PBL Topic 3 Case 9 Flashcards
What is skeletal muscle composed of?
- Muscle fibres
What is the range of diameters of a muscle fibre?
- 10-80 micrometers
What is the sarcolemma?
- Cell membrane of the muscle fibre
- Composed of a plasma membrane
- And an outer polysaccharide coat containing collagen
What is a myofibril?
- Contractile threads within muscle fibres
Identify two myofibrils. How many of each are located within a muscle fibre?
- Myosin (1500)
- Actin (3000)
How are the myofibrils arranged in a muscle fibre?
- They interlock
- Giving off alternate light and dark bands?
What is the sarcomere?
- Portion of myofibril between two successive Z lines
What happens to the sarcomere during contraction? What is the importance of this?
- Shortens 2 micrometers
- Actin and myosin filaments overlap more
- Creating greatest force of contraction
What is titin?
- Molecule that maintains side-to-side relationship between actin and myosin
What is the sarcoplasm and what does it contain?
- Space between myofibrils
- Contains large quantities of potassium ions, mitochondria and sarcoplasmic reticulum
Outline the structure of a myosin filament
- Four light chains and two heavy chains
- Heavy chains wrap spirally around each other into a double helix
- Cross bridges formed by protruding arms and heads
Outline the structure of an actin filament
- F-actin double helix protein
- Composed of G-actin molecules which functions as active site
- Tropomyosin wraps spirally around F-actin, covering the active sites
- Troponin attaches to tropomyosin
Identify the three types of troponin
- Troponin I has a strong affinity for actin
- Troponin T has a strong affinity for tropomyosin
- Troponin C has a strong affinity for calcium ions
Outline the process that allows myosin to bind with actin
- Calcium binds to troponin C
- Troponin is inhibited and moves the tropomyosin deeper into the groove between the actin strands
- This uncovers active sites and allows myosin to bind
Outline the walk along theory of contraction
- Myosin head binds to active site
- Cross bridge tilts and moves the actin filament along it (power stroke)
- Myosin head breaks away from active site and binds to another active site further along the actin filament
How is ATP broken down into ADP and Pi during muscle contraction?
- Myosin head possesses ATPase
Identify two ways in which ATP influences the myosin head
- Energy from breakdown of ATP allows myosin head to tilt and move actin filament along
- Binding of ATP after power stroke causes detachment of myosin head in order to bind to another actin filament
Aside from action on the myosin head, identify two other roles of ATP during muscle contraction
- Pumping calcium from sarcoplasm into sarcoplasmic reticulum after contraction
- Pumping sodium and potassium across muscle fibre membrane to maintain ionic environment for propagation of action potential
How is phosphocreatine able to reconstitute ATP?
- Breakdown of phosphocreatine donates a phosphate ion which is able to bind with ADP to form ATP
How is glycogen able to reconstitute ATP?
- During glycolysis
- Breakdown of glycogen to pyruvic acid and lactic acid liberates energy
- That is used to convert ADP to ATP
Identify two advantages of glycogen breakdown during muscle contraction
- Can occur in anaerobic conditions
- Rate of ATP formation is 2.5x that from foodstuffs
From which source is 95% of energy for muscle contraction derived?
- Oxidative metabolism
- In which oxygen reacts with products of glycolysis to liberate ATP
What is the efficiency of muscle contraction and why?
- 25%
- Much of the energy is lost as heat
How is work done calculated?
- W = L x D
- Where W is work done
- L is load (force exerted on the muscle by a weight)
- And D is distance of movement against the load
What is tension?
- Force exerted on a weight by a muscle
What is load?
- Force exerted on a muscle by a weight
What is meant by isometric contraction?
- Contraction that does not involve lengthening or shortening of a muscle
What is meant by isotonic contraction?
-Contraction that does involve lengthening or shortening of a muscle
What is concentric contraction?
- Isotonic contraction that involves shortening of a muscle
What is eccentric contraction?
- Isotonic contraction that involves lengthening of a muscle
What is a type I fibre?
- Slow oxidative fibre
- That combines low myosin ATPase activity
- With high oxidative energy
What is a type IIb fibre?
- Fast glycolytic fibre
- That combines high myosin ATPase activity
- With high glycolytic activity
What is a type Ib fibre?
- Fast-oxidative-glycolytic activity
- That combines high myosin ATPase activity
- With high oxidative capacity
- And intermediate glycolytic activity
How does blood supply differ in slow and fast fibres?
- More extensive in slow fibres for oxygen for oxidative phosphorlylation
What is myoglobin and how does the presence of myoglobin differ in slow and fast fibres
- More extensive in slow fibres for storage of oxygen, giving slow fibres a red appearance
- Less extensive in fast fibres, giving fast fibres a white appearance
Which type of fibre possesses a more extensive sarcoplasmic reticulum?
- Fast fibres
Which type of fibre possesses a more glycolytic enzymes?
- Fast fibres
Which type of fibre possesses a more mitochondria?
- Slow fibres
What is a motor unit?
- All the muscle fibres innervated by a single nerve fibre
The average motor unit possesses how many muscle fibre?
- 80 to 100
What is spatial summation?
- Intensity of muscle contraction can be increased by increasing the number of motor units
- Size principle
What is temporal summation?
- Intensity of muscle contraction can be increased by increasing the frequency of contraction
What is tetanisation?
- As frequency increases, successive contractions fuse together, giving appearance of smooth and continuous contraction
What is the maximum contractile strength of a muscle?
- 3-4kg per cubic centimetre
How is muscle tone maintained?
- Low rate of nerve impulses from spinal cord
What is muscle fatigue proportional to?
- Rate of depletion of glycogen
What is meant by coactivation of antagonist muscles?
- All movements are caused by simultaneous contraction of agonist and antagonist muscles
- Position of limb is determined by relative degrees of contraction of agonist and antagonist muscles
Identify 4 processes that can occur in muscle remodelling
- Increase in number of myofibrils (fibre hypertrophy)
- Addition of sarcomeres to ends of muscle fibres (sarcomere hypertrophy)
- Increase in number of muscle fibres (hyperplasia)
- Destruction of muscle fibres (denervation atrophy)
What is rigor mortis?
- After death, muscles become rigid
- Due to loss of ATP
- Which causes separation of cross bridges from actin filaments
- Lasts 24 hours
Describe the physiological anatomy of a neuromuscular junction
- Large myelinated nerve fibre branch and invaginate into muscle fibre
- Invaginated space known as synaptic trough
- Synaptic trough separated from nerve fibre by synaptic cleft
- Subneural clefts located at bottom of trough
Explain the role of Ca2+ ions on the secretion of ACh from the presynaptic nerve fibre
- Binds to synaptotagmins
- Causing a change in the SNARE complex
- Allowing fusion of ACh vesicles with neural membrane
- Followed by exocytosis
Explain the effect of ACh on the postsynaptic muscle fibre membrane
- Two molecules of ACh bind to ACh receptor
- (Which is composed of two alpha, a beta, a delta and a gamma protein)
- Allowing entry of positive sodium ions into the muscle fibre
Explain why negative ions do not enter the postsynaptic muscle fibre
- Opening possesses a negative field which repels negative ions
What is the end plate potential and what is its value?
- Local potential caused by influx of sodium ions into the end plate
- 50 to 75 millivolts
What are T-tubules and what is their purpose?
- Dihydropyridine voltage gated calcium channel
- That penetrates all the way through the muscle fibre
- Allowing the action potential to penetrate deep enough to the myofibrils to cause maximum muscle contraction
Describe the process of excitation-contraction coupling
- Calcium ions enter the sarcoplasm through the T-tubules
- Which binds to ryanodine receptors on the sarcoplasmic reticulum
- Causing release of many more calcium ions into the sarcoplasm and into the myofibrils
- To bind to troponin C, to relax its inhibitory grip on tropomyosin to allow myosin to bind to actin active sites
What happens to calcium ions following muscle contraction?
- Pumped out of muscle fibres by sodium calcium exchanger
- Into sarcoplasmic reticulum (regulated by phospholambin) and T-tubule extracellular fluid
Outline the effects of training on muscle hypertrophy
- Increase in number of myofibrils, mitochondrial enzymes, ATP, phosphocreatine and glycogen and triglycerides
What is AMPK and its role?
- Adenosine Monophosphate-Activated Kinase
- Increases cellular energy
- By inhibiting anabolic energy consuming pathways and stimulating energy producing catabolic pathways
What is gait?
- Cyclical pattern of musculoskeletal motion that carries the body forward
Identify the two phases of gait and their relative proportions
- Stance occupying 60% of the cycle
- Swing occupies 40% of the cycle
Describe the features of antalgic gait and identify its cause
- ‘Dot-dash’ movement
- Caused by pain
Describe the feature of a gait with limb-length discrepancy
- Tiptoe on shorter side
- Hip and knee flexion on longer side
Describe the features of apraxic gait and identify its cause
- Small shuffling steps (marche a petit pas)
- Caused by frontal lobe damage e.g. hydrocephalus, infarction
Describe the features of myopathic / waddling gait and identify its cause
- Patient bends pelvis forward and walks with a waddle
- Caused by muscle or hip disease
Describe the features of diplegic / scissoring gait and identify its cause
- Patient walks stiffly on the toes and has problems turning
- Bilateral upper neuron lesion causing bilateral spasticity
Describe the features of a Parkinsonian gait and identify its cause
- Stooped posture
- Slow, shuffling gait
- Unilateral arm swing
- Resting tremor
- Caused by basal ganglia dysfunction
Describe the features of a hemiplegic gait and identify its cause
- Flexed upper limbs on affected side
- Extended lower limbs on affected side
- With circumduction of leg
- Unilateral upper motor neurone lesion
Describe the features of ataxic gait and identify its cause
- Broad-based gate
- Poor tandem gait
- Side-swinging
- Caused by a cerebellar lesion, problem with spinocerebellum since there is overshooting of movements hence side-swinging
Describe the features of sensory ataxia and identify its cause
- Slaps the foot down on walking
- Gait is high stepping to allow clearance of weak foot
- Caused by damage to common fibular nerve
What is a stress fracture?
- One caused by recurrent episodes of minor trauma
- Typically in long bones
- Heals after rest
What is a pathological fracture?
- One caused by a disease such as osteoporosis, osteomalacia, rickets, Paget’s disease, malignant tumours
What is a compound fracture?
- One in which broken bone pierces the skin
What is a comminuted fracture?
- One in which bone is broken into more than two fragments
Outline the process of fracture healing
- Blood vessel rupture results in haematoma
- Which is replaced by granulation tissue
- Chondroblasts deposit hyaline over the granulation tissue to form a provisional callus
- Osteoprogenitor cells deposit a layer of woven bone over the provisional callus forming a bony callus
- Bony union achieved when fracture site is completely bridged by woven bone.
Identify four causes of delayed fracture healing
- Excessive movement during healing process
- Poor intrinsic blood supply or interruption of blood supply
- Infection in a compound fracture
- Necrosis in a comminuted fracture
What is the A band composed of, how does it appear?
- Consists of myosin filaments as well as the actin filaments that overlap it
- Appears dark because it is anisotropic to light
What is the I band composed of, how does it appear?
- Consists of actin filaments only on either side of the A band
- Appears light because it is isotropic to light
What is the Z line?
- The lateral part of each actin filament is anchored to a network of interconnecting proteins known as the Z line
What is the H zone?
- Consists of myosin filaments only in the centre of the A band
- Does not contain overlapping actin filaments
What is the M line?
- Dark band composed of proteins that link together the central region of the thick filament
What is meant by fibre hypertrophy?
- Increase in size of muscle fibre
- Due to increased number of myofibrils due to increase use
What is meant by sarcomere hypertrophy?
- Increase in size of muscle fibre
- Due to increased stretching of a muscle
- And increased number of sarcomeres at each end where they attach to muscles
What is meant by fibre hyperplasia?
- Increase in number of muscle fibres
- Rare
What is meant by denervation atrophy?
- As a result of impaired nerve supply to a muscle
- Decrease in size of muscle fibre
- And replacement with fibrous fatty tissue
- Which continues to shorten resulting in contracture
Identify the four stages of stance phase
- Heal strike
- Loading response
- Midstance
- Heal off
Which muscles are used during heel strike? (2)
- Tibialis anterior
- Gluteus maximus
Which muscles are used during loading response? (1)
- Quadriceps femoris
Which muscles are used during midstance? (2)
- Gastrocnemius
- Soleus
Which muscles are performed during terminal stance? (2)
- Gastrocnemius
- Soleus
Identify the three stages of swing phase
- Pre-swing
- Mid-swing
- Terminal swing
Which muscles are used during pre-swing ? (1)
- Rectus femoris
Which muscles are used during mid-swing? (1)
- Iliopsoas
- Rectus femoris
Which muscles are used during terminal swing?
- Hamstrings
- Tibialis anterior
- Ankle dorsiflexors
What is a placebo?
- Inert substance that causes symptom relief
Identify three non-interactive theories of placebo
- Individual trait theories e.g. personality
- Treatment characteristics e.g. size of drug
- Characteristics of health professional
Identify six factors involved in the interactive theory of placebo
- Experimenter bias
- Patient’s expectations
- Reporting error
- Conditioning effects
- Anxiety reduction
- Endorphins
Outline Cognitive Dissonance Theory with relation to placebo
- For a placebo to work they must involve investment
- For someone to invest they must see themselves as rational, in control and be able to justify their behaviour
- If these factors align the person experiences low dissonance resulting in less distress
How does the placebo effect relate to health beliefs?
- Individual needs to believe the intervention is effective
How does the placebo effect relate to illness cognitions?
- Individual needs to believe that the illness can be overcome
How does the placebo effect relate to health related behaviour
- Individuals believe intervention promotes good health
- So take health promoting measures in other areas of their lifestyle
How does the placebo effects relate to stress?
- Reduces stress since the individual believes that they have taken control of their illness
How does the placebo effect relate to pain reduction?
- Related to opiate release or by anxiety reduction by taking intervention
What is Shared Decision Making
- Process in which clinicians and patients work together
- To select tests, treatments, management or support packages
- Based on both clinical evidence and the patients informed preference
What is the importance of Shared Decision Making
- Ethical imperative by professional regulatory bodies
- Patients want to be more involved than they currently are in making decisions about their own health and health care.
What does patient driven decision making involve?
- Physician presents all options
- Physician makes no recommendation
- Patient makes their own choice
What does physician recommendation decision making involve?
- Physician presents all options
- Physician makes a recommendation
- Based on patient’s values and perspective
What is meant by equal partners decision making involve?
- Physician presents all options
- Physicians and patients work together to reach a mutual decision
- Based on patients values and perspectives
What is meant by informed non-dissent decision making?
- Physician determines best course of action
- Based on patients values and perspectives
- Patient has a right to veto a decision
- Silent is construed as tacit consent
What is meant by physician driven decision making
- Only applies to value neutral decisions
- Care must be taken as they do not necessarily know what a patient deems as value neutral
- Physicians should be aware of possible patient perspectives
