L8 Muscle Flashcards
What determines the strength of a muscle contraction?
- Cross Sectional Area
- # and type of fibers within the muscle
- Frequency of muscle fiber stimulation
- Thickness of each muscle fiber
- Resting length of muscle fibers
- Velocity of movement
What kind of muscle design is the strongest?
pennate
Hennemann’s Size Principle
- When a weak contraction is desired, smaller motor units are recruited
- Stronger contraction is wanted, progressively larger motor units are recruited
Is this recruitment pattern the same when using electrical stimulation to cause a muscle contraction?
NO
activate Type 2 motor unit first
Multiple motor unit summation
different numbers of motor units are brought into play to produce gradations of strength
Type of muscle fibers stimulated to contract
- Muscle fiber type is determined by MOTOR NEURON
- Somatic motor neuron innervates only one type of muscle fibers
Slow Twitch Fibers
Type 1
High Oxidative Capacity
Resistant to Fatigue
postural control muscles are mainly type 1
motor neurons have a slower conduction rate
High Oxidative Capacity of Type 1
rich capillary supply
lots of mitochondria and aerobic enzymes
high concentration of myoglobin
Myoglobin
iron and oxygen binding protein
Resistant to fatigue type 1 fibers
postural muscle s
Fast Twitch Fibers
Type 2 Fibers, phasic
Lower oxidative capacity
Fatigue quickly
gastroc, biceps, extraocular muscles are type 2
motor neurons have faster conduction rate
conduction velocity is fast, cell body size is large
Lower oxidative capacity Type 2 Fibers
Fewer Capillaries, mitochondria
Lower concentration of myoglobin
High concentration of glycolytic enzymes and large glycogen stores
Subpopulations of Type 2 Fibers
Type 2a
Type 2b
Type 2x
Type 2a Fibers
slowest, for endurance
Type 2b fibers
don’t exist in humans
Type 2x fibers
fastest, sprint/interval activities
How is it determined what muscle fibers you have?
Specific muscle (gastroc vs core)
genetics
age
training
you cannot convert type 1 to type 2, you can convert between the type 2 subtypes
Structure of skeletal muscle
composed of fibers/cells
multinucleate
Myofilaments–> Myofibrils –> Muscle fiber
Myofibril
subunit of muscle cell that consists of successive sarcomeres
single sarcomere contains many longitudinal myofilaments
Sarcomere
Z to Z, basic unit of striated muscle contraction
H band
thick filaments, composed of myosin
I band
thin filaments, composed of actin
A band
overlap of thick and thin filaments
Z line/disc
center of each I band
What happens to the sarcomere during contraction?
Distance between Z lines shortens
Successive A bands move closer together
I bands slide over and between H bands, decreasing length
Contraction Steps (ATP)
- globular head of myosin splits ATP
- ADP and P bound to myosin until myosin attaches to actin
- Phosphate is released, causing myosin to perform a power stroke
- Filaments slide
- ADP is released when myosin binds to new ATP, breaking cross bridge
- ATP is hydrolyzed, myosin head returns to original conformation
Excitation-contraction Coupling
- Action potential travels down motor neuron to axon terminal
- Voltage gated calcium channels open, so calcium diffuse into terminal
- Rise in intracellular calcium causes release of AcH into NMJ
- Ach binds to nAch receptors in motor end plate, leading to EPP
- EPP leads to action potentials that travel into T-tubules
- AP causes calcium to be released from SR
- Calcium binds to troponin, allows tropomyosin to move, myosin can bind to actin
Motor end plate
specialized region of sarcolemma at the NMJ with increased nACH receptors
binding of ACh to NaCh reeptors produces an end-plate potential, if it is enough, AP is produced
Terminal Cisternae
Calcium in relaxed muscle is stored within expanded portions of the SR
Transverse tubules
terminal cisternae are separated by a narrow gap which are the T-Tubules
narrow membrane tunnels that are continuous with the sarcolemma
Sarcoplasmic reticulum
modified endoplasmic reticulum, consisting of sacs and tubes that surround each myofibril
Structure of thin filament
F actin is composed of subunits of actin called G-actin
F actin are arranged in a double row and twisted to form a helix
Tropomyosin
protein that lies within the groove between the F-actin chains
Troponin
complex of 3 proteins that is attached to tropomyosin, not actin
Role of Ca in muscle contraction
Ca binds to troponin, which triggers tropomyosin to move. Allows actin to be revealed, causing power strokes
How does skeletal muscle relaxation occur?
- Neural stimulation stops
- ACh is broken down by actylcholinesterase within the NMJ
- SR stops releasing calcium and immediately resequesters calcium that was just released
ACh is broken down…
AChE is an enzyme that metabolizes ACh into acetate and choline
AChEIs
drugs that inhibit acetylcholinesterase, allows ACh to stay in the synaptic cleft longer
used in treatment of myasthenia gravis and alzheimer’s disease
ADRs of AChEIs
DUMBBELSS
Diarrhea
Urination
Miosis
Bronchoconstriction
Bradycardia
Excitation
Lacrimation
Salivation
Sweating
Muscle Fatigue
inability of a muscle to maintain the required tension for a given task or generate an expected power when a contraction is sustained
most likely due to accumulation of extracellular K+
Moderate Exercise and Muscle fatigue
Failure at any site downstream or upstream can contribute to development of muscle fatigue
Fatigue is multifactorial
Increased intracellular concentration of lactate, H+, inorganic phosphate
impaired Ca 2+ release
increased production of fatigue reactants
depletion of glycogen
Central Fatigue
NMJ and upstream
fatiguing exercise changes brain concentrations of NTs, which decreases neural drive to muscle
increase perception of effort
decreased motivation
high serotonin, low dopamine
Peripheral Fatigue
produced by changes at or distal to NMJ
AI diseases that target synaptic proteins
muscular dystrophies
Central fatigue pathological conditions
MS, guillain barre, CIDP
S/S of Muscle Fatigue
muscle discomfort, pain, cramping
tremor in contracting muscle
unintentional slowing of movement
altered quality of movement
decline in peak torque, EMG, MMT
Muscle Fatigue Treatment
No single agreed upon treatment , fatigue must be evaluated based on many variables
massage, compression, NSAIDs, e-stim all treat the S/S
light aerobic exercise is more effective than total rest
Endurance training increases…
lactate threshold
means it prolongs the time until an increasing proportion of energy must be derived from anaerobic glycolysis
Muscle adaptations to aerobic/endurance training
- Increases # of mitochondria and aerobic enzymes in all muscle fiber types
- Increases # of type 2a fibers and decreases # of type 2B fibers
- Does NOT increase size of muscles
Other adaptations of endurance training on muscles
improved ability to obtain ATP from OP
increased size and # of mitochondria
less lactic acid produced
increased myoglobin
increased triglyceride content
increased lipoprotein lipase
increased energy derived from fat
Lower rate of glycogen depletion
improved efficiency of O2 use
decreased type 2B fibers
Muscle adaptations to resistance training
Hypertrophies type 2 muscle fibers
Increase in size and number of MYOFIBRILS
cell # increases mainly in animals, minimal in humans
Smooth Muscle
found in almost every organ
major component of the walls of hollow organs
Smooth muscle contraction
-arranged in circular or longitudinal layers
-lack of sarcomeres, no striations
-more thin vs thick (16:1)
-actin and myosin contract by unique regulatory mechanism
thin filaments/actin attach to either plasma membranes, dense bodies
Myosin Heads of Smooth muscle fiber
arrangement of myosin heads to thin filaments is required for proper smooth muscle function
can still contract at very stretched lengths
must be able function when greatly stretched
Urinary bladder
smooth muscle cels stretch 2.5 times their resting length
Uterus
smooth muscle cells stretch 8 times their original length by end of pregnancy
Differences between smooth and skeletal muscle
- smooth muscle cells produce graded depolarizations and contractions w/out producing APs
- Extracellular Ca enters through special channels in smooth
- Ca binds with calmodulin in smooth
- Calmodulin joins with myosin kinase
Similarities between Smooth and Skeletal
- Depends on sharp rise in intracellular free Ca
- Greater the depolarization of smooth muscle, more Ca will enter the cell, stronger the contraction
- Myosin head binds with actin
- Relaxation occurs when Ca concentration decreases
Single-unit smooth muscles
Most smooth muscles
many gap junctions
only some cells receive ANS innervation
have intrinsic electrical activity and contract in response to stretch
ANS innervation of smooth muscles
entire surface of a smooth muscle contains receptors
ACh released along a stretch of an autonomic nerve fiber located a distance from smooth muscle cells
Varicosities
regions of autonomic fiber that release NT
Multiunit smooth muscles
contraction requires nerve stimulation
few if any gap junctions
arrector pili muscles in in skin
ciliary muscles attached to eye lens
