Neuro Physiology - Muscle Structure and Function Flashcards
Describe the structure of the sarcomere
IMAGE
The contractile unit is called the sarcomere, with thin actin and thick myosin filaments.
Tropomyosin helps regulate the excitation-contraction coupling
Z line: Actin junction between sarcomeres
M line: The myosin junction at the middle of the sarcomere, with no cross-links
A (Anisotropic) band: The length of the thick myosin filament
I (Isotropic) band: Area of actin filaments that are not overlapping with myosin
H band: The area within the A band, where myosin filaments are not overlapping actin
How does skeletal muscle contract?
Image
Excitation-Contraction coupling converts an action potential into mechanical force.
The AP spreads across the post-synaptic membrane and down into the T-tubule system, introducing a conformational change in the co-located DHPr (Dihydropyridine) receptor.
The DHPr interacts with the ryanodine receptor on the sarcoplasmic reticulum, triggering calcium release, which triggers further calcium-induced calcium release
Calcium binds to the C subunit of troponin (I subunit is bound to actin, and T subunit to tropomyosin), next to the myosin binding sites on actin, inducing a conformational change, rotating tropomyosin, and exposing myosin binding sites.
Myosin binds to actin, pivoting the myosin heads and pulling the strands past each other.
Timed inactivation of calcium channels causes calcium levels to drop, and the NaCa exchange pump moves calcium back into the sarcoplasmic reticulum and out of the cytoplasm.
ATP binds to myosin, releasing the actin. ATP is then hydrolysed, activating the myosin head for the next cycle, and the low calcium concentration enables tropomyosin to rotate back over the actin binding site.
Discuss the monosynaptic reflex
Stretch reflex to maintain posture
The muscle spindle detects strech via the afferent nerve, travelling to the dorsal root and synapsing both with interneurons and directly with the efferent motor neuron in the anterior horn.
The efferent motor neuron triggers contraction of the stretched muscle
Interneurons inhibit the antagonistic group of muscles, to facilitate more efficient contraction (reciprocal innervation)
Discuss the structure and function of a muscle spindle
Muscle spindles contain two types of stretch detectors (intrafusal fibres), and are innervated by gamma motor neurons. Split into:
Chain fibres - static fibres that detect degree of stretch
Nuclear bag fibres - dynamic fibres that detect changes in degree of stretch
Extrafusal fibres are the main muscle fibres and are innervated by alpha motor neurons
When the spindle is stretched, extrafusal fibres contract to maintain the length of the muscle, opposing the stretch.
Tremors generally occur at around 10Hz as this is the frequency of this feedback loop
What is purpose of the golgi tendon organ?
Structures located in the tendons of skeletal muscle, modulating muscle tension rather than muscle length.
They are located in-series between muscle fibres, tendons and bone, and as such, transmit the force generated.
A negative feedback mechanism leads to muscle relaxation, and causes contraction of antagonistic muscle groups.
Compare and contrast the different types of muscle
Smooth Muscle
Mitochondria: Very few
Fibre structure: Sheets of cells
Response type: Graded contraction response
Innervation: Autonomic
Pacemaker function: Slow
Calcium binding site: Calmodulin
Capable of tetany: Yes
Cardiac Muscle
Mitochondria: Many
Fibre structure: Branched cylinders with gap junctions (functional syncytium)
Response type: All or nothing
Innervation: Autonomic
Pacemaker function: Fast
Calcium binding site: Troponin
Capable of tetany: No
Skeletal Muscle
Mitochondria: Few
Fibre structure: Long cylindrical fibres with visible striations
Response type: All or nothing
Innervation: Voluntary
Pacemaker function: None
Calcium binding site: Troponin
Capable of tetany: Yes
What are the clinical features of lower and upper motor neuron injury?
Lower motor neurons innervate muscle directly, while upper motor neurons are found higher up in the motor pathywa, either in the cortex, extrapyramidal or cerebellar tracts.
Lower motor neuron:
Flaccid paralysis, fasciculation, absent reflexes, muscular atrophy
Upper motor neuron:
Hypertonia, spastic paralysis (greater risk of HyperK with Suxamethonium)
What happens in muscular dystrophy?
Dystrophin is a scaffold protein that connects myofibrils to the membrane of the sarcolemma, giving muscles their shape and structural integrity
Duchenne muscular dystrophy is caused by the absence of dystrophin
Becker’s muscular dystrophy is an X-linked recessive mutation of the very large dystrophin gene, resulting in progressive weakness and heart failure
