muscles mw %% (+ Flashcards
Muscle
- Generate force & movement
- Allow us to express & regulate ourselves
- 3 types – skeletal, smooth & cardiac
- Another type of excitable tissue
Types of Muscle
- Striated: Skeletal (voluntary muscles, diaphragm) and Cardiac (heart).
- Smooth: blood vessels, vas deferens (the duct which conveys sperm from the testicle to the urethra), airways, uterus, GI tract, bladder etc.
Skeletal Muscle
- Skeletal muscle cell = muscle fibre
- Multinucleate
- Form in utero from mononucleate myoblasts
- Increase fibre size during growth
- Myoblasts do not replace cells if damaged
Features of skeletal Muscle
- 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
Contraction: Sliding Filaments
Myosin Cross-Bridge pic
The Cross-Bridge Cycle pic
Troponin, tropomyosin + Ca2+
- 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
Muscle Mechanics definitions
- 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)
Twitch Contractions
- 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,
Tetanus
•Tetanic tension greater than twitch tension since [Ca2+] never gets low enough to allow troponin/tropomyosin to re-block myosin binding sites.
Tetanic tension definition
- 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.
Fused and unfused tetanus
- 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.
Length-Tension Relationship
- Less overlap of filaments = less tension
- Too much overlap = filaments interfere with each other
- Muscle length for greatest isometric tension = OPTIMAL LENGTH (lo)
Flexors and Extensors
- 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.
Energy for Contraction - ATP
- Hydrolysis of ATP energises X-bridges
- ATP also powers Ca2+- ATPase in sarcoplasmic reticulum (SR)
–Ca2+ pumped back into SR
–Contraction ends
Fatigue
- 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)
Factors causing fatigue
•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.
Factors causing fatigue 2
•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”.
Skeletal Muscle Fibre Types
•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
OXIDATIVE FIBRES
– 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
GLYCOLYTIC FIBRES
–Few mitochondria
– Increase in glycolytic enzymes and glycogen
–Lower blood supply
–White fibres with larger diameters
3 types of muscle fibres
- 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
Muscle fibre recruitment
- 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
Neural control of muscle tension depends on what?
–Frequency of AP’s to motor units
–Recruitment of motor units
Exercise type determines type of muscle fibres you have
- 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
Smooth muscle
- 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)
Smooth muscle features
- 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
Smooth Muscle X-bridge cycle activation
- 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
Sources of Cytosolic Ca2+
•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)
Differences in skeletal muscle and and smooth muscle
•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
Factors affecting contractile activity
•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
Smooth Muscle Types
- 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
