Download Musculoskeletal System 3 Functions: Support, Protection, and

Musculoskeletal System
3 Functions: Support, Protection, and Movement
3 Types of skeletons: Endo, Exo, and Hydrostatic
Hydro – consists of fluid held under pressure in a closed body compartment. – use muscles to change
the shape of fluid-filled compartments (cnidarians, flatworms, nematodes, and annelids.)
Exo – mollusks (calcareous shell) insects (chitin)
Endo –structures buried within soft tissues. Sponges are reinforced by hard spicules of inorganic
material or softer fibers made of protein. Echinoderms have hard plates called ossicles (magnesium
carbonate or calcium carbonate crystals bound by protein fibers.) Chordates have endo of cartilage,
bone, or combo of these.
Vertebrate Skeleton divided into 2 parts:
1.
Axial Skeleton – skull, vertebral column, and rib cage
2. Appendicular Skeleton – limb bones, pectoral and pelvic girdles.
Joints can also be classified as Immovable (sutures in skull); Slightly Movable (bones bridged by cartilage
– vertebrae); Freely Movable (aka synovial…synovial capsule with lubricant)
The Freely Movable Joints are:
1.
Ball and Socket Joints (hip and shoulder – rotation and mvmt)
2. Hinge Joints (humerus and head of ulna – mvmt in single plane)
3. Pivot Joints (rotate forearm at elbow; head)
Skeletal Muscle is attached to bones and responsible for their movement.
Skeletal Muscle has an origin and an insertion.
Origin – the attachment of a muscle that remains stationary during a contraction.
Insertion – the other end of the muscle; attached to bone and moves during contraction.
Muscle relationships: some muscle work together for a motion (synergists) and some oppose
(antagonist)
Structure of Muscle Fiber
1. Consists of a bundle of long fibers running parallel to the length of the muscle.
Each fiber is a bundle of smaller myofibrils arranged longitudinally.
Myofibrils are composed of 2 kinds of myofilaments: thin and thick
Thin – consist of 2 strands of actin and one strand of regulatory protein coiled around
one another.
Thick – are staggered arrays of myosin molecules
Skeletal Muscle is striated b/c the pattern of myofilaments occur as alternating light and dark bands.
Thick Myofilaments (Myosin) together produce dark bands “A-Bands”
Thin Myofilaments (Actin) together produce light bands “I-Bands”
Each “I-band” is divided by a protein disc forming a “Z-Line”.
Each repeating unit is called a sarcomere (the basic contractile unit of a muscle) from Z-line to Z-line.
Muscle contraction is determined by the sliding action of the thick and thin filaments alongside each
other. – SLIDING FILAMENT MECHANISM
Sliding occurs as a result of cross-bridging between myosin and actin. Bridges are formed by myosin
heads. Actin proteins twist into a double helix structure.
Before myosin heads bind to the actin, they act as ATPase enzymes, splitting ATP into ADP and Pi. This
cocks the heads so they can bind to for the cross bridges. Once binding occurs, the myosin head
undergoes a conformational change pulling the thin filament toward the center of the sarcomere.
POWER STROKE. At the end of the stroke, myosin head binds to a new molecule of ATP…this allows the
head to detach from the actin fibers.
Role of Calcium Ions – When myosin heads are cocked, they are unable to immediately bind to actin b/c
they are blocked by protein called tropomyosin. Tropomyosin is removed when bound by regulatory
protein Troponin….which is regulated by Ca conc. In muscle cytoplasm.
Ca is obtained through diet, but stored in sarcoplasmic reticulum. Electrical impulses from nerves travel
through transverse tubules (T tubules) which triggers release of Ca. Excitation-Contraction Coupling.
1. Acetylcholine (neurotransmitter) is released by motor neuron.
2. ACh stimulates muscle fiber to produce own electrochemical impulses.
3. These spread through T-tubules which channel impulses to SR.
4. Ca is released from SR.
5. Ca binds to troponin, which then binds to tropomyosin which removes tropomyosin from actin.
The borders of the sarcomere (Z-Lines) are lined up in adjacent myofibrils. Thin filaments are attached
to the Z-lines and project toward the center of the sarcomere, thick filaments are centered in the
sarcomere.
At rest, thick and thin do not overlap completely, and the area near the edge of the sarcomere where
there are only thin filaments is called I band. The A band is the broad region that corresponds to the
length of the thick filaments. The thin filaments do not extend completely across the sarcomere, so the
H Zone in the center of the A band contains only thick filaments.
Sliding Filament Model – neither the thin filaments nor the thick filaments change in length when the
sarcomere shortens – instead the filaments slide past each other longitudinally, producing more overlap
between the thin and thick filaments.