Download ANPS 019 Beneyto-Santonja 10-10

ANPS 019 Beneyto-Santonja 10/10/12
Sliding Filament Theory
 Explains the relationship between thick and thin filaments as contraction proceeds
 Cyclic process beginning with calcium release from SR
o Calcium binds to troponin
o Troponin moves, moving tropomyosin and exposing actin-active site
o Myosin head forms cross bridge and bends toward H zone
o ATP allows release of cross bridge
Tension
 Created when muscles contract
 Series of steps that begin with excitation at the neuromuscular junction
o Calcium release
o Thick/thin filament interaction
o Muscle fiber contraction
o Tension
Control of skeletal muscle activity occurs at the neuromuscular junction
 Action potential arrives at synaptic terminal
 ACh is released into the synaptic cleft
 ACh binds to receptors on the post-synaptic membrane of the muscle cell
o Action potential in sarcolemma
Specific steps involved in the process of cholinergic transmission
1. Arrival of an action potential at the synaptic terminal
2. Release of acetylcholine – vesicles in the synaptic terminal fuse with the neuronal
membrane and dump their contents into the synaptic cleft
3. ACh binding at the motor end plate – the binding of ACh to the receptors increases the
membrane permeability to sodium ions. Sodium ions then rush into the cell
4. Appearance of an action potential in the sarcolemma – An action potential spreads across
the surface of the sarcolemma. While this occurs, AChE removes the ACh.
5. Return to initial state
Vescicle snares (v-snare) and target snare (t-snare) bring vesicles to the presynaptic membrane
Excitation/contraction coupling
 This process describes the link between arrival of the action potential and the obligatory
release of calcium in the sarcoplasm
 Action potential along T-tubule causes release of calcium from cisternae of SR
 Initiates contraction cycle
o Attachment
o Pivot
o Detachment
o Return
Contraction Cycle
1. Active-site exposure
2. Cross-bridge attachment
3. Pivoting of myosin head
4. Cross-bridge detachment
5. Myosin reactivation
Relaxation
 Acetylcholinesterase breaks down ACh
 Limits the duration of contraction

Why is it that muscles become “stiff as a board” after death? Rigidity of muscles increase
at certain temperature; no influx of oxygen; No ATP  cross-bridges form and myosin
cannot detach; called ‘rigor mortis’
Skeletal Muscle Mechanics
Structural Hierarchy of Skeletal Muscle
 Muscles  Muscle fibers  muscle fiber  myofibril  sarcomere
 A little less that half of the body’s mass is composed of skeletal muscle, with most
muscles linked to bones by tendons through which the forces and movements developed
during contractions are transmitted to the skeleton.
Tension Production by Muscle Fibers
 All or none principle
 Amount of tension depends on number of cross bridges formed
 Skeletal muscle contracts most forcefully over a narrow ranges of resting lengths
 Twitch
o Cycle of contraction, relaxation produced by a single stimulus
 Treppe  Repeated stimulation after relaxation phase has been completed
 Summation  Repeated stimulation before relaxation phase has been completed
o Wave summation = one twitch is added to another
o Incomplete tetanus = muscle never relaxes completely
o Complete tetanus = relaxation phase is eliminated
Tension production by skeletal muscles
 Internal tension generated inside muscle fibers
 External tension generated in extracellular fibers
 Motor units
o All the muscles innervated by one neuron
o Precise control of movement determined by number and size of motor unit
 Muscle tone
o Stabliizes bones and joint
Motor Units: motor neuron and the muscle fibers it innervates
 The smallest amount of muscle that can be activated voluntarily
 Gradation of force in skeletal muscle is coordinated largely by the nervous system
 Recruitment of motor units is the most important means of controlling muscle tension
 Since all fibers in the motor unit contact simultaneously, pressures for gene expression
(e.g. frequency of stimulation, load) are identical in all fibers of a motor unit
 To increase force:
o Recruit more M.U.s
o Increase freq. (force-frequency)
Contractions
 Isometric
o Tension rises, length of muscle remains contstant
 Isotonic
o Tension rises, length of muscle changes
 Resistance and speed of contraction inversely related
 Return to resting lengths due to elastic components, contraction of opposing muscle
groups, gravity.
Three potential actions during muscle contraction:



Shortening (Isotonic: shortening against fixed load, speed dependent on M: ATPase
activity and load)
Isometric
Lengthening (Eccentric)  Most likely to cause muscle injury