💪 Muscle

Stop a moment.

Look at your hand.

And think the thought ‘Bend that finger’…

Metre long nerve cells stretch out their membranes from our brain to our spinal cord. Synapse. Hand on the spark. Spinal cord to muscle.

Brain to muscle. Head to toe. 2x (or sometimes 3x) nerve cells. In perhaps 1/50^th second. Fast, but finite.

100m sprinters know this. Do NOT false start. Go on the 'B' of the 'BANG'. Not before. Shape shifting proteins. Shifting ions. Nerve speed. We know how long it takes.

So...

‘Bend that finger’

Sodium channels shape shift. Spark arrives.

Nerve releases neurotransmitter.
Transmitter diffuses across the gap.
The gap between nerve and muscle cell.

Do 2x transmitter molecules bind 2x receptors?
2x receptors on the same shape shifting protein?

Yes. And the muscle cell protein stretches open.
Sodium ions rush in. An action potential sparks.
Muscle electricity.

The nerve agent sarin halts transmitter breakdown.
In the gap between nerve and muscle cell chaos ensues.

Death. In under 10 minutes.
Even at low doses.
Enough of that.

Muscle cells

Up to 4cm long. Less than 0.1mm wide.
Stretched versions of typical cells.

Packed with thick and thin proteins. Highly organised.
Thin protein filaments pull on the cell structure.
Thick filaments interdigitate in between.
Like multiple hands clasped in prayer.

No signal. Thick filament binding sites are covered. The muscle sits relaxed.

The sarcoplasmic reticulum (SR)
(Moulded flesh, fine network- Latin + Greek)

Incoming spark. Action potential spreads out. Across the muscle cell membrane. Across the sarcoplasmic reticulum (SR).
Another membraned compartment.
Calcium ions pour out.

SR wraps around the filaments. Thin and thick. Throughout the muscle cell.
Sparks turn to action. All simultaneously.

Calcium ions simultaneously uncover binding sites on the thick filament.
Thick protein, powered by ATP energy, can grab hold of the next binding site.
Paddling along the thin filament. Like an Olympic rower.

Thick pulls on thin. Thin pulls on the cell structure.
One nerve signals to 150 muscle cells.
All acting together. All co-ordinating.
One muscle pulls as one.
Joints move.

‘Bend that finger’. Thought to action. In a fraction of a second.

Enough ATP energy and calcium. And that muscle can keep pulling. Keep pumping.

Calcium pumps do their job. Pumping calcium back.
Back into the SR compartment.

No more signal, binding sites cover up.
Thick no longer binds thin.
Muscle relaxes.

Movement complete.

ATP energy

Adenosine Triphosphate is the currency of cellular energy.

ATP energy free in the cell lasts for seconds.
ATP energy stores in the cell lasts maybe 15 more..

ATP energy from splitting sugar (glycolysis) in oxygen (aerobic respiration).

1x glucose (from blood or storage) releases 2x ATP + 2x pyruvate molecules

Pyruvate burns again in oxygen (the aerobic bit) inside the mitochondria.

Mitochondria: the super-efficient powerhouses of the cell.
The evolutionary result of 1x bacterium jumping inside another. Sometime around 2 billion years ago.

Life got the energy kick it needed to evolve complex cells.
Spontaneous combustion. Like a well-controlled, slow-burning fire.

1x glucose molecule here releases 30x ATP energy molecules.

BOOM!

30x ATP molecules from slow burning respiration in the mitochondria...
2x ATP released from simple glycolysis.

We get a flavour of that efficiency gap when we run fast.
Oxygen demand quickly outstrips supply.
The slow burning fire goes out.
But keep running..

Not enough oxygen

Anaerobic respiration takes over.

1x glucose molecule releases 2x ATP energy molecules.
Lactic acid builds up... FATIGUE

Lactic acid lets us know we need to slow down. Or stop moving. If we don’t take the hint...

ATP demand exceeds supply.
Muscle stops contracting.

We can either turn off the nerve stimulated release of calcium. Stop trying. And sit down. Or...

ATP runs out. Thick filament binding sites cover up. Muscles relax. We fall over.

Sit down or fall down. We can't outrun our remarkable physiology.

Run fast and we quickly use up our free ATP. Our stored ATP. We split all of our muscle glycogen.

We burn through our pyruvate with enough oxygen. The oxygen supply dries up. Fatigue sets in. Lactic acid builds up. Our ATP runs out.

Supply of and demand for oxygen. The supply of ATP. Each determines how far we can run.

Run flat out. Make it halfway around a track you are doing well.

But, in everyday life, supplies are plenty. We can 'bend a finger' or balance upright.