KIN 103 (Chp: 12) Flashcards
What happens durring a muscle contraction microscopically?
Contraction- filaments slide, sarcomeres shorten
Relaxation- calcium transported back into SR, filaments uncouple, sarcomeres lengthen
At what rate does tension occur?
Tension increases rapidly and dissipates slowly
How are contractions generated?
Contractions are generated by the summation of repetitive twitches of many muscle fibres (section 1.3).
What do amplitude and rate depend on in muscle fibers?
Amplitude and rate depend on muscle fiber type (section 1.2)
When do muscles need a steady supply of ATP
- Muscles require a steady supply of ATP
- during contractions for cross bridge movement and release
- during relaxation to pump Ca back into sarcoplasmic reticulum
- after contractions to restore and maintain balance of Na and K
Where does the ATP come from?
- There is only enough “free” ATP to produce 8 twitches
- The next source is high-energy phosphate bonds in phosphocreatine
- Next source is metabolism of carbohydrates (glucose)
Muscles at rest (ATP formation)
ATP (from metabolism) + creatine -> ADP + phosphocreatine
Working muscle (Use of ATP)
Phosphocreatine + ADP -> Creatine + ATP
Contraction Fatigue (Central fatigue can be caused by)
psychological factors, failure of control neurons in brain and spinal cord
Contraction fatigue (Peripheral fatigue can be caused by)
- extended submaximal exercise leads to depletion of glycogen stores
- short-duration maximal exertion leads to increased levels of Pi
- maximal exercise leads to ion imbalances
K leaves muscle fiber, leading to increased extracellular K altering the membrane potential
Skeletal muscle classification depends on
Mechanical: maximal shortening velocity (fast vs slow)
maximal force generating capacity
Metabolic: major pathway used to make ATP
Oxidative vs glycolytic
Three types of muscle fibers (slow oxidative)
Slow-oxidative (SO, type 1)
fatigue-resistant
slow, sustained movements (e.g. posture)
Three types of muscle fibers (fast oxidative)
Fast-oxidative-glycolytic (FOG, type IIa)
Three types of muscle fibers (fast glycolytic)
Fast-glycolytic (FG, type IIb)
fatigue rapidly
fast, powerful movements (e.g. a sprint)
What is important to remember about all three muscle types
Most skeletal muscles include all three types.
Slow oxidative (Metabolic properties?)
Slow-Oxidative fibers: get most of their ATP via oxidative phosphorylation
- lots of mitochondria
- strong blood supply and cells contain lots
of myoglobin giving them a dark red color
- smaller in diameter
Fast glycolytic (metabolic properties?)
Fast-Glycolytic fibers: get most of their ATP via anaerobic glycolysis
- few mitochondrion, lots of glycogen
- few blood vessels and little myoglobin (white appearance)
- larger diameter (more myofibrils=stronger)
Slow oxidative muscles (Mechanical properties)
Slow-oxidative skeletal muscle responds well to repetitive stimulation without becoming fatigued; muscles of body
posture are examples
Fast oxidative muscles (Mechanical properties)
Fast-oxidative skeletal muscle responds quickly to repetitive stimulation without becoming fatigued; muscles used in walking are examples.
Fast glycolytic muscles (Mechanical properties)
Fast-glycolytic skeletal muscle is used for quick bursts of strong activation, such as muscles used to jump or to run a short sprint.
Muscle twitches types (4 types)
Single twitches: single twitch
Summation: stimulations together that add
Unfused tetanus: enough apart for relaxing
Complete tetanus: no relaxation between
Force-frequency relationship (what is it?)
Force-frequency relationship- force generated by a muscle or muscle fibre depends on firing frequency
Functional electrical stimulation (FES) (What is it?)
-generate contractions to assist activities of daily living (standing, walking, grasping) or enable participation in exercise programs (bicycle ergometer, rowing machine)
Length-tension relationship (Sarcomere length)
Maximum tension a muscle fiber can develop depends on sarcomere length due to the overlap of the thick and thin filaments.
Length-tension relationship (Cross bridges)
The amount of tension that can develop is directly proportional to the number of cross bridges that can form
Length-tension relationship (filament overlap)
Maximal tension develops when there is optimal overlap of thick and thin filaments
Length-tension relationship (longest state)
At the longest muscle lengths there is minimal opportunity for cross bridges to form.
Load-velocity relationship
The speed of contraction depends of the load.
(Low load, high speed of contraction)
(High load, low speed of contraction)
A motor unit (What is it?)
A motor unit is a motor neuron and the muscle fibers it innervates
- A single motor neuron innervates several muscle fibers (5-2000)
Are all the fibers in a motor unit the same?
- All the fibers in a motor unit are of the same type
Rate coding (What is it?)
increasing firing frequency (rate coding).
Isotonic contraction (what is it?)
Muscle contracts, shortens and produces enough force to move load
- Elastic elements are stretched and sacromeres shorten more
Isometric contraction (What is it?)
Muscle contracts, but does not shorten as the load does not move
- Elastic elements are tightened
Electromyography (EMG) (What is it?)
- Recording of the the electrical activity occurring during muscle contraction
- Measuring the action potentials travelling across the sarcolemma
- An electrode (measures voltage) is placed near the excitable membrane and records action potentials as they pass
- Provides a “window” into the nervous system
Du Bois-Reymond (1849) (What did he discover?)
Was the first to report EMG as we more or less know it today
Recorded weak electrical signals during voluntary contractions using electrodes made of cloth soaked in saline placed on the skin of the forearm
Two EMG methods (Surface EMG?)
Surface EMG
Good: for studying contraction, timing
Bad for: lack of sensitivity, cant track single unit
Two EMG methods (Intramuscular needle EMG)
Good: for studying details of synaptic inputs to motoneurons
Bad for: invasive, lack of stability, intensive labor
Recruitment (What is it?)
More units are recruited as contraction progresses.
Smooth muscle (characteristics)
- Found in walls of hollow organs and tubes
- Develops tension differently than skeletal muscle
1. slower to develop tension and to relax
2. sustained contractions without fatigue - Some contract tonically (maintain tension)
How is smooth muscle classified? (3 categories)
By location
By contraction pattern
By communication with neighboring cells
Single unit smooth muscles (characteristics)
-electrically coupled by numerous gap junctions
-rapid, unified wave of depolarization and contraction
- can be activated by stretch
- forms walls of viscera (e.g. blood vessels, ureters, intestinal tract)
Multi unit smooth muscle (Characteristics)
-not linked electrically, individual activation is necessary
-fine, graded contractions
-not activated by stretch
-iris (ciliary body), hairy skin
Smooth muscle cells (anatomy)
- Cells are spindle shaped
- Actin are form lattice around muscle (net)
- Actin filaments are connected via dense bodies on the plasma membrane
- contraction doesnt result in shortening like skeletal muscle, instead its globular
- They dont have sacromeres
Smooth muscle contraction steps (1-3)
- intracellular Ca2+ concentrations increase as it enters the cell via the sarcoplasmic reticulum
- Ca2+ binds to CaM
- Ca2+ activates myosin light chain kinase (MLCK)
Smooth muscle contraction steps (4-5)
- MLCK phosphorylates light chains in myosin heads and increases ATPase activity
- Active crossbridges slide along actin and create muscle tension
Smooth muscle relaxation steps (1-3)
- free Ca2+ in cytosol decreases when Ca2+ is pumped out of the cell or back into the sarcoplasmic reticulum
- Ca2+ unbinds from CaM. MLCK activity decreases
- Myosin phosphatase removes phosphate from myosin light chains, which decrease myosin ATPase activity
- Less myosin ATPase activity results in decreased tension
Latch state in smooth muscle?
Dephosphorylated myosin may remain attached to actin for a period of time, no ATP needed.
Membrane potentials in smooth muscle
Can vary between -40 and -80mV due to spontaneous ion channel opening/closing
Smooth muscle potentials (slow wave potentials)
-cyclic de- and repolarization of membrane potential
-sometimes only subthreshold, but, if above threshold AP and contraction follow
Smooth muscle potentials (pacemaker potentials)
-regular depolarization always reaching threshold, AP and contraction
-rhythmic contractions
smooth muscle potentials (pharmacochemical coupling)
Occurs when chemical signals change muscle tension without a change in membrane potential
What are sympathetic muscles controlled by?
Many are controlled by sympathetic and parasympathetic neurons as well as hormones and paracrines
What does histamine do to smooth muscle
Histamine constricts smooth muscle of airways
What does nitric oxide do to smooth muscle?
Nitric oxide relaxes smooth muscles of blood vessels
