Muscles Flashcards
(156 cards)
1
Q
What are the three primary types of muscle tissue mentioned, and are they all classified under the same tissue type?
A
- Skeletal muscle
- Smooth muscle
- Cardiac muscle
- They are all classified as muscle tissue
2
Q
Roughly what percentage of the body’s weight do muscles constitute, and does this vary between individuals?
A
- Approximately 50% of body weight
- More for some people and less for others
3
Q
Is the exact number of muscles in the human body universally agreed upon, and what range is mentioned?
A
- Not universally agreed upon
- Depends on who you ask
- A range of 600–900 muscles
4
Q
What is a unique fact about the tongue?
A
- Only muscle attached at only one end,
5
Q
What system activates skeletal muscle, and what are motor neurons?
A
- The somatic nervous system activates skeletal muscle
- Motor neurons are nerve cells that activate skeletal muscle
6
Q
What is the neuromuscular junction (NMJ), and how is it defined?
A
- The synapse between a motor neuron and a muscle fiber
- Includes the motor neuron’s axon terminal and the muscle fiber
7
Q
Where do motor neurons synapse with muscle fibers, and what is this process called?
A
- At the neuromuscular junction (NMJ)
- Which is the site of chemical signaling between the two
8
Q
What is a motor unit, and what does it include?
A
- The functional grouping of a single motor neuron and its associated muscle fibers
- It can include few or many muscle fibers
9
Q
How do muscle fibers within a single motor unit behave during contraction?
A
All the muscle fibers in a motor unit contract together
10
Q
Are all muscle fibers in a motor unit the same type, and can a single motor neuron innervate different types (e.g., fast and slow twitch)?
A
- All fibers in a motor unit are of the same type
- A single motor neuron cannot innervate both fast and slow twitch fibers
11
Q
What two factors determine the smoothness and precision of movement, and is activation of motor units random?
A
- The number and timing of motor units activated determine movement quality
- No, activation is not random
- It follows a specific pattern
12
Q
Which motor units are activated first during muscle contraction, and why?
A
- Small motor units are activated first
- Because small motor neurons are more easily excited than large ones
13
Q
What do small motor units allow for, and what about large motor units?
A
- Small motor units allow for more precise, controlled movements
- Fewer but larger motor units allow for greater force development
14
Q
What happens to motor units as individuals age, and how does this affect movement?
A
- Motor units tend to get larger with age
- Making fine motor movements more difficult
15
Q
What is a muscle composed of, and how are its components structured?
A
- Fascicles
- Bundles of many muscle fibers
16
Q
How long do muscle fibers extend, and what are they made up of?
A
- Extend the length of the muscle from tendon to tendon
- Made up of myofibrils
17
Q
What is the “bubble of fibers” in the diagram referring to, and what surrounds it?
A
- It refers to a fascicle
- Which is surrounded by connective tissue
18
Q
What additional structures are shown in the fascicle diagram?
A
- Nerves
- Blood vessels
- Individual muscle fibers
- Visible nuclei
19
Q
What are the contractile units in skeletal muscle, and how are they arranged?
A
- Sarcomeres are the contractile filaments
- Organized in a regular overlapping arrangement that gives skeletal muscle its striated appearance
20
Q
What is the sarcolemma, and is it shown in the diagram?
A
- The sarcolemma is the plasma membrane of a muscle fiber
- Shown surrounding the muscle fiber
21
Q
What are T-tubules, and what is their role?
A
- Invaginations of the sarcolemma
- Extend deep into the muscle fiber to transmit action potentials
22
Q
Are T-tubules internal or external structures, and do they stay on the surface?
A
- Internal structures
- Do not stay on the surface
- They go into the interior
23
Q
What do T-tubules allow the muscle fiber to do, and what signal do they help transmit?
A
- Respond by transmitting action potentials deep into the cell
24
Q
What is the sarcoplasmic reticulum (SR), and where is it located?
A
- An intracellular organelle specialized for calcium storage
- Located inside the muscle fiber
25
Which muscle type has the best-developed SR, and how does it compare to the others?
- Skeletal muscle has the best-developed SR
- Compared to smooth and cardiac muscle
26
What does the well-developed SR in skeletal muscle contribute to?
- To skeletal muscle having the fastest contraction time among the three muscle types
27
What does the SR store, and is it important for skeletal muscle function?
- Calcium (Ca++)
- Yes, it's essential for skeletal muscle contraction
28
How many broad categories of muscle fibers/motor units are there, and what are they based upon?
- 3
- Based upon their histochemical characteristics
29
What are the three broad categories of muscle fibers mentioned?
- Slow twitch
- Fast twitch
- An intermediate type
30
What are the characteristics of slow-twitch oxidative fibers?
- Contract slowly
- Have many mitochondria
- Rely on oxidative metabolism
- Resistant to fatigue
- Generate small maximal force
31
What are examples of activities and muscles that use slow-twitch oxidative fibers?
- Walking and maintaining posture
- The soleus muscle is a specific example
32
What are the characteristics of fast-twitch glycolytic fibers?
- A fast twitch time
- Produce large amounts of tension
- Fatigue rapidly
- Have few mitochondria
- Rely on glycolytic metabolism
33
What are examples of activities that use fast-twitch glycolytic fibers?
- Sprinting
- Jumping
- Weightlifting
34
What are fast-twitch oxidative glycolytic fibers like?
Intermediate characteristics between slow and fast twitch fibers
35
What type of motor neurons innervate each fiber type?
- Small diameter motor neurons innervate slow oxidative fibers
- Large diameter motor neurons innervate fast glycolytic fibers
36
Which motor neurons are more easily excited, and what excites them?
- Small diameter motor neurons are more easily excited by summation of EPSPs (excitatory postsynaptic potentials)
37
How does excitatory input differ for motor unit recruitment?
- Slow oxidative motor units are recruited by relatively low levels of excitatory input
- Fast glycolytic units require high levels
38
Can small diameter motor neurons innervate fast glycolytic fibers, or vice versa?
No
39
What is the neuromuscular junction (NMJ) and where does interaction occur?
- The synapse between a motor neuron and a muscle fiber
- Where the two interact
40
What is the terminal bouton and the motor end plate?
- The axon terminal of the motor neuron
- The motor end plate is the specialized muscle membrane opposite the terminal bouton
41
What neurotransmitter is released at the NMJ, and what receptors bind it?
- Acetylcholine (ACh) is released
- Binds to nicotinic receptors
42
Are synapses at the NMJ excitatory or inhibitory?
Excitatory
43
What is the first step when you decide to move your finger?
Send a command from the motor cortex
44
What system activates skeletal muscle for contraction?
The Central Nervous System (CNS)
45
What does the activation of a motor neuron depend on?
Summation of EPSPs/IPSPs
46
What initiates communication at the NMJ and what happens next?
- An action potential arrives at the terminal bouton
- Opening voltage-gated calcium channels
- Leading to calcium influx
47
What does calcium influx trigger and where does ACh go during NMJ synaptic events?
- Triggers the release of ACh
- Which diffuses across the synaptic cleft
48
What does ACh bind to and what ions move as a result during NMJ synaptic events?
- Binds to nicotinic receptors
- Opening channels for small cations (Na+ and K+)
49
What is the net effect of this ion movement during NMJ synaptic events?
- Depolarization
- Due to the net movement of positive charge into the cell
50
What happens after ACh binds to its receptor and depolarizes the muscle cell?
- Sodium (Na+) moves into the muscle cell
- Causing depolarization
- Which triggers an action potential
51
Where does the action potential spread after it's generated in the muscle?
- Spreads throughout the muscle
- Including deep inside via the T-tubule system
- Which is made of invaginations of the sarcolemma
52
What does the spreading action potential ultimately result in?
- Muscle contraction
- The final outcome of communication at the NMJ
53
What protects the brain from blood-borne toxins, and are muscles similarly protected?
- The blood-brain barrier protects the brain
- But muscles (are peripheral tissues) are exposed to circulating toxin
54
Can toxins affect the NMJ?
- Yes
- Several toxins can block its function
55
What is the first type of toxin that blocks the NMJ, and what is an example?
- Nicotinic receptor blockers
- An example is curare, found in poison darts
56
How do curariform drugs like curare interfere with NMJ function?
- Prevent ACh from binding and stop cation channels on the end plate from opening
- Making it difficult to generate an action potential in the muscle fiber
57
What effect do curariform drugs have on muscle movement and certain exams?
- Blocks reflexive skeletal muscle action
- Keeping structures like the rectum or esophagus dilated during examination
58
What is the second type of NMJ-blocking toxin and what’s an example?
- Exocytosis blockers
- Botox
59
What is the mechanism of Botox at the NMJ?
- Inhibits vesicle release from the motor neuron
- Preventing acetylcholine (ACh) from being released
60
What happens when ACh isn’t released with Botox?
- The motor end plate can’t depolarize
- No action potentials are generated in the muscle
- Contraction doesn’t occur
61
What is Botox considered and where else does it appear?
- Most potent neurotoxin to humans
- The cause of botulinum food poisoning
62
What is the third NMJ-blocking toxin type and examples?
- ACh-esterase inhibitors
- Nerve gases and pesticides
63
What is the normal role of ACh-esterase and what happens when it’s inhibited?
- Breaks down acetylcholine (ACh)
- When inhibited, ACh is not broken down at the NMJ
64
What happens when ACh stays in the synapse too long?
- The end plate stays depolarized
- Leading to continued depolarization
65
What is the effect of continuous depolarization?
- Paralysis occurs because sodium channels remain inactivated
- The signal can no longer be stopped
66
What is the mechanism by which muscles generate force?
- The crossbridge cycle
- The process by which muscles generate force
67
What happens after an action potential is generated in the muscle?
The muscle generates force via the crossbridge cycle
68
What is a sarcomere, and how are actin and myosin arranged in it?
- The basic unit of muscle contraction
- Where actin and myosin are arranged in an overlapping pattern
69
What are the two main myofilaments in skeletal muscle, and what are they made of?
- Actin (thin filament): made of two strands of actin molecules
- Myosin (thick filament): made of myosin molecules
70
What does each actin molecule have a binding site for, and what does the thick filament contain?
- Each actin has a binding site for myosin
- The thick filament contains many cross bridges formed by myosin heads
71
What is the function of the cross bridges?
- They attach to actin
- Create contraction
72
What causes muscle contraction at the sarcomere level, and does it involve a change in filament length?
- Sarcomere shortening caused by sliding of actin and myosin
- The filaments do not change length but slide past each other
73
What is a myosin molecule described as, and what does its head contain?
- A dimer of two subunits
- The head has an actin binding site and an ATP binding site
74
What part of myosin moves and what powers its movement?
- The myosin head
- Powered by ATP hydrolysis
75
What are the two forms of the myosin head, and what are their affinities for actin?
- High-energy form (high affinity for actin)
- Low-energy form (low affinity but remains bound after ADP release)
76
What protein covers myosin binding sites on actin in the absence of calcium, and what holds it in place?
- Tropomyosin covers the sites
- Troponin holds tropomyosin in place
77
What is the muscle's state when calcium is absent and tropomyosin covers actin?
- Relaxed
- No crossbridge formation
78
What happens when calcium binds to troponin?
- Tropomyosin moves
- Exposing myosin binding sites on actin
- Allowing crossbridge formation and muscle contraction
79
What initiates the crossbridge cycle?
- Calcium binding to troponin
- Which exposes actin’s binding sites
80
What happens when myosin binds strongly to actin?
The power stroke occurs
81
What is the power stroke and its result?
- The movement of the myosin head
- Pulls the thin filament toward the center of the muscle
82
What molecule is released at the end of the power stroke, and what happens to the myosin head?
- ADP is released
- The myosin head enters the low-energy state
- But remains bound to actin
83
What happens next in the cycle after ADP release?
- The myosin head detaches
- Returns to its initial position
- The cycle restarts
84
What is the crossbridge cycle analogous to in rowing?
- Power stroke = pulling oar in water
- Detachment = oar breaks water contact
- Return = oar moves to new position
85
What is excitation-contraction coupling and what is it dependent on?
- The process where an action potential in the sarcolemma causes muscle contraction
- Requires neural input from a motor neuron and calcium release
86
What is released from motor neurons at the NMJ and where does it bind?
- Acetylcholine (ACh)
- Which binds to nicotinic receptors on the motor end plate
87
What happens after ACh binds to its receptors on the muscle fiber?
- Sodium (Na⁺) enters the cell
- Causing depolarization
- Initiating a muscle action potential
88
Where does the action potential arrive in the motor neuron and what diffuses across the synaptic cleft?
- The terminal bouton
- ACh diffuses across the cleft
89
What happens to the motor end plate after ACh binding?
Becomes depolarized
90
What happens to the DHP receptor when an action potential arrives in the T-tubule, and what is it?
- Changes conformation
- A calcium channel (dihydropyridine receptor)
91
What is the RyR channel and how is it related to the DHP receptor?
- The ryanodine receptor calcium release channel in the sarcoplasmic reticulum
- Physically linked to DHP
92
What happens when DHP activates RyR?
Calcium is released from the sarcoplasmic reticulum into the cytoplasm
93
What causes the sarcoplasmic reticulum to release calcium?
The action potential traveling down the T-tubules
94
What does calcium bind to in order to allow muscle contraction, and where are these binding sites located?
- To troponin
- The calcium binding sites are located on troponin
95
What does calcium binding to troponin cause?
- Tropomyosin to move off the myosin binding sites on actin
- Allowing actin-myosin binding and the formation of crossbridges
96
What must happen to calcium to terminate muscle contraction, and what is used to achieve this?
- Calcium must be removed from troponin
- Ca²⁺ ATPase, located in the sarcoplasmic reticulum, uses energy to pump calcium back into the SR
97
What happens when calcium is no longer present in the cytosol?
- Tropomyosin covers the myosin binding sites on actin again
- Preventing crossbridge formation and halting contraction
98
What are the two main myofilaments in a skeletal muscle fiber, and what are they made of?
- Actin (thin filament) made of two strands of actin molecules
- Myosin (thick filament) made of myosin molecules (dimers of 2 subunits)
99
What happens after the power stroke?
- ADP is released
- The myosin remains bound to actin in a low-energy, low-affinity state
100
What molecule is required for myosin to detach from actin, and what happens after binding?
- ATP
- When ATP binds, myosin detaches and hydrolyzes the ATP
- Re-cocking the head into a high-energy, weak-affinity position
101
When does the power stroke begin again?
- When calcium is present
- Tropomyosin has moved
- Allowing strong myosin-actin binding
102
What is the rigor state, and what is a real-life example of it?
- Tight binding between actin and myosin after the power stroke
- With no ATP present
- A real-life example is rigor mortis
103
What happens to muscles during rigor mortis and why?
- Muscles become stiff and stay contracted
- There is no ATP available to detach crossbridges
104
What eventually happens to the muscles in rigor mortis, and what can this phenomenon help determine?
- The muscles break down
- Rigor mortis can help determine the time of death
105
What is a muscle twitch, and what does it refer to?
- A single contraction-relaxation cycle
- Refers to the tension or force generated, not the action potential
106
What are the three phases of a muscle twitch?
- Latent period
- Period of contraction
- Period of relaxation
107
What happens during the latent period?
- Excitation-contraction coupling occurs:
1. The action potential travels down the T-tubules
2. Calcium is released from the SR
3. Calcium binds to troponin to initiate the crossbridge cycle
- No contractile force is present yet
108
What is the duration of a muscle action potential, and how long can tension last?
- About 2 milliseconds
- Tension can last up to 100 milliseconds
109
What causes the delay between the action potential and muscle tension?
The latent period
110
What happens during the period of contraction?
- Intracellular calcium levels are high
- Crossbridge cycling occurs
- Muscle tension or force increases
111
How is the period of contraction seen on a tension graph, and what is happening with calcium?
- Tension rises steadily
- Calcium levels are high and stable
112
What happens during the period of relaxation?
- Intracellular calcium levels fall as Ca²⁺ is pumped back into the SR
- Muscle tension decreases
113
Does crossbridge cycling require high or low calcium?
High calcium
114
Is a single muscle twitch useful for generating significant force?
No, multiple twitches must work together to generate meaningful force
115
What varies with muscle fiber type regarding a twitch?
Rise-time and duration of twitch force
116
What are the rise time and duration of a twitch for fast glycolytic fibers?
- Rise time is ~50 milliseconds
- Duration is ~120 milliseconds
117
What phase follows the period of contraction, and what happens during it?
- The period of relaxation
1. Intracellular calcium levels fall
2. Calcium is pumped back into the sarcoplasmic reticulum
3. Fewer crossbridges interact with actin
4. Muscle tension gradually falls to zero
5. The muscle is relaxing
118
What causes the decline in force during the period of relaxation?
Calcium being pumped back into the sarcoplasmic reticulum
119
What happens when successive twitches occur due to increased action potential frequency in skeletal muscle?
- They fuse with each other in a process called summation
- Leading to increased contractile force
120
What happens if there is no time for the muscle to relax between stimuli?
Tension rises
121
Why can multiple action potentials occur in a muscle fiber before tension ends?
Because muscle action potentials are much shorter than the duration of tension
122
What is tetanus, and what does the muscle generate during it?
- One continuous contraction from repeated stimulation
- During which the muscle generates maximum force (max isometric force)
- Shown as a plateau region on a graph
123
What is muscle fatigue, and what causes it?
- A complex physiological (protective) process
- May be caused by hyperexcitability due to excessive firing
124
What is the effect of muscle fatigue on tension?
Results in a rapid decrease in muscle tension
125
What is Charlie’s horse, and how might stretching help?
- A painful sustained contraction
- Caused by hyperexcitability of motor neurons
- Stretching may help alleviate it by inhibiting the firing of motor neurons
126
Where is smooth muscle found, and how is it classified?
- Found in internal organs and blood vessels, including the GI, urinary, reproductive, respiratory, vascular (blood vessels), and ocular (e.g., pupil) tracts
- Classified by both location and communication with neighboring cells
127
Is smooth muscle under voluntary control, and what nervous system controls it?
- It's involuntary
- Controlled by the autonomic nervous system (ANS)
128
Do smooth muscles have sarcomeres, and how are actin and myosin arranged?
- Do not have sarcomeres
- Actin and myosin are not organized in parallel
- Run in several directions
- Which allows smooth muscle to operate over a range of lengths and have a longer contraction range
129
Does smooth muscle contract quickly, and how efficient is it?
- It contracts and relaxes much more slowly than skeletal muscle
- Uses less energy
130
What are the two main types of smooth muscle based on communication?
- Single-unit (visceral) smooth muscle
- Multi-unit smooth muscle
- Some overlap between them
131
Where is single-unit smooth muscle found, and how does it behave?
- Found in the intestinal tract and blood vessels
- Has spontaneous activity
- Activated by the ANS
- Can exert tension without external stimulation
- Contracts when stretched
132
Why is stretch-activated contraction important in single-unit smooth muscle?
- Helps maintain tension in tissues like artery walls to regulate blood pressure
133
How are the cells in single-unit smooth muscle connected, and how do they function?
- Connected by gap junctions
- Contract together as a single unit
134
Where is multi-unit smooth muscle found, and how does it behave?
- Found in large airways, arteries, and the ciliary muscle in the eye
- Each fiber acts individually, is not electrically linked
- Can be stimulated independently
- Heavily innervated
- Contracts only when its nervous supply is stimulated
135
Where does calcium come from to initiate contraction in smooth muscle, and what is the main source?
- Both extracellular fluid (ECF) and the sarcoplasmic reticulum (SR)
- Though the SR only contributes a small amount
- Calcium influx through membrane channels also triggers calcium-induced calcium release from the SR
136
How does calcium initiate contraction in smooth muscle?
- Binds to calmodulin
- Forming a complex that activates myosin light chain kinase (MLCK)
- MLCK phosphorylates myosin light chains
- Enabling crossbridge cycling and contraction
137
Is smooth muscle contraction faster or slower than skeletal muscle, and why is relaxation slower?
- Slower in smooth muscle
- Relaxation is also slower
- Due to the slower removal of calcium
138
How is calcium removed during smooth muscle relaxation, and what other processes are involved?
- Removed by Ca-ATPase pumps and Ca-Na counter-transport
- Phosphatases remove phosphate from myosin
- Ending contraction
- These phosphatases are counterparts to kinases like MLCK
139
What two types of cells are found in cardiac muscle, and what do they do?
- Contractile cells: generate force
- Conductile cells: conduct excitation
140
How are contractile elements arranged in cardiac muscle, and what does this cause?
- Arranged in sarcomeres
- Giving cardiac muscle a striated appearance
141
How developed is the sarcoplasmic reticulum in cardiac muscle?
- Intermediately developed compared to skeletal muscle
142
What structural feature allows synchronous beating in cardiac muscle?
Gap junctions
143
What nervous system modulates cardiac muscle activity?
The autonomic nervous system (ANS)
144
What is the duration of an action potential in cardiac ventricles, and what causes the plateau phase?
- About 300 ms
- Caused by slow calcium (Ca²⁺) channels
145
What is the function of the plateau phase in cardiac muscle action potentials?
- Allows enough time for a forceful contraction to occur from a single action potential
- Lasting 20–50× longer than in skeletal muscle
146
What does the shape and duration of a cardiac AP reflect?
Changing permeability to Na⁺ and Ca²⁺
147
Can cardiac muscle undergo tetanus or summation? Why or why not?
- No
- Due to a long refractory period
- Even if another AP fires in quick succession, the muscle will relax instead of summating
148
Can cardiac muscle increase force of contraction through motor unit recruitment or frequency?
No, cardiac muscle cannot increase force by recruiting motor units or increasing stimulation frequency
149
How does cardiac muscle increase force of contraction, and what law describes this?
- By increasing muscle length
- Starling’s law
- As the volume of blood in the chambers increases during diastole, the force of contraction increases due to more optimal overlap of actin and myosin filaments
150
Where does calcium for contraction come from in cardiac muscle?
- Most comes from the extracellular fluid (ECF)
- Rest comes from the sarcoplasmic reticulum (SR)
151
What powers contraction in cardiac muscle?
- Contractile proteins, in the presence of increased cytosolic Ca²⁺, power contraction (systole)
152
How is calcium removed for relaxation (diastole)?
- Ca²⁺ pumps in the SR
- The Na⁺/Ca²⁺ membrane exchanger
153
What is Digitalis used to treat, and how does it help?
- Treats certain heart conditions like congestive heart failure
- By increasing intracellular calcium levels and strengthening the heartbeat
154
What enzyme does Digitalis inhibit, and what is the effect on intracellular sodium?
- Inhibits the sodium-potassium ATPase
- Leading to increased intracellular sodium
155
How does increased intracellular sodium affect the Na⁺/Ca²⁺ exchanger?
- Reduces Na⁺ influx and Ca²⁺ efflux
- Causing more calcium to remain in the cell
156
What is the overall result of Digitalis’s action?
- Increased intracellular calcium
- Stronger heart contractions
