Motor units and skeletal systems

Learning Objectives

  1. Define muscle ‘twitch’ and muscle tetanus
  2. Define motor units and explain how the nervous system regulates graded muscle contractions (muscle tension)
  3. Define and explain the physiological differences between fast-, slow-, and intermediate-twitch muscle fibers
  4. Compare and contrast hydrostatic skeletons, exoskeletons, and endoskeletons

Control of Muscle Tension

Skeletal muscle contraction occurs when the cross-bridge cycle of actin-myosin binding is activated; activation of the cross-bridge cycle occur when the muscle cell receives action potentials from an efferent neuron. When the muscle contracts, it pulls on connective tissue which connects the muscle to bones; the result is skeletal movement.

Each action potential causes one unit of muscle contraction, called a twitch, which results in a pull or tension exerted by the muscle. The amount of force created by this tension can vary. In other words, muscles contractions are graded (unlike the action potentials which regulate them, which are all-or-nothing events). The maximum tension possible for a given muscle is called tetanus.

Because muscle contractions are graded (rather than binary or all-or-nothing), the same muscle can both gently move very light objects and forcefully move very heavy objects. (If you’ve ever expected that an object would be a lot heavier than it actually turned out to be and accidentally flung it into the air when you tried to pick it up, then you can probably appreciate the benefits of graded muscle contraction.) The amount of tension produced in a muscle contraction depends on two factors: the number of muscle fibers activated, and the frequency of neural stimulation to the muscle fibers.

  • Number of muscle fibers activated: How do you only activate only some of the muscle fibers in a muscle? By activating varying numbers of motor units. Remember that a muscle fiber is the same thing as a muscle cell. A single efferent neuron will typically control multiple muscle fibers, and there are many many neurons that control different muscle fibers in a single muscle. A single efferent neuron and all of the muscle fibers that it controls is called a motor unit. Thus each muscle contains many motor units, and not all motor units are necessarily activated at the same time. The more motor units are active, the greater the number of muscle fibers that contract, and the greater the degree of muscle contraction.
  • Frequency of neural stimulation: Recall that action potentials cannot vary in magnitude or speed, but the number of action potentials per second (frequency of action potentials) does vary. Each action potential causes one unit of muscle contraction, called a twitch, and the muscle typically begins to relax as soon as the action potential is over. Just like excitatory post-synaptic potentials (remember those?), muscle twitches are additive: if there is a long pause between action potentials, the muscle can fully relax, but if action potentials are rapid enough, the muscle does not have the time to relax and will continue to contract to a greater and greater degree with each new action potential. With frequent enough action potentials, the muscle will reach the maximum tension possible for that muscle, called tetanus.
A motor unit is defined as a single efferent neuron and all of the muscle fibers it controls. Each muscle contains multiple motor units. 1. axon of efferent neuron; 2. neuromuscular junction; 3. muscle fiber; 4. muscle cross-section showing myofibrils. By Synapse_diag3.png: User: DakeMusculus_diagram.svg: *Skeletal_muscle.jpg: User:Deglr6328derivative work: Marek M (talk)derivative work: Marek M (talk) – Synapse_diag3.pngMusculus_diagram.svg, CC BY-SA 3.0,

This video describes the concept and implications of motor units:

Amount of muscle tension is determined by frequency of action potentials. A single action potential results in a single muscle “twitch.” To action potentials in quick succession results in summation of twitches, where the muscle does not relax but engages in a second round of twitching before, which doubles muscle tension. Continuous action potentials will result in fused tetanus, or maximal muscle tension. Image credit: modified from: Daniel Walsh and Alan Sved;

This video explains twitch summation and tetanus:

And his video (beginning at 3:36) describes control of muscle tension:

Types of Skeletal Muscle

All muscle fibers require ATP for the cross-bridge cycle to occur, and depletion of ATP causes muscle fatigue (exhaustion). Different types of skeletal muscle fibers fatigue at different rates due to (among other things) different sources of ATP:

  • Oxidative muscle fibers rely on oxidative phosphorylation to generate ATP. Since oxidative phosphorylation occurs in mitochondria and requires oxygen, oxidative muscles tend to:
    • have high concentrations of mitochondria
    • appear to be deep red due to high concentrations of myoglobin, which delivers oxygen to the mitochondria from the bloodstream.
    • be slow to fatigue (become exhausted): oxidative phosphorylation is comparatively slow for producing ATP, but it is also relatively inexhaustible; it generally takes a very long time to run out of ATP in oxidative muscles.
  • Glycolytic muscle fibers rely on glycolysis to generate ATP. Since glycolysis occurs in the cytoplasm and does not require mitochondria (and thus do not require oxygen for contraction), glycolytic muscles tend to:
    • have low densities of mitochondria
    • appear white due to the comparatively lower concentration of myoglobin
    • be quick to fatigue: glycolysis is comparatively fast for producing ATP, but it is also a rapidly-exhausted source of ATP; glycolytic muscles typically run out of ATP very quickly.

These properties impact the rate of “twitch” and the rate of ATP depletion in a muscle type:

  • Fast-twitch muscles provide brief, rapid, and powerful contractions; they are adapted for bursts of activity and tend to be present in muscles required for short-lived activities such as running. They tend to:
    • be composed of glycolytic muscle fibers
    • contain fewer mitochondria
    • appear white due to lower concentrations of myoglobin
    • be very quick to fatigue.
  • Slow-twitch muscles are capable of maintaining long contractions but are slower to contract. Slow-twitch oxidative muscles are adapted for endurance activities, and tend to be present in muscles required for long-lived activities such as supporting the body core. They tend to:
    • be composed of oxidative muscle fibers
    • contain many more mitochondria
    • appear red due to higher concentrations of myoglobin
    • be very slow to fatigue.
  • Intermediate-twitch muscles (also called moderate fast-twitch fibers) have varying contractile properties due to a mix of oxidative and glycolytic fibers. Most skeletal muscle contain both slow- and fast-twitch fibers in varying ratios, depending on the specific muscle. They can appear pink to red and have ranges of intermediate properties between fast- and slow-twitch muscles, based on the relative abundance of oxidative and glycolytic fibers present in a particular intermediate muscle.

The video below reviews the three types of skeletal muscle fibers:

Skeletal Systems

The information below was adapted from OpenStax Biology 38.1

All the muscle in the world can’t accomplish movement unless the muscle interacts with a skeletal system. A skeletal system is necessary to support the body, protect internal organs, and allow for the movement of an organism. All skeletal systems have one important feature in common: muscles function in antagonistic pairs:

  • Functioning on its own, a muscle can only contract (pull); there is no mechanism within a muscle to cause it to extend (push).
  • Extension of a muscle therefore is accomplished by the contraction of an antagonistic muscle, or a muscle that pulls in the opposite direction.
  • In our own skeletons, you can visualize this from the muscles of the arm: contracting the bicep pulls your forearm toward your shoulder; contracting the tricep pulls your forearm back down to your side. In hinge-based skeletal systems (endoskeletons and exoskeletons), opposing muscles are called flexors or extensors. Flexors (like the bicep) pull two bones toward each other; extensors (like the tricep) straighten two bones out.
All skeletal systems rely on pairs of muscles to move joints. Flexors pull two parts of the skeleton closer together; extensors straighten the skeleton out. Adapted from work by Sheldahl – Own work, CC BY-SA 4.0,

This video, beginning at 1:32, explains the roles of extensors and flexors in movement of endoskeletal systems (watch through 2:54):

There are three fundamentally different types of skeletal systems that can each perform the required functions of as skeleton (support the body, protect internal organs, and allow for the movement of an organism):

  • A hydrostatic skeleton is a skeleton formed by a closed, fluid-filled compartment within the body, called the coelom.
    • The organs of the coelom are supported by the aqueous fluid, which also resists external compression. This compartment is under hydrostatic pressure because of the fluid and supports the other organs of the organism.
    • This type of skeletal system is found in soft-bodied animals such as sea anemones, earthworms, Cnidaria, and other invertebrates. Although a hydrostatic skeleton is well-suited to invertebrate organisms such as earthworms and some aquatic organisms, it is not an efficient skeleton for large terrestrial animals.
    • Movement in a hydrostatic skeleton is provided by muscles that surround the coelom. The muscles in a hydrostatic skeleton contract to change the shape of the coelom; the pressure of the fluid in the coelom produces movement.
    • As in other skeletal systems, the muscles are always paired. For example, earthworms move by waves of muscular contractions in the called peristalsis, which alternately shorten and lengthen the body. Lengthening the body by contracting the longitudinal muscles causes the anterior end of the organism to extend. Shortening the body by contracting the opposing circular muscles then draws the posterior portion of the body forward, resulting in forward movement.
    • Hydrostatic skeletons place no constraints on the contraction width of the muscle, as there is no hard physical barrier that limits the size of the muscle.

This video provides an overview of muscle arrangement in hydrostatic skeletons and how organisms with hydrostatic skeletons move:

  • An exoskeleton is an external skeleton that consists of a hard encasement on the surface of an organism.
    • Exoskeletons provides defense against predators, support the body, and allows for movement through the contraction of attached, opposing muscles.
    • Exoskeletons are present in arthropods and mollusks. Arthropods such as crabs and lobsters have exoskeletons that consist of 30-50 percent chitin, a polysaccharide derivative of glucose that is a strong but flexible material. The exoskeleton is further strengthened by the addition of calcium carbonate in organisms such as the lobster.
    • Because the exoskeleton is not composed of cells and cannot grow larger, arthropods must periodically shed their exoskeletons (and form a new one) as their bodies grow larger.
    • Unlike in a hydrostatic skeleton, muscles must cross a joint inside the exoskeleton to effect movement. Shortening of the muscle changes the relationship of the two segments of the exoskeleton, drawing the segments together (flexion) or moving them apart (extension).
    • Another difference from the hydrostatic skeleton is that an exoskeleton physically constrains the contraction size of a muscle; the muscle can only increase in diameter so much before it physically runs out of room due to the hard shell of the exoskeleton.
  • An endoskeleton is a skeleton that consists of hard, mineralized structures located within the soft tissue of organisms.
    • Endoskeletons are present in sponges (yes, sponges!), echinoderms, and chordates. Mammalian skeletons have on the order of over 200 bones, including some that are fused and some that are connected by ligaments at joints to allow movement.
    • The skeletal system in vertebrates is divided into the axial skeleton (which consists of the skull, vertebral column, and rib cage), and the appendicular skeleton (which consists of the shoulders, limb bones, the pectoral girdle, and the pelvic girdle).
    • Like the exoskeleton (and unlike the hydrostatic skeleton), endoskeletons are jointed and have opposing flexor and extensor muscles.
    • But like the hydrostatic skeleton (and unlike the exoskeleton), endoskeletons do not constrain the diameter of a muscle during contraction (if it did, bodybuilding couldn’t be a thing.)

The video below provides an overview of some of the tradeoffs between endoskeletons and exoskeletons (with a clear human-centric bias toward endoskeletons):