Skeletal Muscles Flashcards
What is the primary function of all muscle?
To generate force and/or movement in response to a physiological stimulus
-Body movement
-Maintenance of posture
-Respiration
-Production of body heat
-Communication
-Constriction of organs and vessels
-heartbeat
-In all muscle types the generaction of force depends on the conversing of chemical energy (ATP)
Explain the placement and movement of skeletal muscle
Describe thin filaments (actin) and the function of tropomyosin
-White dots are myosin binding sites where thick filament can grab onto thin filament to generate force
-In resting state, tropomyosin sits over top of myosin binding sites
What is the troponin complex?
Describe the structure of thick filaments (myosin)
What are the additional proteins in a sarcomere and their functions?
Describe the structure of a sarcomere in terms of zones and bands
What is the sliding filament model?
What are the brain regions involved in voluntary movement?
Primary motor cortex:
-basal ganglia
-premotor cortex (motor association)
-thalamus
-cerebellum
-midbrain
What are the steps to initiate skeletal muscle contraction?
What are the events in the CNS that initiate skeletal muscle contraction?
Descending lateral corticospinal pathway
-Corticospinal tract: descending tract (ventral and interior lateral white matter)
-Upper motor neuron: brain to brainstem or spinal cord
-Alpha (lower) motor neuron: spinal cord (or brainstem) to muscle (efferent neurons)
Describe the alpha (lower) motor neuron
-From spinal cord or brainstem to muscle
-A single motor neuron and all the muscle fibres it innervates is known as a MOTOR UNIT
-One muscle fiber will only contact one motor neuron but motor neurons can interact with many muscle fibers
-The area where the motor neuron makes synaptic contact with the muscle fibre is known as the NEUROMUSCULAR JUNCTION
Describe neuromuscular junction and explain the synaptic process
-Region of sarcolemma at the neuromuscular junction is referred to as the motor end plate
-Junctional folds on sarcolemma increase surface area
-Motor neuron vesicles contain acetylcholine
-Muscle sarcolemma contains nicotinic acetylcholine receptors
Explain how nicotinic acetylcholine receptors function
Describe excitatory end-plate potential
-Happens in middle of muscle fibre, moves outward
What are the functions of T-tubules, SR and terminal cisternae?
Explain the steps of muscle action potential and excitation-contraction coupling
The process by which electrical excitation of the surface membrane triggers an increas of Ca2+ in muscle
How does the increase in calcium trigger contraction?
-Ca2+ binds low affinity sites on troponin C (TnC) which induces a conformational change in the troponin complex
-Causes the troponin complex as well as tropomyosin to move revealing the binding site on the actin
Explain tight binding in the rigor state
How does the muscle terminate contraction?
How does the SR maximize Ca2+ uptake?
Explain why rigor mortis happens
Development of rigid muscle several hours after death
-Ca2+ leaks into the sarcoplasm and binds troponin
Since ATP production stops:
-Ca2+ cannot be removed (SERCA pump is ATP powered)
-ATP is needed to release myosin head from actin
-Remains in latched cross bridge formation until muscles begin to deteriorate
What is the timing of E-C coupling?
What is ATP needed for in muscles and what are the sources?
ATP is needed for:
-Myosin ATPase (contraction)
-Ca2+ ATPase: SERCA (relaxation)
-Na+/K+ ATPase (after AP in muscle fibre)
Sources:
-Free intracellular ATP (few sec)
-ATP formed from phosphocreatine (10 sec)
-Anaerobic metabolism
-Aerobic metabolism
Describe the anaerobic metabolic process
-Occurs in the absence of oxygen, take place in sarcoplasm of muscle
-Large amount of glucose is stored in muscle cells in the form of glycogen (glycogenesis)
-When ATP is needed, glycogen is then converted back to glucose (glycogenolysis)
-One glucose molecule can then be broken down to pyruvated by glycolysis and produces 2 ATP molecules
-Pyruvate is further converted to lactate
Describe the oxidative (aerobic) metabolic process
-Occurs in mitochondria and oxygen is necessary
-After glycolysis, pyruvate then enters the citric acid cycle producing two more molecules of ATP as well as high energy electrons and H+
-High energy electrons and H+ combine with O2 in the electron transport chain to produce an additional 26-28 molecules of ATP
-
What is the relative speed and amount of ATP for each source?
What is central fatigue?
-Feeling of tiredness and a desire to cease activity (precedes physiological cell fatigue)
-A decrease in muscle tension as a result of previous contractile activity that is reversible with rest
-Low pH from acid production during ATP hydrolysis may influence the sensation of fatigue perceived by the brain (likely only the case during maximal exertion)
What is peripheral fatigue?
-Failed excitation-contraction coupling at the t-tubule (increase in extracellular K+)
-Accumulation of phosphate, acid and ADP:
-Decreases in the rate of Ca2+ release, reuptake, and storage by the SR
-Decreased activation of thin filament proteins by Ca2+
-Direct inhibition of the binding and power stroke motion of the myosin cross-bridges
What are the classifications of skeletal muscles?
Maximal velocity of shortening: Depends on ability to hydrolyze ATP
-Slow fibres contain myosin with slower ATPase activity (Type 1)
-Fast fibres contain myosin with more rapid ATPase activity (Type 2)
Type of enzymatic machinery available for synthesizing ATP
-Oxidative fibres contain a large amount of mitochondria and have a high capacity for aerobic (oxidative) metabolism (surrounded by blood vesses and contain a large amount of myoglobin)
-Glycolytic fibres contain a few mitochondria but an abundance of glycolytic enzymes and a large store of glycogen
What are the determinants of muscle force in individual muscle cells and the entire muscle?
Force/muscle cell
-Fibre diameter
-Fatigability
-Initial resting length
-Frequency of activation
Force/entire muscle
-Number of muscle cells activated which depends on:
-Number of muscle cells per motor unit
-Number of motor units activated
How does muscle length influence tension development?
How can the force developed by a muscle fibre be increased?
How is muscle twitch summation possible?
-A single AP does not cause release of the entire Ca2+ store from the SR
-The Ca2+ released from one AP may NOT lead to all troponin complexes being activated for a sufficient amount of time, some regions of actin may be re-covered by tropomysoin before crossbridge cycling can actually begin
-A second AP causes a second wave of Ca2+ that may keep additional troponin complexes activated allowing for more cross bridges to be formed
-Ca2+ remains elevated for a longer period of time allowing increased cross bridge cycling and further shortenin of the sarcomeres
What is tetanus?
A maintained contractile response to repeated stimuli
-Unfused tetanus: reaches a steady state of contraction by stimuli are far enough apart that the muscle fibre slightly relaxes between stimuli
-Fused tetanus: stimulation rate is fast enough that the fibre does not relax, instead it reaches maximum tension and remains there
-One way to increase tension developed by a single muscle fibre is to increase the rate at which action potentials occur in the fibre
What is the size principle and what is the reason for order of recruitment?
Motor unit: a single motor neuron and all the muscle fibres it innervates, one motor neuron innervates one fibre type
Motor neuron pool: the group of all motor neurons innervating a single muscle
Size principle: as the stimulus onto the motor neruon pool increases, additional larger motor neurons are recruited
-Type 1
-Type 2A
-Type 2X
What is asynchronous recruitment?
-during submaximal contraction, the CNS modulates firing rates of upper motore neurons to allow different motor units to maintain contraction in order to prevent fatigue
what is tension and the difference between isotonic and isometric contraction?
Tension: the force tending to pull the attachment points of a muscle toward one another
-Isotonic: the muscle contracts, shortens, and createds enough force to move the load (concentric and eccentric)
-Isometric: the muscle contracts but does not shorten, force created cannot move the load
How do the sarcomeres shorten during isometric contraction without the muscle changing length?
What are two proposed mechanisms by which muscle mass may be increased?
-Hypertrophy (increase in diameter) or hyperplasia (increase in number)
-Myosatellite cells involved in muscle repair may form new muscle fibres in development
What is muscle hypertrophy?
When skeletal muscle is subjected to an overload stimulus, it causes perturbations in muscle fibres and the related extracellular matrix. This sets off a chain of myogenic events that ultimately leads to:
-Increase in size of sarcomeres and number of contractile proteins (myosin and actin)
-Increased number of sarcomeres within a single muscle length, increased myofibrils
-Increased sarcoplasmic storage (glycogen)
What are skeletal muscle reflexes?
-Involved in almost all movements
-Receptors sense changes in joint movements, muscle tension and muscle length and feed info into the CNS which responds in one of two ways:
1. If muscle contraction is needed the CNS acitvates motor neurons to the muscle fibres
2. If relaxation is needed sensory input activates inhibitory interneurons in CNS which inhibit activity in motor neuron leading to relaxation
What is the difference between monosynaptic and polysynaptic reflexes?
What are proprioceptors?
What are muscle spindles?
What are muscled spindles doing when relaxed?
What is a muscle spindle reflex?
Describe the function of muscle spindles without gamma motor neurons and with alpha-gamma coactivation
What are golgi tendon organs and their function?
Describe stretch reflexes and reciprocal inhibition control
