Muscle Anatomy and Contraction Flashcards
Motor neuron to muscle
Axons of motor neurons extend from the spinal cord to the muscle. There each axon divides into a number of axon terminals that form neuromuscular junctions with muscle fibres scattered throughout the muscle
Action potential travels down the motor neuron to the neuromuscular junction. Acetylcholine is then released from the neuromuscular junction by which the ACh then binds to ligament based receptors on the sarcolemma. When they bind to the receptors they open up voltage gated channels which then allow for the interchange of sodium and potassium which creates a chemical gradient which allows the action potential to start propagating along the sarcolemma
Organisation Hierarchy of Skeletal Muscle
Fascicles = composed of muscle fibres
Muscle Fibres/cells = composed of myofibrils
Myofibrils = composed of sarcomeres
Sarcomeres = composed of thick and thin myofilaments
Mysium - Sheath of connective tissue
Endomysium = encapsulates single muscle fibre
Perimysium = encapsulate group of muscle fibres into fascicle
Epimysium = encapsulates entire muscle
Sarcolemma, Transverse tubule & Sarcoplasmic Reticulum
Sarcolemma = Propagation of action potential along muscle fibre, transport metabolites, maintains acid balance
T-tubules = Transmit action potential into fibre via sarcoplasmic reticulum (push the action potential deep into the muscle cell )
Sarcoplasmic Reticulum = Release and uptake of calcium to myofibrils
Process of muscle contraction process
- Action potential arrival at terminal of NMJ
- Release of ACh to receptors
- Change in ion level = depolarization of membrane
- Action potential travels along sarcolemma
- AP travels down T-tubules
- Release of Calcium from sarcoplasmic reticulum
- Calcium binds to troponin - moves tropomyosin which exposes actin site
- Myosin heads bind to actin (start contraction)
What could be a rate limiter for speed of contraction
- Stimulation strength and frequency from axon terminal
- Calcium kinetics (release and uptake)
- Affinity of calcium to Troponin
- Myosin ATPase
- Ability of Myosin machinery to produce force
- Energy supply and type
Influencing Factors Contractility
Four major types : Type 1, 2a, 2x, 2b (little evidence for 2b in humans)
Identified using: histochemical, immunological (reaction with MHC proteins) and twitch characteristics.
Generally good agreement between approaches in identifying
Twitch - Isometric
The speed of a twitch for a fast twitch muscle fibres is faster while the speed of a twitch for a slow twitch muscle fibre is slower
Twitch - Calcium kinetics
- Sarcoplasmic reticulum - release and uptake of calcium is greater in type 2 fibres
- The force of the contraction to finish (decay) is slower in slow twitch muscle fibres (type 1) whereas the fast twitch is up and finishes quickly. The relates the calcium uptake
Twitch - Wave Summation
Force frequency relationship
Force Frequency Relationship
- The soleus takes less stimulation frequency to get to almost full saturation or peak force of that motor unit (slow twitch)
- It takes a higher stimulation frequency to full saturate the fibres in the EDL (fast twitch)
- Soleus has mainly slow twitch fibres while the EDL has mainly fast twitch fibres
Twitch - Isotonic
Factors effecting the ability to produce power?
Faster twitch muscle fibres are able to generate higher velocities at a given force when compared to slow twitch fibres
A number of factors ultimately effect the ability to produce power.
- Frequency of stimulation
- Fibre types
- Number of motor units
- Architectural factors (parallel vs series)
- Fibre length relative to optimal length (length tension relationship)
Length tension relationship
- At short and elongated lengths (Lo denotes resting length) the ability to produce force diminishes due to reduced number of binding sites available on the actin filament
- Short length, too much overlap
- Elongated, sarcomeres detached too much
Length tension and force velocity are interlinked
- Force velocity curve and length tension curve are connected
- At higher shortening velocities, the weight would be less
- As weight increase, shortening velocity decreases
- At zero shortening velocity, a isometric contraction occurs
- Stronger during the eccentric phase, we can tolerate higher loads
Fibre types and the nervous system
Motor unit.
- Alpha motor neuron and the muscle fibres it innervates.
- Fibre types within a motor unit have been shown to be uniform in most cases
- Smaller motor units innervates less muscle fibres
- Larger motor units innervates more muscle fibres
E.Henneman size Principle
- Frequency of motor unit use is directly related to the size and ease of triggering AP in the soma (neuron cell body). (smaller MU will get requited more frequently than larger MU)
- Is additive not sequential. (smaller MU continue to get used while larger MU are requited)
- Recruitment better related to force than contraction velocity
Architectural influencers
Parallel = muscle fibres are almost equivalent
Pennate = fibres are on an angle. Allows fo more stacking of muscle fibres
