Muscle 2 Flashcards
Where does the energy for contraction come from?
Hydrolysis of ATP energises cross bridges -ATP binds to myosin -Dissociates bridges bound to actin -New cycle may begin ATP also powers Ca ATPase in SR -Ca pumped back into SR -Contraction ends
Muscle fatigue
Repeated muscle stimulation
What is fatigue dependent on?
- Fibre type
- Length of individual contraction
- Fitness of individual
What does fatigue prevent?
Prevents muscles using up vast amounts of ATP which would cause rigor
How does high intensity, short duration exercise cause fatigue?
- Conduction fails due to increase in[K] leading to depolarisation
- Increased [lactic acid] leads to acidified proteins
- Increased [ADP] and [Pi] inhibits cross bridge cycle, delaying myosin detachment from actin filaments
How does long term, low intensity exercise cause fatigue?
- Decrease in muscle glycogen
- Decrease in blood glucose
- Dehydration
What is central command fatigue?
Cerebral cortex cannot excite motor neurones
How are skeletal muscle fibres characterised?
Based on
- Fibres are fast or slow shortening
- The oxidative or glycotic ATP forming pathways are used
Describe ATPase activity in fast shortening skeletal muscle fibres.
Myosin has high ATPase activity
Describe ATPase activity in slow shortening skeletal muscle fibres.
Myosin has low ATPase activity
Describe oxidative fibres.
- Increase in mitochondria leads to increase in oxidative phosphorylation
- Increase in vascularisation to deliver more O2 and nutrients
- Contain myoglobin which increases O2 delivery
- fibres are red and have low diameters
Describe glycolytic fibres.
-Few mitochondria
-Increase in glycolytic enzymes and glycogen
-lower blood supply
White fibres with larger diameters
What are the 3 types of muscle fibres?
- Slow oxidative (I) which resist fatigue
- Fast oxidative (IIa) which have intermediate resistance to fatigue
- Fast glycolytic (IIb) which fatigue quickly
Describe muscle fibre recruitment.
- Increased load means and increased need to activate more motor units
- Increased number of active motor units means recruitment
What is the order of muscle recruitment?
- Slow oxidative
- Fast oxidative
- Fast glycolytic
What does neural control of muscle tension depend on?
- Frequency of AP to motor units
- Recruitment of motor units
What causes denervation atrophy?
Destruction of nerve/NMJ
What causes disuse atrophy?
Muscle not used
What causes hypertrophy?
Exercise
What does aerobic exercise cause?
- Increase in mitochondria
- Increase in vascularisation
- Increase in fibre diameter
What does anaerobic exercise cause?
- Increase in diameter
- Increase in glycolysis
How are smooth muscles identified?
Contain no striations
What are smooth muscles innervated by?
Autonomic nervous system
How are their mechanisms similar and different to skeletal muscle?
- Similar: has a cross bridge cycle and uses Ca ions
- Different: filaments and excitation-contraction coupling are different
What are the features of smooth muscle?
- Spindle shaped
- Mononucleate and divide throughout life
- Thick myosin and thin actin filaments
How are the filaments arranged in smooth muscle?
Diagonally across cells and are anchored to membranes and cell structure by dense bodies (like Z lines)
Describe the cross bridge cycle in smooth muscle.
- Increase in calcium
- Calcium binds to calmodulin
- Ca-calmodulin binds to myosin light chain kinase
- Kinase phosphorylates myosin cross bridges with ATP
- Phosphorylated cross bridges bind to actin filaments
- Contraction and tension
How does smooth muscle relax?
Via the action of myosin light chain phosphatase which dephosphorylates the cross bridges
How can tension be maintained in smooth muscle for a long time with low ATP consumption?
- Persistent stimulation and increased [Ca] in smooth muscle
- Phosphorylated cross bridges may be dephosphorylate when still bound to actin
- Decreases the rate of ATP splitting
- Slows cross bridge cycle
What are the 2 sources of cytosolic Ca ions?
Sarcoplasmic reticulum
-Less SR in smooth muscle than in skeletal, no T-tubules + randomly arranged
Extracellular Ca
-Voltage activated Ca channels
How is Ca removed from the cytosol?
It is pumped back into the SR and out of the cell by Ca ATPase
How much Ca is released by 1 AP in skeletal muscle?
Enough to saturate all troponin sites
How is contraction graded in smooth muscle?
Dependent on the number of APs that reach the cells as only some sites are activated
What is meant by smooth muscle having tone?
A basal level of Ca ions in cells causes a constant level of tension
What factors affect contractile activity?
- Spontaneous electrical activity in muscle membranes (Pacemaker activity)
- Autonomic neurotransmitters from varicosities
- Hormones
- Local factor
- Stretch
What are the 2 types of smooth muscle?
Single or multiunit smooth muscle
What are examples of single unit smooth muscles?
- GIT
- Uterus
- Small blood vessels
Describe single unit smooth muscles.
- Many cells linked by gap junctions
- Signals travel between cells
- Contract synchronously
- May contain pacemaker cells
- Stretch evokes constriction
What are examples of multiunit smooth muscles?
- Heart
- Large arteries
- Hairs
Describe multiunit smooth muscles.
- Few or no gap junctions
- Richly innervated by ANS
- Don’t respond to stretch
What are most smooth muscles in organs?
A mixture of single and multiunit populations of cells
