RK Goit, Lecturer

Department of Physiology

Muscle Contraction

Isometric contraction Isotonic contraction

Length of the muscle

Remains same Shortening of the muscle

Tension ↑ during the contraction

No change

Mechanism Sarcomere which shorten do so by stretching those which do not

Shortening of individual sarcomeres adds up to the shortening of the whole muscle

External work No external work down

Work down

Example Trying to lift heavy weights (when the weights are not actually lifted)

Lifting of weights

Energetics of Muscle Contraction

when a muscle contracts against a load, it performs work

energy is transferred from the muscle to the external load to lift an object

energy required to perform the work is derived from the chemical reactions in the muscle cells during contraction

muscle contraction depends on energy supplied by ATP required to actuate the walk-along mechanism

pumping Ca++ from sarcoplasm into SR

pumping Na+ & K+

concentration of ATP in muscle fiber is sufficient to maintain full contraction for only 1 to 2 s

ADP is rephosphorylated to form new ATP within another fraction of a second

There are several sources of the energy for this rephosphorylation.

Phosphocreatine

phosphocreatine is instantly cleaved, & its released energy causes bonding of a new phosphate ion to ADP to reconstitute the ATP

the total amount of phosphocreatine in the muscle fiber is also very little (only about five times as great as the ATP)

the combined energy of both the stored ATP & the phosphocreatine in the muscle is capable of causing maximal muscle contraction for only 5 to 8 seconds

Glycogen

breakdown of glycogen to pyruvic acid & lactic acid liberates energy that is used to convert ADP to ATP

ATP can be used directly to energize additional muscle contraction & also to re-form stores of phosphocreatine

importance of glycolysis mechanism is

glycolytic reactions can occur even in the absence of O2

rate of formation of ATP is about 2.5 times as rapid as ATP formation in response to cellular foodstuffs reacting with O2

glycolysis also loses its capability to sustain maximum muscle contraction after about 1 min

Oxidative metabolism

means combining O2 with the end products of glycolysis & with various other cellular foodstuffs to liberate ATP

↑ 95 % of all energy used by muscles for sustained, long term contraction is derived from this source

for extremely long-term maximal muscle activity the greatest proportion of energy comes from fats, but for periods of 2 to 4 hours, as much as one half of the energy can come from stored carbohydrates

Thermal Changes

the energy expenditure of muscle differ markedly at rest as compared to that during activity

although an unstimulated muscle produces heat, heat production increases during & immediately after contraction

Resting heat

heat produced in unstimulated muscle & reflects energy metabolism in the resting muscle

a resting muscle needs energy for the vital processes of life, especially for operating the sodium pump to maintain the resting membrane potential

Activation heat

heat produced in stimulated muscle before shortening

energy spent on release of calcium form the terminal cisternae, binding of calcium to troponin

Shortening heat

heat associated with shortening

energy spent on the ratchet mechanism involving myosin cross bridges & the active sites on actin filaments

Maintenance heat

heat produced during tetanus

partly made up of the activation heat associate with each stimulus & partly of the heat produced due to actin-myosin interaction

Relaxation heat

relaxation heat is associated with relaxation

energy expenditure associated with uptake of calcium by the terminal cisternae

Recovery heat or delayed heat

additional heat spent over & above the resting heat after contraction & relaxation are over

this is due to restoration of the resting state

References

Textbook of Medical Physiology, 12/E Guyton & Hall

Understanding Medical Physiology, 4/E Bijlani & Manjunatha

Thank You