Lecture 12 Musculoskeletal 2 Flashcards
Excitation contraction coupling NMJ ACh
1) ACh released by axon of motor neurone, crosses cleft and binds to receptors on motor end plate
2) action potential generated in response to binding ACh and subsequent end plate potential propagated across surface membrane and down transverse (T) tubules of muscle
3) action potential in T tubule triggers Ca2+ release from sarcoplasmic reticulum
4) Ca2+ ions released and bind to troponin allowing actin myosin cross bridge to form
5) myosin cross bridge attached to actin, pulls filaments towards sarcomere
6) Ca2+ active reuptake by sarcoplasmic reticulum after local action potential
7) with Ca2+ no longer bound to troponin tropomyosin slips back to block position over acting binding site
- return to resting position
Sarcoplasmic reticulum
Consists of fine network of interconnected compartments surrounding each myofibril
Segments wrapped around each A band and each I band
Terminal cisternae ( lateral sacs)
Transverse (T) tubules
Membranous perpendicular extensions of surface membrane run from surface of muscle cell into central portions of the muscle fibre
Since membrane is continuous with surface membrane action potential of surface membrane also spreads down into T tubules
Spread of action potential down a T tubule triggers release of Ca2+ from SR into cytosol
Dihydropyridine receptors
Voltage gated channels connecting SR to T tubule opened by change in action potential releasing Ca2+ ions
Rigor Mortis
When we die cytosolic calcium leaks and binds to troponin - forming cross bridges. Muscles grow stiff as no new ATP is available. After 12 hours protein breakdown causes muscles to relax
Motor units and skeletal muscle
Muscles: groups of fibres bundled together and attached to bones
Connective tissue covering muscle divides muscle internally into bundles
Connective tissue extends beyond end of muscles to form tendons that attach muscle to bone
Contraction of muscles
Contraction of whole muscle can be of varying strength
2 primary factors can be adjusted to accomplish gradation of whole muscle tension
1) no. Of muscle fibres contracting within the muscle
2) tension developed by each contracting fibre
Motor units
No. Of muscle fibres pet motor units and no. Of motor units per muscle vary widely
Muscles that produce precise, delicate movements contain fewer fibres per motor unit
Muscles performing powerful, coarsely controlled movement have larger no. Of fibres per motor unit
Asynchronous recruitment of motor units helps delay or prevent fatigue
Factors influence extent to which tension can be developed:
-Frequency of stimulation
-Length of fibre at onset of contraction
-Extent of fatigue
-Thickness of fibre
Twitch summation and tetanus
If a muscle is restimulated after it has completely relaxed the second twitch is the same magnitude as the first
If a muscle is restimulated before it has completely relaxed the second twitch is added to the first resulting in summation
If a muscle fibre is stimulated so rapidly that it does not have an opportunity to relax at all between stimuli a maximal sustained contraction known as tetanus occurs
Muscle tension and length
Tension produced in sarcomeres
Tension must be transmitted to bone via connective tissue and tendons before it can be moved (series elastic component)
Muscle is attached to at least 2 diff bones across a joint - origin and insertion
Contraction types
Isotonic- tension remains constant as muscle changes length. Can be concentric or eccentric.
Isometric - constant length
Levers
Levers = bones
Fulcrums= joints
E.g. lever ratio of an arm 1:7 (5cm:35cm)
Power of arm lever 5cm
Load of arm lever 35cm
Downward force of load (item held)
Hand velocity 7cm/units of time
Distance moved by hand 7cm
(See diagram)
Energy requirements for contraction and relaxation
Energy for power stroke of cross bridge comes from ATP splitting by myosin ATPase
At end of power stroke new ATP must bind to myosin for it to release
For relaxation to occur active transport of Ca2+ back into SR required
Energy sources : creatine
Creatine synthesised mainly in liver from amino acids
Creatine is phosphorylated to give phophorylcreatine - effectively an energy store in muscle
During exercise reaction reversed to form ATP from ADP
ATP is prime source of energy for muscle contraction
Glycolysis supports anaerobic of high intensity exercise
Intensity and duration of contraction
Determines fuel required
100m - Ussain Bolt 9.69sec
(85% anaerobic)
5000m Kenensia Bekele 12 min 57.8sec
(15-20% anaerobic)
41 195m Samuel Wanjiru 2hr 6min 32sec
(<5% anaerobic)
Fatigue
Muscle fatigue - protects muscle from reaching a point where it cannot continue to produce ATP
Central fatigue- happens when CNS cannot adequately activate motor neurones to working muscles. May be psychological.
Mechanism for both types of fatigue unclear
Types of muscle fibre and characteristics
Type 1 - slow oxidative
Low myosin ATPase activity
Slow speed of contraction
High resistance to fatigue
High oxidative phosphorylation capacity
Low in enzymes for anaerobic glycolysis
Many mitochondria
Many capillaries
High myoglobin content
Red fibre
Low glycogen content
Type 2a - fast oxidative
High myosin ATPase activity
Fast speed of contraction
Intermediate resistance to fatigue
High oxidative phosphorylation capacity
Intermediate in enzymes for anaerobic glycolysis
Many mitochondria
Many capillaries
High myoglobin content
Red fibre
Intermediate glycogen content
Type 2x - fast glycolytic fibres
High myosin ATPase activity
Fast speed of contraction
Low resistance to fatigue
Low oxidative phosphorylation capacity
High in enzymes for anaerobic glycolysis
Few mitochondria
Few capillaries
Low myoglobin content
White fibre
High glycogen content
Muscle spindles
Control of motor movement from 3 level input
1)Afferent input from sensory endings
2)Alpha motor neuron output to skeletal muscle and stretch reflex pathway
3) Gamma motor neuron output to contractile end portions of spindle
Contraction w/spindle fibres gives even muscle contraction.
Spindles are groups of specialised muscle fibres known as intrafusal fibres
- lie withing spindle-shaped connective tissue capsules parallel to extrafusal fibres
- each spindle has its own private efferent and afferent nerve supply
- play key role in stretch reflex
