Chapter 10: Muscle Tissue Flashcards
Properties of muscle tissue include:
- Electrical excitability – ability to respond to a stimulis by producing an action potential that travels along the muscle sarcolemia
- Contracility – ability to contract forcefully when stimulated by an action potential
- Extensibility – can stretch without damage (to a certain extent)
- Elasticity – ability to return to the original length/ shape
CT coverings extend from fascia & include:
Endomysium → surrounds individual muscle cells
Perimysium → surrounds a bundle (fascicle) of muscle cells
Epimysium → encircles entire muscle (many fascicles)
Muscle Organization, made up on ___ which are made up of 100s of ____ which contain even smaller structures
- Muscle cell (fiber) is elongated & multinucleated
- Cell cytoplasm (sarcoplasm) contains 100s – 1000s of threadlike myofibrils extending the length of the cell
- Each myofibril consists of even smaller structures – myofilaments, which are the contractile proteins
2 types of myofiliments
Thick myofilaments
– composed of the protein myosin
– has a tail & 2 globular heads
– during contraction, myosin head projects towards thin filaments & forms cross bridges
Thin myofilaments
– composed of the globular protein actin & regulatory proteins (troponin & tropomyosin)
– tropomyosin blocks myosin-binding sites on actin
– troponin binds to actin, tropomyosin, & Ca2+
Myofilaments parts
- Are organized into sarcomeres
- Arrangement of myofilaments results in striations
- Z discs (boundaries of sarcomere).
- A band (region in the centre and has thick and thin myofilaments, areas with both thick/thin and then an area with only thick. The A band is the length of the thick myofiliment)
- H zone (In the center of the myofilament where theres only the thick filament. Region that overlaps the A filament.)
- I band (in either side of A band and only has thin filament. )
- M line (the area where the 2 thick filiments are stiched together to form one long thick filament)
sarcoplasmic reticulum (SR)
- specialized smooth ER
- Functions to store & release Ca2+
- Has a series of channels that run longitudinally along the myofibril
- Has dilated end sacs called terminal cisterns at the A band/I band junctions
- A T-tubule & the 2 cisterns on either side of it forms a triad
Complete muscle contraction
- A nerve action potential in a somatic motor neuron triggers the release of Ach
- Ach binds to receptors in the motor end plate, triggering muscle action potential
- Acetylcholinesterase destroys Ach so another muscle action potential doesn’t start unless more Ach is relased from somatic motor neuron
- Muscle action potential traveling alonf a transverse tubule triggers a channel in the voltage gated Ca2+ channgels that causes the Ca2+ release channels to open, allowing the relase of calcium ions into the sarsoplasm
- Ca2+ binds to troponin on thin filament, exposing the myosin binding site on actin
- Contraction. Myosin heads bind to actin, undergo power stroked and release think filament are pulled toward center of sarcomere
- Ca2+ relase channels close and CA2+-ATPase pumps use ATP to restore low level of Ca2+ in the sarcoplasm
- Tropomyosin slide back into position where it blocks the muosin-binding sites on actin
- Muscles releax
What uses more motor units, fine or overall motor skills?
- Fine motor skills (move of lips for talking) = less motor units
- overall motor skill = more motor units
Muscle Stimulation
- Neurotransmitter acetylcholine (ACh) is used to cross the synaptic cleft
- ACh diffuses across cleft & binds to ACh receptor sites on the muscle sarcolemma
- Opens ligand-gated Na+ channels & Na+ diffuses into muscle cell
- Triggers an action potential over entire surface of sarcolemma
Excitation-Contraction Coupling
- action potential on the sarcolemma (excitation) to the muscle contraction
- Action potential travels down T-tubules & causes SR to release Ca2+ into sarcoplasm
- Ca2+ binds to troponin, which changes shape & in turn moves the tropomyosin away from the myosin-binding sites on actin
- Binding sites are “freed”
Sliding Filament Mechanism
Once binding sites on actin are exposed, the following 4 steps occur:
- ATP hydrolysis. Myosin head has an ATP binding site & ATPase enzymes that split ATP → ADP + P
Myosin head is reoriented & energized - Cross bridge attachment. Energized myosin head attaches to the myosin binding site on actin. P is released
- Power stroke. Myosin cross bridge rotates toward the centre of the sarcomere & generates force. Releases ADP. Thin filament slides toward M line
- Cross bridge detachment. A new ATP molecule binds to the myosin head & the cross bridge detaches from actin
- Thin myofilaments progressively slide past the thick myofilaments & the sarcomere shortens
- Z discs come closer together
- The amount of overlap b/t thick & thin filaments increases
- Once nerve action potentials cease, Ca2+ channels in muscle are closed & myosin-binding sites are blocked once again by tropomyosin
Force of Muscle Contraction
A motor unit is one motor neuron & all the muscle cells it innervates
An ↑ in stimulus strength will ↑ the number of motor units responding to ↑ the force of contraction
An ↑ in stimulus frequency will ↑ the force of contraction
The muscle is partially contracted & 2nd stimulus releases more Ca2+ so contractions are summed
