muscle Flashcards
muscles allow us to
Generate force & movement
Allow us to express & regulate ourselves
types of muscle
smooth
skeletal
cardiac
what muscles are striated
skeletal (voluntary muscles diaphragm)
cardiac (heart)
Smooth muscle is found in
blood vessels vas deferens airways uterus GI tract bladder etc.
in skeletal muscle the nucleus
is on the top of striations (many nucleus)
in cardiac muscle the nucleus is
in the middle of striations
in smooth muscle the nucleus is
in teh middle of a cell and looks like an eye
Skeletal muscle cell =
muscle fibre
- Multinucleate
skeletal muscle is formed in
utero from mononucleate myoblasts
Increase fibre size during growth
- Myoblasts do not replace cells if damaged
Muscles are bundles of
fibres encased in connective tissue sheaths.
- Attached to bones by tendons
if muscle cells are injured they are replaced by
satellite cells
Satellite cells differentiate to form new muscle fibres
- Other fibres undergo hypertrophy to compensate
- Muscle will never completely recover
the cross bridge cycle
- cross bridge binds to actin (Ca2+ rises)
- crossbridge moves
- ATP binds to myosin causing cross bridge to detach
- hydrolysis of ATP energises cross bridges.
roponin, tropomyosin + Ca2+
Tropomyosin partially covers myosin binding site (like wire)
- Held in blocking position by troponin
- Co-operative block
Calcium binds to troponin
Troponin alters shape – pulls tropomyosin away
Remove calcium – blocks sites again
relaxed muscle =
less calcium
muscle is energy hungry that why so much
mitochondria is spread out across it
muscles have double membrane so that
if the first one is damaged then the second one will protect the contents from leaking out as contents have potential to do harm
muscles contain loads of capiliries as
then its easier for oxygen to reach the muscle and also to get rid of waste products easily
muscle contains
sarcoplasmic reticulum - mesh like myofibrils cytosol plasma membrane mitochondria (lots) lateral sacs transverse tubules (between lateral sacs)
tropomyocin contains
troponin (calcium binding site)
in contraction and relaxation of skeletal muscle
- muscle action potential propogated
- DHP receptor and Ryanodine receptor cause release of Ca2+ from lateral sac
- Ca2+ binding to troponin removes blocking action of tropomyosin
- cross bridge move using ATP
- Ca2+ removal from troponin restores tropomyosin blocking action
- ATP used to take up Ca2+ into lateral sac
Motor Units is made up of
Motor neurons + muscle fibres
- Muscle fibres within a unit may be scattered throughout muscle
Muscle Mechanics:
TENSION
Force exerted by muscle
Muscle Mechanics:
LOAD
Force exerted on muscle
Muscle Mechanics:
ISOMETRIC
Contraction with constant length
e.g. weightlifting
Muscle Mechanics:
ISOTONIC (or concentric)
Contraction with shortening length
e.g. running
Muscle Mechanics:
LENGTHENING
Contraction with increasing length
e.g. sitting down
Twitch Contractions
Single AP —> Muscle fibre —> TWITCH
Latent period =
time before excitation contraction starts
Contraction time occurs
between start of tension and time when we have peak tension
Muscle fibres have different
contraction times.
Contraction time depends on [Ca2+]
Isometric has shorter
latent period, but longer contraction event
As load increases,
contraction velocity and distance shortened decreases,
Tetanus
- AP is 1-2ms long, but twitch may last up to 100ms
- May get more AP’s during contraction
These add up = SUMMATION
Tetanic tension greater than
twitch tension since [Ca2+] never gets low enough to allow troponin/tropomyosin to re-block myosin binding sites
Length-Tension Relationship
Less overlap of filaments =
less tension
Length-Tension Relationship
Too much overlap =
filaments interfere with each other
Length-Tension Relationship OPTIMAL LENGTH (lo)
Muscle length for greatest isometric tension
Flexors and Extensors
Movement around a limb requires
2 antagonistic groups of muscles – 1 flexes, the other extends(straightens)
Flexors and Extensors
Muscles are arranged in
lever systems
- Muscles exert far more force than the load they support
Lever system amplifies muscle shortening velocity producing increased maneuverability
Energy for Contraction - ATP:
Hydrolysis of ATP energises X-bridges
ATP binds to myosin
Dissociates bridges bound to actin
New cycle may begin
note- X bridge is the same as cross bridge
Energy for Contraction - ATP:
ATP also powers Ca2+ - ATPase in SR
Ca2+ pumped back into SR
Contraction ends
Fatigue is caused by
Repeated muscle stimulus.
this depends on:
- fibre type
- length of contraction
- fitness of individual
If muscle is rested, can contract again
Fatigue prevents
muscles using up vast amounts of ATP which would cause rigor (ie muscles would not be able to activate new cross bridge cycles)
Factors causing fatigue
During high intensity, short duration exercise:
- Conduction failure due to increase in [K+] which leads to depolarisation
- increase in [lactic acid] which acidifies proteins
- increase in [ADP] and [Pi] inhibits X-bridge cycle, delaying myosin detachment from actin filaments
Factors causing fatigue
During long-term, low intensity exercise:
- decrease in muscle glycogen
- decrease in blood glucose
- dehydration
central command fatigue is when the 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 shortening skeletal muscle fibre means
myosin has high ATPase activity
slow shortening skeletal muscle fibre means
it has low ATPase activity
oxidative fibres
- increased mitochondria leads to increased oxidative phosphorylation
- increased vascularisation to deliver more O2 and nutrients
- contain myoglobin which increases O2 delivery
- fibres are red and have low diameters
Glycolytic Fibres
- few mitochondria
- increased glycolytic enzymes and glycogen
- lower blood supply
- white fibres with larger diameteres
what are the three types of muscle fibres and how do they react to fatigue
Slow Oxidative (I) resists fatigue
Fast Oxidative (IIa) has intermediate resistance to fatigue
Fast Glycolytic (IIb) fatigues quickly
Muscle fibre recruitment
- An increase in load means we have to increase the need to activate more motor neurons
Muscle fibre recruitment =
increase in the number of active motor units
- An increase in load causes and increase in need to activate more motor neurons.
slow oxidative fibres are activated first followed by fast oxidative and lastly fast glycolytic fibres.
neural control of muscle tension depends on
- frequency of action potentials to motor units
- recruitment of motor units
Denervation atrophy =
nerve is destroyed/ NMJ is destroyed
disuse atrophy =
happens when muscle is not used
muscle mass can be decreased by
denervation atrophy
disuse atrophy
exercise causes
hypertrophy (increase in mass)
aerobic exercise causes
increase in mitochondria which leads to increased vascularisation which causes increase in fibre diameter
Anaerobic (strength) exercise causes
increased diameter and increased glycolysis
the types of muscle fibres you have is determined by
the type of exercise you do
smooth muscle unlike skeletal and cardiac muscles have
No striations
smooth muscle is
innervated by autonomic nervous system not the somatic nervous system
smooth muscle has a
X-bridge cycle and uses Ca2+
Filaments and excitation-contraction coupling are different
smooth muscle 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
how are the filaments in smooth muscle different
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 to calmodulin
- Ca2+ calmodulin binds to myosin light chain kinase
- kinase phosphorylates myosin cross bridges with ATP
- phosphorylated cross bridges bind to actin filaments
- contraction and tension occurs
smooth muscle relaxed via the action of
myosin light chain phosphatase.
this dephosphorylates the cross bridges
persistant stimulation and increase in Ca2+ in some smooth muscle means
- Phosphorylated X-bridges may be dephosphorylated when still bound to actin
- there is a decreased rate of ATP splitting
- Slows X-bridge cycle
- Means you can maintain tension for long time with low ATP consumption
- Useful in blood vessel walls that have to stay open for long periods
Sources of Cytosolic Ca2+
Sarcoplasmic Reticulum (SR)
- less sarcoplasmic reticulum in smooth muscle than in
skeletal, no T-tubules + randomly arranged
extracellular Ca2+
- voltage activated Ca2+ channels (VACC’s)
Ca2+ is removed from cytosol by
pumping back into the sarcoplasmic reticulum and out of cells by Ca2+ ATPases (slower process than in skeletal muscle)
In skeletal muscle:
1 action potential
releases enough Ca2+ to saturate all troponin sites
in smooth muscle:
1 action potential
causes some sites to be activated
- you can grade contraction depending on the number of action potentials that reach the cells
smooth muscle has tone.
this means
a basal level of Ca2+ in cells causes a constant of tension
Factors affecting contractile activity
Dynamic balance of all the following:
- Spontaneous electrical activity in muscle membranes = Pacemaker activity
- Autonomic neurotransmitters from varicosities
- Hormones (e.g. oxytocin)
- Local factors (paracrine agents, pH, O2, osmolarity, ions, NO)
- Stretch
Smooth Muscle Types
single
multiunit
single unit smooth muscle found in
gasto intestinal tract
uterus
small blood vessels
single unit smooth muscle structure and function
- many cells linked by gap functions
- signals travel between cells
- contract synchronously
- may contain pacemaker cells
- stretch evokes contraction
multiunit smooth muscle found in
airways
large arteries
hairs
multiunit smooth muscle structure and function
- few or no gap junctions
- richly innervated by the autonomic nervous system
- don’t respond to stretch
most smooth muscle in organs are
a mixture of single unit and multiunit population of cells.
this means that an organ can have a mixture of properties in different areas.
