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uitgebreide Samenvatting hoofdstuk 12 silverthorn human physiology
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Fleur ch 12CHAPTER 12: MUSCLES
Muscles have 2 common functions: to
generate motion & to generate force.
Skeletal muscles also generate heat and
contribute significantly to homeostasis
of body temp. when cold conditions
threaten homeostasis, brain may direct
our muscles to shiver, creating
additional heat.
The human body has three types of
muscles: skeletal muscle, cardiac
muscle and smooth muscle.
Most skeletal muscles are attached to
the bones of the skeleton, enabling
these muscles to control body
movement.
Cardiac muscles, found only in heart and moves blood through circulatory system.
Skeletal & cardiac muscles are classified as striated muscles, bc of their alternating light and dark
bands seen under light microscope.
Smooth muscle is primary muscle of internal organs and tubes, like stomach, urinary bladder, blood
vessels. Primary function is to influence the movement of material into, out of, and within the body.
Bv. Passage of food through gastrointestinal tract. Smooth muscle lacks cross-bands of striated
muscles. Results from less organizes arrangement of contractile fibers within the muscle cells.
Skeletal mujscle is voluntary, smooth & cardiac muscle are involuntary. But this is not always true.
Skeletal muscles can contract without conscious direction, and we can learn certain degree of
conscious control over some smooth & cardiac muscle.
Skeletal muscles are unique, bc they contract only in response to a signal from a somatic motor
neuron. They cannot initiate their own contraction, and their contraction is not influenced directly by
hormones. Cardiac & smooth muscles have multiple levels of control. Their primary extrinsic control
arises through autonomic innervation, but some types of smooth & cardiac muscle can contract
spontaneously, without signals form the CNS.
The activity of cardiac & some smooth muscle is subject to modulation by the endocrine system.
Despite these differences, smooth & cardiac muscle share many properties with skeletal muscle.
All 3 muscle type have certain properties in common. The signal to initiate muscle contraction is
intracellular Ca2+ signals and movement is created when a motor protein called myosin uses energy
from ATP to change its conformation. Details of these processes vary with the different muscle types.
12.1 skeletal muscle
Skeletal muscle make up the bulk of muscle in the body and constitute about 40% of human body
weight. They position and move the skeleton. They are usually attached at the bone through
,tendons. The origin of a muscle is the end of the muscle that is attached closest to the trunk or the
more stationary bone. The insertion of the muscle is the more distal or mobile attachment.
When 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.
Most joints in the body have both flexor and extensor muscles, because a contracting muscle can pull
a bone in a direction but cannot move it back. Flexor extensor pairs are called antagonistic muscle
groups because they exert opposite effects. Bv. Antagonistic muscles in the arm: biceps branchii,
which acts as flexor, triceps branchii, which acts as extensor. When 1 muscle contracts and shortens,
the antagonistic muscle must relax and lengthen.
Muscles function together as a unit. A skeletal muscle is a collection of muscle cells, or muscle fibers,
like the 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 muscles fibers are the largest cells in the body, created by the fusion of individual embryonic
muscle cells. Committed stem cells called satellite cells lie just outside the muscle fiber membrane.
Satellite cells become active 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 bunded 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 muscle sheath that is continuous with the connective
tissue around the muscle fibers and fascicles and with the tendons holding the muscle to underlying
bones.
The cell membrane of a muscle fiber is called a 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 muscle fibers also contain extensive sarcoplasmic
reticulum (SR), a form of modified ER that wraps around
each myofibril. The SR consists of longitudinal tubules with
enlarged end regions called terminal cisternae. The SR
concentrates/keeps Ca2+ with the help of Ca2+-ATPase in
the SR membrane. Calcium release from the SR creates
Calcium signals that play key role in contraction in all types
of muscles.
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 cisternae are called a triad. Membranes
of t-tubules are a concentration of the muscle fiber membrane, which makes the lumen of t-tubules
continuous with the ECF.
, T-tubules allow action potentials to move rapidly
from the cell surface into the interior of the fibers 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 AP through the cytosol, a
slower/less direct process that would delay the
response time of the muscle fiber.
The cytosol between the myofibrils contain many
glycogen granules and mitochondria. Glycogen, the
storage form of glucose found in animals, as a
reserve source of energy.
Mitochondria contain the enzymes for oxidative
phosphorylation of glucose and other biomolecules
so they produce much of the ATP for muscle
contraction.
1 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 organized into repeating contractile
structures called sarcomeres.
Myofibril proteins include: the motor protein myosin, which forms thick filaments & the
microfilament actin which creates thin filaments & the regulatory proteins tropomyosin and troponin
& two giant accessory proteins: titin and nebulin.
Myosin is a motor protein with the ability to create movement. Various isoforms of myosin occur in
different types of muscle and help determine the muscle’s speed of contraction. 1 myosin molecule is
composed of two identical protein chains, each with one large heavy chain + two smaller light chains
The heavy chains of the myosin molecule are organized into 3 regions: pair of tadpole-like heads, stiff
rodlike sections that intertwine to form a tail, elastic neck region that joins the head to the tail.
Heads form the motor domain that uses energy from the ATP to create movement.
Myosin acts as an enzyme to hydrolyze ATP, it is considered myosin ATPase. The heavy chains of
myosin heads also contain the binding sites for actin.
In skeletal muscle, about 250 myosin muscles join to create a thick filament. Each thick filament is
arranged so 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), which polymerize to form long chains to form F actin.
The parallel thick and thin filaments are connected by myosin cross bridges, that span the space
between the filaments. Each G-actin molecule has a single myosin-binding site, and each myosin
head has 1 actin-binding site.
Cross bridges form when the myosin heads of thick filaments bind to actin in the thin filaments.
Crossbridges can be in two stages: low force (relaxed muscles) and high force (contracting muscles).
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