Muscle Physiology Flashcards
(83 cards)
1
Q
Characteristics of Muscles?
A
• Excitability - responds to stimuli
• Conductivity- impulse propagation
• Contractility - able to shorten in length
• Extensibility - stretches when pulled
• Elasticity - tends to return to original length after contraction or extension
2
Q
Muscle Tone?
A
• Tone is the tension in the muscles due to partial contraction of muscle
• Resting muscle tone-(partial contraction of muscle at rest)
3
Q
Two ways to categorise muscle
A
- striated and unstriated
- voluntary and involuntary
4
Q
How do we know if a muscle is under voluntary or involuntary control?
A
• Depending on whether they are innervated by the somatic nervous system and are subject to voluntary control
• Or are innervated by the autonomic nervous system and are not under conscious control
5
Q
Features of the Skeletal Muscle Cell.
A
Skeletal Muscle Cell- (Muscle Fiber)
1. Elongated and multinucleated cells
2. No mechanical / electrical interconnection with adjacent cells
3. Sarcoplasm is rich in glycogen,myoglobin, mitochon dria
4. Each muscle fiber contains myofibrils that consist of thin and thick myofilaments
5. Each Myofibril is bathed in sarcoplasm and surround ed by a sarcoplasmic reticulum from which it gets c alcium for contraction ( a modified endoplasmic reti culum).
6. Plasma membrane (sarcolemma) has circular tubular extensions (transvers tubules) encircling the sarcoplasmic reticulum (SR)
6
Q
Describe th membrane that carries impulses in the muscle cell membrane.
A
• This membrane, like that of neurons, has a
membrane potential
• RMP= -90mv
• Impulses travel along muscle cell membra nes just as they do along nerve cell memb ranes.
• The impulses bring about contraction.
7
Q
Describe the Sarcoplasmic Reticulum.
A
• Muscle fibrils are surrounded by structures made up of membranes.
• They appear as vesicles and tubules under microscope.
• This is known as Sarcoplasmic Reticulum (SR)
• T system are transverse tubules continues with
the sarcolemma of the muscle fiber
• Smooth endoplasmic reticulum (SR) surrounds each myofibril and fused with each other at A/I junctions
• SR is well developed in Skeletal muscles
• It stores Ca++ ( SR membrane is impermeable to Ca++)
• Maintain a high Ca++ level in SR and low Ca++ in the cytoplasm
• The membrane has special Ca++ channels for release and reuptake of Ca++
8
Q
Myofibrils consists of
A
• Actin thin filaments
• Myosin thick Filament
9
Q
Describe the structure of Myosin?
A
Myosin protein has a golf club like shape
1. Head (cross bridges) that can bind to the actin filaments and use ATP
2. Tail (shaft of the thick filament)
Myosin Head has three important features
• ATP-binding sites
• ACTIN-binding sites
• has a “hinge” or neck to move head back and forth.
10
Q
What are the three types of contractile proteins?
A
Actin
• spherical shape protein and arrange as Helical chains
• can bind with myosin heads
Tropomyosin
• rod-like protein that helps to stiffen actin structure
• In a relaxed muscle it covers the myosin binding site on the actin
Troponin
• Globular protein capable to bind Ca++ to regulate actin/myosin binding
11
Q
Describe the three subunits TC, TI and TT
A
It has three subunits TC, TI and TT
• TroponinT binds the other troponin comp onents to tropomyosin
• Troponin I inhibits the interaction of myosi n with actin
• Troponin C contains the binding sites for t he Ca2+ that initiates contraction
12
Q
What are the two types of Ca2+ channels in skeletal muscles?
A
- Dihydropyridine receptors (DHPR)
- Ryanodine receptor (RyR)
13
Q
What are the Dihydropyridine receptors
A
voltage-gated Ca2+ channels in the TT membrane
(which open and close depending on change in electrical potential difference near the channel (I.e. Action potential)
14
Q
Describe the Ryanodine Receptor (RyR)?
A
ligand-gated Ca2+ channel in the SR
group of transmembrane ion channels that are opened or closed in response to the binding of a chemical messenger (i.e., a ligand).Ca2+ is the ligand
15
Q
How is Ca2+ is released from the SR in skeletal muscles?
A
Physical interaction between the sarcolemmal-bound DHPR and the SR-bound RyR allows for Ca2+ release from the SR in skeletal muscles
16
Q
Describe the nature of impulse in a relaxed muscle.
A
• At rest these gates are closed
Ca++ cannot pass & remains in SR
Very high Ca++ levels in SR
Very low concentration in sarcoplasm
• When an impulse travels along the membrane of the SR, the calcium “gate” open (ryanodine receptor) & Ca++ diffuses out of the SR into the sarcoplasm.
17
Q
Features of Skeletal Muscle cells?
A
- Well developed cross striations
- Requires nervouse stimualtion for contraction
- Under Voluntary control
18
Q
How is cardiac muscle different from skeletal muscle?
A
Functionally syncytial
Pacemaker in the myocardium allows rhythemic cotraction without external innervation
19
Q
Two tyoes of smooth muscle and difference?
A
- Unitary: syncytial + pacemakers - irregular
- Multiunit: graded contractile ability - not spontaneous
20
Q
How are muscles connected to bone?
Why is this done?
A
they begin and end with a tendon
so that th force of contraction is additive
21
Q
Basic morphology of muscle cell?
A
multinucleated, long, cylindrical and aurrounded by sarcolemma
Myofibrils are divisible into idividual filaments
22
Q
What are the three main proteins for the contraction of muscle cells?
A
- Mysoin II
- Actin
- Tropomyosin
- Troponin - I, T and C
23
Q
What are the thin filaments made of?
A
- Actin
- Tropomysoin
- Troponin
24
Q
Under the electron microscope, how do we see the thick filaments in a transverse section of the A band?
A
Surrounded by 6 thin filaments in a hexagonal fashion
25
Differentiate between the three types of troponin.
Troponin T - binds to tropmosin
Troponin I - inhibits the myosin interaction with Actin
Troponin C - Binding sites for Ca2+
26
Describe the role of Actinin, Titin and Desmin?
Acitinin - binds actin to Z lines
Titin - connect Z mine to M line + provide scaffolding for sarcomere
Desmin - binds Z line to the plasma membrane to add structure
27
Describe the sarcotubular system
- made up of T tubules and the sarcoplasmic reticulum
- T tubules- continuous with the sarcolemma
- sarcoplasmic reticulum - has enlarged terminal cisterns at junction between A and I bands
28
Explain the contraction mechanism of a skeletal muscles cell.
1. An action potential travels along a motor nerve to its endings on muscle fibers (NMJ)
2. At each ending, the nerve secretes a small am ount of neurotransmitter called acetylcholine.
3. Acetylcholine acts on a local area of the muscl e fiber membrane (nicotinic receptor) resulting infl ux of sodium
4. Generation of muscle action potential
5. Conduction of AP through T-Tubules
6. Stimulate voltage- gated Dihydropyridine recep tors in T-Tubules, sensed by Ryanidine (non volta ge gated) receptors in SR
7. Ca++ released through Ryanodine receptors fr om sarcoplasmic reticulum
8. Ca++ binds to TnC region of Troponin
9.Uncovering-Troponin changes shape, moving,tropom yosin exposing binding site of Myosin head on actin fil ament
10. Attachment - (cross-bridge with thin filament actin).
11. Power Stroke - (myosin heads rotate, move the attached actin and shorten the muscle fiber).
12.Sliding thin filaments over thick filaments.
29
Explain the relaxation of a skeletal muscle cell.
1. ATP-Dependent Ca++ Pumps pump the Ca++ back into the sarcoplasmic reticulum
2. Low Ca++ levels occurs in sarcoplasm
3. Troponin/Tropomyosin blockade of actin and muscle relaxes.
(Muscle relaxation is also an energy dependent process)
4. Motion continues until no more ATP is present or Ca++ completely re-uptake into SR
30
Differentiate between the relaxed and contracted state of a skeletal muscle.
RELAXED STATE:
No myosin-actin cross bridge formation
CONTRACTED STATE
Sliding of filaments via myosin-actin cross bridg e formation leading muscle get shorter
31
Define Motor Unit.
• a single motor neuron and all of the muscle fibers innervated by it.
32
Define Muscle Twitch.
• Response of a skeletal muscle to a single stimulation (or action potential)
33
Explain Graded Response.
• Skeletal muscles can vary the degree of contraction.
• Muscles do this by
1. Motor Units recruitment
2. Wave Summation
34
Explain the recruitment of Motor Unit in a graded response.
• Muscles composed of many motor units
• Each stimulated by a single motor neuron
• Recruitment - More motor units are added as stimulus strength increases resulting higher force in the muscle
• Large motor units have more muscle cells per motor neuron
• Recruitment follows “size principle”
35
Explain Motor units and how they function.
There are three types of motor units based on duration of their twitch contraction
–S (slow)
–FR (fast, resistant to fatigue),
- FF (fast, fatigable)
• Recruitment follows “size principle”
• First S then FR and finally FF motor units
36
Explain the size principle in the action of Motor Units.
- Motor units will be recruited in order of size from smallest to largest motor unit.
- The small units do not produce much force, they are slow to act, and they are resistant to fatigue.
- It ensures the efficiency in muscle action .
- So larger units which recruit last, produce a lot of force, they react quickly, but they are easily fatigued allowing for smoother, more controlled movements.
37
Explain the concept of Tetanus.
• If a muscle fiber is stimulated so rapidly that it does not relax at all between stimuli, a smooth, sustained contraction occurs
• This is wave summation
• The resulted sustain contraction in the is called tetanus
38
Whata are the two types of tetanus
• There two types of tetanic contractions
1. Complete tetanus 2. Incompletetetanus
39
Explain the relationship between contraction and refractory period.
If action potential is prolonged like the mechanical response the absolute refractory period will also get prolonged resulting ARP extending into the relaxation phase of the muscle
So no sustain contracting even stimuli frequency is too high.
40
Explain the relationship between contraction and refractory period for skeletal muscle fibers.
For skeletal muscle fibers, ARP typically ends during the early part of the contraction period.
41
Describe the length tension relationship
• A muscle fiber is stimulated maximally at various lengths
• Then tension develop is recorded at each length
A muscle fiber is stimulated maximally at various length and tension develop is recorded at each length
• Active tension is the amount of tension actually generated by the contractile process alone through excitation contraction coupling
• Active tension = (Total tension - passive tension)
42
What is active tension?
• Active tension is the amount of tension actually generated by the contractile process alone through excitation contraction coupling
• Active tension = (Total tension - passive tension)
43
Describe the resting length of a muscle
The length of the muscle at which maximal overlap between actin and myosin occurs is the ideal length of a muscle at which they can gener ate greatest force (tension)
This length is observed when muscle is at rest in the anatomical position of the body
So optimal length that produces maximal tension in the skeletal muscles are called resting length
44
What is Muscle Atrophy?
Explain why it might occur?
Decrease in the mass of the muscle resulting weakness
It may be due to
– Malnutrition
– Lack of blood supply
– Denervation (loss of nerve supply)
– Disuse (due to lack of use/exercise)
– Intrinsic disease
45
What are the similarities between skeletal and muscle tissue?
• Structurally ( both are striated muscle)
• Functionally ( similar excitation contraction coupling)
46
Whata are the two components of the sliding mechanism?
1. Covering/ Inhibition of the binding site
2. Uncoverin - power stroke
[Refer diagram]
47
Explain the general similarities and differences of cardiac muscle when compared with striated muscle.
• Presence of striation / sarcomere
• T-Tubules (but larger and at Z lines)
• Sarcoplasmic Reticulum
• Contractile proteins
• But it is under involuntary control (innervated by the Autonomic Nervous System)
48
What are structural features in Muscle Tissue that makes it different?
1. Endomysium
2. Intercalated Disc
3. Z lines and A bands
4. Branching point of the cardiac muscle cell.
5. cross striations
6. Myocytes
49
What are the 2 ways that cardiac muscle cells differ from skeletal muscles cells?
1. Cell Morphology
2. Functions as a syncitium
50
how is the cell morphology of a cardiac muscle cell different from a skeletal muscle cell?
Cell morphology
• Cardiac muscles have single nucleated cell • Have a branching net work
• Intercalated Disc present
– Where one fiber abuts on other
– Can transmit the force from one fiber to another
• Gap junction in adjacent cell membrane (low resistance bridges to transmit the excitation )
51
What does it mean to say that cardiac muscles fucntion as a syncytium .
All these features result in cardiac muscles
Contracting in syncytial fashion
But still have phasic contraction
Ie. Arial contraction /Ventricular contraction separately
52
What is the differences in the excitation-contraction coupling between skeletal and cardiac muscles?
1. automaticity
2. Needs an efflux of Extracellular Ca2+ through the Dihydropyridine receptor to trigger the Ca2+ release from SRs
3. Prolonged action potential with longer refractory periods (>200ms)
4. cardiac muscles cannot be tetanized
5. Shape of Cardiac action potentials
6. Excitation spread from one cell to another
53
Explain the Automaticity of cardiac muscles.
Automaticity
Ability to generate spontaneous pacemaker potentials without external nervous stimulation. This is due to the presence of specialized conduction tissue of the heart called pacemaker tissue.
[Refer to pacemaker action potential]
54
Why do cardiac muscles Need influx of Extracelluar ca++ through Dihydropyridine receptor to trigger the ca++ release from SRs)
• This is called Ca ++ -induced Ca ++ release
• As a result, the force of contraction can vary with
the amount of Ca ++ entering the cell.
• Several mechanisms involve in regulating Ca2+ entry during the action potential.
• There is Na+-Ca++ exchange antiport in cardiac muscles for the removal of Ca++ during relaxation
[Refer to diagram]
55
What is the Na+/Ca2+ exchange antiport?
• Antiport in the cardiac cell membrane that expel one Ca++ from the cell in return of three Na+ into the cell .
• This antiport is important to regulate intracellular Ca2+
- The function of this exchange protein is tied to the Na +,K+ ATPase pump
56
Explain the effect of Digitalis on the Na+/Ca2+ exchange antiport.
–if the Na+, K+ pump is inhibited, function of this exchanger is reduced & more Ca++ is accumulated in the cardiac cell resulting more Ca++ available for contraction.
–drugs that inhibit the Na, K ATPase in the cardiomyocytes can be used to increase the force of cardiac contractions (i.e Digitalis)
[Diagram]
57
Explain: “Prolonged action potential with longer refractory periods (>200ms)”
help to have a single contraction for pumping the blood and prevent tetanization in the heart muscles
58
Explain the difference in muscle refractory perio d between skeletal and cardiac muscles.
in skeletal muscle cells, refractory period is short, allowing muscle to re-contract even before Ca+2 cycle is complete
in cardiac muscle cells, refractory period is so long that muscle is always in relaxing stage when ARP is over to prevent arrhythmia
For skeletal muscle fibers, ARP typically ends during the early part of the contraction period.
[Diagram]
59
Explain: “cardiac muscles cannot be tetanized”
• Due to longer refractory period, the cardiac muscle cannot be stimulated before it complete its contraction phase, thus the cardiac muscle cannot be tetanized.
• When refractory period is over muscle is always in the relaxing phase
• i.e cardiac muscles cannot be tetanized!!!
60
Explain: Shape of Cardiac action potentials
Plateau type AP Action potential in cardiac muscle
Pacemaker potential SA node
61
Explain: Excitation spread from one cell to another
Consequences?
Excitation spread from one cell to another
• This has two important consequences in myocardial contraction
– Ventricles (atria) contract as coordinated fashion (phasic)
– Force of energy cannot be increased by motor recruitment or through tetanization.
– Length tension relationship (Straling’s law of the heart) is more important
• Resting length of cardiac muscle fibers
62
Length tension relationship in cardiac muscle?
• The relationship is quite similar to skeletal muscles
• In the heart the initial length of the cardiac fibers are depend on the amount of venous return (diastolic filling of the heart)
• The resting length of cardiac muscle is not fixed as in skeletal muscles
• The pressure developed in the ventricle is proportional to the volume of the ventricle at the end of the filling phase (Starling’s law of the heart).
• The developed tension increases as the diastolic volume increases until it reaches a maximum, then tends to decrease
• So cardiac muscles has no fixed resting length ( optimal length at which maximal tension developed in the cardiac muscles)
• It varies with the amount of ventricular filling ( larger the amount of blood return to ventricles longer the fiber length)
63
Active tension in the cardiac muscles?
The force of contraction of cardiac muscle can be also increased by catecholamine's ( norepinephrine and epinephrine)
• This occurs without a change in muscle length.
• This is called positive ionotropic effect
• Catecholamine's mediates via innervated β1 – adrenergic receptors via cyclic AMP in the autonomic nervous system
64
Length-Tension Relationship of a cardiac muscle?
A myofiber develops its greatest tension when there is an optimal zone of overlap between the actin and myosin.
The force of muscle contract’n depends on the length of the fiber before contract’n begins.
Maximum tension occurs when the zone of overlap between actin and myosin extends from the edge of the H-zone to an end of the myosin myofilament.
65
What is Staring’s law of the heart?
Initial length of the fibers is proportionate to the tension developed in the cardiac muscle
• This is called the Starling's law of the heart.
• The developed tension increases as the diastolic volume increases until it reaches a maximum
• Greater the heart muscle is stretched during filling, the greater is the force of contraction and the greater the quantity of blood pumped into the aorta
[Diagram]
66
Whta are the 5 phases of a cardiac action potential?
Phase- 0
Na+ channel opening (Na+ influx)
• Phase -1
Na + channel inactivation & opening of one type of K+ channel
• Phase- 2
Opening of slow and prolonged Ca 2+ influx due to opening of L-type Ca 2+ channels)
• Phase- 3
closing of Ca 2+ channels continuation of delayed K + efflux.
• Phase-4
Establishing of RMP through ATPase Na/K pump
67
What are smooth muscles?
• Involuntary muscle
• innervated by the Autonomic Nervous System
• found primarily in the walls of hollow organs & tubes
• typically arranged in sheets
• Has poorly developed SR/T-tubules
• Single central nucleus with spindle shape cells, but the amount of cytoplasm is less as compared to cardiac muscle, i.e. the nucleus takes up a great space in the cell in smooth muscle.
• Less mitochondria (heavily depend on glycolysis for energy)
• Have spontaneous electrical activity in the cell membrane (without nervous stimulation)
• Cell is not striated, as actin and myosin are not arranged in linear fashion.
- Smooth muscle contraction is not through the sliding mechanism.
68
What are the type sof smooth muscle Action Potentials?
1. Spike
2. Plataeu
3. Slow waves
69
Explain the excitation and contraction coupling in smooth muscles.
1.Binding of Ach to muscarinic receptors in muscle cell membrane
2.increase influx of Ca2+ into cell from ECF or from SR
3.Formation of “Ca-calmodulin complex”
4.Activation of calmodulin dependent myosin light chain kinase
5.Phosphorylation of myosin
6.Myosin ATP activity & bind myosin to Actin49
7. Contraction
70
Relaxation in visceral smooth muscles.
Dephosphorylation of myosin by myosin light chain phosphatase
This results in relaxation of smooth muscles
or
sustained contraction due to the latch bridge formation
• A Calcium Pump Is Required to Cause Smooth Muscle Relaxation.
• Calcium ions must be removed from the intracellular fluids for smooth muscle relaxation
• This is done by an ATP dependent calcium pump
• It is slower action in comparison with the fast-acting
sarcoplasmic reticulum pump in skeletal muscle.
• Therefore, a single smooth muscle contraction lasts longer
71
Compare Smooth muscles to skeletal muscles.
• Poorly develop SR • No T-Tubules
• No sarcomere
• No striations but are made up of thick & thin myofilaments.
• Need of formation of Ca-calmodulin complex
• Thin filaments in smooth muscle do not contain troponin.
• Ca++ does not bind to troponin but a protein called calmodulin
• Ca++ for the contraction is coming from ECF
• Need phosphorylation of myosin head for contractions
• Slow and sustain (tonic) contractions (slow cycling of the myosin cross-bridges)
• Low energy requirement for smooth muscle contraction when compared with other muscle type
• Plasticity
• No direct prelateship between length and the tension of smooth muscle fibers
72
Compare duration of muscle contraction in three types of muscle:
[Diagram]
73
Explain: “Slow and sustain (tonic) contractions (slow cycling of the myosin cross-bridges)”
• The speed of cycling of the myosin cross-bridges in smooth muscle-that is, attachment to actin, then release from the actin, and reattachment for the next cycle-is much slower than in skeletal muscle
• A reason for the slow cycling is that the myosin heads have far less ATPase activity than in skeletal muscle, resulting slowing of smooth muscle contractions.
• The cross-bridges between myosin and actin not immediately separated and remain attached to the actin filaments for sometime (Latch bridges formation).
• So smooth muscle contractions are sustain or tonic
74
Explain how smooth muscles utilise less energy
• One reason for the low energy requirement to sustain smooth muscle contraction is Latch bridges formation
• Latch bridges formation - myosin cross bridges remain attach to actin even after Ca++ is removed from the cytoplasm
• The importance of the latch mechanism is that it can maintain prolonged tonic contraction in smooth muscle for hours with little use of energy
• So energy consumed to maintain contraction is often very small
Low energy requirement for smooth muscle contraction
Only 1/10 to 1/300 as much energy is required to sustain the same tension of contraction in smooth muscle as in skeletal muscle.
• This low energy utilization by smooth muscle is important because organs such as the intestines, urinary bladder, gallbladder, and other viscera has to maintain tonic muscle contraction every time of the day (almost indefinitely).
1. Support tubes to maintain continues flow
2. Regulation of flow (prevent backflow and controlled release ( guarding sphincters/valves)
75
Explain: “Advantage of sustain contractility with minimum energy” in smooth muscles
1. Support tubes to maintain continues flow • Air flow in respiratory system
• Blood flow in blood vessels
• Gastrointestinal tract / Urinary system
• Regulation of flow (prevent backflow and controlled release ( guarding sphincters/valves)
E.g.
Lower oesophageal sphincter Anal sphincter
• Control pupil diameter to adjust amount of light reaching the retina
– Sustain contraction
– Wall composed of smooth muscles has a resting
tone ( partially contracted)
– Prevent of collapse tubes /hollow organs helping continuous flow of fluid (blood urine, intestine)
76
Explain advantage of: “Slow and sustain contraction of Smooth Muscles make them highly efficient”
• Slow sustain contractions
– Little fatigue
– Low O2 use
77
Explain why: “No direct prelateship between length and the tension of smooth muscle fibers”
Give example.
• No significant relationship between the length of the muscle fibers before contraction and the tension that can be developed
• The range of lengths over which a smooth muscle fiber can develop maximal tension is much larger than for skeletal/cardiac muscle.
– This is because the thin filaments still overlap the much longer thick filaments even in the stretched position, so cross-bridge interaction and tension development can still take place.
• In contrast, when skeletal muscle is stretched even only three fourths longer than its resting length, the thick and thin filaments are completely pulled apart and can no longer interact and contractility is drastically reduced
•The ability of a considerably stretched but retaining the ability to contract and produce tension is important for their functions
• Stomach as a storage of food till digested and release in to duodenum completely after completing the digestion in the stomach or urinary bladder
• Smooth muscle allows maximal distension within the optimal length and still develop tension at fully stretched situation
• This feature is due to Plasticity of smooth muscles
78
Explain the Plasticity of a smooth muscle.
• Smooth muscle have the ability to return to nearly its original force of contraction seconds after it has been stretched
• So smooth muscles behave like plastics.
• I.e., a sudden increase in fluid volume in the urinary bladder
• stretching the smooth muscle in the bladder wall, causes an immediate increase in pressure in the bladder.
• However soon after this despite continued stretch of the bladder wall, the pressure returns almost back to the original level.
• When the volume is increased by another step, the same effect occurs again.
• Because of this plasticity there no direct relationship between length and tension in the smooth muscle
• Advantages of smooth muscle plasticity is that – Bladder can store more urine
– Blood vessels can accommodate more blood
– Stomach can store more food
without increasing the wall tension
79
Explain: “Plasticity and human urinary bladder”
• The consequences of plasticity can be demonstrated in human urinary bladder
• Tension exerted by the smooth muscle walls of the bladder can be measured at different degrees of distention as fluid is infused into the bladder via a catheter.
• Initially, tension increases relatively little as volume is increased because of the plasticity of the bladder wall.
• However, a point is eventually reached at which the bladder contracts forcefully.
• Advantage is the plasticity increses the bladder capacity for storing urine
80
What are the two types of Smooth muscles?
1. Visceral smooth muscles
2. Multi-unit smooth muscles
81
Whata re visceral msooth muscles?
• Fibers are arranged in sheets or bundles
• Have interconnections among fibers
• Fibers adherent to one another at multiple points so that force generated in one muscle fiber can be transmitted to the next.
• Have gap junctions between adjacent cell membranes through which action potentials can travel from one fiber to the next and cause the muscle fibers to contract together (contract as a syncytium).
• found in the walls of hollow organs
(e.g., blood vessels, digestive tract, urinary system, & reproductive system)
• self-excitable (generate spontaneous action potentials & contractions without external nerve stimulation
• unitary smooth muscle is stimulated by non-nervous stimuli like stretch as well circulating neurotransmitters, hormones.
• Ability to contract following stretch without activation of extrinsic nerve innervations is an unique feature of unitary smooth muscles
• Stretch in unitary SM leads to
– Reduce membrane potential
– increase the frequency of spikes
– increase muscle tone.
• This allows to contract automatically and rhythmically.
• E.g. passage of food bolus in intestine
– intestinal contents triggers local automatic contractions often set up peristaltic waves that move the contents away from the overfilled intestine, usually in the direction of the anus.
82
Explain Multiunit smooth muscles.
• Composed of discrete, separate smooth muscle fibers.
• Each fiber operates independently of the others and often is innervated by a single nerve ending, like in skeletal muscle fibers.
• Each fiber can contract independently of the others, and their control is exerted mainly by nerve signals (unlike in unitary smooth muscle which is through non-nervous stimuli.
• found in Irish of the eye
• base of hair follicle (the 'goose bump‘)
• Contraction is non-syncytial fashion
• contractions do not spread between fibers.
• Contractions more discrete, fine, and localized
83
Explain: “Smooth muscle when compared with other two types of muscles”
• Slow to initiate
• Prolonged; sustain (tonic) • Slow relaxation
• Less energy utilization
• Has the plasticity
• But produce higher force
