Muscles Flashcards
What are myocytes and sarcoplasms?
Myo—> myocyte—> muscle cell
Sarco—> sarcoplasm(cytoplasm), sacrolemma (plasma membrane
What are myofilaments?
Muscle tissue contains filamentous cytoplasmic organelles —> myofilaments which gives the tissue its contractile property
What’s the function of muscle?
Contraction of muscle tissue produce movement of body parts and changes in volume and shape of internal organs and vessels
What does the arrangement of myofilaments?
The arrangement of myofilaments allows for morphological classification based on the appearance under light microscope
What are striated muscle?
These tissue subtypes have visible cross striations due to regular arrangement of contractile organelles within their cells
Describe smooth muscle
No cross striations
What are the types of striated muscle?
Skeletal- voluntary
Cardiac- involuntary
Visceral- voluntary
Where is skeletal muscle found?
Somatic/body wall
Where is visceral muscle found?
Soft tissue origin. Tongue,pharynx, larynx, diaphragm and upper esophagus
What are the locations for cardiac muscle?
Heart and roots of great veins that empty into heart
Where are smooth Muscle located?
Walls of visceral organs, stomach, gut tube
Compare smooth, cardiac and skeletal muscle
Skeletal muscle- multiple nuclei peripherally nucleated, long cylindricAl cells, striations
Cardiac muscle-intercalated cells, centrally located nucleus, branched cells striations
Smooth- spindle-shaped cells, centrally located nucleus
What is dystrophin?
A rod-shaped cytoskeletal protein which links to ECM proteins laminin & argin found in the external lamina of the myocyte —> Clinical route: Duchenne & Becker’s muscular dystrophy
What does dystrophin do?
Forms a complex with two groups of transmembrane proteins:
- Dystroglycans —> links dystrophin and laminin of the ECM
- Sarcoglycans—> associated with membrane dystroglycans—> clinical correlate: limb girdle associated dystrophy
Congenital muscular dystrophy: is another group of muscular dystrophy associated with ECM components
What causes duchenne muscular dystrophy ?
Results from a defect in the gene coding for dystrophin associated proteins on X chromosome —> muscular fiber fragility
- Most common inherited myopathy
- more prevelant in males
- X-linked recessive
What are the symptoms of Duchenne Muscular Dustrophy?
Patients are unable to stand unaided in early childhood and develop progressive muscle weakness, becoming wheelchair-bound by their mid-teens and typically dying in early adult life
What are the other muscular dystrophy?
- Becker muscular dystrophy
- limb bridle dystrophy
- congenital muscular dystrophy
What is the triad?
1 T tubule + 2 terminal cisternae at the A-I junction
What are sarcoplasmic reticulum?
Forms a network arounf the myofibrils (calcium reservoir)
-Extends-from one A-I junction to the next A-I
Forms the terminal cisterna at the end of each network
What is a transverse tubule?
Invagination of the sarcolemma at the A-I junction.
Have voltage sensor protein/ channels activated by membrane depolarization
Where. Is a myofibrils arranged?
Arranged in the center 9f the cell, surrounded by mitochondria
How does the transverse tubule work?
Depolarization of t tubule membrane triggers release of Ca2+ from terminal cisterna & initiates muscle contraction cycle
Describe the neuromuscular Junction
Neuromuscular junction or motor end plate
-Contact between the terminal branches of an axon and muscle fiber
- Axons branch as they near the muscle and give rise to twigs that end on individual muscle fibers= motor unit
- All muscles within a motor unit are of the same type (I,IIa, or IIb etc.)
Covered by thin portion of external Lamina
What is involved in a muscle contraction physiologically?
1 neuromuscular junction per muscle fiber
Enters close to the origin of the muscle fiber
Depolarization is propagated along the entire length in a sequential manner
-Each sarcomere contracts independently allowing for smooth singular action in a particular direction
Explain the struct7ure of the neuromuscular junction
Presynaptic membrane : synapt8c vesicles contain ACh can be observed
Synaptic cleft(SnC) where acetylcholine is released
Post synaptic membrane (of the muscle): junctional folds with acetylcholine receptors
NMJ is covered by Schwann cell external lamina
What is a muscle spindle?
- encapsulated sensory receptors
- specialized stretch receptor located in the muscle belly
- Senses changes in muscle length or stretch
- Contains modified muscle fibers or spindle cells
What is the Golgi tendon organ?
- encapsulated prioceptor located in the myotendinous junction
- senses tension in the muscle
- Sensiry component of the Golgi tendon reflex
What 8s myasthenia grav8s?
- myasthenia Gravis is an autoimmune disease which affects neuromuscular junction
- common cause: ACh receptor antibodies, which block and attack ACh receptors in the postsynaptic membrane.
Fewer ACh receptors results in:
-Fluctuating weakness and fatigue of skeletal muscles
-OCCULAR, bulbar, limb and respiratory muscles are affected
What are the requirements for skeletal muscle?
Myofibril
Sarcoplasmic reticulum
Transverse tubules
Mitichondria
What are the structural requirements f9r contraction in cardiac muscle?
Sarcoplasmic reticulum
Transverse tubule
Diad
What is the function of the sarcoplasmic reticulum in cardiac muscle?
- single network along the sarcomere
- extends fromZ line to Z line
- Less developed than in skeletal muscle
- Terminal cisterna
- Contain Ca2+ release channels
- Release Ca2+ into sarcoplasm
- Contain Ca2+ release channels
Describe the transverse tubular system in cardiac muscle
Located at Z line (compare to skeletal muscle)
-T tubules contain voltage-sensor proteins
What is the Diad in cardiac muscle?
Complex of o;e t tubule and one aadjacent terminal cisternae at the Z line
Describe intercalated discs in cardiac muscles
Intercalated discs are attachment sites between adjacent cardiac myocytes
The transversely oriented parts of the intercalated disk (T) which is at right angle to the myofibril like the risers of a stairway
The longitudinal or lateral parts (L) are parallel to myofibrils like the steps of a stairway
Mitochondria(m) are abundant in card8ac muscle due to high metabolic demands of these cells
Describe the “arterial component of the intercalated d7sc
Gap junctions (communicating junctions)
Macula adherens(desmosomes)
Describe the transverse component in intercalated disks
Fascia adherents (adhering junctions) -Major structural element
- Binds cardiac muscle cells at their ends
- Serves as attachment site for thin filaments in terminal sarcomere
(MA) Macula adherens (desmosomes)
-Bind individual muscle cells to each other
- Reinforce fascia adherens
- Found in both transverse & lateral components
What are Purkinje fibers?
Large, modified muscle cells located just deep to the endocardium in the subendicardial connective tissue
Specialized to conduct impulses of the A-V valve bundle and allow synchronization of ventricle contraction
Abundant in mitichondria
Why are Purkinje fibers pale staining ?
- few myofibrils (these are located peripherally)
- Large amount of glycogen
How are smooth muscles connected?
Interconnected by gap junctions or nexus
- Communicating junctions
- Small molecules & ions pass from cell to cell
- Provode communication links to regulate contraction of entire bundle or sheet
Sarcoplasmic reticulum
Pinocytic vesicles
Dense bodies
Explain smooth muscles secreting connective tissue matrix
- Well developed rER & Golgi
- Synthesize both type IV collagen and type III collagen
- Elastin, proteoglycans, and multi-adhesive glycoproteins
- Vascular and uterine SM also secrete large amounts of type 1 collagen and elastin
What do the thick filaments of smooth muscle contain?
- Myosin II scattered throughout the sarcoplasm
- Side polar vs striated muscle where it is oriented towards the center of the sarcomere
What do thin filaments contain in smooth muscle?
- Actin
- Tropomyosin (no associated protein)
- Smooth muscle specific proteins Caldsmon and Calponin which bind to Actin proteins blocking the myosin binding site
What are the accessory proteins of smooth muscle?
- Myosin light chain kinase(MLCK): initiates contraction
- a-actinin (see dense bodies)
- Calmodulin- Ca2+ binding protein
- Ca2+ calmodulin complex binds & activates MLCK
Describe dense bodies of smooth muscles
Cytoplasmic densities) which attach thin and intermediate filaments
-Contain a-actinin
-analogs of Z lines in striated muscle
- Distributed throughout the sarcoplasm in a network of intermediate filaments
- Desmin
- Vascular smooth muscles contains vimentin in addition to desmin
Describe smooth muscles having no T tubules systems
-Analogous system of:
Caveolae—> invaginations of cell membrane
Sarcoplasmic reticulum
Cytoplasmic vesicles
Summarize smooth muscle contraction
- Contractile myofilaments are oriented obliquely to the long axis of the myocyte
- These are anchored to cytoplasmic and cell membrane densities
- Therefore during concentration there is a net shortening of the cell
- The cell adopts a globular shape and the nucleus adopts a “cork screw” shape
What are the steps in smooth muscle contraction
Contraction regulated by the Ca2+-calmodulin-MLCK system
- Increase in Ca2+ concentration
- Ca2+ binds to calmodulin forming the Ca2+ -calmodulin complex
- Ca2+ xcalmodulin complex binds MLCK
4 . MLCK-phosphorylates regulatory light chain of myosin
- Actin-binding site kf myosin head is activated & attaches to actin
How is smooth muscle contraction regulated?
Mechanical: passive stretching of vascular smooth muscle activates myogenic reflex
Electrical: neural stimulation leading to release of ACh and NE
Chemical: Angiotensin II, vasopressin, thromboxane A2- uses second m3ssanger pathways such as IP3, and NO-cGMP pathways
Why can the force of smooth muscles be sustained for such long periods of time ?
Latch state when the myosin head is unable to detach from actin filament
Which muscles undergo hypertrophy?
All muscle types - cardiac in hypertensive cardiomyopathy
Which muscle types with injury repair?
Cardiac and skeletal don’t undergo hyperplasia
Smooth muscle undergoes hyperplasia, the uterine proliferates during the normal menstrual cycle and pregnancy
SM of blood vessels increase in size as well as number due to hypertensive conditions etc.
How does skeletal muscle respond to atrophy?
Skeletal- limited capacity
Satellites cells are myogenic precursor cells located between the sarcolemma of a muscle fiber and its external lamina
Satellite cells are responsible for skeletal muscle regeneration
Regenerative capacity is limited
How does cardiac muscle respond to injury?
No regenerative capacity
After the death of cardiac muscle cells, the tissue is replaced with fibrous connective tissue. Cardiac function is lost at site of injury
How does smooth muscle respond to injury?
Smooth muscle of blood vessels divide regularly differentiating from Mesenchymal stem cells in the adventitia or from division and differentiation of endothelial cells and pericytes in capillaries and post-capillary venues during the repair process following a vascular injury
