Muscular System
by: Daniel Gable
An overview of the muscular
system
Muscles move skeletal parts by
contracting
• Muscles always contract
• Muscles must be attached in antagonistic pairs
so each muscle can work against each other
• Skeletal muscle is attached to the bones and is
responsible for their movement, it is
characterized by a hierarchy of smaller and
smaller parallel units
• A skeletal muscle consists of a bundle of long
fibers running the length of the muscle
Each fiber is a single cell with many nuclei, reflecting its formation by the fusion of many
embryonic cells. Each fiber itself a bundle of smaller myofibrils arranged longitudinally.
The myofibrils in turn are composed of two kinds of myofilaments. Thin filaments
consist of two strands of actin and one strand of regulatory protein coiled around one
another, while thick filaments are staggered arrays of myosin molecules. Each
repeating unit is a sarcomere, the basic contractile unit of the muscle.
Interactions between myosin and actin
generate force during muscle contractions
• When a muscle contracts the length of each
sacomere is reduced
• This is known as the sliding-filament model
where neither the thin or thick filaments change
length but rather slide past each other
longitudinally
• The myosin head is the center of bioenergetic
reactions that power muscle contractions
• Muscle cells typically only store enough ATP for
a few contractions
Calcium ions and regulatory
proteins control muscle contraction
• A skeletal muscle contracts only when stimulated by a
motor neuron
• When at rest the myosin binding sites on the actin
molecules are blocked by the regulatory protein
tropomyosin
• The troponin complex controls the position of
tropomyosin on the thin filament
• When calcium concentration in the cytosol falls, the
binding sites of actin are covered, and contraction stops
• Calcium concentration in the cytosol of the muscle cell is
regulated by the sarcoplasmic reticulum, a specialized
endoplasmic reticulum
• The action potential spreads deep into the interior of the
muscle cell along infoldings of the plasma membrane
called T (transverse) tubules
Diverse body movements require
variation in muscle activity
• Muscles act in twitches and if the rate of
stimulation is fast is enough, the twitches will
blur into one smooth and sustained contraction
called tetanus
• A motor unit consists of a single motor neuron
and all the muscle fibers it controls
• Tension in a muscle can be progressively
increased by activating more and more of the
motor neurons controlling the muscle, a process
called recruitment of motor neurons
Fast and Slow Muscle Fibers
• Fast muscle fibers are used for short,
rapid, powerful contractions
• Slow muscle fibers, often found in
muscles that maintain posture, can sustain
long contractions
• Mygoglobin binds oxygen more tightly
than hemoglobin so it can effectively
extract oxygen from the blood
Other Types of Muscle
• Vertebrate cardiac muscle is found in only one
place-the heart
• The junctions between cardiac muscles contain
specialized regions called intercalated discs,
where gap junctions provide direct electrical
coupling among cells
• Smooth muscle lacks the striations of skeletal
and cardiac muscle because the actin and
myosin filaments are not all regularly arrayed
along the length of the cell.