Lesson 3: Muscle Structure and Function Flashcards
Muscles
Smooth muscle is the muscle tissue found in the walls of
the digestive system, arteries and other internal organs. Responsible for involuntary body activities.
Cardiac muscle forms the contractile tissue of the heart.
Under involuntary control.
Skeletal or striated muscle is the muscle that causes
movement of the joints.
Skeletal Muscle
Skeletal muscle 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 ( formed through the fusion of many embryonic cells) surrounded by a plasma membrane called the sarcolemma.
Each fiber is a bundle of myofibrils arranged longitudinally surrounded by a system of membranes called the sarcoplasmic reticulum.
Myofibrils
Myofibrils are made up of repeating units called sarcomeres.
Sarcomeres produce the stripped appearance of the muscle fiber and are responsible for contraction.
Myofibrils are composed of two kinds of myofilaments.
Thick Filaments – consist of myosin (contractile protein) molecules arranged in a
staggered formation.
Thin Filaments – consist of two strands of actin (contractile protein) and the regulatory proteins, troponin and tropomyosin. Coiled around one another.
Between the myofibrils are many mitochondria in order to meet the energy needs of these cells.
Sarcomeres
The sarcomere is the functional unit of the muscle.
The borders of the sarcomeres (Z lines) are lined up in adjacent myofibrils.
Thin filaments are attached to
the Z lines and project towards the center of the sarcomere, while thick filaments are centered in the sarcomere.Where myosin is present the myofibril has a dark appearance and a light band is seen where there is only actin present.
Muscle Contraction
Muscle contraction is explained using the sliding filament theory.
This theory describes how actin and myosin filaments slide over one another to shorten the muscle.
At rest the thick and thin filaments do not completely overlap. The area Where there are only thin filaments is called the I band.
The A band is the region that corresponds to the length of the thick filament.
The thin filament does not extend all the way across the sarcomere, so the H zone in the center contains only thick filaments.
Mechanism of Muscle Contraction
Nerve impulses travel along the sarcolemma and are carried into the fiber through T-tubules. This changes the permeability of the sarcoplasmic reticulum which causes the release of Ca2+ ions.
Before contraction, the binding sites on actin for the myosin heads are blocked by troponin and tropomyosin. The arrival of calcium ions cause the troponin and
tropomyosin to change shape and expose the myosin binding sites. Forming cross-bridges.
The myosin heads are ATPase
molecules which hydrolyse ATP to ADP + Pi. During rest, ADP + Pi is bound to the myosin head.As the cross-bridges form ADP + Pi are
released. The myosin heads bend towards the center of the sarcomere, pulling the actin filaments inwards by about 10nm. This produces a “power stroke”.
New ATP molecules bind to the myosin heads , breaking the cross-bridges and detaching them from the actin filaments. ATP is used and the myosin heads return to the start position.
If the muscle receives further stimulation the process repeats.
The Sarcomere During Contraction
The actin and myosin
filaments do not change in
length when a muscle
contracts.
The appearance of the
banding pattern of the
sarcomere does change. The
light band (I band) is reduced
and the Z lines are closer
together.
