SN7 SHORT NOTE 5M

Write a short note on muscle fatigue. Add a note on oxygen debt.

Physiology · 1st Professional — Pre-Clinical

Muscle fatigue = the reversible decline in the ability of a muscle to generate force following prolonged or repeated activity.

Site	Fatigues?	Remarks
Nerve fibre	Practically never	Indefatigable — energy needs are tiny
Neuromuscular junction	Yes	ACh stores deplete — the first site to fail in the isolated nerve–muscle preparation
Muscle fibre	Yes	Depletion of glycogen and creatine phosphate; lactate accumulation
CNS	Yes	"Central fatigue" — the first site to fail in the intact human

Depletion of energy stores — ATP, creatine phosphate, muscle glycogen
Accumulation of metabolites — lactic acid (↓ pH inhibits myosin ATPase and Ca2+ release), inorganic phosphate, K+
Impaired Ca2+ release from the sarcoplasmic reticulum
↓ Blood flow — sustained contraction above 15% of maximum occludes the vessels
Depletion of ACh at the neuromuscular junction

Type	Site	Feature
Central fatigue	CNS — motivation, motor cortex	Sense of tiredness; force returns with strong encouragement or electrical stimulation
Peripheral fatigue	NMJ and muscle fibre	True failure of the contractile machinery

Oxygen debt = the extra oxygen consumed after exercise, over and above resting requirement.

Used to — resynthesise ATP and creatine phosphate, convert lactate back to glucose (Cori cycle), replenish myoglobin and haemoglobin oxygen stores
Maximum about 11.5 litres in a trained athlete

Plasticity (stress relaxation) — tension falls back toward normal after sustained stretch. Allows the bladder and stomach to fill without a rise in pressure
Rhythmicity — slow waves and spike potentials generated by the interstitial cells of Cajal, the pacemaker of the gut
Tone — maintained partial contraction, as in sphincters and arterioles
Response to stretch — stretch itself causes contraction (myogenic response); basis of autoregulation of blood flow
Sensitive to chemical agents — hormones, local metabolites, pH, O2; contracts even when denervated

Training increases mitochondrial density, capillary supply and glycogen stores → delays fatigue
McArdle disease — muscle phosphorylase deficiency → cannot use glycogen → early fatigue and cramps with no rise in blood lactate on exercise
Myasthenia gravis — pathological fatigability at the neuromuscular junction

All questions in Nerve & Muscle Physiology