Lecture 14 - Muscles (2) (Part 1) Flashcards
Describe the main similarities and differences between cardiac and smooth muscle.
Similarities:
- Nuclei are central in both (peripheral in skeletal)
- Only have 1 contractile cell type
- Both act as syncytium
- Both have gap junctions
Differences:
- Cardiac muscle is striated, smooth is not
- Smooth doesn’t contain sarcomeres
- Smooth doesn’t contain troponins
Both cardiac and smooth muscle fibres have 1 central (cardiac sometimes 2), how can you distinguish between the two in H&E stains?
- Cardiac muscle has clear intercalated discs.
How is cardiac muscle innervated?
What are the different types of innervation and how do they affect HR?
- Via the ANS (sympathetic + parasympathetic branches)
- Para innervates atrium, sympathetic innervates atrium and ventricles
- Para = decreased HR, sympathetic = increased HR
- Sympathetic increases HR by activating DHP which leads to increased Ca2+ release through RyR’s.
How is smooth muscle innervated?
How does smooth muscle contract?
- Via the ANS, which causes NT release from varicosities which are released at pre-synaptic clefts.
- Ca2+ entry occurs and binds to calmodulin. The Ca2+/calmodulin complex activates MLCK which phosphorylates myosin light chains in order for actin to bind and contraction to occur.
- Contraction continues until there is a decrease in IC Ca2+
How is skeletal muscle innervated?
How can recruitment of fibres per motor unit change muscle power?
- Skeletal muscle is innervated by release of ACh at the neuromuscular junction is response to nerve impulses. This binds to sarcolemmal receptors initiating an AP which is propagated along the muscle.
- Less units recruited per motor unit = less power (therefore useful for fine control muscles), more units = more power (e.g.: for gastrocnemius).
What is the role of Kranocytes in the NM junction?
Resides over the terminal Schwann cell on pre-synaptic junction and anchors nerve to the muscle cell.
Describe the series of events that occur in the build up to skeletal muscle contraction.
- Nerve impulse along motor axon leads arrives at NM junction, leading to ACh release which binds to post-synpatic receptors.
- This opens Na+ channels and influx of Na+ leads to DP of sarcolemma which spreads to t-tubules.
- Voltage sensor proteins in T-tubule undergo conformational change and release Ca2+ into sarcoplasm via activation of VG Ca2+ channels
- Ca2+ binds to TnC and contraction cycle is initiated.
- Ca2+ ions return to terminal cisternae of SR
What is Myasthenia Gravis and what does it lead to?
- Autoimmune disease, antibodies block ACh receptor, leads to reduced synaptic transmission and thus intermittent muscle weakness.
- Symptoms include Ptosis (drooping eyelid)
Describe the basic structure of actin + myosin
- Myosin is a rod-like structure with 2 protruding heads.
- Actin is a thin filament protein that is composed of F-actin fibres and G-actin globules, with tropomyosin-troponin complex covering binding sites. (therefore actin, tropomyosin and troponin together from the thin filament).
- Tropomyosin coils around actin helix and troponin complex is attached to each tropomyosin molecule.
Describe the structure of the functional contraction unit
- Thick filaments run through the middle, thin filaments on the outside, with tropomyosin coiling round the actin helix and a troponin attached to each tropomyosin.
- Myosin heads extend towards actin filaments in regions of overlap. Therefore centre of sarcomere (M-line) is devoid of myosin heads.
How is Ca2+ involved in muscle contraction?
- Ca2+ binds to TnC leading to a conformational changes that moves tropomyosin away from actin’s binding sites, allowing myosin heads to bind and contraction to occur.
- Tropomyosin sits in cleft of G-actin spheres
Describe the cyclic sequence of events that occurs during skeletal muscle contraction.
- Myosin heads bind to actin forming cross bridge
- ADP & Pi release allowing working stroke to occur, myosin head pivots and pulls actin towards M line.
- New ATP attaches at myosin head level, so cross bridge is detached.
- ATP hydrolysis starts the process again.
What happens to the length of actin, myosin, sarcomeres and Z-line, I band, A band and M-line when muscle contracts?
Actin & Myosin- stays the same length Sarcomere - Shortens Z-lines - come closer together I-band - shortens A-band - stays the same length M-line - stays the same length (A+M slide over each other)
What is the origin and insertion points of a muscle?
Origin = muscle start point (typically a bone and proximal) - attaches to a stable non-moving bone Insertion = muscle re-attachment point (bone, tendon or connective tissue, typically distal). - attaches to a bone that can move.
What are the roles of agonist, antagonists, synergists, neutralisers and fixators in movement?
Agonist = primary mover Antagonist = oppose primary movers Synergists = assist primary movers Neutralisers = prevent unwanted movements agonist can perform Fixators = holds a body part immobile whilst another part is moving.
