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MUSCLE PHYSIOLOGYAss. Prof. Dr. Emre Hamurtekin
EMU Faculty of Pharmacy
SKELETAL MUSCLE
STRUCTURAL PROPERTIES
STRUCTURAL PROPERTIES
STRUCTURAL PROPERTIES
STRUCTURAL PROPERTIES
SARCOTUBULAR SYSTEM• T system and sarcoplasmic
reticulum• T system of transverse tubules
is continous with the sarcolemma
• The SR has enlarged terminal cisterns at the junctions between the A and I bands.
• Triad• T system provides a path for
the rapid transmission of action potential
• SR is an important store of Ca
SARCOTUBULAR SYSTEM
ELECTRICAL and ION CHARACTERISTICS of SKELETAL MUSCLE
• Resting membrane potential: -90 mV• Action potential lasts: 2-4 ms• Conducted along the muscle fiber : 5 m/s• Ion distribution is similar to that across the
nerve cell membrane.
CONTRACTILE RESPONSES
• Muscle twitch: A brief contraction folllowed by relaxation which is caused by a single action potential.
MOLECULAR BASIS OF CONTRACTION• In resting muscle:
Troponin I covers the sites where myosin heads interact with actin Myosin head contains tightly bound ADP
• Step 1: Following an action potential, cytosolic Ca increases and binds to Troponin C
• Step 2: Weakening of Troponin I interaction with actin and myosin-actin cross bridges occur.
• Step 3: Upon formation of the cross-bridge, ADP is released.• Step 4: ADP release causes a conformational change and myosin head
moves the thin filament (power stroke).• Step 5: ATP binds to the free site on the myosin. This leads to a
detachment of the myosin head from the thin filament.• Step 6: ATP is hydrolyzed and cycle is completed.
MOLECULAR BASIS OF CONTRACTION
• Excitation-contraction coupling: The process that a depolarization of a muscle fiber initiates a contraction
• Ca is reduced in the muscle cell by SERCA and this causes a cessation of interaction between actin and myosin
• Pumping of the Ca back into the sarcoplasmic reticulum causes relaxation
TYPES OF CONTRACTION
ISOMETRIC ISOTONIC(same length) (same tension)
SUMMATION OF CONTRACTIONS
• Contractile mechanism does not have a refractory period.• Summation of contraction: Additional contractile
response that is added to the already present contraction.
• Tetanic contraction (tetanus): Because of the activation of contractile mechanism occurs repeatedly before any relaxation occurs, individual responses fuse into one continuous contraction.
• Tetanic contraction:– Complete tetanus– Incomplete tetanus
SUMMATION OF CONTRACTIONS
MOTOR UNIT
• Each single motor neuron and the muscle fibers it innervates constitute a motor unit.
• The number of muscle fibers in a motor unit varies.
CARDIAC MUSCLE
MORPHOLOGY• The striations are
similar to those in skeletal muscle.
• Z-lines are present.
• There are large numbers of mitochondria.
• The muscle fibers branch and interdigitate.
• Intercalated disks
Muscle fiber resmi
MORPHOLOGY
• Along the sides of the muscle fibers next to the disks, the cell membranes form gap junctions.
• Cardiac muscle function as if it was a syncytium: Intercalated disk Gap junctions
• T-system in cardiac muscle is located at Z-lines (not at the A - I junction).
ELECTRICAL PROPERTIES
• The resting membrane potential: -80 mV.• A plateau is present before the membrane
potential returns to the baseline.• Cardiac myocytes contain at least 2 types of Ca
channels (T- and L-types), but the Ca current is mostly due to opening of slower L-type Ca channels.
ELECTRICAL PROPERTIESPhase 0
initial rapid depolarization and the overshoot
Phase 1
the initial rapid repolarization
Phase 2
prolonged plateau
Phase 3 to Phase 4final repolarization to resting membrane potential
opening of voltage-gated Na channels.
closure of Na channels and opening of one type of K channel.
slower but prolonged opening of voltage-gated Ca channels.
closure of Ca channels and a slow delayed increase of K efflux through various types of K channels.
MECHANICAL PROPERTIES & METABOLISM
• The contractile response of cardiac muscle lasts about 1.5 times as long as the action potential.
• The cardiac muscle can’t be tetanized!!!• Abundant blood supply, numerous mitochondria, high
content of myoglobin.• Normally less than 1% of the total energy is provided by
anaerobic metabolism.• Under basal conditions caloric needs of the heart are
provided by,– 35% by carbohydrate– 5% by ketones and amino acids– 55% by fat
SMOOTH MUSCLE
MORPHOLOGY
• No visible cross-striations.• Actin & myosin II are present and slide on
each other to produce contraction .• Instead of Z-lines, there are dense bodies.• Contains tropomyosin but troponin is absent.• Sarcoplasmic reticulum is less extensive.• Contain few mitochondria.
MORPHOLOGY
TYPES
• A) Unitary (visceral) smooth muscleB) Multiunit smooth muscle
• Unitary (visceral) smooth muscle– i.e. intestine, uterus, ureter– occurs in large sheets– has many low-resistance gap-junctional connections (syncytial
function)• Multiunit smooth muscle
– i.e. iris of the eye– individual units (few or no gap junctional bridges)– Each multiunit smooth muscle cell has endings of nerve fibres.
TYPES
ELECTRICAL & MECHANICAL ACTIVITY
• Unitary smooth muscle is characterized by the instability of its membrane potential.
• Continuous, irregular contractions (tonus)• Resting potential: from -20 mV to -65 mV.• Excitation-contraction coupling in unitary
smooth muscle is a very slow process.• Contractions of multiunit smooth muscle are
more discrete, fine and localized than unitary smooth muscle contractions.
CONTRACTION & RELAXATION
• Source of Ca increase in unitary smooth muscle:– influx through voltage or ligand-gated plasma
membrane channels.– efflux from intracellular stores through the RyR– efflux from intracellular stores through the IP3
receptor Ca channel. • The lack of troponin: myosin must be
phosphorylated for activation of myosin ATPase.– Calmodulin-dependent myosin light chain kinase
CONTRACTION & RELAXATION
CONTRACTION & RELAXATION
CONTRACTION & RELAXATION
Contraction
Dephosphorylation of myosin by myosin light chain
phosphatase
Relaxation, or sustained contraction due to the latch
bridge and other mechanisms
CONTRACTION & RELAXATION
• Unitary smooth muscle contracts when stretched in the absence of any extrinsic innervations unlike other types of muscle.
• Intestinal smooth muscle preparation:Norepinephrine (relaxation) X acetylcholine (contraction)
• NO-nitric oxide- (released from endothelial cells) leads to relaxation of blood vessel smooth cell.
THE END