Muscular Tissue

Muscular tissue embodies 5 properties. The first property is excitability, the muscle is able to respond to electrical changes in the membrane. When a muscle cell is exited, it causes a widespread effect, embodying the second characteristic of muscular tissue, conductivity. The stimulation of a muscle cell can cause it to shorten or stretch. Contractility occurs when the muscle shortens and pulls on bone. The opposite effect can also occur and a muscle can lengthen exhibiting the characteristic of extensibility. While a muscle can stretch and shorten, it can also recoil demonstrating elasticity.

Skeletal Muscle

Skeletal muscle are voluntary striated muscle that is attached to one or more bones. Striations present in skeletal muscle occur as a result of alternating light and dark bands. The dark bands of skeletal muscle are made up of thick filaments laying side by side. The thick filaments make up the A band. The light striations are known as I band. Each I band is bisected by a line known as the Z disc or Z line. The distance between two Z discs make up a sarcomere, a functional contractile unit of muscle fiber. A muscle contracts when Z discs are pulled closer together.

Muscle Fibers

Skeletal muscle cells are known as muscle fibers or myofibers due to the length of the cell. The skeletal muscle fiber is covered by a fibrous connective tissue. Endomysium covers each muscle fiber, perimysium bundles muscle fibers into fascicles, and the epimysium covers the whole muscle. The plasma membrane of a muscle fiber is known as the sarcolemma and its cytoplasm is known as sarcoplasm. The sarcolemma has tubular infoldings called t tubules. The t tubules form a triad with two terminal cisternae. The t tubule signals the sarcoplasmic reticulum to release to release calcium in order for muscle contractions to occur.

Muscle fibers are made up parallel protein microfilaments called microfilaments. Microfilaments can either be thick, thin, or elastic. Thick filaments are made up of protein molecules known as myosin. A myosin has two chains that intertwine to form a tail and head protruding at an angle. Thin filaments are composed of F actin, intertwined strands of proteins. The proteins that make the actin are known as globular, or G actin . G actin can bind to the myosin of a thick filament through the active site. Tropomyosin blocks the active site when muscle fibers are relaxed and each tropomyosin has troponin, a calcium binding protein. Elastic filaments anchor the thick filament to the disc and M line. It is made up of titin which stabilizes the thick filaments and keeps it center to thin filaments.

How Your Muscle Knows When to Move

Somatic nerve cells communicate with the muscle cells, connecting the nervous system with the muscular system. A single nerve cell branches out to different muscle cells, causing muscle cells to contract and relax in unison. The nerve cell and the muscle cells it communicates with make a motor unit.

Nerve cells connect with muscle cells at the neuromuscular junction or motor end plate. Nerve fibers end in a bulb like structure called the synaptic knob. The synaptic knob contains synaptic vesicles filled with acetycholine. Acetylcholine is a neurotransmitter that is released when an electrical signal travels down the nerve fiber. It is released because an electrical signal cannot cross the synapse. Muscle fibers receive the Ach through the acetycholine receptors located on the junctional folds, maximizing the amount of Ach released by increasing surface area. The basal lamina, a covering of the muscle fiber, and the sarcolemma contain an enzyme called acetylcholinesterase. The enzyme breaks down acetylcholine in order to relax muscles and end muscle contraction.

The Science of Bicep Curls (and other muscle contractions)

The Science of a Bicep Curl

Biceps curls are a simple workout to tone your biceps. You start by holding a dumbbell in each hand at arm’s length while keeping elbows close to the torso and having palms facing forward. Bring the weight to the top of your shoulder using your biceps, then return to the starting position. Through this workout, you just contracted and relaxed your bicep. Seems simple, right? But when you take a closer look as to how a bicep curl can happen, the process is not as simple as you would think.

To start a bicep curl or any other contraction of a muscle, the muscle cell must be excited by a nerve fiber. Read the next steps to find out how this happens…

A nerve signal arrives at the synaptic knob causing voltage gated channels to open for calcium to be released.
Calcium stimulates synaptic vesicles release Ach.
Ach binds to receptors on the sarcolemma of a muscle cell.
As a response, ligand regulated ion gates open, and causes a rapid change in voltage.
Voltage regulated ion gates open, creating an action po

Once a muscle cell is excited, excitation-contraction coupling occurs! This allows a nerve signal to activate the myofilaments that make a sarcomere, the contractile unit of a muscle.

5. Action potentials reach down T tubules.

6. Calcium is released from the terminal cisternae.

7. Calcium binds to troponin of thin filaments.

8. The troponin-tropomyosin changes shape as a result of the calcium, exposing the active binding site.

Now that excitation and contraction coupling occurs, the bicep is ready to curl. The bicep curl or contracts as a result of fibers shortening. The fibers shorten as a result of sliding filament theory, the thin filaments slide over the thick ones.

9. Hydrolysis of ATP to ADP and P activate and cocks the myosin head.

10.The cocked myosin binds to an active site on the thin filament forming a cross bridge.

11. Myosin releases the ADP and P and tugs the filament, known as the power stroke.

12. ATP binds to myosin, breaking the cross bridge.

These steps allow a muscle to contract, such as curling your biceps. When you’re contracting your muscles, you also need to relax them. To relax your muscles, the body follows these steps:

13. When you want your muscle to relax, nerve signals are no longer released which causes Ach to stop being released.

14. AChe breaks down the acetylcholine

15. Calcium ions are reabsorbed by the sarcoplasmic reticulum.

16. Calcium ions are unbounded from troponin.

17. Tropomysosin blocks the active site of actin.

Which Energy Source Do You Use?

All muscle contractions depend on ATP, the energy source, which can be made in different ways. The type of energy used to contract a muscle depends on the length of time that muscle is being used. Read more to find out which energy source you use for your favorite exercise…

If you like 100 meter sprints, then you are using your immediate energy system. Your immediate energy system, also known as aerobic respiration, provides ATP for 10 seconds. Muscle fibers supply oxygen for aerobic respiration which quickly runs out. Myokinase and creatine kinase transfers phosphate to ADP to make ATP, giving your leg muscles the ability to contract and sprint 100 meters in 10 seconds!

If you like weight lifting, then you are using the phosphagen system. The phosphagen system is made up of ATP and Creatine Phosphate. This energy system is used to perform exercises that are brief but rquire great amounts of efforts. It will provide energy to lift weights for about 1 minute.

If you like playing basketball, then you are using the glycogen lactic acid system. Muscles obtain glucose from blood and glycogen, a glucose storage. The glucose is broken down, creating a net gain of 2 ATP for each glucose molecule. This energy source will let you run across a basketball court for 30 to 40 seconds.

If you like to run long distance, you are using anaerobic fermentation. Unlike the two previously mentioned energy systems, this system is anaerobic. This means that energy is made without the presence of oxygen. It can produce up to 30 ATP per glucose, making it an efficient system of providing energy.

Exercises to Increase Muscle Size

Muscle size can be increased through resistance exercise which contracts muscles against a load that resists movement. Resistance exercise promotes muscle growth through the enlargement of muscle cells. Muscle cells increase in size as fibers make more myofilaments and myofibrils grow thicker. Some exercises that can increase muscle size are arm raises, leg adduction, planks, and any other weight lifting exercises.

Exercise to Decrease Fatigue

To resist muscle fatigue, the muscle needs to enhance delivery and use of oxygen. This can be accomplished through endurance exercises. Exercises that require oxygen cause muscle fibers to produce more mitochondria and glycogen. It also increases red blood cell count and oxygen transport capacity of blood. Exercises that can promote fatigue resistance are jogging and swimming

For best results, try cross training! Cross training incorporates both resistance and endurance exercises.