeded to give Tony a proper kiss on the lips there and then, but eh, now that Tony was able to suck in his own oxygen, that wasn’t going to happen. All he got was another reassuring shoulder rub.

He actually felt a little sorry it was over. That was all kinds of messed up. Only a tiny bit, of course. Mostly he just felt relieved beyond words. Which was fine because he couldn’t really do words yet.

It took another five minutes or so—estimating time was actually more difficult without those breaths—before he had regained enough control of his jaw, tongue and lips that he thought he might be able to talk, at which point he realized he didn't know what to say. He’d been going wild with the internal monologue for the best part of an hour, and Steve had been keeping him alive all that time, sharing the very air he breathed. How did you even thank someone for that?

As it was, the words he finally picked were the amazingly eloquent, slightly slurred, “Steve. Thank you.”

“Hey!” Steve said, smiling warmly at him. “How do you feel?”

“Weird. Still can’t move.” Also still couldn’t talk normally. He sounded like he’d had some demented dentist go wild with the anesthesia, his tongue way too big for his mouth.

“You’ll get there soon enough, I’m sure. I should let the others know, they were pretty worried, too.” Steve shuffled towards the door.

The thought of Steve leaving him even for a short while when he still couldn’t as much as twitch a finger brought up an almost absurd amount of dread. “Do you have to?”

“No, of course not, if you’d rather have me here,” Steve said, and settled back to his earlier position.

“Good. I would.” Tony wasn’t about to admit that he was afraid he’d lose it completely if he had to stay here alone.

“If there’s anything else I can do, just ask,” Steve offered. “Anything to make you more comfortable while we wait for the venom to wear off. A blanket? More pillows?”

Okay, this was his window of opportunity. Tony was obviously traumatized and irrational and possibly sort of unhinged after this awful ordeal, that was the excuse he was going to use if the next thing he said backfired on him. It might. Still, it was now or never. He had to give it a shot.

“Could you do the mouth-to-mouth thing one more time?”

Steve's eyes narrowed in instant concern. “You still feel like you can't breathe?”

“No, it's not that,” Tony said as quickly as he could, because he wasn't going to use his apparent infirmity to trick Steve, that would be wrong. “I was counting. Got up to 399. That's bugging me like—“

“Tony, are you asking me to kiss you?” Steve didn't look or sound incredulous. More like—cautiously hopeful? That might've been just wishful thinking on Tony's part, but maybe it wasn't. Maybe he really was onto something here.

“Possibly. Would that be bad?”

Steve didn't reply, but instead, for the 400-plus-nth time, crouched over Tony's motionless form and locked their lips, only without the fingers pinching Tony's nostrils shut. Since Tony had the full use of his mouth to himself again, he was able to respond, making it a proper—and oh, whoa there, Cap, quite enthusiastic—kiss.

He did feel a little breathless afterwards.

“That's better,” Tony declared.

“Only you, Tony,” Steve said, shaking his head and grinning. “Only you could pull through what's got to be about the most horrific experience there is, and turn that into a pick-up line.”

“Well, it worked! Assuming you'll still want to kiss me once I can move again,” Tony said.

“Just you wait,” Steve said. “I may want to do a little more than that.”