Muscle Tissue Flashcards
1
Q
What are the functions of muscle?
A
- movement of body and body fluids (blood, lymph, food, waste, etc)
- maintain posture and body position
- support soft tissues (blood vessels, intestines, urogenital tract, etc)
- sphincters at entrances and exits of certain organ systems
- maintain body temperature
2
Q
What is muscle specialized for?
How does this process occur?
A
- contraction
- contraction occurs: when thin actin microfilaments (in muscle also referred to as actin myofilaments) and thick myosin filaments (aka myosin myofilaments), organized into structures called myofibrils in the cytoplasm, slide past one another
3
Q
What are the 3 types of muscle tissue differing in their anatomic distribution, structure, and function?
A
- skeletal muscle: MSK system, striated (due to highly organized myofilaments) and voluntary, movement and posture, makes up 40% of body mass, cells are multinucleated
- cardiac muscle: heart, striated and involuntary, pumps blood through CV system, cells have one nucleus, are branched, and are connected to one another by intercalated discs
- smooth muscle: non-striated and involuntary but still have myofilaments that drive cell contraction, visceral organ tone and movement, walls of hollow viscera, and blood vessels (vasodilation and vasoconstriction)
4
Q
What is the anatomy of skeletal muscle?
A
- cells can be long, ranging from few cm to dozens of cm
- cells are called myofibers or myocytes that group to form fascicles
- three layers of CT: 1) myofibers are separated by loose CT called endomysium, 2) fascicles are separated by CT called perimysium, 3) dense irregular CT layer, epimysium, surrounds entire muscle
- all CT sheaths are continuous w/ tendons that insert onto bones (trasmit entire contractile force of entire muscle onto bone)
- CT also distributes blood/nerves to myofibers
5
Q
What is the microanatomy of skeletal muscle?
A
- cells are multinucleated cylinders (myofibers)
- nuclei located in periphery, cytoplasm primarily occupied by myofibrils
- myofibril composition: actin microfilaments and thick myosin filaments form repeating functional units (sarcomeres), myofibrils = 80% total vol of cell
- each cell has thousands of sarcomeres
- plasma membrane (sarcolemma) and smooth ER (sarcoplasmic reticulum (SR)) provide Ca2+ storage and sleeves around each myofibril
- transverse tubules (t-tubules): invaginations of sarcolemma
- terminal cisternae: dilated ends of SR that release Ca2+ (trigger muscle contraction)
6
Q
- combination of sarcolemma T-tubules w/ two abutting dilated expansions of specialized SR (terminal cisternae)
- trigger muscle contraction: during depolarization of the T-tubule through a signal from motor neurons, the T-tubules signal release of Ca2+ from terminal cisternae and SR
A
skeletal muscle triad
7
Q
What is the structure of a sarcomere?
A
- sarcomeres extend from one Z-line to next Z-line
- when a muscle contracts, every sarcomere shortens
- Z-line: anchor site for actin (thin) microfilaments
- M-line: anchor site for thick myosin filaments
- I-band: only contains actin filaments
- H-zone: contains only thick myosin filaments
- A-band: overlap of thick and thin filaments plus H-zone
8
Q
What is the mechanism of sliding that produces sarcomere contraction?
A
- when a muscle cell contracts, the thin filaments slide past thick filaments toward the center of the sarcomere, bringing Z-lines closer together
- every myofibril in a muscle cell shortens at the same time, thus entire muscle cell contracts
- thin and thick filaments do not change in length, the degree of overlap shortens
9
Q
What is the biochemical mechanism of sarcomere contraction?
A
- signal to contract is driven by release of calcium from sarcoplasmic reticulum (SR)
- calcium influx exposes sites of actin, allowing myosin head to bind to actin
- when myosin binds actin, power stroke is invoked
- ATP binds myosin and is hydrolyzed, generating energy used to release myosin head from actin and re-cock myosin for another power stroke
- this process is repeated until calcium is removed and is transported back into SR
- each single nerve impulse results in a ~30 msec cycle of contraction/relaxation
(rigor mortis: occurs shortly after death, loss of source of ATP prevents myosin head from detaching from actin, thereby locking muscles in place)
10
Q
What are the regulatory proteins involved in muscle contraction?
A
- tropomyosin: runs in groove formed by F-actin strands, binds to troponin complex
- troponin: troponin T (binds complex to tropomyosin), troponin I (inhibits binding of myosin to actin), troponin C (binds Ca2+)
11
Q
What are the parts and function of the chemical synapse?
A
- parts: 1) presynpatic knob (axon terminal), 2) synpatic cleft, 3) presynaptic membrane, 4) postsynaptic membrane
- synaptic vesicles in presynaptic knob contain NT that is released into synaptic cleft by exocytosis
- NT binds to receptors on postsynaptic membrane (NT ex: acetylcholine and noradrenaline)
- binding of NT stimulates postsynaptic cell
12
Q
How is skeletal muscle innervated?
A
- motor neurons initiate muscle contractions
- every skeletal muscle cell receives an axon terminal, not all cardiac or smooth muscle cells are directly innervated
- neuromuscular junction (or motor end plate) is a specialized area where muscle and nerve ending meet
- the axon of motor neuron branches near its end to innervate a number of muscle fibers (the group of muscle fibers an axon innervates is referred to as a motor unit)
13
Q
What is the biochemical mechanism of skeletal muscle innervation?
A
- neuron fires, axon terminal release vesicles containing NT’s (acetylcholine) into synaptic cleft, NT’s diffuse across cleft to bind to receptor moleules on muscle cell sarcolemma
- acetylcholine binds to its receptors and induces release of calcium from SR, which initiates muscle contraction
- acetylcholinesterase breaks down acetylcholine right after single muscle contraction or any leftover acetylcholine is taken up into axon terminal by receptor-mediated endocytosis
14
Q
What is the microanatomy of cardiac muscle?
A
- cells usually have one nucleus, but some have two
- nucleus centrally located in cytoplasm
- cross-striations visible in cytoplasm
- most cardiac muscle cells are not innervated, contracts rhythmically w/o innervation (spontaneous), rate of contraction controlled by hormones or cardiac conduction system
- junctions, intercalated discs, are sites of cell-cell adhesion and low electrical resistance (combination of gap and desmosomal junctions)
- cells are branched (i.e. connect to more than one cell via intercalated discs)
- arranged in cardiac diads (structurally similar to skeletal triad), rather than abutting along the entire length of t-tubules, terminal cisternae abut via focal spots along t-tubules
- extensive capillary blood supply
15
Q
- combination of 1 t-tubule and 1 SR cisterna in cardiac muscle cells
- found at Z-lines
- permits uniform contraction of myofibrils within a single cardiomyocyte
A
cardiac diad T-tubules
16
Q
- junctions within cardiac muscle that are sites of special cell adhesions and low electrical resistance
- at sites where cells meet end-to-end, always coincide w/ Z-lines
- 3 types of junctions: fascia adherens (transverse region, perpendicular), desmosome (macula adherens), gap (nexus) junctions (longitudinal region, parallel)
- TLDR: special cell adhesions within cardiac muscle; bind cells, transmit forces of contraction, and allow spread of excitation
A
intercalated discs
17
Q
What are the 3 types of intercalated disc junctions?
A
- fascia adherens (transverse region, perpendicular): actin filaments at the ends of terminal sarcomeres insert into junction, transmit contractile forces b/w cells, most predominant
- desmosome (macula adherens): provide anchorage for the intermediate filaments of the cytoskeleton
- gap (nexus) junctions (longitudinal region, parallel): sites of low electrical resistance, allows excitation to pass b/w cells
18
Q
- type of muscle that is found in walls of viscera and blood vessels, but also in the integument
- non-striated, involuntary muscle that mediates visceral organ tone and movement
- in blood vessel walls, it mediates vasodilation and vasoconstriction
A
smooth muscle
20
Q
What is the specificity of contraction within smooth muscle?
A
- contraction/relaxation are slow but can be sustained and resistant to fatigue
- innervated by autonomic nervous system (involuntary), impulse to contract is passed onto neighboring cells via gap junctions
- contraction can also be stimulated by hormones (e.g. uterine contraction during labor)
- possess a contractile apparatus of thin and thick filaments that are anchored to cytoplasmic densities/dense bodies comprised of desmin and vimentin intermediate filaments
- tension transmitted via densities to the membrane
- cells contract as one unit and assume a globular shape
21
Q
Compare the tissue type on the left vs the right:
A
- left: dense regular CT
- right: smooth muscle
