Muscle

Three basic types: Skeletal = striated = voluntary Smooth = non-striated = involuntary = visceral Cardiac

What does muscle do?

Mainly, generate tension. Depending on arrangement, this can a. Flex or extend a limb b. Constrict the diameter of an opening or a tube c. Change the dimensions or shape of a sheet They are mechanical devices, and can do work. Therefore, they require energy. No real mechanical device is 100% efficient, so not all the energy they use is converted to work. Some is dissipated as heat.

Heat generation by muscles is why body temperature rises during exercise. Sweating dissipates the heat through evaporation, keeping body temperature in a safe range. Shivering is an important mechanism for generating extra heat when body temperature falls.

Skeletal Muscle Basics

Some vocabulary: Cell membrane = sarcolemma Endoplasmic reticulum = sarcoplasmic reticulum Cytoplasm = sarcoplasm Muscle cell = muscle fiber Muscle fiber contains myofibrils Myofibrils consist of thick and thin filaments (bundles of myosin and actin, respectively)

The repeating unit is called a sarcomere. In a resting muscle, sarcomere length is about 2 microns (0.002 mm).

Source of energy for muscle is ATP, so there are lots of mitochondria in myofibrils.

Contraction of Skeletal Muscle

It was once believed that thick and or thin filaments shortened when muscle cells contract. In fact, neither changes length. They slide across each other, reducing sarcomere length.

Excitation-Contraction Coupling

Sarcolemma has projections (T-tubules) that dip deeply into muscle fibers and make close contact with sarcoplasmic reticulum.

Action potentials are transmitted via T-tubules to all of sarcoplasmic reticulum. This triggers release of Ca ions into sarcoplasm, which triggers “sliding” of filaments. When action potential is over, Ca is taken back into sarcoplasmic reticulum, muscle relaxes.

Muscle Mechanics

1. Develop tension without shortening = isometric (constant dimension)

Pushing against a wall = isometric contraction2. Shorten with nearly no change in tension over the distance

shortened = isotonic (constant force) Lifting a pencil = isotonic contraction3. Tension increases during shortening = auxotonic Stretching a spring = auxotonic contraction4. Tension decreases during shortening = meiotonic Playing a piano = meiotonic contractions

Virtually every mechanical act we perform is some combination of these.

Isometric Length-Tension Relation

This illustrates what happens if you hold a muscle at various fixed lengths, give it a brief stimulus at each length and record the response to each stimulus ( = active tension) and the total tension ( = active tension + passive tension resulting from elastic properties of the connective tissue).

Notice that active tension increases, reaches a maximum at around the resting length, then decreases and eventually falls to zero.

Who cares?

The now-obsolete theory that muscles contract because actin and/or myosin filaments shorten can’t be reconciled with this length-tension relation, and this is why it was abandoned. The sliding filament theory fits it perfectly.

Notice how the tension simply reflects the extent of overlap between crossbridge-forming areas of the thick and thin filaments.

Force-Velocity Relations

Now let’s look at what happens to the velocity with which muscles contract as the load varies.

Not surprisingly, velocity is greatest with small loads. As the load increases, velocity decreases and the contraction finally becomes isometric.

Since no work (= force x distance) is done under isometric conditions, the work output falls to zero with very high loads, and the power output (= work/velocity) does as well. The power output reaches a maximum at around the resting length of a muscle, at about one-third of the maximum load.

Twitches and Sustained Contractions

If a muscle is given a single, brief stimulus, it responds with a single, brief contraction (= twitch). If more stimuli are applied at not-too-frequent intervals, they elicit more twitches.

If we increase the frequency of the stimuli, we eventually reach a point at which the muscle can’t fully relax between stimuli. This is incomplete tetany.

If we continue to increase the stimulus frequency, we eventually reach a frequency at which the muscle can’t relax at all between stimuli. This is called tetany; the lowest frequency that causes it is called the tetanic frequency.

The tetanic frequency in humans is around 60/second. Since wall current is at 60 cycles/second, if you grab a live wire, you can’t let go, and you die. The rest of the world uses 50 cycle current, greatly reducing the risk of electrocution.

Why the heck do we use 60 cycle current?

In the early days of electricity, congress commissioned an engineering society to recommend the best frequency for wall current. The engineer who studied the question mistakenly believed that lights would appear to flicker at frequencies below 60 cycles/second. He recommended 60 cycle current for that reason.

In fact, you won’t see flicker at frequencies as low as 30 cycles per second (that’s the frame rate of movies – no flickering at all).

The moral of the story is that basing public policy on science results in policies that are no better than the science on which they’re based.

How do we control the duration of a muscle contraction?

The duration of a muscle contraction is controlled by the duration of a tetanic stimulus applied to it. Varying duration of tetany = varying duration of contraction.

How do we control contraction strength?

We accomplish this by varying the number of muscle cells that are contracting. The larger the number, the stronger the contraction.

This brings us to motor units.

Motor Units

A muscle cell is innervated by one motor neuron, but a motor neuron can innervate up to 500 muscle cells.

A motor neuron and all the muscle cells it innervates = motor unit. An action potential in a motor neuron results in ACh release from axon terminals on every muscle cell it innervates, so they all contract and relax simultaneously.

The strength of a muscle contraction is controlled by the number of motor units activated during the contraction.

Some muscles, the quadriceps, for instance, have very large motor units. Thus, gradations of contraction by the quadriceps are coarse. If the motor unit size is around 500 muscle cells, it can contract cells in multiples of 500 – no intermediate gradations are possible. That’s why you can’t play the piano very well with your heels.

Other muscles, those controlling the fingers, for instance, have small motor units. That’s why they can make very finely graded contractions.

Muscle Metabolism

Muscle uses a lot of energy, in the form of ATP. Muscle mitochondria use oxygen for ATP synthesis, at least during aerobic contractions. This applies to conditions in which the muscle is generating up to about 2/3 of its maximum power. Beyond that point, muscles don’t receive oxygen fast enough to generate all their ATP aerobically.

When muscles generate more than 2/3 of their maximum power, they use anaerobic pathways (glycolysis) in addition to aerobic pathways to generate ATP. This provides ATP from glucose stored in muscle as glycogen. One end product of anaerobic glycolysis is lactic acid. Lactic acid accumulation is a major factor in muscle fatigue. That’s why anaerobic exercise can only go on for short periods. Resting the muscle allows lactic acid to be metabolized to carbon dioxide. Panting is a mechanism for increasing oxygen uptake and carbon dioxide elimination. We pant after anaerobic exercise for that reason.

Types of Skeletal Muscle Fibers

It’s a bit of a simplification to say that there are two main kinds of skeletal muscle fibers: red and white. The difference is a result of one type (red) having large amounts of a red protein, myoglobin, in the sarcoplasm. Red fibers come in two types, fast and slow twitch.

Myoglobin is a close relative of hemoglobin (the protein that gives blood its color). Like hemoglobin, it binds oxygen reversibly. So, red muscle has a reservoir of oxygen in it. For that reason, it fatigues slowly. Muscles that remain contracted for long durations (postural muscles, for instance) are mainly slow twitch red muscle fibers.

Having lots of myoglobin leaves less room for actin and myosin. For that reason, red muscle twitch velocities are slower than white.

White muscle fatigues easily but can generate great power for short periods.

White Muscle and Red Muscle

Different species of animals have different proportions of red and white muscle, and their lives reflect that.

Cats are primarily white muscle animals. They are capable of great speed, but only for short distances. Hence, they hunt by stealth. Dogs are primarily fast twitch red muscle animals. They have great stamina, and hunt by running the prey to exhaustion.

Different muscles in the same animal have different proportions of red and white fibers. The turkey on your Thanksgiving table has red muscle legs, the dark meat. It has white muscle on the breast (flight muscles), the white meat.

The flight muscles of birds generate enough power to keep the bird aloft. Why don’t they quickly fatigue, dropping the bird like a rock? Watch a bird in flight and you’ll see the answer.

Muscle Atrophy and Hypertrophy

Use and disuse -> hypertrophy and atrophy, respectively.This is the basis for training and body building.

Marathon runners promote hypertrophy of red muscle. Sprinters promote hypertrophy of white muscle.

Aerobic exercise also promotes growth of the vasculature in muscles, resulting in greater rate of supply of oxygen and removal of carbon dioxide. VERY important in heart muscle (more about that later).

Immobilization (in a cast, for instance) results in very rapid, dramatic disuse atrophy and loss of muscle tone.

Smooth Muscle

Occurs in viscera, blood vessels, diaphragm. Not subject to voluntary control via somatic branch of peripheral nervous system. Lacks striations because myofibrils aren’t aligned the way they are in skeletal muscle cells.

Contractions are slower than in skeletal muscle, and range of lengths is greater.

Smooth muscle cells, unlike skeletal muscle cells, transmit action potentials from cell to cell. Thus, stimulating one cell in a smooth muscle tissue results in contraction propagating in all directions from the cell that was stimulated.

Rate of action potential propagation in smooth muscle cells is slow, so the contractions originating in a single cell propagate as waves of contraction and relaxation.

Smooth muscle contraction and its rate and direction of propagation are usually modulated by neural influences, sometimes by circulating chemical messengers (hormones) as well.