Session 8: Muscle Flashcards
What is the sarcolemma?
The outer membrane of a muscle cell
What is the sarcoplasm?
The cytoplasm of a muscle cell
Muscle tissue take on which two forms?
Striated muscle
Non-striated muscle
What two types of muscle are striated?
Skeletal muscle
Heart muscle
What muscle type is non-striated?
Smooth muscle
What are the three fibre types present in skeletal muscle?
Red
Intermediate
White
Skeletal muscle forms what connections?
Fascicle bundles
Tendons
Cardiac muscle forms what connections between cells?
Junctions join cells end to end
Smooth muscle forms what connections?
Connective tissue
Gap Junctions
Desmosome-type Junctions
Skeletal muscle is under what nervous control?
Somatic motor neurons
Voluntary control
Cardiac muscle is under what nervous control?
Intrinsic rhythm
Involuntary autonomic modulation
Smooth muscle is under what kind of control?
Involuntary
Autonomic
Intrinsic activity
Local stimuli
What kind of power is achieved through skeletal muscle contraction?
Rapid, forceful
What kind of power is achieved through cardiac muscle contraction?
Lifelong, variable rhythm
What kind of power is achieved through smooth muscle contraction?
Slow, sustained (rhythmic)
How do red filaments compare to white filaments in skeletal muscle in relation to their diameter, vascularisation and innervation?
Red filaments are smaller in diameter
Red filaments have a rich blood compared to white filaments
Red filaments have fewer neuromuscular junctions than white filaments
Red filaments have how many mitochondria relative to white filaments?
Red filaments have numerous mitochondria compared to white filaments
Red filaments have how much myoglobin compared to white filaments?
Red filaments have a rich supply of myoglobin compared to the white filament’s poor supply
What kind of contraction is achieved through contraction of red filaments relative to white filaments (Speed, strength)?
Red: Slow, repetitive, weaker
White: Faster, stronger
What enzymes are red skeletal fibres rich in?
Oxidative enzymes
What enzyme (s) are white skeletal fibres rich in?
ATPase
Myoglobin is present in which of the three muscle types?
Skeletal and cardiac, not smooth
What is myoglobin?
A red protein containing haem, which functions as an oxygen storing molecule, providing oxygen to working muscles
Under what conditions does haemoglobin especially give oxygen up to myoglobin?
Lowered pH (acidic conditions)
The individual muscle fascicles are wrapped in what?
Perimysium
The individual muscle fibres are wrapped in what?
Endomysium
What is the name of the layer of connective tissue that covers the entire muscle?
Epimysium
What are the 7 ways in which skeletal muscle can be arranged?
Circular Convergent Parallel Unipennate Multipennate Fusiform Bipennate
Extrinsic muscles of the tongue are responsible for what?
Protruding, retracting and moving the tongue from side to side
Intrinsic muscles of the tongue are responsible for what?
Allowing the tongue to change shape
Do extrinsic muscles of the tongue attach to bone?
Yes
Do intrinsic muscles of the tongue attach to bone?
No
What accounts for the mobility of the tongue?
Plasticity and strength of the connective tissue
Multidirectional orientation of the muscle fibres
Skeletal muscles of the tongue often terminate by what?
Interdigitation with collagen and ECM of surrounding connective tissues
What is a muscle fibre?
A striated muscle cell
What is the sarcomere?
What does it consist of?
The structural unit of a myofibril in striated muscle
Segment from Z line to Z line
Made up of actin and myosin filaments
What are the A bands of the sarcomere?
How do they appear histologically?
The thick myosin filaments
They form the dark bands
What are the I bands of the sarcomere?
How do they appear histologically?
The thin actin filaments with no overlap
They form the light bands
I for “Isotropic’’
What is the M line of the sarcomere?
How do they appear histologically?
The middle of the sarcomere
What is the H zone ad how does this appear histologically?
The H band or zone are the non-overlapping myosin filaments
What are the Z lines?
How do they appear histologically?
The end of each sarcomere, attaching one sarcomere to the next
They appear as dark bands on histology
Which molecules come together to form the thin filaments of skeletal and cardiac muscle?
Actin
Tropomyosin
Troponin complex
Troponin assays are a useful tool for diagnosing what?
Cardiac Ischemia
The troponin complex is made up of what?
Troponin I
Troponin C
Troponin T
Troponin is released from ischaemic cardiac muscle within what time frame? And it must be measured within what time frame?
Released within: An hour
Measured within: 20 hours
True of false: Quantity of troponin is proportional to the degree of damage
False, not necessarily
Which enzyme did troponin supersede as a marker for myocardial infarction?
Creatine Kinase (CK)
True or false: The amount of CK is proportional to the infarct size
True
What is a major disadvantage of using CK as a marker for ischaemic cardiac muscle?
The enzyme is also released into the blood by damaged skeletal muscle and brain, not necessarily specialised to damage to cardiac muscle
Name three things that can result in a rise in plasma CK
Intramuscular injection Vigorous physical exercise A fall (especially in the elderly) Muscular dystrophy Acute kidney injury Rhabdomyolysis (severe muscle breakdown)
Describe the structure of an individual myosin molecule
Rod-like structure with two heads that protrude
Describe the structure of a myosin filament
Many individual myosin molecules whose heads protrude at opposite ends of the filament
Describe the structure of the actin filament
The actin filament forms a helix around which tropomyosin coils around. The troponin complex is attached to the tropomyosin molecule
In the centre of the sarcomere, the thick myosin filaments are lacking what?
Myosin heads
When in its high energy configuration, what is the myosin head doing?
Attaching to the actin myofilament forming a cross bridge
What is the importance of ionic calcium in muscle contraction?
Increased ionic calcium binds to troponin C of troponin which leads to a conformational change and moves tropomyosin away from myosin binding sites on actin, which means myosin can then bind
What occurs during the “working stroke” of muscle contraction?
The myosin head pivots and bends as it pulls on the actin filament, sliding towards the M line
What happens when the myosin head is in “low-energy” configuration?
The new ATP attaches to the myosin head and the cross bridge detaches
When ATP is hydrolysed into ADP and P what happens during contraction?
The myosin head detaches from actin and cocks back to starting position
What is essential for the releasing of tropomyosin’s blockage of actin’s active sites?
Ionic calcium
What is the neuromuscular junction?
Small terminal swellings of the axon that contain vesicles of acetylcholine (Ach)
What happens during initiation of contraction of skeletal muscle?
The nerve impulse travels along the motor neuron axon and arrives at the neuromuscular junction
What happens at the NMJ when a nerve impulse reaches it?
It releases acetylcholine (Ach) into the synaptic cleft causing local depolarisation of the sarcolemma
During skeletal muscle contraction, what happens once the sarcolemma is depolarised by Ach release?
Voltage-gated Sodium channels open and sodium ions enters the cell
General depolarisation of the sarcolemma and into T tubules causes what?
The voltage sensor proteins of the T tubule membrane to change their conformation and activates Ca2+ release from terminal cisternae into the sarcoplasm
Once Ca2+ is released into the sarcoplasm, what happens?
Ca2+ binds to troponin C (TnC) and the contraction cycle is initiated
Where is ionic calcium then return to following the initiation of the contraction cycle during muscle contraction?
The terminal cisternae of sarcoplasmic reticulum
What is muscle fibrillation?
The contraction of individual muscle fibres (muscle cells)
What is muscle fasciculation?
The contraction of whole muscle fascicles, often under the innervation of a single motor neurone (As in motor neurone disease)
What are absent from cardiac muscle that are distinctly present in skeletal muscle?
What are formed instead?
Myofibrils
Myofilaments of actin and myosin form continuous masses in the cytoplasm
____________ and _____________ penetrate through the cytoplasm between myofilaments
Mitochondria
Sarcoplasmic reticulum
Longitudinal sections of cardiac muscle fibres show what?
Striations
Centrally positioned nuclei (1 or 2 per cell, no more than 2)
Intercalated discs
Branching
Transverse sections of cardiac muscle fibres show what?
Centrally positioned nuclei
Endomysium
Lobular profiles of branching fibres
Cardiac muscle fibres have what instead of the Z lines that are seen in skeletal muscle?
Intercalated discs
Where are the intercalated discs of cardiac muscle?
What two types of junctions do they have?
Where the cells meet, end to end
Gap junctions and adherens- type junctions
What is the purpose of the gap junctions at the intercalated discs?
Electrical coupling between cells to ensure that the cells contract together
What is the purpose of the adherens- type junctions at the intercalated discs?
Provide anchorage for actin filaments
What is different with regard to the arrangement of T-tubules in cardiac muscle compared to skeletal muscle?
The T-tubules of cardiac muscle are in lie with the Z disc rather than the A-I band junction
What is a diad? Are these found in skeletal or cardiac muscle?
The point at which the t-tubule meets a single terminal cisterna of the sarcoplasmic reticulum. Cardiac muscle
Where is the diad located in the cardiac myocyte?
At the Z line
What is a t-tubule?
A “transverse” tubule that is a deep invaginated of the sarcolemma
What is a triad?
The point at which the t-tubule meets a terminal cisterna of the sarcoplasmic reticulum on either side of it. They are found in skeletal muscle
Where is the triad typically located in skeletal muscle?
Along the A-I junction
What is the purpose of triads and diads in skeletal and cardiac muscle fibres?
Allows close association of t-tubules and sarcoplasmic reticulum which permits the release of ionic calcium into the sarcoplasm and subsequent muscle contraction
What is cell hypertrophy?
Enlargement of individual cells without an increase in number of cells
What is cell hyperplasia?
An increase in the number of cells due to mitosis
What is heart atrophy?
When the heart is smaller than the normal heart due to smaller individual cells that make up the heart
What is heart hypertrophy?
When the heart is bigger than the normal heart due to the larger individual cells that make up the heart
What are natriuretic peptides?
Peptide hormones that are synthesised by the heart, brain and other organs in response to atrial and ventricular distension, usually in response to heart failure
What are the main physiological actions of natriuretic peptides?
To reduce arterial pressure by decreasing blood volume and systemic vascular resistance
What is ANP?
When would it be released?
Atrial natriuretic peptide
When the atria are distended
Do normal hearts secrete extremely small or large amounts of ANP?
Extremely small amounts
In what patients would you find elevated levels of ANP?
Patients with left ventricular (LV) hypertrophy and mitral valve disease
(During hypervolemic states which occur in congestive heart failure)
What is BNP?
Where is it synthesised?
Brain-type natriuretic peptide
In the ventricles and the brain
Both BNP and NT-pro-BNP are sensitive, diagnostic markers for what?
Heart failure in patients
In response to cardiac distension, ANP causes what?
The kidney to raise glomerular filtration rate therefore excreting more sodium and water to reduce blood volume. This leads to reduction in cardiac output and systemic blood pressure
In response to cardiac distension, BNP causes what?
Vasodilation which causes blood pressure to lower as well as increased natriuresis and diuresis (sodium and water excretion)
Natriuretic peptides serve as a counter-regulatory system for what?
The renin-angiotesin-aldosterone system
In the heart, action potentials are generated where?
In the sinoatrial node (SA node)
Action potentials pass from the SA node to where?
To the atrioventricular node (AV node)
The impulses pass from the AV node to where?
Ventricles
What are the specialised myocardial cells that carry impulses to the ventricular muscle called?
Purkinje fibres
Purkinje fibres relatively are small or large cells?
Large
Purkinje fibres are abundant in what?
Glycogen
Purkinje fibres have sparse what?
Myofilaments
Purkinje fibres have extensive what between cells?
Gap junctions
Do purkinje fibres conduct action potentials more or less rapidly than cardiac muscle fibres?
More rapidly (3-4m/s compared to 0.5m/s)
What does the rapid conduction of action potentials by purkinje fibres enable?
Contraction of ventricles in a synchronous manner
Smooth muscle cells are described as what shape? Where is the nucleus located?
Fusiform (Spindle-shaped)
In the centre of the cell
Is smooth muscle striated?
No
Does smooth muscle contain sarcomeres?
No
Does smooth muscle contain T tubules?
No
Smooth muscle contraction relies on what?
Actin-myosin interactions
Why do we not see the regular banding of actin and myosin filaments in smooth muscle?
They are not lined up in a regular way
Compared to other muscle types, smooth muscle contraction is what?
(In relation to speed, duration and energy-requirement)
Slower
More sustained
Requires less ATP
Smooth muscle can remain contracted for _____ or ______
hours
days
Is smooth muscle capable of being stretched?
Yes
Smooth muscle responds to stimuli in the form of what?
Nerve signals
Hormones
Drugs
Local concentrations of blood gases
Smooth muscle can form _____, bundles or ________ containing thousands of cells.
Sheets
Layers
Smooth muscle often forms contractile walls of passageways or cavities in order to do what?
Modify the volume passing through them
Where might you find smooth muscle?
Vascular structures
The gut
The respiratory tract
The genitourinary system
Is smooth muscle voluntary or involuntary?
Involuntary
When smooth muscle contracts with pathological affect it can cause what conditions?
Asthma High blood pressure Atherosclerosis IBS Detrusor muscle instability
Modified smooth muscle can occur singly as which two cell types?
Myoepithelial cells
Myofibroblast cells
What are myoepothelial cells?
Stellate (Star-shaped) cells that form a basketwork around secretory units of some exocrine glands (e.g.sweat,salivary,mammary)
Myoepithelial cells found in the ocular iris contract to do what?
Dilate the pupil
What are myofibroblasts?
Sites of wound healing that produce collagenous matrix but also contract (they contain actin and myosin)
In what processes are myofibroblasts prominent?
Wound contraction
Tooth eruption
Most smooth muscles are innervated by what?
The autonomic nervous systeem
What are varicosities?
Enlarged regions of autonomic nerves that release neurotransmitters into a wide synaptic cleft
How are smooth muscle thick and thin filaments are arranged?
Diagonally within the cell spiralling down the long axis
How do smooth muscle cells contract?
In a twisting way
What do intermediate filaments of smooth muscle attach to?
Dense bodies
Where are dense bodies found in smooth muscle?
Throughout the sarcoplasm and occasionally anchor to the sarcolemma
Are calcium ions necessary for contraction of smooth muscle?
Yes
What is the nature of repair possible in a mature skeletal muscle?
Cells cannot divide but the tissue can regenerate by mitotic activity of satellite cells
Hyperplasia follows muscle injury
How does skeletal muscle undergo hypertrophy?
By satellite cells fusing with existing muscle cells to increase mass
What is the nature of repair possible in mature cardiac muscle?
Cardiac muscle is incapable of regeneration
What happens to cardiac muscle following damage?
Fibroblasts invade, divide and lay down scar tissue
What is the nature of repair possible in mature smooth muscle?
Smooth muscle cells retain their mitotic activity and can form new smooth muscle cells
Where is smooth muscles ability to form new smooth muscle cells particularly evident?
In the pregnant uterus where the muscle wall becomes thicker by hypertrophy and by hyperplasia
