PHGY 209 Muscle physiology.doc

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17 Mar 2012
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Muscle
3 types of muscle:
-Skeletal muscle is used for posture and locomotion. This is the muscle that enables our arms and legs to contract,
under our conscious control.
-Cardiac muscle is responsible for the rhythmic contractions of the heart.
-Smooth muscle causes involuntary contraction in blood vessels, gut, bronchi and the uterus.
-Thin cells constituting muscles are called muscle fibers (can be 1 ft long)
-Muscle is attached at each end to tendons, which in turn attach to bone on both sides of a joint.
Contraction of skeletal muscle pulls on the tendons resulting in flexion of the joints.
-Muscle fibers are generated during development by the fusion
of a large number of small precursor cells called myoblasts.
Each myobast has a single nucleus, whereas the fiber is a
multinucleated cell.
-The striations within each myofibril are caused by alternating
light I-bands and dark A-bands. In the center of each light
band is a dark line called the Z-line. These structures
delineate the sarcomere, the contractile unit of skeletal
muscle.
-Each sarcomere consists of two sets of parallel and partially
overlapping protein filaments: thick filaments extending
from one end of the A band to the other, and thin filaments,
attached to the Z lines and extending across the I band and
part way into the A band.
-Thin filaments are physically attached to Z lines.
-Thin filaments consist of actin. Each actin filament is formed
from two chains of globular actin subunits, twisted into a helix.
The myofibril is a lattice of thick and thin filaments.
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-Muscle contraction occurs when the thin filaments slide over the
thick filaments. Neither the thick or thin filaments change in length.
-The thin filaments are pulled over the thick filaments by
the myosin headgroups, which repeatedly grab, pull and
release the thin filaments. The reaction is driven by ATP
hydrolysis.
-Headgroups: Reach out – Grab - Pull – Release
-The sarcomere contracts when thin filaments slide over
thick filaments.
-Contraction of the sarcomeres shortens the entire
myofibril.
-Actin molecules have binding sites for
myosin headgroup
-When binded, change of conformation
occurs and leads to release of ADP+Pi.
-Power stroke is caused by the release
of ADP+Pi
-When headgroup binds ATP, it is able to
let go.
-Motor unit=motor neuron+all the
muscle fibers it innervates.
-Size varies: number of fibers innervated
by a single motor neuron may range from
10 (ex. extraocular muscles) to 100
(muscles of the hand) to several
thousand (large flexor&extensor muscles
of leg).
-Motor neurons are found in spinal cord
(arms, legs…) and the brain stem
(face…).
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