Flashcards in muscle Deck (122):
What is myalgia?
What is myasthenia?
-Weakness of the muscles
What is myocardium?
-The muscle component of the heart
What is myopathy?
-Any disease of the muscles
What is myoclonus?
-Any sudden spasm of the muscles
What is sarcolemma?
-The outer membrane of a muscle cell
What is sarcoplasm?
-Cytoplasm of muscle cells
What is sarcoplasmic reticulum?
-The smooth endoplasmic reticulum of muscle
What are two characteristics of all types of muscle?
What are the three types of muscle?
Which types of muscle are striated?
Which type of muscle is non-striated?
Where are skeletal muscle cells derived from?
-Mesodermally-derived multipotent myogenic stem cells
How do multinucleated skeletal muscle cells develop?
-Myogenic stem cells-> myoblasts which fuse together to form primary myotube with a chain of multiple central nuclei
Why are skeletal muscle nuclei displaced to the periphery?
-The synthesis of actin and myosin filaments displace the nuclei
What are the three types of muscle fibre?
Which muscle fibre is small in diameter?
Which muscle fibre has poor vasculature?
Which muscle cell has a poor store of myoglobin?
Which muscle fibre has the most mitochondria?
What are the contractions like in red skeletal muscle fibres?
-Slow, repetitive, weak
What are the contractions like in white skeletal muscle fibres?
-Fast and strong
How do the red and white skeletal muscle fibres fatigue?
Which skeletal muscle fiber has the most neuromuscular junctions?
Give an example of where red and white skeletal muscle fibres are found
-Red-> postural muscles of the back
-White-> extraocular muscles and the fingers
Describe the structural organisation of muscle units from bone to myofilaments
-Myofilaments (actin and myosin)
How can you tell a fascicle from a muscle fibre?
-The fibre will have multiple nuclei on its sarcolemma
What is epimysium?
-Connective tissue sheath which surrounds the entire muscle
What is perimysium?
-Connective tissue sheath which surrounds fascicles
What is endomysium?
-Connective tissue sheath which surrounds muscle fibres
What is another name for a muscle fibre?
What are the different anatomical arrangements of muscles?
What is a myotendinous junction?
-Where the muscle and tendon meet
How is skeletal muscle arranged at a myotendinous junction?
-The fibres interdigitate with tendon collagen bundles
What two types of skeletal muscle are found in the tongue?
-Extrinsic and intrinsic muscles
What are the extrinsic muscles of the tongue attached to?
-The have insertions into bone and cartilage
What are the functions of the extrinsic tongue muscles?
-To protrude, retract and move the tongue from side to side
What is the function of the intrinsic muscles of the tongue?
-Allow the tongue to change shape but not position
How are nuclei arranged in muscle fibres?
-In rows at the periphery of the cell
What structures does the perimysium contain?
What structures do endomysium contain?
What are myofibrils made up of?
-Myofilaments (thin actin and thick myosin)
How are myofilaments arranged?
-In a strict banding pattern of A and I bands
What shade is the A band of a myofibril?
What shade is an I band of a myofibril?
Where is the Z line located?
-In the centre of an I band
What is a sarcomere?
-A contractional unit from Z line to Z line
What myofilament(s) do the A binds contain?
-Both actin and myosin as they overlap
What myofilament(s) do the I bands contain?
What is the H zone?
-The segment within an A band which only contains myosin
Where is the M line?
-In the centre of an H zone
What is a pneumonic for remembering these bands?
-MHAZI (M is in H in A, Z is in I)
Which bands decrease in size during contraction and which stay the same length?
-I and H zones get smaller
-A remains the same
What happens to the size of the sarcomere during contraction?
What is the structure of an actin myofilament?
-Contains actin, troponin and tropomyosin which form a helix by troponin attaching to tropomyosin which then wraps around actin
What can troponin be used as a marker for?
Describe the structure of myosin
-An individual myosin molecule has a rod-like structure from which two heads protrude
-Each thick filament is made up many myosin molecules in such a way that the 'heads' protrude in a multidirectional way
-The filaments are devoid of myosin heads in the centre of the filament (where there is no overlap)
-The heads extend towards the actin filaments
What is the role of calcium in muscle contraction?
-Influx of calcium into the the muscle fibril
-Binds to troponin causing a conformational change leading to dissociation from the tropomyosin, and causing tropomyosin to change position, exposing the active site of actin
-This allows the myosin head to bind to actin and contraction to begin
Describe the sequence of events in the sliding filament model
-Myosin head attaches to actin filament as ATP is hydrolysed
-Working stroke-> myosin head pivots and bends as it pulls on the actin filment, sliding it towards the M line
-Release of ADP and Pi
-A new ATP molecule binds to myosin causing the myosin cross-bridge to detach
-ATP hydrolysis causes the cycle to repeat
What is the cause of rigor mortis?
-Lack of ATP results in myosin heads staying bound to actin
What is a neuromuscular junction?
-The point at which an axon meets muscle containinf a small terminal swelling of the axon and a synaptic cleft
Describe the mechanism of innervation of muscle and excitation contraction coupling
-Nerve impulse along motor neurone arrives at neuromuscular junction
-Influx of calcium at the synaptic bulb causes the release of acetylcholine
-Acetylcholine diffuses across the synaptic cleft and fuses with nicotinic receptors located on the sarcolemma of the muscle causing depolarisation
-This cause voltage-gated Na+ to open, causing the action potential to enter
-Action potential (depolarisation) spreads along sarcolemma down T tubules and to two sarcoplasmic reticulum
-This causes the opening of voltage gated calcium channels at the terminal cisternae of the two adjacent sarcolasmic reticulum; calcium is released into the sarcoplasm
-Resulting in the sliding filament model
What happens to the calcium after the action poteltial is complete?
-Returned to the sarcoplasmic reticulum by active transport
What is the triad in skeletal muscle contraction?
-One T tubule causes calcium release from two sarcoplasmic reticulum
Describe the histology of cardiac muscle in longitudial section
-Centrally positioned nuclei(one or two per cell)
Why does cardiac muscle contain intercalated discs?
-For electrical and mechanical coupling with adjacent cells
Describe cardiac muscle in transverse section
-Centrally positioned nuclei
-Endomysium bearing rich supply of capillaries
-Some lobular profiles which represent branching
Are myofibrils arranged in bands in cardiac muscle?
-Yes however myofilaments of actin and myosin form continuous masses in cytoplasm
What other structures are present in the cytoplasm of cardiac muscle?
How are intercalated discs associated with Z lines?
-Intercalated discs lay over the Z lines (Z lines become obscured in LM)
Why does cardiac muscle contain many gap junctions?
-For electrical coupling
What is the function of adherens-type junctions in cardiac muscle?
-To anchor the cells and provide anchorage for actin filaments
Where in skeletal muscle are the T tubules located?
-At the A I band junction
Where do T tubules lie within cardiac muscle?
-At the points of intercalated discs and Z lines
What is meant by cardiac muscle having a diad not a triad?
-One T tubule serves on sarcoplasmic reticulum
How does muscle contraction occur in cardiac muscle?
-Same as skeletal muscle except electrical impulse is generated from the heart itself
Where are purkinje fibre located?
-What generates the action potential in the heart and how does this travel to the ventricles?
-Passes to AVN into ventricles
What is the function of the purkinje fibres?
-Transmit the action potential from the AVN to the ventricles to allow rapid conduction enabling ventricles to contract in a synchronous manner
Describe the characteristics of the purkinje fibres
-Extensive gap junctions
Describe the histology of smooth muscles
-Have a central nucleus
-Non striated, no sarcomeres, no T tubules
What is the mechanism of contraction of smooth muscle? (simply)
How do contractions of smooth muscle differ from cardiac and skeletal contractions?
-Require less ATP
Which of the muscle types is capable of being stretched?
What generic stimuli cause smooth muscle to contract?
What conformation do lots of smooth muscle cells make?
-Where is smooth muscle most frequently located?
-The contractile walls of passageways or cavities
eg vascular structures, the GI, respiratory and genitourinary tract
When can smooth muscle become clinically significant?
-Abnormal gut mobility
What are myofibroblast cells?
-Specialised smooth muscle cells involved in wound healing which produce a collagenous matrix but also contract the wound
What are myoepithelia?
-Specialised smooth muscle cells known as satellite cells which form a basketwork around the secretory units of some exocrine glands to help secretion via contraction eg sweat, salivary, mammory and iris
Outline the contraction of smooth muscle
-Thick and thin myofilaments are arrananged diagonally within a cell, spiralling down the long axis
-Smooth muscle contracts in a twisting way
-Requires Ca2+ (binds to CaM, binds to MLCK, phosphorylates myosin-> sliding filament model)
-Intermediate filaments attach to dense bodies in the sarcoplasm
Describe the nature of repair of skeletal muscle
-Can regenerate-> mitotic activity of satellite cells result in hyperplasia after muscle injury
-Satellite cells fuse with existing muscle to increase muscle mass
Describe the nature of repair of cardiac muscle
-Not capable of regeneration
-Following damage fibroblasts invade, divide and lay down scar tissue
Describe the nature of smooth muscle repair
-Retain mitotic activity and form new smooth muscle cells
-Evident in pregnant uterus which undergoes hypertrophy and hyperplasia
Describe the process of skeletal muscle remodelling
-The contractile proteins actin and myosin can increase/decrease in number
-This is known as remodelling and is a continual process
-All the myofilaments can be replaced within two weeks
How does atrophy of skeletal muscle occur?
-Destruction of the myofilaments during remodelling is greater than replacement
How does hypertrophy of skeletal muscle occur?
-Replacement of the myofilaments during remodelling is greater than destruction
What is the cause of hypertrophy in skeletal muscle?
-Increased demand, ie exercise
What happens to skeletal muscle cells when they undergo hypertrophy?
-Increase metabolic demand
-Sarcoplasmic reticulum swelling
-Increased Z line width
-Increased density of T tubules
-Increased contractile proteins
What are the main causes of atrophy?
-Immobility, age, denervation
How long after denervation does re-innervation need to occur for full use?
What increases uptake of acetylcholine at the neuromuscular junction?
-Motor neurone is embedded into the muscle with the muscle having a highly-folded endplate to increase surface area
What terminates acetylcholine in the synaptic cleft?
What causes myasthenia gravis?
-Autoimmune destruction of the endplate ach receptors, specifically by IgG
What does autoimmune destruction of the achR in myasthenia gravis do to the endplate?
-Causes the loss of junctional fold resulting in widening of the synaptic cleft
What are the signs and symptoms of myasthenia gravis?
-Fatiguability and sudden falling
-Drooping upper eyelids (ptosis)
Why does sudden falling occur in myasthenia gravis?
-Muscles fail die to lack of excitation caused by the lack of AchR
Why are the eyes one of the first structures to be effected in myasthenia gravis?
-Have a low amount of muscle and therefore are effected easier
Why do the symptoms in myasthenia gravis fluctuate?
-Based on general state of health
What are the treatment options for myasthenia gravis?
-Acetylcholinesterase inhibitors (pyridostigmine)
How can acetylcholinesterase inhibitors treat myasthenia gravis?
-Ach not as quickly broken down so it can stimulate muscles for longer
How can plasmapheresis treat myasthenia gravis?
-Removal of specific IgG from serum
How can a thymectomy treat myasthenia gravis?
-Thymus is producing abnormal lymphocytes, removal will stop production
How does botulism disrupt the neuromuscular junctions?
-Reduces acetylcholine release and thus decreases excitation of muscles
How does organophosphate poisoning disrupt neuromuscular junctions?
-Decreased production of acetylcholinesterases; ach stays active for longer; overexcitation of muscles
What type of disorder is muscular dystrophy?
How is muscular dystrophy characterised?
-By progressive muscle weakness and wasting
What is the cause of muscular dystrophy?
-Complete absence of the protein dystrophin (protein for stength) causing the muscles to tear apart upon contraction
What is the consequences of muscle fibres tearing?
-The enzyme creatine phosphokinase is liberated into the serum causing calcium to enter the cell resulting in necrosis
-Pseudohypertrophy (swelling) occurs before fat and connective tissue replace muscle fibres
Outline the pathophysiology of malignant hyperthermia
-Large quantities of calcium released from sarcoplasmic reticulum of skeletal muscle due to altered calcium channel gated kinetics so ca2+ is not taken back up into SR
-Causes hypermetabolic state
-Sustained Ca2+ elevation allows excessive stimulation of glycolytic metabolism
-Causes respiratory and metabolic acidosis
-Causes rigidity due to all muscles contracting at once
-High heat generation due to hypermetabolic state
What type of disease and what are triggering agents of malignant hyperthermia?