Chapter 12 Phisio
What is occurring during the contraction time?
1. ACH is released by a motor neuron and initiates an AP in the muscle cell 2. AP is carried into the central portions of the muscle fiber by transverse (T-tubules) 3.AP stimulates the release of calcium ions from lateral sacs of the sarcoplasmic reticulum 4. The calcium ions bind to troponin, which moves tropomyosin off the myosin binding sites on the actin molecules 5. Exposed actin molecules bind with the myosin head groups 6. Binding of actin & myosin causes the myosin heads to bend (powerstroke) which pulls the actin filaments toward the M line. This causes the sarcomere to shorten
What is occurring during the relaxation time?
1. ADP & Pi are released during the powerstroke 2. A fresh ATP binds to the myosin head, which causes it to detach from the actin 3. ATP is split by myosin ATPase & is energized (cocked) & ready for another contractile process 4. (A) If calcium ions are still present, the myosin binding sites on actin are still exposed & myosin is allowed to interact again & cycle another contraction 4. (B) f calcium ions are re-sequestered back into the lateral sacks, troponin returns to its normal shape, allowing tropomyosin to slip back into its blocking position 5. Cross-bridge cycling is stopped. Muscle relaxes
What is happening during the the relaxation time?
1. Calcium is being pumped back into the lateral sacs & the cross bridges are slowly disengaging. 2. As long as the next AP is generated before the concentration of calcium returns to normal, the residual calcium is combined with the calcium released from the new AP, resulting in an overall higher calcium concentration & more available time for cross bridge cycling to occur 3. all of the cross-bridges that were formed during the first AP do not slip back to their relaxed state, thus the second contractile event builds upon the first & overall tension increases
What is each myosin molecule made up of?
2 heavy and 4 light chains -combine to for 2 intertwined golf clubs
How long does muscle AP last? How long does the contracting and relaxing take?
2ms 100ms or longer
Neuromuscular fatigue
ACH production cannot meet the muscle's demand
What are regulatory proteins?
Actin and Myoisn
What are thin filaments composed of?
Actin, Troponin, Tropomyosin
What are the two binding sites for the myosin head?
-actin -myosin ATPase
What is the difference between slow and fast oxidative fibers?
fast fibers have a higher myosin ATPase activity than slow fibers
What are sarcomeres?
filament that is the smallest functional contractile unit of skeletal muscle
What do actin molecules do?
form the backbone of the thin filament by forming two intertwining strands -spherical shape
Twitch summation
frequency based increase in fiber tension, similar to temporal summation -single AP results in a twitch -if the twitch is allowed to go back to its initial relaxed state, another AP will result in another twitch of the same magnitude
What are the two myosin heads composed of?
heavy and light chains
What is hypertrophy?
increase in diameter bc of myosin and actin synthesis -fast fibers only
Hyperplasia
increase in number of cells -no mitosis -cells increase in size and split lenghtwise
What is hyperplasia?
increase in the number of cells -cells increase in size and split lengthwise -no mitosis
How do you increase the development of tension in a single-unit smooth muscle?
increase the amount of calcium -you can't use recruitment as a means of increasing tension (no using other motor units, just one)
What does the abundant amount of myoglobin do to oxidative fibers?
increases the rate of oxygen transfer from blood and imparts a red color to the fibers
What are the dense bodies anchored throughout the cell by? What are the attached to?
intermediate filaments the cell membrane
What are the two myosin tails composed of?
intertwined heavy chains
What is the development of tension in a muscle fiber related to?
length -number of actin and myosin cross-bridges that can form
What happens if a muscle is stretched?
less than optimal number of cross-bridges that can be formed -less than optimal tension being developed
What do muscle spindles do?
maintain optimal muscle length
What are M lines?
mark the middle of sarcomere -anchor thick filaments
What do glycolytic fibers rely upon?
massive quantities of stored glycogen within the fibers
What does the interaction between the myosin head and the actin molecules form?
mechanism in which skeletal muscle contract
What can ATP synthesizing change?
mitochondria content and vascularization to fibers (increase)
psychological fatigue
motor neuron activity is below desired contractile state
What happens when the number of muscle fibers involved in a particular contraction is below maximal?
motor units will "cut in" & "cut out" to give each other a rest
What is a muscle fiber?
muscle cell
Isometric contraction
muscle tension develops at a constant muscle length
Isotonic contractions
muscle tension remains constant as the muscle changes length -ex: as soon as you are able to lift a dumbbell, no further tension is needed and the muscle shortens
What are within each muscle fiber?
myofibril
What are thick filaments made of?
myosin (protein)
What does each actin molecule posses?
myosin binding site
denervation atrophy
nerve supply to muscle is lost
What is denervation atrophy?
nerve supply to muscle is lost -decrease in diameter
How long can muscle contraction be maintained?
It depends upon the number of motor units needed for the given contraction.
What do oxidative fibers posses an abundant amount of? What does this do?
Myoglobin -increases the rate of oxygen transfer rate -add a red color to the fibers
What is the difference between oxidative and glycolytic fibers?
Oxidative fibers produce more ATP and no lactic acid and are more resistant to fatigue Glycolytic fibers rely upon massive quantities of stored glycogen within the fibers -don't require oxygen, little myoglobin (white fibers)
What specific branch would be for voluntary movement?
Pyramidal branch
What is each myofibril composed of?
Thick and thin filaments
What level is smooth muscle regulation on?
Thick filament or Myosin
What level is skeletal muscle regulation on?
Thin filament
Are all of the muscle fibers within a motor unit the same fiber type?
Yes
Can fast fibers switch between oxidative and glycolytic pathways?
Yes
Can the fast fibers switch between oxidative and glycolytic pathways?
Yes
What is pacemaker activity?
a membrane potential that gradually depolarizes due to shifts in passive ionic fluxes, resulting in an AP
What is a motor unit?
a single motor neuron and all of the muscle fibers that innervates (supplies with nerves) -when a motor neuron unit fires an AP all of the muscle fibers that innervates will contract simultaneously
What is tetanus?
a smooth, sustained contraction of maximal strength
What is a multiunit smooth muscle?
a type of smooth muscle that are separated into discrete units (motor units) and are nuerogenic (stimulated by a nerve)
What happens if a muscle is shortened too much?
actin and filaments overlap, cross-bridges reduced, thick filaments forced into the Z lines, preventing further shortening, calcium released inhibited and ability of calcium to bind to troponin and remove tropomyosin is also inhibited
What kind of filaments are attached to the dense bodies?
actin filaments
What are thin filaments made of?
actin, troponin, tropomyosin
What is a gap junction?
allow intercellular communication -once a single smooth muscle cell contracts it will send the wave to the next cell and will cause a falling domino effect
What are slow wave potentials?
alternating depolarizing and hyperpolarizing of the membrane potential -if the depolarization is significant AP generation is achieved
How does a single unit smooth muscle contract?
as a single unit -myogenic (will contract on its own) -connected by gap junctions -it is self excitable -main smooth muscle unit
What specific branch would be for involuntary movement?
Extrapyramidal
What kind of movements does the Primary Motor Cortex allow?
Fine, discrete, voluntary movements
What is steric hindrance?
basically when something gets in the way of something else
Hypertrophy
Increase in diameter of muscle fibers due to an increase in myosin and actin synthesis -only fast, glycolytic fibers
What allows tropomyosin to slide away from its blocking position on actin?
calcium ions binding to troponin and a conformational change in troponin's structure -this allows actin and myosin to interact causing contraction
What kind of movements does the Multineural Motor System allow?
Involuntary movements -Regulation of overall body posture
What are A bands?
consist of overlapping bands and thick filaments
What are H zones?
consist of thick filaments
What are I bands?
consist of thin filaments
What happens as you increase the frequency of the AP's in the muscle?
contraction stacks upon each other until tetanus is reached
Disuse atrophy
decrease in muscle mass due to inactivity
What is disuse atrophy?
decrease in muscle mass due to inactivity -decrease in diameter
What are the skeletal muscle fiber types?
slow oxidative fibers, fast oxidative fibers and glycolytic fibers
What can smooth muscle cells be described as?
small and unstriated
What are Z lines?
the outer periphery of a sarcomere -anchoring point for thin filaments
What is cross-bridge cycling?
the shortening due to the interaction of myosin and actin when calcium ions are present
What is each myosin head pointed to?
the thin filament surrounding it
What happens as thin filaments slide inward?
they pull on the Z lines, shortening the sarcomere -since all the muscle fibers shorten simultaneously the entire fiber contracts
What kind of filaments interact with thin actin filaments?
thick myosin filaments
What three filaments in smooth muscle do not contain myofibrils or sarcomeres?
thick myosin, thin actin, intermediate filaments
Which filaments slide inward toward the M line, during contraction?
thin filaments
Relaxation time
time period between peak tension and complete relaxation
What is contraction time?
time period between the beginning of contraction to peak contraction strength (tension)
What is the latent period?
time period between the beginning of stimulus and the beginning of contraction
When is the muscle at its optimal length?
while at rest -maximum number of actin and myosin cross-bridges formed
At what three outputs is control mediated?
1. spinal cord 2. Primary motor cortex -voluntary movement 3. Multineural motor system -involuntary movement -regulation of overall body posture stopped on slide 25
Why does fatigue occur?
1. Lactic acid accumulation (inhibits enzymes) 2. Depletion of energy reserves 3. Neuromuscular fatigue (not enough ACH) 4. Phycological fatigue (neuron activity below desired contractile state)
ATP is required at these following stages:
1. Splitting of ATP by myosin ATPase 2. Binding of ATP to the myosin to detach it from the actin 3. Active transport of calcium back into the lateral sacs
How can muscle contractions be varied in strength?
1. The number of muscle fibers contracting within a muscle - motor unit recruitment 2. The tension developed by each contracting fiber
What three pathways provide extra ATP during contraction?
1. creatine phosphate -> ATP 2. Oxidative phosphorylation/Electron transport system (if oxygen supply is met) 3. glycolysis (if oxygen supply is not met, ATP for contracting muscle)
How can tension of individual muscle fibers vary?
1. frequency of stimulation 2. length of the fiber at the onset of contraction 3. extent of fatigue 4. thickness of the fiber
How are the muscle fibers classified?
1. speed of contraction 2. primary mechanism for generating ATP
Why does the contraction cycle outlast the muscle AP cycle?
?
Are slow and fast fibers interchangeable? Why?
No If you had a motor unit made up of 50% fast and 50% slow muscle fibers, would they all contract at the same rate? or jerky movement? -with finite control muscle contraction can be smooth -if you had a muscle fiber with mixed fast and slow fibers it would not be good
What happens if another AP is generated before the muscle fiber is completely relaxed?
The AP generates another contraction even before the muscle fiber is relaxed. The tension developed by this second stimulus is added on to what ever is left from the first contraction, thus increasing the tension developed by the muscle fiber
What does troponin consist of?
Three polypeptide units -one that binds to tropomyosin -one that binds to actin -one that binds to calcium ions
How does muscle contraction happen?
Tropomyosin...
How are the muscle fibers of a motor neuron distributed?
uniformly throughout the muscle -to give an even contraction -when more tension is needed more motor units are recruited and more muscle fibers are stimulated to contract
