Skeletal and Smooth Muscles Flashcards
Skeletal Muscle
-A little less than 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.
- Under voluntary control.
- Biological transducers.(ATP movement)
Organization of skeletal muscle
A muscle is covered by Epimycium, then muscles fascicles (Bundle of fibers) are covered by Perimycium, Each muscle fiber is covered by Endomycium.
Ultrastructure of muscle fiber.
-A muscle fiber Is a bundle of other fibers called Myofibrils, covering is by sarcolemma. Made up of thick and thin filaments, Surrounding the myofibril is the Sarcoplasmic reticulum.
Define and describe Sarcomere
The sarcomere is the fundamental unit of contraction and is defined as the region between two Z-lines. Each sarcomere consists of a central A-band (thick filaments) and two halves of the I-band (thin filaments).
The thick filament is Myosin
The thin filament is Actin
Myosin
-Each molecule has two heads and filamentous tails- chain 480kDa
- Hydrolytic site located below the head - a molecular motor.
- There are many forms of myosin
- Approx. 200 group together to form that thick filament.
Actin
- Note that actin is a twin sranded filament. 46 kDa
- Has active sites
- 6 actin filaments/ z disc
- Association with T-T complex [Troponin-tropomyosin]
- Troponin(globular) comprises 3 subunits = TnC, TnT, TnI
- Steric effect of calcium.
Filaments interdigitate and active sites are shielded.
-NB the myofilaments interdigitate
-Active sites are shielded by troponin-
tropomyosin complex @ rest
-TnT point of attachment of troponin to
tropomyosin
-TnC binds Ca2+ → active sites there after
exposed.
-Crossbridge formation → hydrolysis of
ATP @ myosin → changes head angle
The myofilaments are associated with other proteins.
- Nebulin helps align actin
- Titin provides elasticity and stabilizes myosin
Motor units
Motor neuron and the muscle fibers it innervates.
The smallest (minimum) 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 contract simultaneously, pressures for gene expression (e.g frequency of stimulation load) are identical in all fibers of a motor unit.
TO INCREASE FORCE:
- Recruit more motor units
- Increase frequency (force-frequency)
Motor unit remodeling with aging
- Fewer motor units
- More fibers/motor unit???
Events that cause muscle contraction
- Axon terminal of somatic motor neuron = Acetylcholine is released at the neuromuscular junction
- Net entry of Na through Acetylcholine receptor-channel initiates a muscle action potential.
- The action potential moves down the T tubules, activates the DHP receptor, which leads to stimulation of Ryanodine receptor (because the DHP receptor is linked to Ryanodine receptor) on the sarcoplasmic reticulum, leading to calcium being released into sarcoplasm.
A single Powerstroke = one of many!
Resting muscle
1. Energized cross-bridge (ADP+Pi)
=The calcium concentration rises, this causes the exposure of active site on actin, Cause the cross-bridge to bind to actin. [calcium conc. has to be greater than 0.1 microMoles]
- The cross-bridge moves [ loses ADP+Pi]
- ATP binds to myosin, causing cross-bridge to detach. [at this point the calcium concentration is has decreased to less than 0.1 microMoles]
- Hydrolysis of ATP energizes cross-bridge. [Mg 2+? = cofactor??]
When the person is dead, no ATP so the muscle goes into rigor mortis [cross-bridge never detaches, remains in a contracted manner]
When muscle contracts it shortens the length between that z discs but the actin and myosins remain the same lengths.
Difference between action potential and muscle twitch
When an action potential travels down the motor neuron, it will result in a contraction of all of the muscle fibers associated with that motor neuron. The contraction generated by a single action potential is called a muscle twitch.
Muscle twitch takes longer to diminish than the action potential.
Three potential actions during muscle response:
- Shortening (concentric) - Shortening against fixed load, speed-dependent on M·ATPase activity &load
- Eccentric - Most likely to cause muscle injury.
- External force»_space; tension developed by muscle. - Isometric - Length remains constant
Length - tension relationship
- Active tension + resistance to stretch depend on cross bridges. Tone?
- Length of overlap determines the number of cross-bridges that can form.
- Sarcomeres stretch between 1–4 μm, thus varying the number of cross-bridges that can be formed.
- The number of cross-bridges affects the force of contraction.
- Optimum force generated @ at resting length of 2.2μm
- Refer to graph (slide 17) showing sarcomeres of 1.2μm & 3.8μm in length and respective tension that is developed!
- Explains in part the variable force that muscles can produce.
The neuromuscular junction
- Motor neuron action potential
- Ca2+ enters voltage-gated channels
- Acetylcholine release
- Na entry
- Local current between the depolarized endplate and adjacent muscle plasma membrane
- Muscle fiber action potential initiation
- Propagated action potential in the muscle plasma membrane
- Acetylcholine degradation.
