Muscles + action potentials Flashcards
What is the peripheral nervous system?
all axons + ganglia outside CNS
Subdivisions of the autonomic nervous system?
parasympathetic
sympathetic
Subdivisions of the somatic nervous system?
efferent motor nerve
afferent sensory nerve
How is resting membrane potential generated + maintained?
cell membrane relatively permeable to potassium ions and relatively impermeable to sodium ions
sodium ATPase pump actively transports 2 potassium ions into cell and 3 sodium ions out of cell
maintains concentration gradient of potassium ions and sodium ions
-70mV
How is an action potential initiated and generated?
- resting potential raised slightly by small ionic change (or other changes)
- threshold voltage for opening of VgNa reached (voltage-gated sodium channels), VgNa channels open
- sodium ions flow into cell down electrochemical gradient, depolarising cell membrane potential from -70mV –> +30mV
- VgK channels open + VgNa channels close
- potassium ions flow out of cell down electrochemical gradient through open VgK channels
- cell becomes repolarised back near resting membrane potential
- following AP (action potential), Vg channels become inactive + refractory
- duration before another AP generated = refractory period
Propagation of an action potential
- unidirectional due to refractory period of VgNa channels
- propagated signal does not vary in amplitude + is a digital signal (all or none)
- largely dependent on frequency of firing rates
What does nerve conduction velocity depend on?
- rate at which membrane ahead can reach threshold
- which depends on longitudinal conductance of cable
- which depends on cable diameter
What is saltatory conduction?
action potential skips from node to node down length of axon
- insulated axons of large diameter
- schwann cells produce myelin
- nodes of ranvier = sodium channels clustered at each node
- reduces losses, speeds conduction + saves energy
Describe what happens at a neuromuscular junction (synapse)
- arriving AP triggers VgCa channels to open at nerve terminal
- calcium ions enter cell + triggers a reaction cascade
- vesicles of acetylcholine integrate with presynaptic membrane
- contained acetylcholine released
- acetylcholine neurotransmitter molecules bind to (nicotinic) acetylcholine receptor (AChR) - ligand-gated ion channel on post-synaptic muscle membrane
- various ions, mainly sodium ions, flow in and depolarise muscle membrane (same way as in axon)
- muscle action potential propagated over muscle cell membrane (sarcolemma) through T (transverse) tubules to inner aspects of muscle fibre (similar to nerve action potential)
Define muscle
bundle of fibrous tissue that can contract to produce movement - voluntary or involuntary
What are the 3 muscle types?
striated (skeletal) muscle - locomotion + posture
smooth muscle - peristalsis
cardiac muscle - heart
Define contraction
Shortening
Define elasticity
Returning to resting state
Define hypertrophy
Increase in size
Define hyperplasia
Increase in number (of muscle cells)
Describe the structure of skeletal muscle
- muscle attached to bone by tendon
- epimysium = surrounds muscle
- epimysium folds inwards to form perimysium which separates muscle chunks into fascicles
- bundles of muscle fibres within fascicles, fibres separated by endomysium
- muscle fibres = multinucleated, multicellular structures
- muscle cells develop from myoblasts (myo = muscle, blast = immature/precursor cell)
Describe a skeletal muscle fibre
filled with myofibrils
sarcolemma analogous to plasma membrane
sarcoplasm analogous to cytoplasm
sarcoplasmic reticulum analogous to smooth endoplasmic reticulum
transverse tubular system (TT) = invaginations of sarcolemma
What are transverse tubular systems?
invaginations of sarcolemma
What is a triad?
2 terminal cisternae of sarcoplasmic reticulum (SR) and transverse tubule (TT) in close proximity
What is a sarcomere?
unit of contraction of myofibril
Define Z-line
either end of sarcomere
thin filaments insertion
Define M-line
origin of thick filaments (middle of sarcomere)
Define A-band
overlap of thick and thin filaments
Define I-band
only thin filaments
What are sarcomere thick filaments?
myosin
What are sarcomere thin filaments?
actin
Describe the structure of the thick filaments
multiple myosin molecules
head = actin binding site
tail = 2 intertwined heavy chains
2 regulatory light chains = ATPase activity
2 alkali light chains = stabilise myosin head
hinge = movement of myosin head
Describe the structure of thin filaments
chain of intertwined actin molecules
tropomyosin = block myosin receptors on actin molecules
troponin = controls tropomyosin position
Describe muscle contraction
myosin head attached to actin filaments
greater overlap between thick and thin filaments
Z-lines closer together = shortening of sarcomere - occurs along length of muscle fibre
Describe excitation-contraction coupling
plasma membrane invaginates into transverse tubules
sarcoplasmic reticulum = storage organelle for calcium ions
action potential motor nerve end plate propagates along membrane and down T tubules
membrane depolarisation opens L-type Ca2+ channels on T-tubules
[L-type Ca2+ channel (AKA dihydropyridine (DHP) receptor) blocked by antihypertensive drugs]
coupling between DHP receptor and calcium ion release channel opens calcium ion channels
calcium ions released into myofibril - activates troponin C + cross-bridge cycling
Describe initiation of cross-bridge cycling
Ca2+ modulates contraction through regulatory proteins rather than direct interaction with actin + myosin
tropomyosin blocks myosin binding site
troponin C = binding of calcium ions to high affinity sites causes conformational change in troponin complex
troponin I = moves away from actin filament
troponin T = pushes tropomyosin away from myosin binding site on actin
myosin head binds to actin
What are the 3 types of tropomyosin molecule?
C = binds Ca2+ I = anchors complex to actin T = binds to tropomyosin
What are the 5 stages of cross-bridge cycle in skeletal muscle?
1) ATP binding - ATP binds to myosin head, causes dissociation of actin-myosin complex
2) ATP hydrolysis - ATP hydrolysed causes myosin heads to return to resting conformation
3) Cross-bridge formation - cross-bridge forms and myosin head binds to new position on actin
4) Release of Pi from myosin - myosin heads change conformation resulting in power stroke - filaments slide past each other
5) ADP release
Describe contraction termination
Calcium ions must be removed from cytoplasm for contraction to cease and relaxation to occur
minor = Na-Ca exchanger (NCX) - remove Ca2+ from cell by Ca2+ pump in plasma membrane
major = Ca2+ reuptake into sarcoplasmic reticulum by SERCA-type Ca pump
Calsequestrin = major Ca-binding protein in skeletal muscle (located primarily at triad junction)
What does the amount of force generated by a muscle depend on?
number of active muscle fibres cross-sectional area of muscle initial resting length of muscle rate at which muscle shortens frequency of stimulation
What is isometric contraction?
muscle length fixed
stimulation of muscle causes increased tension but no shortening
eg. holding weight in hand of outstretched arm
(contraction against resistance where length of muscle remains the same)
What is isotonic contraction?
muscle length not fixed
stimulation of muscle causes increased shortening provided tension generated is greater than opposing load
eg. holding weight in hand and lifting hand, bending at elbow
(contraction against resistance where length of muscle changes)
concentric or eccentric
Describe length-tension relationship
muscle can be stretched
passive tension = tension measured before muscle contraction (elasticity)
at any fixed length, if muscle is contracted, active tension develops due to cross-bridge formation
Describe force-length relationship
muscles elastic due to titin
muscles held at resting length
maximum tension = thick + thin filaments overlapping 80-120%
Describe force-velocity relationship
speed of change in length
power = force=velocity
velocity increases, force decreases
Describe summation in single muscle fibres
one action potential = singular skeletal muscle twitch
as muscle twitch longer than AP, a 2nd AP can be initiated before 1st contraction subsided - called summation
Describe slow twitch muscle fibres
type 1 fatigue resistant red (due to myoglobin) oxidative respiration lots of mitochondria low glycogen levels prologned endurance
Describe fast twitch muscle fibres (2a)
type 2a fatigue resistant red (due to myoglobin) oxidative respiration higher levels of mitochondria abundant in glycogen endurance or rapid force
Describe fast twitch muscle fibres (2b)
type 2b fatigable white (less myoglobin) glycolytic metabolism fewer mitochondria high levels of glycogen rapid force production
What are long skeletal muscle fibres used for?
rapid movement
What are short skeletal muscle fibres used for?
large forces
What is concentric muscle contraction?
isotonic contraction in direction of contraction
What is eccentric muscle contraction?
isotonic contraction opposite to direction of contraction
Effect of smoking on muscle fibres
decreased cross-sectional area of muscle fibres
decreased type 2 fibres
decreased exercise capacity
How is creatine phosphate created
ATP + creatine combined in resting muscle to produce creatine phosphate + ADP
What is the function of creatine phosphate?
acts as reservoir within muscles for ATP
exercise initiated - creatine kinase breaks down creatine phosphate - produces creatine + ATP that can be used in muscle contraction
Describe glycolysis
1st step in respiration
does not require oxygen (anaerobic)
net gain = 2 ATP + 2 pyruvate
Describe citric acid cycle (Krebs cycle)
pyruvate enters
net gain = 1 ATP, 3 NADH + 1 FADH2 per turn
Describe the electron transport chain
most energy produced during oxidative process
NADH moved along ETC within mitochondria of muscle cells
NADH reduced - loss of H+ ion - form ADP - converted to ATP by ATP synthase
requires oxygen (oxygen = final electron acceptor - produced water)
Describe the function of lactate in respiration
pyruvate from glycolysis can enter aerobic or anaerobic systems
pyruvate - lactic acid - liver - ATP
increased lactate build up in athletes
endurance athlete = longer aerobic component
What is gluconeogenesis?
lactate transported back to liver
converted back to pyruvate - citric acid cycle
then either used for energy production or converted back to glucose (gluconeogenesis)
Define oxygen debt
amount of oxygen needed after finished exercising to return systems to back where they started
What is used for energy in intense short-term exercise?
1st 15 seconds = creatine phosphate, ATP
2 minutes = glycogen -> glucose-6-phosphate
What is used for energy in longer + less intense exercise?
glycogen from circulation
glucose from plasma
increased hepatic glucose production (short term glycogenolysis) (longer term gluconeogenesis - muscle proteolysis, glucagon + insulin, fatty acid release)
Define VO2 max
oxygen usage under maximal aerobic activity
Define EPOC
excessive post-exercise oxygen consumption
What are the fast and slow components of recovery?
fast = resting levels of ATP + CP restored slow = lactic acid converted to glucose in liver, lactic acid converted to pyruvic acid
How does the respiratory system change to meet oxygen demand during exercise?
increase ventilation rate
increase tidal volume
Do blood gases change during exercise?
arterial oxygen and venous carbon dioxide do not change significantly during exercise
respiratory system can provide adequate aeration
How does oxygen consumption change during exercise?
increased oxygen consumption
reaches steady state where lactic acid accumulation is minimal
Describe changes to VO2 max with age, sex and activity
increase in exercise past VO2 max results in increased lactic acid production
VO2 max improves with activity (endurance training increases it)
decreases after age 25
lower in females
How does exercise affect alveolar diffusion
increased oxygen and carbon dioxide diffusion capacity
related to increased perfusion more than ventilation
Cardiac changes with exercise
increased cardiac output
increased stroke volume
increased heart rate
Define stroke volume
volume of blood pumped per contraction
Cardiac output formula
stroke volume x heart rate
What is Starling’s Law?
more full heart is, harder it will contract, increased stroke volume
(force of contraction related to how stretched the cardiac muscle is)
Benefits of exercise
lower BP
increases circulating HDL and lowers triglycerides
changes in arterial wall homeostasis = decreases atherosclerotic disease
increased aortic valve function, decrease in calcification
increased ventricular wall thickness
increased red cells (to a point)
changes in cardiac vasculature to increase oxygen availability
(can also decrease insulin resistance (lower risk of type 2 diabetes))
How does exercise affect depression?
moderate clinical effect in decreasing depression
no more effective than psychological/pharmacological treatments
How does exercise affect RA?
increase muscle mass + strength = decrease cachexia
weight-bearing exercise = reduce risk of osteoporosis
resistance training = increase tendon stiffness + strengthen connective tissue
cyclic loading increases cartilage integrity + joint lubrication
mobility exercises = increased ROM
Describe epimysium
surrounds muscle
Describe perimysium
inward folds of epimysium
separates muscle chunks into fascicles
Describe endomysium
separates muscle fibres within fascicles
