Biology 211Anatomy & Physiology I

Dr. Thompson

Physiology of Muscle

Saladin describes the chemical events of excitation, contraction, and relaxation in Chapter 11

You will not be responsible for the details of these events, but you should review and understand some general concepts we recently discussed, so let’s review:

1. When resting, the plasma membrane of a myocyte is polarized. Sodium ions are concentrated on its outer surface, while potassium ions are concentrated on its inner surface.

2. The motor neuron secretes the neurotransmitter acetylcholine, which binds to the sarcolemma of the myocyte

3. This causes the sarcolemma to depolarize by allowing ions to flow across it, into and out of the cell.

4. This depolarization spreads along the sarcolemma and is carried deep into the cell by transverse tubules, while lie side-by-side with the sarcoplasmic reticulum.

5. Depolarization of the transverse tubules stimulates the sarcoplasmic reticulum to release large amounts of calcium ions (Ca++) into the cytoplasm of the myocyte

6. This calcium binds onto troponin of the thin myofilament, which moves the tropomyosin to expose active sites on actin

Myosin head groups can now bind to the actin, forming cross bridges, after which they flex to move the thin filament

7. This movement of thin filaments causes the sarcomeres to shorten, generating the force of muscle contracton

8. If the motor neuron keeps stimulating the sarcolemma to depolarize, the sarcoplasmic reticulum can not remove calcium from the cytoplasm, so contraction continues.

but:

If the motor neuron stops stimulating the sarcolemma, thena) it can repolarizeb) sarcoplasmic reticulum pulls Ca++ out of the cytoplasmc) cross-bridges between thin & thick myofilaments breakd) sarcomeres relax and the muscle stops producing force

Breaking of cross-bridges also requires the input of energy, so if myocytes run out of energy they will stay contracted.

The easiest way to visualize and understand muscle contraction is to consider it a series of muscle twitches.

One twitch: A very brief stimulus from the motor neuron causes a single, brief contraction of the myocyte, after which it relaxes and stays relaxed until it is stimulated again

Muscle contractions very rarely consist of individual twitches (full contraction followed by full relaxation)

In normal use, myocytes are repeatedly stimulated by their motor neurons many times per second.

Depending on how frequently they are stimulated, myocytes either contract repeatedly or remain contracted.

Depending on how frequently they are stimulated, myocytes either contract repeatedly or remain contracted.

If the stimuli from the motor neuron are long enough apart, the myocyte has time to relax completely so a series of twitches occur

Depending on how frequently they are stimulated, myocytes either contract repeatedly or remain contracted.

If the myocyte is stimulated again before it has had time to relax completely from the stimulus before, the contractions "piggyback" on each other and get stronger each time. This is called summation.

Depending on how frequently they are stimulated, myocytes either contract repeatedly or remain contracted.

If the myocyte is stimulated so rapidly that it can not even begin to relax, it will remain fully contracted. This is called tetany or tetanus

Recall: Contraction and relaxation require the input of energy, which is produced in mitochondria by burning fuel molecules such as glucose or fatty acids together with oxygen.

This produces waste products in the myocyte, primarily carbon dioxide and lactic acid, which inhibit further contraction.

If a myocyte remains in tetanus: a) It will eventually run out of fuel molecules b) It will eventually run out of oxygen c) Carbon dioxide & lactic acid accumulate inside myocyteand it loses the ability to contract. This is called fatigue - the myocyte stops producing force even though the motor neuron continues to stimulate it

If all (or most) of the myocytes in a muscle are contracting at the same time, the entire muscle will fatigue when those myocytes fatigue.

However: if only a few of the myocytes are contracting at the same time, the entire muscle will not fatigue when individual myocytes fatigue.

This is because rested myocytes can take over contraction when the earlier cells fatigue and stop contracting. This is called recruitment.

Each muscle consists of a mixture of muscle cells, some of which will contract more quickly and strongly than others.

The proportions of each type determine the characteristics of the muscle as a whole.

Fast-twitch muscle cells: Contract and relax very quickly Large diameter Very strong Mostly anaerobic metabolism (glucose lactic acid) Fatigue easily

Fast-twitch myocytes: Contract and relax very quickly Large diameter Very strong Mostly anaerobic metabolism (glucose lactic acid) Fatigue easily

Slow-twitch myocytes: Contract and relax more slowly Smaller diameter Less strong Mostly aerobic metabolism (glucose carbon dioxide) (fatty acids Do not fatigue easily

Also: intermediate forms between these two

Proportions of Slow- and Fast-twitch Myocytes in the Quadriceps Femoris Muscle of Male Athletes

Slow-Twitch Fast-Twitch

Average Males 45% 55%

Swimmers 74% 26%

Marathon runners 82% 18%

Sprinters & jumpers 37% 63%

Contraction = the generation of force within a myofibril, myocyte, or muscle by interaction between thin myofilaments and thick myofilaments.

It does not necessarily get shorter.

When a muscle first begins to contract, the force it produces is less than the mechanical load resisting movement.

The muscle develops more force but does not get shorter.

This is called isometric contracton.

As the muscle continues to contract (it generates more force), this amount of force eventually equals and then exceeds the resistance of the mechanical load.

The muscle stops developing more force and becomes shorter.

This is called isotonic contracton.

In other words:

Contraction of a muscle does not always produce shortening of that muscle

because

Contraction always occurs against a mechanical load and begins as isometric contraction

and

The muscle does not shorten until the force it produces is at least equal to the force of this load against it, after which it uses isotonic contraction.