Skeletal Muscle

Skeletal muscles typically attached to one or more bones via a bundle of collagen fibers called tendons. Skeletal muscles can contract in size which results in a pulling force being applied to the bone. This may cause the bone to move or may cause other surrounding tissues to move as will occur when you smile.

The somatic nervous system, which comprises a nerve network that is under conscious or voluntary control by the brain, controls skeletal muscles. Therefore you can move skeletal muscles via conscious thought. As a result skeletal muscles are usually classified as voluntary muscles.

The number of skeletal muscles in the human body varies greatly depending on who is counting. There are at least 650 different skeletal muscles, but some suggest there are over 800.  By any measure, that represents a substantial number of muscles that you can consciously control.

Skeletal Muscle Composition

Multiple bundles of muscle tissue form each skeletal muscle. A dense layer of connective tissue called the Epimysium surrounds each muscle. The epimysium constrains the muscle tissues so they can both contract more strongly and maintain their structure. The epimysium also isolates the muscle from surrounding tissues so the muscle can move independently.

Muscles are further organized into bundles, called fascicles, that each contain many muscle fibers. Surrounding each fascicle is the perimysium. The perimysium performs a function similar to the epimysium on behalf of a fascicle, but also allows an individual fascicle to undergo contraction independently of the other fascicle in a muscle. This allows the nervous system to activate only a specific fascicle within a muscle when required.

Muscles also contain a large quantity of blood vessels that deliver nutrients, oxygen and minerals and also remove waste materials. Muscles also contain a complex network of nerve tissues that initiate muscle contractions and help a person identify the location or position of a limb.

Each fascicle contains multiple muscle fibers that run the entire length of the fascicle. Each muscle fiber is a single separate muscle cell. These are very thin and long cells. Within the adult human body the longest of these cells is nearly 12 inches (30 cm) in length.

The following diagram depicts the composition of a single muscle fiber (cell).

Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. (Own work) [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

Each muscle fiber contains one or more nucleus, an outer covering called the Sarcolemma, multiple mitochondria, and a great many myofibrils. There are usually between 30 and 40 myofibrils per muscle fiber (cell). A structure called the Sarcoplasmic Reticulum surrounds each myofibril.

The Sarcolemma has many holes that connect to the T-tubule (transverse tubule) between two sarcoplasmic reticulum. This junction of two sarcoplasmic reticulum and a T-tubule forms the triad.

Each myofibril contains numerous sarcomere. A myofibril may have perhaps 5,000 sarcomere for every 1 cm (0.4 inches) of its length. This means there are about 175,000 sarcomere in every 1 cm of a typical muscle fiber (35 myofibril * 5,000 sarcomere/cm).

The sarcomere have the innate ability to contract. A muscle can generate a significant pulling force when contractions occur in unison across its entire area.

Sarcomere

Sarcomere make up the largest proportion of a muscle cell. These are flexible structures that cause muscles to contract.

Richfield, David (2014). “Medical gallery of David Richfield”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.009. ISSN 2002-4436. (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons

A sarcomere has several different types of protein components. The thick central myosin filament (shown in red) remains stationary during contractions. The thinner actin filament (shown in blue) moves over top of the myosin filaments during contraction. This arrangement leads to definition of several noteworthy bands. The I-Band is the section between the end of the sarcomere and the start of the myosin filament (essentially the length of the Titin). An I-band exists on both ends of the sarcomere when the muscle is in a relaxed condition. Between the two I-bands is an area referred to as the A-band (not depicted). It represents the total area occupied by the myosin. The H-Zone represents that portion of the A-Band where the actin and myosin filaments do not overlap.  Finally, a Z-disc occurs where the CapZ from multiple sarcomere intersect in a muscle myofibril.

Note that the Titin, a third protein-component, is not part of the myosin filament. It functions to keep the myosin straight (in line) during contractions and to act as a spring when the contraction is complete so that the sarcomere returns to its normal relaxed state.

By OpenStax [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

The diagram at right shows composition of a muscle fiber and the various banding found in striated muscle.You may notice a couple of things from this diagram. The first is that the I-band and A-band areas alternate. This yields an alternating pattern of dark and light areas that repeats throughout the muscle fiber. This striation or striped pattern is what led to skeletal muscles being classified as striated muscles.

The sarcoplasmic reticulum is responsible for storing and then releasing calcium ions into the sarcomere to facilitate contraction. The sarcoplasmic reticulum extends along and around the entire length of each myofibril. When a contraction begins, the sarcoplasmic reticulum releases stored calcium ions into the myofibril and all of its sarcomere. This is commonly called a calcium spark and it initiates the contraction process throughout all affected sarcomere.

The sarcoplasmic reticulum also pumps calcium ions from surrounding tissues to store these ions for use in future contractions.

Skeletal Muscle Movement

By OpenStax [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

Muscle movement occurs when motor neurons send signals to the sarcolemma. These signals travel down the holes in the sarcolemma and into the T-tubule. From there these signals transfer into the two adjacent sarcoplasmic reticulum causing them to release calcium ions into the sarcomere in the myofibril. The calcium ions initiate chemical reactions within the sarcomere that cause the multiple myosin heads to repeatedly go through the following cycle.

  1. The myosin head extends and attaches itself to a molecule in the actin filament.
  2. The myosin head changes its orientation and effectively drags the actin filament inward and over the myosin filament, narrowing the H-zone.
  3. The myosin head detaches from the actin and chemically prepares for a subsequent attachment and movement cycle.

Each myosin head repeats this process multiple times during a contraction. There are always multiple myosin heads in contact with and pulling on the actin filament at any time during a contraction.

The primary byproduct of these chemical reactions is heat generation. Fully 80% of the energy expended during muscle contractions is in the form of heat. The remaining 20% results in muscle movement. An important benefit of blood flow through muscles is removal of this heat. Blood transports this heat to surface areas of the body for cooling.

When a sarcomere is fully contracted the I-bands will disappear completely and the H-zone will be quite narrow or non-existent. These actions will also significantly compress the titin.

When a contraction ends the titin will push the Zcaps apart, returning the sarcomere to its normal relaxed state. Pumps within the sarcoplasmic reticulum retrieve and store calcium ions in preparation for future contractions. This helps stop contraction activities within the muscle.

As millions of connected sarcomere contract they quickly shorten the overall length of the entire muscle. This exerts a strong united pulling force sufficient to move bone and lift heavy loads.

Energy Use

Energy is a necessary component to support the actions of the myosin heads. The energy for this action comes from adenosine triphosphate (ATP).  The many mitochondria within each muscle fiber produce ATP. Muscles fibers store a small amount of ATP for immediate use, but for longer-term activity muscles require the ongoing production of ATP provided by mitochondria.

A contraction begins when calcium ions cause a chemical change in the actin and an ATP molecule binds to a myosin head. For the purposes of our discussion this cycle begins at (d) in the following diagram. For additional clarification, the Thin Filament is the actin and the Thick Filament is the myosin.

By OpenStax [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

The ATP molecule then undergoes a transformation resulting in an adenosine diphosphate (ADP) molecule and an inorganic phosphate atom (or put another way, the ATP loses a phosphate atom and becomes ADP plus an inorganic phosphate atom) as depicted in the diagram at step (e). This enables the myosin head to activate and attach to molecules in the actin (which can only happen while calcium ions are present) as depicted at step (b). When this occurs the phosphate atom releases from the myosin head and the myosin head becomes more strongly attached to the actin.

Now the ADP molecule releases from the myosin head and the myosin head pivots to pull the actin inward as shown in step (c). The myosin head remains attached to the actin until another ATP molecule binds to it. At that time the myosin head will release in preparation for the next cycle. You can find multiple videos that animate this process to offer a clearer understanding should you wish to study this process further. A simple search for ‘ATP myosin head video’ will yield many informative videos.

ATP is essential to support muscle contractions. Muscles store a small amount of ATP to support immediate contraction needs, but this ATP will only last a few seconds before depletion. So muscles need a way to quickly acquire more ATP. After the first few seconds muscles begin to break down glucose stored in the muscle. A chemical chain reaction converts one glucose molecule into two ATP molecules (and several other byproducts, including two lactic acid molecules). This is an anaerobic process (not requiring oxygen). It is fast, but not an especially efficient use of glucose. This will support short or intense bursts of muscle activity.

Longer-term muscle use requires a more efficient and sustainable energy supply. This comes through a separate aerobic process (utilizing oxygen) that converts one glucose module to 36 ATP molecules. This conversion also produces carbon dioxide and water as byproducts (not lactic acid). This conversion occurs within the mitochondria in the muscle fiber and requires abundant supplies of oxygen. This provides the energy necessary for long endurance activities. In that case the body may use existing blood glucose stores. The body may also convert stored fat into glucose to sustain long-term muscle activity.

In reality muscles will use all three sources of energy over a given unit of time. Short term ATP supports sudden short bursts of heightened activity. The same thing applies for anaerobic processes where a sudden uptick in energy use leaves a muscle unable to supply enough oxygen to support adequate aerobic glucose conversion processes. The muscle may also use some aerobic conversions even during short-term contraction cycles.

Fast Twitch vs. Slow Twitch Muscle Fibers

The muscle fibers in a muscle are not all the same. There are various types of muscle fibers that determine how a muscle responds during a contraction event.

Type I

Type I muscle fibers are commonly called slow twitch muscle fibers. These fibers primarily use aerobic processes to generate ATP. Since this process takes longer than anaerobic processes, these muscle fibers respond less quickly to a contraction event.

Type IIA

Type IIA muscle fibers rely on both anaerobic and aerobic processes for ATP production. As a result these muscles have relatively fast response times and more prolonged endurance.

Type IIB

The Type IIB classification refers to fast twitch fibers. These fibers rely largely on anaerobic processes for ATP production. As a result these muscles have smaller blood vessels and fewer mitochondria than the other muscle fiber types. This means these muscle fibers have limited endurance but respond very quickly to a contraction event.

 

Muscle Composition

Most skeletal muscles are fast twitch muscles. They can respond quickly (perhaps three times faster than slow twitch muscles) but also rapidly tire. Slow twitch muscles are found in the large muscles that support body posture and provide locomotion. Most muscles contain a mixture of fast twitch and slow twitch muscle fibers. The percentage of each fiber type is often determined by genetics and muscle function. However, repeated training can change the mixture of muscle fiber types in a muscle thereby changing its response in different situations.

Support Systems

Muscle tissues enjoy a rich blood supply that delivers nutrients, oxygen, and minerals and removes waste materials. Muscles require constant and ample blood supplies while undergoing sustained activity. An extensive nerve network also provides contraction initiation and proprioception information. This latter function serves to allow a person to know limb position and rate of movement information without direct observation.

 

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