Muscle Physiology

Unit Overview: In this section you will learn about the skeletal, smooth, and cardiac muscles. For example how the cardiac muscle and smooth muscles are involuntary and the skeletal muscle is voluntary. You will also read about the golgi tendon organs and their function which is to monitor the tension that the muscles that it is in exerts on its tendons. Later on you will also learn about the fact that M lines are produce by protein filaments.

1. Skeletal Muscles
In your body you have many different muscle of different shapes and sizes. They are attached to bone by strong connective tissue tendons. By contracting muscles you are able to make motions at joints, making you functional. Your skeletal muscle are striated voluntary muscles. Where the muscle inserts is usually the most movable spot. While on the other hand the most fixed spot is where the muscle originates. In the sense of the muscles moving, the muscles usually move from the insertion towards the origin. When you contract your flexor muscles you are decreasing the angle of a joint. When you extend your muscles you are increasing your angle of that particular joint.

Skeletal Muscle
Agonist muscle is the name for any muscle that is a prime mover of any skeletal muscle. Antagonistic muscle is the name for the muscles that produce opposite functions at the same joint. The order of the breakdown of all the layers that make up the muscles are from superficial to deep is: Tendon, fascia, skeletal muscle, epimysium, perimysium, fasciculus, endomysium, muscle fiber, sarcolemma, striations, sarcoplasm, myofibrils, and filaments. A motor unit consists of somatic motor neuron that innervates the muscles fibers causing them to contract. Normally there are 23 muscle fibers per every one neuron. Larger motor units innervate bigger muscles while the smaller motor units are used for smaller muscles. Small motor units are the most commonly used motor unit.

Cardiac and Smooth muscles
Cardiac muscles are not voluntarily controlled. But they are striated and found around the heart. They are able to contract by means of the sliding filament mechanism. At each end of the muscle it is attached to an adjacent muscle by an electrical synapse also known as a gap junction. Gap junctions in cardiac muscles have dark stains in lines called intercalated discs. Cardiac muscles are able to move automatically because there are specialized cells called the pacemaker that produces the action potential.


Cardiac Muscle

Smooth muscles are lined in a circular pattern. They are also unvoluntary but they are not striated. They also contain very long muscle cells that are able to stretch up to two and a half times their size. This for example happens around your bladder. For another example by the time you are at the end of your pegnacy the smooth muscle can be stretched up to eight times its own size. They are found in the tubular digestive tract, ureters, ductus deferentia, and the uterine tubes. With the way that smooth muscles lay they move every thing in these tubes in one way (downward) by peristaltic waves. Smooth muscles dont need the presence of action potentials in order to move like cardiac muscles do.

Smooth Muscle

Smooth muscles are also split into two different classes: single-unit and multiunit. Single-units are the ones that have gap junctions like the cardiac muscles have and usually contain pacemaker function.They are the ones found in the digestive tract and uterus. Multiunit smooth muscles are in need of nerve stimulation in order to contract. They are located in the arrector pili muscles and the ciliary muscles.

2. Mechanisms of Contraction & Contractions of Skeletal Muscle

Within each muscle fiber, the A bands are composed of thick filaments and the I bands contain thin filaments. Tension and shortening is caused by cross bridges that extend from the thick to the thin filaments. Availability of Ca2+ regulates the activity of cross bridges. It is increased by action potentials in the muscle fibers.
Myofibrils are a subunit of striated muscle fibers. They consist of successive sarcomeres. Myofibrils run parallel to the long axis of the muscle fiber. The pattern of their filaments provides the striations characteristic of striated muscle cells. Myofibrils are so densely packed that other organelles are restricted to the narrow cytoplasmic spaces that remain between adjacent myofibrils. Each myofibril contains smaller structures called myofilaments. Myofilaments the thick and thin filaments in a muscle fiber. Protein myosin composes the thick filaments. Protein actin composes the thin filaments. Thick and thin filaments overlap at the edges of each A band, which gives the A band a darker appearance.


A thin dark Z line is in the center of each I band. Thick and thin filaments between a pair of Z lines forms a repeating pattern that is the basic subunit of striated muscle contraction. These subunits are know as sarcomeres. A sarcomere is equal to the distance between two successive Z lines. M lines are produced by protein filaments that are located at the center of thick filaments in a sarcomere. The purpose of these is to anchor the tick filaments and help them stay together during a contraction. Titin is a type of elastic protein that runs through the thick filaments from the M lines to the Z discs. Titin contributes to the elastic recoil of muscles that helps them return to their resting length during muscle relaxation.


Summary of the Sliding Filament Theory of Contraction

  • A myofiber shortens by movement of the insertion toward the origin of the muscle.
  • Shortening of the sarcomeres causes shortening of the myofibrils. The distance between discs is reduced.
  • Shortening of the sarcomeres is accomplished by sliding of the myofilaments. The length of each filament remains the same during contraction.
  • Sliding of the filaments is produced by asynchronous power strokes of the myosin cross bridges. This pulls the thin filaments over the thick filaments.
  • The A bands remain the same length during contraction, but become pulled toward the origin of the muscle.
  • Adjacent A bands are pulled closer together as the I bands shorten between them.
  • The H bands shorten during contraction as the thin filaments on the sides of the sarcomeres are pulled toward the middle.

The action of many cross bridges produce sliding of the filaments. These cross bridges are a part of the myosin proteins that extend from the axis of the thick filaments. When the myosin head binds to actin, which forms a cross bridge, the myosin head becomes dephosphorylated. The result of this is a change in the myosin which causes the cross bridges to produces a power stroke. A power stroke is the force that pulls the thin filaments toward the center of the A band.


3.Energy Requirements of Skeletal Muscles, Neural Control of Skeletal Muscles

Energy Requirements of Skeletal Muscles

Aerobic cell respiration is ultimately required for the production of ATP needed for cross-bridge activity.
  • Resting muscles and muscles performing light exercise obtain most of their energy from fatty acids.
  • During moderate exercise, just below the lactate threshold, energy is obtained about equally from fatty acids and glucose.
  • Glucose, from the muscle's stored glycogen and from blood plasma, becomes an increasingly important energy source during heavy exercise.

  • New ATP can be quickly produced from the combination of ADP with phosphate derived from phosphocreatine.


  • Muscle fibers are of three types.relative_abundance_of_different_muscle_fiber_types_in_different_people.jpg

    • Slow-twitch red fibers are adapted for aerobic respiration and are resistant to fatigue.

    • Fast-twitch white fibers are adapted for anaerobic respiration.

    • Intermediate fibers are fast-twitch but adapted for aerobic respiration.


Muscle fatigue may be caused by a number of mechanisms.

  • Fatigue during sustained maximal contraction may be produced by the accumulation of extracellular K+ as a result of high levels of nerve activity.
  • Fatigue during moderate exercise is primarily a result of anaerobic respiration by fast-twitch fibers.
    • The production of lactic acid lowers the intracellular pH, which inhibits glycolysis and decreases ATP concentrations.
    • Decreased ATP inhibits excitation-contraction coupling, possibly due to a cellular loss of Ca2+.

Physical training affects the characteristics of the muscle fibers.

  • Endurance training increases the aerobic capacity of all muscle fiber types, so that their reliance on anaerobic respiration, and thus their susceptibility to fatigue, is reduced.

  • Resistance training causes hypertrophy of the muscle fibers due to an increase in the size and number of myofibrils.

If damage to a muscle occurs the remaining healthy fibers can't divided to replace the ones damaged.
  • Satellite cells (stem cells) are activated at the site muscles are injured which fuses the damaged fibers to produce new muscle fibers. New nuclei is also created to support the new muscle fibers.
    • Satellite cells decrease with age. One reason is an increase of myostatin which inhibits satellite cell function and muscle growth.

New sarcomeres and the growth of myofibrils are formed using 3 giant proteins.
  • Titin- attaches to both Z-discs and the M-band
  • Nebulin- is in the actin of the I bands
  • Obscurin- surrounds the sarcomeres around the Z-discs and M-bands.

These proteins act as molecular scaffolding during muscle growth and repair of the new sarcomeres, among other functions.

Neural Control of Skeletal Muscles

The somatic motor neurons that innervate the muscles are called lower motor neurons.

  • Alpha motoneurons innervate the ordinary, or extrafusal, muscle fibers. These are the fibers that produce muscle shortening during contraction.
  • Gamma motoneurons innervate the intrafusal fibers of the muscle spindles


Muscle spindles function as length detectors in muscles.

  • Spindles consist of several intrafusal fibers wrapped together. These spindle fibers are in parallel with the extrafusal fibers.

  • Stretching of the muscle stretches the spindles, which excites sensory endings in the spindle apparatus.
    • Impulses in the sensory neurons travel into the spinal cord in the dorsal roots of spinal nerves.
    • The sensory neuron makes a synapse directly with an alpha motoneuron within the spinal cord, which produces a monosynaptic reflex.

    • The alpha motoneuron stimulates the extrafusal muscle fibers to contract, thus relieving the stretch. This is called the stretch reflex.

  • The activity of gamma motoneurons tightens the spindles, thus making them more sensitive to stretch and better able to monitor the length of the muscle, even during muscle shortening.

The Golgi tendon organs monitor the tension that the muscle exerts on its tendons.

  • As the tension increases, sensory neurons from Golgi tendon organs inhibit the activity of alpha motoneurons.
  • This is a disynaptic reflex, because the sensory neurons synapse with interneurons, which in turn make inhibitory synapses with motoneurons.

A crossed-extensor reflex occurs when a foot steps on a tack.

  • Sensory input from the injured foot causes stimulation of flexor muscles and inhibition of the antagonistic extensor muscles.
  • The sensory input also crosses the spinal cord to cause stimulation of extensor and inhibition of flexor muscles in the contralateral leg.

Most of the fibers of descending tracts synapse with spinal interneurons, which in turn synapse with the lower motor neurons.

  • Alpha and gamma motoneurons are usually stimulated at the same time, or coactivated.
  • The stimulation of gamma motoneurons keeps the muscle spindles under tension and sensitive to stretch.
  • Upper motor neurons, primarily in the basal nuclei, also exert inhibitory effects on gamma motoneurons.

Neurons in the brain that affect the lower motor neurons are called upper motor neurons.

  • The fibers of neurons in the precentral gyrus, or motor cortex, descend to the lower motor neurons as the lateral and ventral corticospinal tracts.
    • Most of these fibers cross to the contralateral side in the brain stem, forming structures called the pyramids; this system is therefore called the pyramidal system.
    • The left side of the brain thus controls the musculature on the right side, and vice versa.

  • Other descending motor tracts are part of the extrapyramidal system.
    • The neurons of the extrapyramidal system make numerous synapses in different areas of the brain, including the midbrain, brain stem, basal nuclei, and cerebellum.
    • Damage to the cerebellum produces intention tremor and degeneration of dopaminergic neurons in the basal nuclei produces Parkinson's disease.


Essential Questions:

-Describe the sliding filament theory of muscle contraction and why it is called the sliding filament theory. Describe the action of the cross bridges that cause a power stroke. What is the role of calcium and ATP in muscle contraction and relaxation? -Include these key words in your description: actin, myosine, myosine head, ATP, ADP+P, power stroke, cross bridges, tropomysosin, troponin, Ca2+, sarcoplasm, sarcoplasmic reticulum.

When you contract a muscle, you are shortening sarcomeres that slide between two filaments, one thick and one thin. The sliding is produced by cross bridges that come out from both the myosin and actin. Myosine heads are located on each end of a sarcomere which pulls actin towards the center. The myosine heads split up ATP into ADP and P. After the myosine heads attach to a actin protien it forms a cross bridge causing dephosphorylated making a change in the myosin creating a power stroke. This adds to the force pulling the thin filament towards the center. More cross bridges are made to make the force stronger. Attached to actin is a protein called tropomyosin which then has a protein called tropinin attached to it. They are the switch that controls wheather the muscle is going to contract or relax. In order to relax, tropomyosin has to be removed and it can only be removed by tropinin and Ca2+ working together. Each muscle cells contains a cytoplasm which in turn contains the sarcoplasm. The sarcoplasm of each cell has to get to a certain level for it to either contract or relax. The sarcoplasmic reticulum has the space in needed to store Ca2+ when a muscle is relaxed.

-How this applies to PTA:

As PTA's we need to be aware of how the muscle stretches and how the muscle reacts to stimuli and their muscle reflex. Knowing how a muscle contracts at a molecular level can us gauge how far we can stretch a muscle. When someone gets a "knot" we know that means that not all the sarcomeres are relaxing after a contraction and over time more and more sarcomeres are getting stuck in a contraction that creates what we call the "knot." By knowing this we will be able to work the tissues loose by either massage, manual traction, or applying a TEN's unit or some other form of electrical stimulation to force the stuck sarcomeres to relax.

Using TEN's unit, other electrical stimulation or Ultrasound can be used to interrupt the muscle spasm cycle and help the patient relieve pain caused by tight muscles.

1. Fox, Stuart I. "Human Physiology." New York: McGraw-Hill, 2011. Print.