Skeletal muscle Flashcards
1
Q
Where are Skeletal muscle cells orginate?
A
- Mesodermal origin
2
Q
What are mature cells in adult skeletal muscle called?
A
- Myotubes
- these are differentiated multinucleated cells formed by cytoplasmic fusion of immature mononuclteated myoblasts
3
Q
What are the muscle fibres bounded by ?
A
- Plasma membrane - sarcolemma
- they have cytoplasm- sarcoplasma
4
Q
What is the name for many muscle fibres grouped together?
A
- Fascicles
- the fascicles are the smallest unit of structure visible to the naked eye
- It is the ability if the fascicle to contract that determines the character of the muscle
5
Q
What is the endomysium?
A
- Connective tissue that surrounds individual fibres
6
Q
What is the perimysium?
A
- connective tissue that encloses the functional fascicular unit
7
Q
What is the epimysium?
A
- The connective tissue that surrounds the muscle in its entirety
8
Q
A
9
Q
What is surrounding each myofibril ?
A
- A membranous sac = sarcoplasmic reticulum
- it serves as a repository for calcium which is released to stimulate contraction
10
Q
What do T tubules do?
A
- They connect at a membranous junction with the sarcoplamic reticulum of each myofibril
- the function is to carry depolarization of the surface membrane deep inside the muscle fibre
11
Q
How does the arteriole entry the muscle fibre?
A
- Perimysium - where the arterioles penetrate the sheath surrounding the fascicle
- the arterioles enter obliquely or at 90 degrees to the muscle fibres then run parallel
- terminal capillaries are associated with muscle fibre nuclei
12
Q
what are the major contractile proteins in skeletal muscle?
A
- Actin
- Myosin
13
Q
What is the grouped functional unit of actin/myosin filaments?
A
- Myofibril
14
Q
Myofibrils are segmented into functional contractile units called what?
A
- Sarcomeres
- visible under electron microscope
- 2-2.5 microns in length
- 1 micron in diameter
- length varies with muscle activity but shows a variance along the length of the myofibril
- sarcomeres in the myotendinous junction tend to be shorter
15
Q
What is the A band?
A
- Represents the Myosin filaments
- Anisotrophic on light microscope
- ***“AM”
16
Q
What does the I band represent?
A
- Actin filaments in adjacent sarcomeres where there is no overlap with myosin filaments
- Isotrophic on light microscope
- **IA**
17
Q
What does the H band represent?
A
- Myosin filament segment where there is no interdigitiating actin filaments
18
Q
What do the M lines represent?
A
- Connections between adjacent mysoin filaments in their central region
- these are termed M band proteins
19
Q
What do Z discs represent?
A
- Attachment of adjacent sarcomeres
- the disc lies between sarcomere
20
Q
What is the arrangement of actin and myosin filaments?
A
- Hexagonal lattice in the centre of the sarcomere
- ie each myosin filament is bounded by 6 actin filaments
- this becomes more square towards the end of each sarcomere
21
Q
What other important proteins have a role in maintaining the structure of the sarcolemma?
A
- Dystrophin ( absent in duchenne muscular dystrophy)
- as a group called structural proteins as they maintain the overall architecture of the sarcomere during contraction
22
Q
What is actin?
A
- A globular protein (molecular weight 42 000)
- chief constituent of the thin filament of the sacromere
23
Q
What other proteins constitute the thin filament of the sarcomere?
A
- Tropomyosin
-
Troponin subunits
- troponin C
- troponin T
- troponin I
24
Q
What does tropmyosin do?
A
- It exends across seven actin subunits
- Blocks the bindings sites of the myosin head unit until unblocked by calcium binding to the troponin C subunit ( tropomysoin moves & mysoin- binding site exposed)
- the activated troponin C subunit counteracts the inhibitory effect of the troponin I subunit
- Troponin T assists toponin C binding to tropmyosin
25
What is the myosin structure?
* Made up of **6** distinct subunits
* **2 heavy chains**
* **S1** segment/**cross bridge**= articulate with actin
* **S2** segment= forms **flexible neck** which moves to allow articulation of the S1 head segment
* **4 light chains** - uncertain function
26
Describe the sliding filament contraction/ cross bridge theory by Huxley?
1. in rush of Ca 2+
2. Ca 2+ binds to Troponin C
3. Activated troponin C displaces inhibitory Troponin I from site on tropomyosin/actin complex
4. Troponin T assists
5. Tropomyosin undergoes conformational change to allow myosin head enagagement
6. Myosin head engages
7. Forced generation occurs due to S1 head segement rotation
27
What is the force of a muscle is porportional to?
* **Cross sectional area**
* the more sarcomeres that are acting parallel to each other the higher the force generated
28
What is speed of muscle related to ?
* **Length of the muscle**
* Upon stimulating a muscle all sarcomeres contract at the same time.
* for **long muscle** there will be a greater change in length per unit time ie the **greater the muscle velocity**
* the sarcomeres act in series
29
What is the power of the muscle a product of ?
* **Force x velocity**
* short fat muscles produce high force but low max velocity
* long thin muscles produce low force but high max velocity
30
What is the name given to all muscles fibres innervated by the same motor neuron?
* **Motor unit**
31
Can you draw a neuromuscular junction?
32
Describe what happens at the Neuromuscular junction?
* Motor neurone, action potential propagates an influx of Ca2+ thru voltage- sensitive channels
* **Increased concentrations of intracellular Ca2+** cause preformed **vesicles of acetylcholine** to **fuse** with presynaptic nerve membrane
* **Acetylcholine** in the synaptic cleft **binds** to the **postsynaptic receptors** on the **sarcolemma**
* Binding of acetylcholine **depolarises the muscle fibre membrane**
* Depolarisation is dependent on the amount of acetylcholine releases into the synaptic cleft
* Also dependent on the rate of release of acetylcholine into the cleft as it is broken down rapidly by cholinerases from the post synaptic membrane
33
What is the myotendinous junction?
* **An area where insertion of every skeletal muscle fibre into its tendon occurs**
* specific morophology
* shorter sarcomere lengths
* greater no of organelles per cell
* greater synthetic ability
* interdigitation of the cell membrane
* extracellular connective tissue
* high degree of membrane folding- increases resistance to stress by increasing the surface area and reducing the angle of the force vector applied
* net result **junction very strong**
34
Describe the different types of muscle?
* **Type 1 "Slow Red Ox"**
* **Slow** fibres
* Large conc of **myoglobin** ( **red** in colour)
* **Oxidative**
* Very fatigue Reisistance
* **Type 2a**
* **Fast** fibres
* **Oxidative**
* **Glycolytic** ( realtively **white** in colour)
* _Fatigue resistant_
* **Type 2b**
* **Fast** fibres
* **Glycolytic** ( white in colour)
* **Fatiguable**
35
What is the single most important factor in fibre type expression?
* **Pattern of activity** imposed on the muscle
* ie atheletes who train for endurance can reach proportions of up to 80% of type 1 fibres in their muscles
36
What causes damage to muscle cells in truama?
How does this continue?
* **Raised calcium concentrations**
* Once cell damaged the process continues via a calcium-activated enzymes - **Proteases** and **phospholipids**
* also by **free-radicals and oxidation** of liberated **free fatty acids**
37
What are the modes of muscle injury?
* Muscle belly tear
* Muscle laceration
* Musculotendinous junction injury
* Ischaemic damage and compartment syndrome
* Denervation
* Crush injury and rhabdomyolysis
* **Malignant hyperpyrexia**
* skeletal muscle reaction to halothane with prolonged muscle contraction -\> metabolic distintegration of muscle
* K is released first so risk of MI, followed by myoglobin release and renal failure
* Delayed muscle soreness
* muscle soreness that develops 24-72hrs following intense exercise
38
What is the prognosis of a proximal belly tear?
* **Worse the prognosis**
* As more bullk is denervated
39
What do muscle lacerations result in ?
* **Dense fibrous scars**
* Myotubes regenerate across scar tissue in small no
* Partial lacerations have better functional outcomes cf complete belly lacerations
* complete lac- distal portion wastes rapidly.
* muscle regeneration can occur with a nerve supply but permanent muscle atrophy will develop if regeneration fails
* A complete lac in mid-substance can recover only 50% of previous force that was generated by the muscle
40
Where do complete tears normally occur?
* **Myotendinous junction**, with a segement of adjacent muscle avulsed when stretched to failure
41
Where do incomplete tears occur?
* **Near** the **myotendinous junction** in an area of relatively stiffer sarcomeres
42
Describe the EMG of skeletal muscle?
* is a recording of electrical activity in skeletal muscle
* Needle electrodes are inserted into muscle to be studied
* The electrode records any spontaneous activity within the muscle are rest and the signal given by firing of motor units when the muscle is activated
* **normally no spontaneous activity at rest** in a **healthy muscle** after the needle is inserted
* the patterns of motor activity ( motor unit action potentials) are related to the nerve supply to the muscle in question
43
What is the EMG findings of acute degenerative injury?
* **Spontaneous** activity of **Sharp waves**
44
What is the EMG findings of chronic degenerative injury?
* Fasiculations
45
What is seen on EMG with early re-innervation?
* **Reduced amplitude** motor action potentials
* **Longer duration**
* **Poor recruitement**
* the new sprouts conduct slowly , producing temporary dispersion ( prolonged MUAP) adn MUAP polyphasicity
46
What is seen on EMG with late re-innervation?
* **Large ampitude, stable consistent firing** Motor action potentials
* Good recruitement of units gives **polyphasic signal**
