Muscle Flashcards
1
Q
What is a SARCOLEMMA?
A
Outer membrane that surrounds the muscle cell (also includes the basal lamina and reticular fibres)
2
Q
What are the 3 types of muscle?
A
- Voluntary SKELETAL
- Involuntary CARDIAC
- Involuntary SMOOTH
3
Q
Describe how skeletal muscles are stimulated to contract
A
- Stimulus from somatic motor neurones to motor end plate
- Action potential triggers release of ACh which diffuses across synapse and binds to sarcolemma causing DEPOLARISATION, which spreads down T tubules and triggers the release of Ca2+ from SR, which leads to contraction
4
Q
Describe the formation of skeletal muscle fibres from myogenic stem cells
A
- Mesenchymal multipotent cells give rise to MYOBLASTS
- Myoblasts fuse synchronously forming a primary MYOTUBE with a chain of central nuclei
- Nuclei displaced to periphery following formation of myofilaments
5
Q
How does the formation of muscle fibres from myoblasts in cardiac muscle differ from that of skeletal muscle?
A
- NO FUSION OF MYOBLASTS
- Gap junctions formed at early stage
6
Q
Describe the characteristics of skeletal muscle
A
- Cell length ranges from 1mm-20cm (width 10-100μm)
- STRIATED (darker A band, lighter I band)
- Long parallel cylinders with multiple peripheral nuclei
- Muscle fibres bundle into fascicles surrounded by perimysium (bundle to form muscle tissue surrounded by epimysium)
- T tubules in line with A-I band overlap junction
7
Q
Describe the action of skeletal muscle
A
- Rapid forceful contractions for movement of skeleton (joined at tendons)
- Controlled by somatic motor neurones (VOLUNTARY)
8
Q
How are muscle cells arranged and joined in cardiac muscle?
A
- Muscle fibres are BRANCHED
- Joined end to end by INTERCALATED DISCS (composed of gap junctions and adherent-like structures called desmosomes)
9
Q
What is the purpose of INTERCALATED DISCS in cardiac muscle?
A
- GAP JUNCTIONS allow coupling of electrical impulses between cells
- ADHERENT-LIKE structures (desmosomes) anchor cells and actin filaments
10
Q
What is the purpose of the ENDOMYSIUM?
A
- Runs between muscle fibres
- Contains blood vessels and nerves
11
Q
Describe the ultrastructural arrangement of cell components in cardiac muscle fibres
A
- IRREGULAR arrangement of myofilaments mitochondria and SR
- Mitochondria and SR penetrate through myofibrils
- Actin and myosin form CONTINUOUS MASSES in sarcoplasm
12
Q
Explain how the arrangement of muscle fibres and connective tissue assist in the mobility of the tongue
A
- Connective tissue (TENDONS) terminate to muscle via interdigitation in multiple directions
- Multidirectional orientation of muscle fibres and plasticity/strength of tendons allows for mobility
13
Q
Describe the regions of the contractile unit of the muscle fibre
A
- A band (region of myosin with some overlapping actin)
- I band (region of only actin)
- H zone (region of only myosin)
- Z disc (runs down centre if I band)
- M line (runs down centre of H zone)
14
Q
What is a SARCOMERE?
A
- Contractile unit of muscle fibre
- Distance between two Z discs
- Shortens during contraction
15
Q
Explain how the SARCOMERE changes during contraction
A
- Z discs move closer together - SHORTENS
- H zone and I band also shorten
- A band remains unchanged
16
Q
Describe the ultrastructure of skeletal muscle
A
- Myofilaments bundled together into myofibrils, each surrounded by sarcoplasmic reticulum
- Abundant mitochondria arranged in rows between myofibrils
- Nucleus displaced to periphery
17
Q
Explain how TROPONIN can be used as a marker for cardiac iscaemia
A
- Released from iscaemic cardiac muscle within 1 hour
- Detection can indicate damage to heart muscle (possibly MI)
- Must measure within 20 hrs
- More useful that using enzyme assays
- QUANTITY OF TROPONIN IS NOT NECESSARILY LINKED TO DEGREE OF DAMAGE
18
Q
Describe the structure of an actin filament
A
- Actin helix
- TROPOMYOSIN wrapped around each helix, reinforcing them (blocks myosin binding sites)
- TROPONIN complexes attachted to tropomyosin
19
Q
Explain the arrangement of myosin in the contractile unit
A
- Centre of contractile unit is devoid of myosin heads (M line)
- Myosin heads protrude at opposite ends of the filament
- Myosin heads extend towards actin filaments in regions of potential overlap
20
Q
Explain the role of Ca2+ ions in the contraction of muscle
A
- Bind to TnC of troponin causing a conformational change of the protein
- Conformational change DISPLACES tropomyosin, exposing the myosin binding site on actin
- Myosin heads can bind and initiate the power stroke
21
Q
Explain the sequence of events involved in the SLIDING FILAMENT MODEL during contraction of muscle
A
- Ca2+ binds to TnC of troponin, which changes shape and moves tropomyosin, exposing the myosin binding site on actin
- Myosin head binds to actin and bends, pulling the actin filament towards the M line - this is know as the POWER STROKE (ADP and Pi are released)
- ATP binds to myosin head, breaking cross bridge
- ATP –> ADP and Pi and myosin head cocks back (resets)
22
Q
Explain the sequence of events that occur at the NEUROMUSCULAR JUNCTION which lead to contraction of muscle
A
- Action potential arrive at presynaptic knob and stimulates vesicles containing ACh to fuse with the presynaptic membrane
- ACh released into neuromuscular cleft and diffuse across to bind with receptors on Na+ channels on motor end plate
- Na+ channels open causing DEPOLARISATION, which passes down T tubules
- Stimulates release of Ca2+ from SR which leads to contraction
23
Q
Explain how RIGOR MORTIS occurs shortly after death
A
- Lack of ATP so myosin heads remain attached to actin (cross bridges cannot be broken)
- Muscle remains in contracted state
24
Q
Describe the arrangement of T tubules and sarcoplasmic reticulum within striated muscle fibres
A
- SR wraps around myofibrils with T tubules running in line with A-I band junction (skeletal) or in line with Z discs (cardiac)
- T tubules in contact with terminal cisternae of SR
- T tubules and SR arranged in TRIAD arrangement in skeletal muscle and DIAD arrangement in cardiac muscle
25
State the main features of cardiac muscle that distinguish it from other muscle types
- STRIATED like skeletal muscle
- BRANCHING of muscle fibres
- INTERCALATED DISCS
- Centrally positioned nuclei (1 or 2 per cell)
26
What is the role of the sarcoplasmic reticulum (SR)?
- Specialised smooth endoplasmic reticulum of muscle cells
- Regulates Ca2+ ion concentrations in sarcoplasm of striated muscle
- Stimulated to release Ca2+ by T tubules receiving wave of depolarisation from motor end plate
27
Explain the contraction exhibited by cardiac muscle and how this is controlled
- Spontaneous, RHYTHMIC
- Action potentials generated by SAN (sinoatrial node) spread to AVN, which sends a wave of excitation down Purkinje fibres, stimulating ventricular contraction
- FREQUENCY of action potentials controlled by MEDULLA OBLONGATA in brain (control of heart rate)
28
Describe the structure of the Purkinje fibres of the heart muscle
Large (light staining) cells with:
- Abundant glycogen
- Few myofilaments
- Extensive gap junction sites
29
Explain the role of the Purkinje fibres in the contraction of the heart muscle
- Carries impulses from atrioventricular node (AVN) down to apex of heart and up the walls of the ventricles
- Depolarisation of cardiac muscle fibres stimulates contraction of ventricles in a synchronous manner
- Speed of conduction is very quick (3-4m/s)
30
Describe the main features of smooth muscle cells
- NON STRIATED
- Spindle shaped with centrally positioned nuclei
- Thousands of cells clump together forming bundles or layers
- NO sarcomeres and NO T tubules
31
How does the contraction of smooth muscle differ from that of skeletal or cardiac?
- SLOWER, requires less ATP
- Still relies on actin and myosin interactions but NO SARCOMERES
- Can be stimulated by hormones, drugs and local blood gas concentrations AS WELL AS NERVE SIGNALS
32
Explain the roles of the different smooth muscle cells
- Form contractile walls of passageways or cavities
- MYOFIBROBLASTS - Important in wound healing (contract to close wound and produce extracellular matrix containing collagen fibres)
- MYOEPITHELIAL - Form basketwork around secretory units of some exocrine glands and ocular iris
33
List 5 places in the body where you would find smooth muscle
- Walls of blood vessels (tunica media)
- Muscularis externa of gut wall (2 layers)
- Ocular iris of eye
- Genitourinary tract
- Respiratory tract (bronchioles, trachea)
34
Describe the arrangement of myofilaments in smooth muscle and how this affects contraction
- Arranged DIAGONALLY and spiral down cell
- Contracts in a TWISTED action, compressing cell
- Intermediate filaments attached by DENSE BODIES to sarcolemma
35
Explain how skeletal muscle can repair itself following injury
_ Muscle cells CANNOT DIVIDE
- HYPERPLASIA due to increased mitotic activity of SATELLITE cells can increase number of muscle fibres
- HYPERTROPHY where satellite cells fuse with existing muscle leading to increase in muscle mass
36
Briefly explain the healing of the heart muscle following iscaemic MI
- Death of muscle tissue due to lack of oxygen
- Cardiac muscle CANNOT REGENERATE
- Fibroblasts invade following damage and lay down SCAR TISSUE which reduces the force of contraction (less muscle present)
37
Explain why smooth muscle cells are able to regenerate
- Retain their mitotic ability so can form new cells
| - Can repair smooth muscle by hyperplasia and hypertrophy of new cells
38
Explain the functions of skeletal muscle
- Movement
- Posture
- Heat generation
- Stability of joints
