12. Skeletal Muscle Disease TBL Flashcards
How are individual sarcomeres protected from excessive stretching?
A. Myosin does not interact with active sites on the edge of F-actin filaments.
B. The muscle spindle protects a skeletal muscle from overstretching.
C. Titin holds the sarcomere intact.
D. Tropomyosin holds the sarcomere intact.
E. Z-disks protect sarcomeres from overstretching.
C. Titin holds the sarcomere intact.
Titin is a large spring-like protein that holds the structure of the sarcomere in place as it spans the sarcomere between the Z disk and the M line. It contains an elastic domain with repeats of the amino acid sequence PEVK (Pro, Glu, Val, Lys) that is important for muscle tension, where it uncoils storing energy and recoils during relaxation.
Type I muscle fibres fatigue slower than Type II muscle fibres. Why is that?
A) Type I muscle fibres don’t carry out glycolysis.
B) Type I muscle fibres have a high concentration of calcium ions.
C). Type I muscle fibres have a high concentration of creatine phosphatase.
D). Type I muscle fibres have a large number of mitochondria.
E). Type I muscle fibres have a reduced concentration of calcium ions.
D). Type I muscle fibres have a large number of mitochondria.
Type I muscle fibres have a larger number of mitochondria compared to type II fibres but smaller endoplasmic reticulum (due to their smaller size) so less calcium ions compared to the type II fibres. This affects the speed of contraction and that is why type I muscle fibres are slower to contract. However, the increased number of mitochondria, increased concentration of myoglobin and increased vascularisation ensures that type I muscle fibres produce up to 32 molecules of ATP through glycolysis and the TCA cycle and they fatigue slower. The amount of creatine phosphatase does differ substantially between the two types of fibres.
What is the role of calcium ions released from the sarcoplasmic reticulum in skeletal muscle fibres?
A). Calcium ions binding on tropomyosin is essential for cross bridge formation.
B). Calcium ions binding on troponin C is essential for cross bridge formation.
C). Calcium ions binding on titin is essential for sarcomere contraction.
D). Calcium ions binding on troponin T is essential for sarcomere contraction.
E). Calcium ions binding on F-actin is essential for myosin interaction.
B). Calcium ions binding on troponin C is essential for cross bridge formation.
Calcium ions released from the sarcoplasmic reticulum bind on Troponin C, leading to a conformational change for the troponin complex, pulling tropomyosin away from the F-actin active sites and making those accessible to myosin for cross bridge formation.
Which of the following best describes hemiparesis?
A). All four limbs are weakened.
B). Both arms are weakened. Usually occurs in spinal cord damage at the lower level.
C). Both legs are weakened. Usually occurs in spinal cord damage at the lower level.
D). One arm and one leg on either side of the body are weakened.
E). One leg or one arm is weakened.
D). One arm and one leg on either side of the body are weakened.
Monoparesis: One leg or one arm is weakened.
Paraparesis: Both legs are weakened usually occurs in spinal cord damage at the lower level.
Hemiparesis: Weakness of one entire side of the body, this commonly occurs in strokes affecting the upper motor neurons.
Tetraparesis/Quadriparesis: All four limbs are weakened due to cervical cord or spinal cord damage at a higher level.
Which condition is characterised by absence of dystrophin?
A). Duchenne muscular dystrophy
B). Malignant hyperthermia
C). Myasthenia gravis
D). Rhabdomyolysis
E). Spinal muscular atrophy
A). Duchenne muscular dystrophy
Absence of dystrophin is responsible for two disorders that cause progressive destruction of muscle, Duchenne muscular dystrophy and Becker’s muscular dystrophy. Complete absence of dystrophin impairs the integrity of the sarcolemma’s membrane, rendering the membrane susceptible to mechanical damage. Presence in a reduced altered form can cause the milder Becker’s variant (BMD). In BMD, the abnormal dystrophin preserves enough function to slow down the progress of the illness.
In malignant hyperthermia, myasthenia gravis, rhabdomyolysis, and spinal muscular atrophy dystrophin is not affected.
On examining a patient’s muscle strength (MRC muscle grading scale) there is active movement against gravity with some resistance. Which grade is this?
Grade 1
Grade 2
Grade 3
Grade 4
Grade 5
Grade 4
The MRC grading for muscle strength is:
0 – No movement
1 – Flicker of movement only
2– Active movement only with gravity eliminated
3 – Active movement against gravity but not resistance
4 – Active movement against gravity with some resistance
5 – Active movement against gravity with full resistance
Two medical students successfully pass their MIS module and indulge in a celebratory high-five. During this voluntary movement:
A). The descending motor signal passes through the dorsal root ganglion.
B). The descending motor signal synapses in the anterior horn.
C). The signal is carried via a reflex arc.
D). The upper motor neuron exits the spinal cord synapsing directly with skeletal muscle.
E). The upper motor neuron passes down the spinothalamic tract.
B). The descending motor signal synapses in the anterior horn.
Voluntary skeletal muscle contract originates in the motor cortex, passes down the upper motor neuron via the descending fibres of the corticospinal tract. The upper motor neuron synapses with the lower motor neuron in the anterior (ventral) horn.
1 - The dorsal root ganglion receives sensory information from the proprioceptors and conveys it to the ascending tracts and relay neurons.
3 – The voluntary contraction of skeletal muscle is not a reflex arc as described here.
4 – The upper motor neuron does not exit the spinal cord.
5 – The spinothalamic tract does not carry descending motor signals.
Preparing for a shoulder operation, you palpate the tip of the coracoid process, an important surface landmark. Which structures are related to this?
A). The axillary nerve
B). The musculocutaneous nerve
C). The shared tendon of triceps and coracobrachialis.
D). The subscapularis muscle insertion.
E). The three parts of the brachial artery.
B). The musculocutaneous nerve
The musculocutaneous nerve enters the conjoint tendon 5-8cm distal to the tip of the coracoid. It can be injured if the conjoint tendon is vigorously retracted.
Option 1 – The axillary nerve passes through the quadrangular space inferior to the teres minor. It is not in proximity to the coracoid process.
Option 3 – The conjoint tendon is made up of the coracobrachialis and biceps brachii, not triceps brachii
Option 4 – The subscapularis tendon is part of the rotator cuff inserting on to the lesser tuberosity of the humerus.
Option 5 – Pectoralis minor denotes the 3 parts of the axillary artery, not brachial artery.
A 25-year-old basketballer sustains a massive rotator cuff tear during a match. What is the main role of the rotator cuff muscles of the shoulder?
A). They create an enclosed synovial space.
B). They deepen the glenohumeral joint to improve stability.
C). They provide dynamic stabilisation of the glenohumeral joint.
D). They provide main power during active shoulder movements.
E). They provide static stabilisation of the glenohumeral joint.
C). They provide dynamic stabilisation of the glenohumeral joint.
Dynamic stabilisers of the glenohumeral joint create a stable fulcrum.
Option 1 – The joint capsule encloses the whole joint providing a closed synovial cavity.
Option 2 – Describes the glenoid labrum
Option 4 - The main power of movement is provided by the extrinsic muscles of the shoulder.
Option 5 - The static stabilisers of the glenohumeral joint are the glenoid labrum, the glenohumeral ligaments, the capsule and the glenoid.
Myasthenia gravis (MG)
Which single pathological process most accurately explains this condition?a) Demyelination
b) Myopathy
c) Neuropathy
d) Neuronal degeneration
e) Space occupying lesion
b) Myopathy – the scenario describes fatigue related weakness affecting several muscle groups. This weakness is due to muscle fibre dysfunction.
MG is an autoimmune condition characterised by the production of autoantibodies that affect the neuromuscular junction (NMJ).
Which site of the neuromuscular junction is affected in myasthenia gravis?
Post-synaptic receptor sites
In those signals which are conducted, for muscles to contract, a specific ion must be released from the terminal cisterne of the sarcoplasmic reticulum. What is this ion?
Ca2+
The MG patient asks you to explain what is “going on” inside her body to be causing these symptoms, how might you explain this to her?
Your immune system normally makes antibodies that fight infections and protect you from harm. However, in myasthenia gravis, your immune system is making antibodies that attack some of the body’s own tissues. In your case, these antibodies are interfering with the transmission signals between the nerves and the muscles. Not all signals are getting through so your muscles are finding it harder to carry out the jobs you want them to.
The definitive diagnosis of an autoimmune condition can often take considerable time to achieve. Suggest some patient-, clinician- and institutional-related factors that may contribute to this.
Patient – broad spectrum of symptoms that may appear unrelated, so these issues are put off or only addressed indirectly e.g., inflammatory joint pain managed solely with NSAIDs.
Multiple visits to health care professionals without resolution of symptoms may cause lack of faith in system. You can also consider that patients are complex and have many parts to their identity that play into what their healthcare experience is like. This includes factors such as race, ethnicity, sex, gender identity, and dis/ability. These elements intersect and this has been shown to correlate with experiences of discrimination in healthcare, variable clinical outcomes, and various degrees of engagement between patients and healthcare professionals.
Clinician – broad differential which takes time to narrow down with lots of available investigations which may be unhelpful.
Time pressures.
Potential lack of knowledge. However, consider how many cases of depression a GP faces with the presenting complaint of fatigue vs an autoimmune condition.
Institutional - Patients are typically “allowed” only one presenting complaint. May have to rebook to discuss other symptoms. Consider the case above and how different the direction of the consultation could take if the patient presented with fatigue vs swallowing difficulties.
Autoimmune conditions are more frequently observed in women, and it is recognised that women often have to wait longer than men for a diagnosis as it is.
Access to secondary care is inconsistent and can take a long time.
What are the overall health priorities of the NHS and how do autoimmune conditions feature vs cancer for instance?
Which tumour is MG associated with?
a) Chondrosarcoma
b) Lipoma
c) Myosarcoma
d) Osteosarcoma
e) Thymoma
e) Thymoma
