Muscle contraction Flashcards
General functions
Movement, heat production, and posture
Functions of skeletal muscle
- Excitability: ability to be stimulated 2. Contractibility: ability to contract or shorten 3. Extensibility: ability to extend or stretch and return to resisting length after contraction
Myoctes
Muscle fibers. Each fiber is made of several cells combined into one and therefore having more than one nucleus
Satellite cells
Stem cells that can fuse with myocites during strength training to make bigger muscle fibers - can become active post injury to produce more muscle fibers
Sarcolemma
Plasma membrane of a striated muscle fiber - transeverse T tubules
Sarcoplasm
Cytoplasm of muscle fibers
Sarcoplasmic reticulum
Network of tubules and saves in muscle fiber; smooth endoplasmic reticulum of muscle cells
Transverse T tubule
Transverse tubules unique to muscle cells; formed by inward extensions of the sarcolemma that allows electrical impulse to move deeper into the cell
Myofibrils
The cytoplasm skeleton of the muscle fiber. Very fine longitudinal fibers found in skeletal muscle cells composed of thick and thin filaments - extend lengthwise
Myofilaments
Ultramicroscopic, threadlike structures found in myofibrils; composed of myosin (thick) and actin (thin)
Sarcomere
Contractile unit of muscle cells; length of a myofibrils between two z disks
Myoglobin
Large protein molecule in the sarcoplasm of muscle cells that attracts oxygen and holds it temporarily
3 types of myobilaments
- Contractile: myosin (thick) and actin (thin) 2. Regulatory: tropomyosin and troponin 3. Structural: several kinds that assist to stabilize the position of thick and thin myofilaments
Actin
Thin, twisted protein strands that have sites that attract myosin heads
Myosin
Thick fibers that have “heads” that cross bridge to actin active sites
Tropomyosin
Thin strands that block actin active sites
Troponin
Globular proteins that hold tropomyosin in place
Excitation of sarcolemma
Motor neuron-specialized nerve cells that release ACh - motor endplate: folded aspect of searcolemma where motor neuron connects - ACH binds to receptors on mm fiber - neuromuscular junction - junction (synapse) between motor neuron and motor endplate - signal travels down motor neuron - signal travels down motor neuron - ACh released from end of motor neuron into synaptic cleft - ACh binds to receptors on motor endplate - electrical impulse is transmitted to the sarcolemma
Contraction
- impulse travels through T tubules to save of SR 2. Ca2+ released into sarcoplasm and binds to troponin (resting mm troponin holds tropomyosin in place) 3. tropomyosin molecules shift and expose active actin sites 4. myosin cross bridges and binds to actin to pull actin filaments toward center of sarcomere 5. myosin heads release and bind to next active site and pull again which results in shortening of mm fiber 6. sliding-filament model or mechanism
Relaxation
- SR starts pumping back Ca2+ into its sacs immediately after releasing them 2. Ca2+ removed from troponin molecules shutting down contraction 3. If no nerve impulse follows, the muscle relaxes
Energy sources for muscle contraction
- skeletal muscle fibers fluctuate between low and high levels of activity based on relaxation and exercise/activity - a small amount of ATP is present inside resting uscle fibers (enough to power contraction for a few seconds) - if more ATP is needed, the muscle has 3 ways to produce it 1. From creatine phosphate 2. Anaerobic glycolysis 3. Aerobic respiration
Sources of ATP
- small amount stored in mm cell- enough for a few seconds of mm activity - creatine phosphate (CP)- enough for 15-20 seconds of mm activity - glycolysis- enough for 15-60 seconds of mm activity (glycogen=glucose stored in mm) - citric acid cycle and ETC- enough for hours of activity
Aerobic respiration
Key components are oxygen and glucose
Anaerobic respiration
- pyruvate from glycolysis is converted into lactic acid - lactic acid dissociates H+ decreasing mm pH causing the burn or ache in mm
Muscle fatigue
An inability of a muscle to maintain force of contraction after prolonged activity - even before true mm fatigue occurs an individual has the sensation or desire to stop activity
Actual muscle fatigue
- failure of the Na/K pumps to maintain ion concentration - inadequate calcium release from the sarcoplasmic reticulum - depletion of creatine phosphate - insufficient oxygen - depletion of glycogen - build up of lactic acid and ADP - failure of neuron to release enough acetylcholine
Motor units
-consist of a somatic motor neuron and all the skeletal muscle fibers it stimulates - a single somatic motor neuron comes in contact with an average of 150 skeletal muscle fibers - all of the muscle fibers in 1 motor unit contact at the same time - typically, the muscle fibers in 1 motor unit are dispersed throughout a muscle (not clustered)
Fine motor
Only a few muscle fibers per neuron - eye movements 10-2- muscle fibers per motor unit - voice box/larynx movement 2-3 fibers per motor unit
Gross motor
Large scale and powerful movements have many fibers per neuron - biceps and gastroc have 2000-3000 fibers per motor unit
Twitch contraction
Brief contraction of all muscle fibers in a motor unit in response to a single impulse - latent phase - relaxation phase
Twitch contraction
Brief contraction of all muscle fibers in a motor unit in response to a single impulse - latent phase - contraction phase - relaxation phase
Latent phase
Nerve impulse to SR to trigger release of Ca++
Contraction phase
Ca++ bind to troponin and sliding of filaments occurs
Relaxation phase
Sliding of filaments ceases. Ca++ is actively transported back to SR, myosin binding sites are covered by tropomyosin
Treppe
Gradual, steplike increase in strength of contraction when series of twitch contractions occur about a second apart - this means a muscle contracts with more force after it has contracted a few times
Tectonic contractions
tetanus - smooth, sustained contractions - muscle doesn’t have time to relax fore the next contraction starts - also calle multiple wave summation
Incomplete tetanus
Tension is not sustained at a constant level
Tonic contraction
Continual, partial contraction in a muscle organ - a small number of fibers in a muscle contract to produce tautness - motor units fire in relays to achieve this tone
Muscle tone
Small amount of tautness due to weak, involuntary contraction of motor units - hypertonic - spastic - flaccid
Graded strength principle
Skeletal muscles contract with varying degrees of strength at different times - allows us to match force with demand - factors 1. Metabolic condition 2. Recruitment of motor units 3. Effect of muscle length on strength 4. Effect of load on strength
Metabolic condition
It is helpful to think of metabolism as its own system - metabolic activity is separate from muscle contraction but can positively and negatively effect contraction - if some fibers are not able to keep up with metabolic requirements to generate force the entire muscle suffers
Recruitment of motor units
The more intense and frequent the stimulus, the more motor units recruited - once maximum contraciton is reached, regardless of strength or frequency of stimulus, the muscle cannot contract more strongly - motor units fire in revolving sequence
Length-tension relationship
Maximal strength in mm can develop bears a direct relationship to the initial length of its fibers
Load on strength
The heavier the load the stronger the contraction
Isotonic contraction
Concentric and eccentric - muscle contraction in which the muscle sustains the same tension or pressure and a change in the distance between two bones occurs
Isometric contraction
Muscle contraction in which muscle does not alter the distance between two bones
Slow twitch fibers
Slow oxidative fibers (red fibers) - relies on oxygen to produce ATP - high concentration of myoglobin (stores O2) - highly vascular - myosin (type I) is slow acting so ATP production is able to keep up - fatigue resistant - found in postural and endurance type mm
Intermediate twitch fibers
Fast oxidative-glycoytic fibers - rely on oxygen and glycogen to produce ATP - some myoglobin - some vascularity - myosin (type IIa) is moderately fast but more fatigue resistant than fast twitch fibers - found in gastrocnemius mm - sprint muscles
Fast twitch (white fibers)
- relies on glycogen to produce ATP (anaerobic process) - low concentration of myoglobin - decreased vasculatity - myosin (type IIb) is fast so rapid depletion of ATP - short duration contractions - found in muscles of fingers and eyes
Endurance training
Results in a switch to oxidative, slow-twitch muscle fibers
Resistance training
Increase te proportion and cross sectional area of glycolysis muscle fibers
PGC-1alpha and PGC-1beta
Potent chemicals that assist in transformation of fibers, regulate metabolic disease
Sedentary lifestyle
A lack of or irregular physical activity - one of the leading causes of preventable deaths worldwide - associated with: DMII, cardiovascular pathologies, obesity, postmenopausal breast cancer and other tumor types, dementia, depression, neurodegenerative events, chronic systematic inflammation
Benies of exercise
- increased lifespan - decreased hyperglycemia - decreased hypercholesterlemia - decreased HTN - improved kidney, heart, brain, and liver function - possible reduction in inflammation and circulating stress hormones
