Muscle Physiology Flashcards
function of muscle
convert chemical energy into mechanical energy
what percent of the total body mass is skeletal muscle?
30-40%
largest muscle
gluteus maximus
smallest muscle
stapedius (inner ear)
muscle generates ____
heat
what percent of the body mass is smooth and cardiac muscle?
10%
no muscle means
no breathing, no chewing, no blinking, no digesting, no circulation, no walking, no talking, no smiling, no sitting up straight
—> NO MOVEMENT
skeletal muscle
- long parallel fibers
- striated
- mutinucleated
- voluntary
cardiac muscle
- short branched fibers
- striated
- uninucleated
- involuntary
smooth muscle
- sheets of cells
- not striate
- uninucleate
- involuntary
whole skeletal muscle
bundle of fascicles
fascicle
bundle of muscle fibers
muscle fiber
bundle of myofibrils (contractile protein bundles)
myofilaments
- contractile proteins
- in the presence of Ca+2 the myosin cross-bridge and actin physically interact to generate tension
scaroplasmic reticulum (smooth ER)
stores Ca+2
transverse tubule (T-tubule)
in foldings of plasma membrane
sarcolemma
plamsa membrane
sarcoplasm
cytoplasm
myofibrils
- bundles of myofilaments
- intracellular contractile structures arranged in repeating units)
thin myofilament
actin
thick myofilament
myosin
sarcomere
- functional units of muscle
- one unit of repeating thin and thick filament pattern within the myofibril
I band
actin only (light under the microscope)
M line
grey under the microscope
H band
myosin only
A band
- both actin and myosin
- dark under the microscope
sarcomere within filament
from one z disk to next z disk
what are the “chaperons” regulatory proteins?
- tropomyosin
- troponin
tropomyosin
- covers active sites on 7 actin molecules
- on/off switch for contracted/relaxed muscle
what are the contractile proteins?
- myosin
- actin
muscle contraction order
- activation of motor neuron
- signal transmitted to muscle at Neuromuscular Junction (NMJ)
- excitation of muscle fiber
- excitation-contraction coupling
- contraction (sliding filament mechanism)
- relaxation
what division is skeletal muscle in?
efferent division
somatic motor
skeletal muscle (voluntary)
autonomic motor
cardiac and smooth muscle
motor unit
one motor neuron and all of muscle fibers it innervates
small motor units
- 3-15 fibers
- fine motor control
- eyes
- hands / fingers
larger motor units
- 100s-1000s
- for strength and power
- arms
- legs
skeletal muscle is always _____
excitatory
neuromuscular junction (NMJ)
alpha motor neurons originating in CNS terminate on skeletal muscle fibers at a specialized synapse
junctional folds
increase SA
motor end place
region of muscle membrane directly under axon terminal
what is the first phase of muscle contraction?
excitation of muscle fiber
excitation of muscle fiber step 1
- arrival of nerve signal
- Ca+2 enters synaptic knob via VG Ca+2 channels
excitation of muscle fiber step 2
- acetylcholine (ACh) release
- via exocytosis of synaptic vesicles
- ACh diffuses across synaptic cleft
excitation of muscle fiber step 3
binding of ACh to receptor
excitation of muscle fiber step 4
- opening of ligand-regulated ion gate; creation of end-plate potential
- Na+ flows in, then K+ flows out
- More Na+
- causes local change in potential = end plate potential (EPP)
excitation of muscle fiber step 5
- opening of voltage-regulated ion gates; creation of APs
- caused by end plate potential
- APs propagate along muscle membrane
- muscle fiber = “excited”
Botulinum toxin
- prevents vesicle exocytosis
- skeletal muscle paralysis
- respiratory arrest
organophosphates
- pesticides
- nerves gases (sarin gas)
1. Inhibits AChE
2. Na+ channels remain inactive (must repolarize to close)
3. ACh receptor become desensitized
4. no additional APs
5. no contraction
6. paralysis
7. respiratory arrest
curare / succinylcholine
- nACh receptor antagonist
- binds to receptor but doesn’t open channel
- no AP
- no contraction
myasthenia gravis
- autoimmune disorder
1. auto-antibodies produced against nACh receptor
2. bind/block/degrades
3. affects muscles in face/throat/eyes first
4. caused muscle weakness and fatigue - can treat with AChE inhibitor that increase available ACh
what is the second phase of muscle contraction?
excitation - contraction coupling
latent period
coupling events occur
excitation-contraction coupling
events linking muscle AP to cross-bridge (CB) formation
skeletal muscle fiber
AP has to travel far/deep inside fiber via T-tubules
Excitation-contraction coupling step 1
muscle AP propagated into T-tubules
- at rest Ca+2 stored in SR
- free Ca+2 in cytoplasm is very low
excitation-contraction coupling step 2
- activated DHP receptor caused ryandine receptor to open
- Ca+2 is released from SR
DHP receptor
VG receptor
Ryanodine receptor
Ca+ channel
excitation-contraction coupling step 3
increased Ca+2
excitation-contraction coupling step 4
binding of calcium to troponin
excitation-contraction coupling step 5
shifting of tropomyosin; exposure of active sites on actin
Cross bridge cycling step 1
hydrolysis of ATP to ADP+Pi activation and cocking of myosin head
- at rest, myosin head is already energized
cross bridge cycling step 2
- formation of myosin-actin cross-bridge
- increased Ca+2
cross bridge cycling step 3
power stroke; sliding of thin filament over thick filament
cross bridge cycling step 4
binding of new ATP; breaking of cross-bridge
cross bridge cycling
- if Ca+2 stays high the process will continue
- can cycle about 5x sex if Ca+2 is present
rigor mortis
- “stiffness of death”
- occurs about 3 hours after death
- peaks at 12 hours
- unit about 48 hours as muscle decomposes
- contraction continues as long as there is ATP and Ca+2
- locked in contracted state because ATP is required to break CB
relaxation phase step 1
cessation of nervous stimulation and ACh release
relaxation phase step 2
ACh breaks down by ACHE
relaxation phase step 3
Ca+2 ATP actively transports Ca+2 back into SR
- this pump is always on
- takes time to pump all Ca_2 back in
- reason why tension generated lasts so long after AP (100 ms)
relaxation phase step 4
loss of calcium ions from troponin
- muscle returns to resting length
relaxation phase step 5
return of tropomyosin to position blocking active sties of actin
muscle returns to resting length by:
- elastic recoil
2. action of opposing muscles
costamere
- transmits contrails forces from sarcomeres on one myofiber to another
- synchronizes contraction of myofiber within a muscle
muscular dystrophy (MD)
- mutation in dystrophin gene (X chromosome)
- Duchenne = dystrophin absent
- Becker = protein truncated
- muscle contraction = muscle degeneration and weakening = death from cardiac
muscular dystrophy symptoms
- becomes evident when begin walking
- waddling gait
- need braces by age 10
- unable to walk by age 12
- respiratory arrest
- cardiomyopathy
Duchenne muscular dystrophy lifespan
15-20 years
Beckers muscular dystrophy lifespan
usually normal
Gower’s sign
affect hip muscles first
load
force exerted by an object on muscle
tension
force exerted by muscle on object
isometric contraction
same length
- CB cycle but bing in same place
isotonic contraction
change length
- work performed
types of isotonic contraction
- concentric
- eccentric
concentric isotonic contraction
- shortening
- tension > load
eccentric isotonic contraction
- lengthening
- tension < load
muscle fiber AP
response of a single fiber to one AP
skeletal muscle metabolism
- creatine phosphate
- glycolysis
- oxidative phosphorylation
- muscles use lots of ATP
- different fiber types use different mixtures of ATP sources
what system do you use for immediate energy?
phosphagen system (direct phosphorylation)
phosphagen system
- borrow phosphate from other molecules to make ATP
- FAST but limited
- about 5-8 seconds
- ATP –> ADP + Pi
what system do you use for short term energy?
glycolysis (anaerobic fermentation)
glycolysis
- uses glycogen from muscle and glucose from blood
- 2 ATP / glucose
- fast, but not efficient
- produces lactic acid
- no oxygen available
- occurs in cytoplasm
- 30-40 seconds of intense energy
- if not intense it will occur longer
what system do you use for long term energy?
aerobic respiration (oxidative phosphorylation)
aerobic respiration
- uses glycogen from muscle and glucose and fatty acids from blood
- 32 / ATP
- slower, but most efficient
- produces CO2
muscle fatigue
decrease ability to produce tension despite continued stimulation
factors associated with fatigue
- decreased glycogen / blood glucose
- increase lactic acid (increase acidity = decrease enzyme function)
- increase K+ in ECF (decrease excitability)
- dehydration / decrease electrolytes (decrease excitability)
- increase ADP/Pi
- -> CB slower to release ADP/Pi if Pi increased
- -> Pi combine Ca+2
- decrease CNS output to muscles
lactic acid
decrease enzyme function
smooth muscle
lines the walls of hollow organs
similarities between skeletal and smooth muscle
- siding filaments (actin, myosin) -> CB’s
- ATP power
- elevated Ca+2 triggers contraction
differences between skeletal and smooth muscle
- organization of contractile filaments
- many inputs can modulate activity
- process of EC-coupling
smooth muscle difference organization of contractile filaments
- small cells in sheets, single nuclei, no striations
- no sarcomeres, T-tubules, or troponin
smooth muscle difference many inputs can modulate activity
- these can be excitatory or inhibitory :
- ANS (SNS and PSNS)
- hormones
- local factors (H+, O2, NO)
- stretch
- pacemaker activity
smooth muscle difference process of EC-coupling
- relies on Ca+2 from both inside and outside the cell
- cross bridge cycling is controlled by Ca+2 regulated enzyme that phosphorylates myosin
smooth muscle contractile properties
- slow cross bridge cycling: 3 sec twitch & slower removal of Ca+2
- slow ATP splitting is economical
- activity can be graded by varying Ca+2
- -> increase Ca+2 = increase tension - broad length-tension relationship
latch state phenomenon
cross-bridge “latch onto” thin filaments for long periods (up to 60s) = pseudo rigor state
- maintain high tension without fatigue
- low ATP consumption = efficient
- –> these two decrease fatigue because less ATP used over time
parallel elastic components function
- absorb tension first
- allows for slow, purposeful development of tension
temporal summation
increase stimulation of frequency causes increase muscle tension
twitch
submaximal tension because not all myosin heads find actin before Ca+2 decrease
temporal summation / incomplete tetanus
- normal
- 10-40 / sec
- physiologically normal stimulus frequency
- increase tension with particle relaxation between stimuli
complete tetanus
- sustained muscle contraction, no relaxation between stimuli
- max = 3-5x twitch strength
overly contracted
- overlapping thin filaments interfere with each other
- Z dics collide with thick filaments
optimum resting length
overlap that maximizes CB’s formed
overly stretched
- decreased overlap between thin/thick filaments
- decreased number of CB’s formed
- decreased tension
what maintains muscle tone in optimal range?
CNS
factors that determine muscle fiber tension that can vary from contraction to contraction
- the frequency of stimulation (temporal summation)
- muscle fiber length (length-tension relationship)
factors that determine muscle fiber tension that do not vary from contraction to contraction
- metabolic capability
- fatigue
- fiber diameter
skeletal muscle fibers are categorized by:
- speed of contraction (depends on myosin isoform) (fast or slow)
- metabolic pathway used to form ATP (ox. phosph. or glycolysis)
what are the three basic types of skeletal muscle fibers?
- type I (slow oxidative)
- type IIA (fast oxidative-glycolytic)
- type IIB (fast glycolytic)
slow oxidative
- or known as slow twitch
- used in posture/antigravity
- slow to fatigue (aerobic resp)
fast oxidative-glycolytic
- intermediate
- used in aerobic exercise activities
fast glycolytic
- known as fast twitch
- fast to fatigue
- used in high power activities
slow oxidative fatigue
fatigue slowly but require longer rest periods
fast oxidative-glycolytic fatigue
intermediate
fast glycolytic fatigue
fatigue rapidly but recover quickly
fiber diameter
SO < FOG < FG
- smaller fibers generate less tension
endurance training
FG –> FOG
strength training
FOG –> FG
adaptations of muscle fibers
- aerobic exercise induces metabolic changes within fibers, which enable muscle to use O2 more efficiently
- -> increase number of mitochondria
- -> increase capillaries - anaerobic, short-duration, high-intensity exercise causes enlargement of fast-glycolytic fibers (muscle hypertrophy)
- -> increase number of myofibrils per cell
- -> increase glycolytic enzymes
factors that determine muscle tension
- tension developed by each fiber
a. AP frequency **
b. fiber length
c. fiber diameter
d. fatigue - number of active fibers
a. number of fibers per motor size (motor unit size) **
b. number of active motor units (motor unit recruitment)
** factors that change to modify muscle tension from each contraction
motor unit
each motor unit contains one muscle fiber type determined by the alpha - motor neuron
motor unit size
SO < FOG < FG
motor unit recruitment
primary means of varying tension in whole muscle
muscle recruitment order
SO –> FOG –> FG
higher centers
intent
higher centers function
give “intention” to move
higher centers structures
sensorimotor areas of cortex and others involved in emotion, memory and motivation
- frontal lobe
middle level control
coordination
middle level control function
create a “motor program” based on input from higher centers (intention) and sensory input from muscles, joints, skin, eyes about starting position, space available to move
middle level control structures
sensorimotor cortex, thalamus, basal nuclei, brainstem, cerebellum
- with practice, the initial “motor program” is more accurate and the movement becomes skilled
local level portal
execution
local level function
provide sensory input and enact the “motor program”
local level structures
- muscle spindles
- Golgi tendon organs
- other receptors
- spinal / brainstem interneurons
- alpha motor neurons
muscle spindles
- stretch receptors inside a muscle for proprioception
- abundant in muscles that control find movement
- concentrated at ends of muscles near tendons
extrafusal muscle fiber
regular skeletal muscle fibers innervated by alpha motor neurons
stretch reflex
muscle spindles provide info on length increase stretch –> contract
golgi tendon organs
proprioceptors in tendon near junction with muscle = nerve endings embedded in collagen fibers
GT reflex
increased tension = decreased contraction
withdrawal reflex
move away from painful stimulus (flexors) = ipsilateral
crossed extension reflexes
shift body weight to maintain balance (contralateral)
