Summary BGZ2025: Use it or Lose it
Case 1
Anatomy of the skeletal muscle
Skeletal muscles are usually attached to bones by tendons made of collagen. When the bones
attached to a muscle are connected by a flexible joint, contraction of the muscle moves the skeleton.
The muscle is called a flexor if the centers of the connected bones are brought closer together when
the muscle contracts, and the movement is called flexion. The muscle is called an extensor if the
bones move away from each other when the muscle contracts, and the movement is called extension.
Muscles function together as a unit. A skeletal muscle is a collection of muscle cells, or muscle fibers,
just as a nerve is a collection of neurons. Each skeletal muscle fiber is a long, cylindrical cell with up
to several hundred nuclei near the surface of the fiber. Skeletal muscle fibers are the largest cells in
the body, created by the fusion of many individual embryonic muscle cells. Committed stem cells
called satellite cells lie just outside the muscle fiber membrane. Satellite cells activate and
differentiate into muscle when needed for muscle growth and repair. The fibers in a given muscle are
arranged with their long axes in parallel. Each skeletal muscle fiber is sheathed in connective tissue,
with groups of adjacent muscle fibers bundled together into units called fascicles. Collagen, elastic
fibers, nerves, and blood vessels are found between the fascicles. The entire muscle is enclosed in a
connective tissue sheath that is continuous with the connective tissue around the muscle fibers and
fascicles and with the tendons holding the muscle to underlying bones.
Skeletal muscle fibers
Muscles function together as a unit. A skeletal muscle is a collection of muscle cells, or muscle fibers
The cell membrane of a muscle fiber is called the sarcolemma, and the cytoplasm is called the
sarcoplasm. The main intracellular structures in striated muscles are myofibrils, highly organized
bundles of contractile and elastic proteins that carry out the work of contraction. Skeletal muscles
also contain extensive sarcoplasmic reticulum (SR), a form of modified endoplasmic reticulum that
wraps around each myofibril
,The sarcoplasmic reticulum consists of longitudinal tubules with enlarged end regions called the
terminal cisternae. The sarcoplasmic reticulum concentrates and sequesters Ca2+ with the help of a
Ca2+-ATPase in the SR membrane. Calcium release from the SR creates calcium signals that play a
key role in contraction in all types of muscle. The terminal cisternae are adjacent to and closely
associated with a branching network of transverse tubules, also known as t-tubules. One t-tubule
and its two flanking terminal cisternae are called a triad. The membranes of t-tubules are a
continuation of the muscle fiber membrane, which makes the lumen of t-tubules continuous with the
extracellular fluid.
T-tubules allow action potentials to move rapidly from the cell surface into the interior of the fiber so
that they reach the terminal cisternae nearly simultaneously. Without t-tubules, the action potential
would reach the center of the fiber only by conduction of the action potential through the cytosol, a
slower and less direct process that would delay the response time of the muscle fiber. The cytosol
between the myofibrils contains many glycogen granules and mitochondria. Glycogen, the storage
form of glucose found in animals, is a reserve source of energy. Mitochondria provide much of the
ATP for muscle contraction through oxidative phosphorylation of glucose and other biomolecules
One muscle fiber contains a thousand or more myofibrils that occupy most of the intracellular
volume, leaving little space for cytosol and organelles. Each myofibril is composed of several types of
proteins
- Myosin is a motor protein with the ability to create movement
Each myosin head has two protein chains: a heavy chain and a smaller light chain.
The heavy chain is the motor domain that binds ATP and uses the energy from ATP’s
high-energy phosphate bond to create movement. Because the motor domain acts as
an enzyme, it is considered a myosin ATPase. The heavy chain also contains a binding
site for actin. In skeletal muscle, about 250 myosin molecules join to create a thick
filament. Each thick filament is arranged so that the myosin heads are clustered at
each end of the filament, and the central region of the filament is a bundle of myosin
tails
- Actin is a protein that makes up the thin filaments of the muscle fiber. One actin molecule is a
globular protein (G-actin). Usually, multiple G-actin molecules polymerize to form long
chains or filaments, called F-actin. In skeletal muscle, two F-actin polymers twist together
like a double strand of beads, creating the thin filaments of the myofibril.
Most of the time, the parallel thick and thin filaments of the myofibril are connected by myosin
crossbridges that span the space between the filaments. Each G-actin molecule has a single myosin-
binding site, and each myosin head has one actin binding site and one binding site for ATP.
Crossbridges form when the myosin heads of thick filaments bind to actin in the thin filaments.
Crossbridges have two states: low-force (relaxed muscles) and high-force (contracting muscles).
Under a light microscope, the arrangement of thick and thin filaments in a myofibril creates a
repeating pattern of alternating light and dark bands. One repeat of the pattern forms a sarcomere,
which has the following elements:
- Z disks. One sarcomere is composed of two Z disks and the filaments found between them. Z
disks are zigzag protein structures that serve as the attachment site for thin filaments.
- I bands. These are the lightest color bands of the sarcomere and represent a region occupied
only by thin filaments
, - A band. This is the darkest of the
sarcomere’s bands and encompasses
the entire length of a thick filament.
At the outer edges of the A band, the
thick and thin filaments overlap.
- H zone. This central region of the A
band is lighter than the outer edges of
the A band because the H zone is
occupied by thick filaments only.
- M line. This band represents proteins
that form the attachment site for
thick filaments, equivalent to the Z
disk for the thin filaments
Slow Twitch (Type 1)
Slow-twitch fibers are designed for
endurance activities that require long-term,
repeated contractions, like maintaining
posture or running a long distance. The ATP
required for slow-twitch fiber contraction is generated through aerobic respiration (glycolysis and
Krebs cycle), whereby 30 molecules of ATP are produced from each glucose molecule in the presence
of oxygen. The reaction is slower than anaerobic respiration and thus not suited to rapid movements,
but much more efficient, which is why slow-twitch muscles do not tire quickly. However, this
reaction requires the delivery of large amounts of oxygen to the muscle, which can rapidly become
rate-limiting if the respiratory and circulatory systems cannot keep up.
Due to their large oxygen requirements, slow-twitch fibers are associated with large numbers of
blood vessels, mitochondria, and high concentrations of myoglobin, an oxygen-binding protein found
in the blood that gives muscles their reddish color. One muscle with many slow-twitch fibers is the
soleus muscle in the leg (~80% slow-twitch), which plays a key role in standing.
Fast Twitch (Type II)
Fast-twitch fibers are good for rapid movements like jumping or sprinting that require fast muscle
contractions of short duration. Unlike slow-twitch fibers, fast twitch-fibers rely on anaerobic
respiration (glycolysis alone) to produce two molecules of ATP per molecule of glucose. While much
less efficient than aerobic respiration, it is ideal for rapid bursts of movement since it is not rate
limited by need for oxygen. Lactate (lactic acid), a byproduct of anaerobic respiration, accumulates in
the muscle tissue reducing the pH (making it more acidic, and producing the stinging feeling in
muscles when exercising). This inhibits further anaerobic respiration. While this may seem counter-
intuitive, it is a feedback cycle in place to protect the muscles from over-exertion and resultant
damage.
As fast-twitch fibers generally do not require oxygenation, they contain fewer blood vessels and
mitochondria than slow-twitch fibers and less myoglobin, resulting in a paler color. Muscles
controlling eye movements contain high numbers of fast-twitch fibers (~85% fast-twitch).
Muscle shapes
, - Circular Muscles: These muscles appear circular in shape and are normally sphincter
muscles which surround an opening such as the mouth, surrounded by Obicularis Oris and
Obicularis Oculi surrounding the eyes
- Parallel muscles: have fibers which, run parallel to each other and are sometimes called
strap muscles. They are normally long muscles which cause large movements, are not very
strong but have good endurance.
- Pennate muscles have a large number of muscle fibers per unit and so are very strong, but
tire easily. They can be divided into:
Unipennate: These muscles have their fibers arranged to insert in a diagonal
direction onto the tendon, which allows great strength. Examples include the
Lumbricals (deep hand muscles) and Extensor Digitorum Communis (wrist and
finger extensor)
Bipennate: Bipennate muscles have two rows of muscle fibers, facing in opposite
diagonal directions, with a central tendon, like a feather. This allows even greater
power but less range of motion. An example is the Rectus Femoris
Multipennate: Multipennate muscles have multiple rows of diagonal fibers, with a
central tendon which branches into two or more tendons. An example is the Deltoid
muscle which has three sections, anterior, posterior and middle.
Fusiform Muscles: these muscles are more spindle-shaped, with the muscle belly being wider than
the origin and insertion. Examples are, Biceps Brachii and Psoas major
Contraction types
- Concentric: causes muscles to shorten, thereby generating force
- Isometric: contractions generate force without changing the length
of the muscle
- Eccentric: contractions cause muscles to elongate in response to a
greater opposing force
- Isokinetic: refers to movement at a constant speed regardless of
the force applied. Muscles contract and shorten at a constant speed
- Isotonic: length changes, tension stays the same
Antagonist: acts as opposing muscle to agonists, usually contracting as a
means of returning the limb to its original resting position.
Agonist: associates with the movement itself, and are sometimes referred to
as prime movers. They contract while another muscle relaxes.
Synergist: This type of muscle acts around a movable joint to produce
motion similar to or in concert with agonist muscles.
Anatomy of the bones
Central/ axial skeleton: The axial skeleton forms the central axis of the
body and includes the bones of the skull, ossicles of the middle ear, hyoid bone of the throat,
vertebral column, and the rib cage. The function of the axial skeleton is to provide support and
protection for the brain, the spinal cord, and the organs in the ventral body cavity. It provides a
surface for the attachment of muscles that move the head, neck, and trunk, performs respiratory
movements, and stabilizes parts of the appendicular skeleton.
Peripheral/ appendicular skeleton: The appendicular skeleton is composed of the bones of the
upper limbs (which function to grasp and manipulate objects) and the lower limbs (which permit
locomotion). It also includes the pectoral girdle, or shoulder girdle, that attaches the upper limbs to
the body, and the pelvic girdle that attaches the lower limbs to the body
