muscles mw %% (+

Question 1

Muscle

Answer 1

• Generate force & movement
• Allow us to express & regulate ourselves
• 3 types – skeletal, smooth & cardiac
• Another type of excitable tissue

Question 2

Types of Muscle

Answer 2

1. Striated: Skeletal (voluntary muscles, diaphragm) and Cardiac (heart).
2. Smooth: blood vessels, vas deferens (the duct which conveys sperm from the testicle to the urethra), airways, uterus, GI tract, bladder etc.

Question 3

Skeletal Muscle

Answer 3

• Skeletal muscle cell = muscle fibre
• Multinucleate
• Form in utero from mononucleate myoblasts
• Increase fibre size during growth
• Myoblasts do not replace cells if damaged

Question 4

Features of skeletal Muscle

Answer 4

• Muscles are bundles of fibres encased in connective tissue sheaths
• Attached to bones by tendons
• Cells replaced after injury by satellite cells
• Satellite cells differentiate to form new muscle fibres
• Other fibres undergo hypertrophy to compensate
• Muscle will never completely recover

Question 5

Contraction: Sliding Filaments

Answer 5

Question 6

Myosin Cross-Bridge pic

Answer 6

Question 7

The Cross-Bridge Cycle pic

Answer 7

Question 8

Troponin, tropomyosin + Ca²⁺

Answer 8

• Tropomyosin partially covers myosin binding site
• Held in blocking position by troponin
• Co-operative block
• Calcium binds to troponin
• Troponin alters shape – pulls tropomyosin away
• Remove calcium – blocks sites again

Question 9

Muscle Mechanics definitions

Answer 9

• Force exerted by muscle = TENSION
• Force exerted on muscle = LOAD
• Contraction with constant length = ISOMETRIC (e.g. weightlifting)
• Contraction with shortening length = ISOTONIC (or concentric) (e.g. running)
• Contraction with increasing length = LENGTHENING (e.g. sitting down)

Question 10

Twitch Contractions

Answer 10

• Single AP to Muscle fibre to TWITCH.
• Latent period is time before excitation contraction starts
• Contraction time occurs between start of tension and time when we have peak tension
• Contraction time depends on [Ca²⁺]
• Isometric has shorter latent period, but longer contraction event
• As load increases, contraction velocity and distance shortened decreases,

Question 11

Tetanus

Answer 11

•Tetanic tension greater than twitch tension since [Ca²⁺] never gets low enough to allow troponin/tropomyosin to re-block myosin binding sites.

Question 12

Tetanic tension definition

Answer 12

• A sustained muscle contraction evoked when the motor nerve that innervates a skeletal muscle emits action potentials at a very high rate.
• During this state, a motor unit has been maximally stimulated by its motor neuron and remains that way for some time.
• This occurs when a muscle’s motor unit is stimulated by multiple impulses at a sufficiently high frequency.
• Each stimulus causes a twitch. If stimuli are delivered slowly enough, the tension in the muscle will relax between successive twitches.
• If stimuli are delivered at high frequency, the twitches will overlap, resulting in tetanic contraction.

Question 13

Fused and unfused tetanus

Answer 13

• A tetanic contraction can be either unfused (incomplete) or fused (complete).
• An unfused tetanus is when the muscle fibers do not completely relax before the next stimulus because they are being stimulated at a fast rate; however there is a partial relaxation of the muscle fibers between the twitches.
• Fused tetanus is when there is no relaxation of the muscle fibers between stimuli and it occurs during a high rate of stimulation.

Question 14

Length-Tension Relationship

Answer 14

• Less overlap of filaments = less tension
• Too much overlap = filaments interfere with each other
• Muscle length for greatest isometric tension = OPTIMAL LENGTH (l_o)

Question 15

Flexors and Extensors

Answer 15

• Movement around a limb requires 2 antagonistic groups of muscles.
• 1 flexes, the other extends(straightens)
• Muscles arranged in lever systems
• Muscles exert far more force than the load they support.

Question 16

Energy for Contraction - ATP

Answer 16

• Hydrolysis of ATP energises X-bridges
• ATP also powers Ca²⁺- ATPase in sarcoplasmic reticulum (SR)

–Ca²⁺ pumped back into SR

–Contraction ends

Question 17

Fatigue

Answer 17

• Repeated muscle stim leads to muscle fatigue
• Depends on fibre type, length of contraction and fitness of individual
• Fatigue prevents muscles using up vast amounts of ATP, which would cause rigor (i.e. muscles would not be able to activate new X-bridge cycles)

Question 18

Factors causing fatigue

Answer 18

•During high intensity, short duration exercise:

–Conduction failure due to increase in ­[K⁺] leads to depolarisation

–­ Increase in [lactic acid] which leads to acidifies proteins

–­Increase in [ADP] and [Pi] which inhibits X-bridge cycle, delaying myosin detachment from actin filaments.

Question 19

Factors causing fatigue 2

Answer 19

•During long-term, low intensity exercise:

–muscle glycogen

–blood glucose

–Dehydration

•Central command fatigue – cerebral cortex cannot excite motor neurons. There is no “will to win”.

Question 20

Skeletal Muscle Fibre Types

Answer 20

•Characterised based on whether:

–Fibres are fast or slow-shortening

–The oxidative or glycolytic ATP forming pathways are used

• FAST means myosin has high ATPase activity
• SLOW means myosin has low ATPase activity

Question 21

OXIDATIVE FIBRES

Answer 21

–­ Increase in mitochondria leads to increase in oxid. phosphorylation

–­ Increase in vascularisation to deliver more O₂ and nutrients

–Contain myoglobin (oxygen-binding protein) leads to increase in ­O₂ delivery

–Fibres are red and have low diameters

Question 22

GLYCOLYTIC FIBRES

Answer 22

–Few mitochondria

–­ Increase in glycolytic enzymes and glycogen

–Lower blood supply

–White fibres with larger diameters

Question 23

3 types of muscle fibres

Answer 23

• Slow oxidative (I) lead to a resisting in fatigue
• Fast oxidative (IIa) leads to an intermediate resistance to fatigue
• Fast glycolytic (IIb) leads to fatigue quickly

Question 24

Muscle fibre recruitment

Answer 24

• ­Increase in load = ­ need to activate more motor units
• ­ ­Increase in number of active motor units = RECRUITMENT
• Slow oxidative fibres activated (first) ⇒ fast oxidative ⇒ fast glycolytic last

Question 25

Neural control of muscle tension depends on what?

Answer 25

–**Frequency** of AP’s to motor units –**Recruitment** of motor units

Question 26

Exercise type determines type of muscle fibres you have

Answer 26

* Destroy nerve/NMJ leads to **denervation atrophy** * Muscle not used leads to **disuse atrophy** * Both cause decreases in muscle mass * Exercise causes hypertrophy (­increase in mass) * Aerobic exercise leads to increase in­ **mitochondria, ­ vascularisation and­ fibre diameter** * Anaerobic (strength) exercise leads to increase in ­**diameter and glycolysis**

Question 27

Smooth muscle

Answer 27

* No striations * Innervated by ANS, not somatic NS * Has a **X-bridge cycle and uses Ca²⁺** * Filaments and excitation-contraction coupling are different * Exists in **hollow organs** (e.g. GI tract, uterus, airways, ducts)

Question 28

Smooth muscle features

Answer 28

* **Spindle-shaped** (2-10mm diameter) * **Mononucleate** and divide through life * **Thick** myosin and thin actin filaments, like skeletal muscle * However, filaments arranged **diagonally** across cells and are anchored to membranes and cell structures by dense bodies (like Z-lines) * Filaments still slide together to **contract cell**

Question 29

Smooth Muscle X-bridge cycle activation

Answer 29

* Increase in [Ca²⁺] * Ca²⁺ binds **calmodulin** * Ca²⁺- Calmodulin binds to **Myosin Light Chain Kinase** * Kinase **phosphorylates** myosin X-bridges with ATP * Phosphorylated X-bridges bind to actin filaments * CONTRACTION + TENSION

Question 30

Sources of Cytosolic Ca²⁺

Answer 30

•Sarcoplasmic Reticulum (SR) –**Less** SR in smooth muscle than in skeletal, no T-tubules & more randomly arranged •Extracellular Ca²⁺ –Voltage-activated Ca²⁺ channels (VACC’s) •Ca²⁺ removed from cytosol by pumping back into SR and out of cell by Ca²⁺-ATPases (slower process than in skeletal muscle)

Question 31

Differences in skeletal muscle and and smooth muscle

Answer 31

•In skeletal muscle: –1 AP releases enough Ca²⁺ to saturate **all troponin sites** •In smooth muscle: –Only **some sites activated** –Can **grade** contraction depending on number of AP’s that reach cells •Smooth muscle has tone i.e a basal level of Ca²⁺ in cells causes a constant level of tension

Question 32

Factors affecting contractile activity

Answer 32

•Dynamic balance of all the following: –Hormones: Oxytocine (a neurotransmitter and a hormone that is produced in the hypothalamus) –Local factors: paracrine agents, pH, O₂, **osmolarity**, ions, NO –Stretch

Question 33

Smooth Muscle Types

Answer 33

* Single or multiunit smooth muscle * Single Unit (GIT, uterus, small blood vessels) –Signals travel between cells –Contract synchronously –May contain pacemaker cells –Stretch evokes contraction