Lecture 5 - Muscle 1 Flashcards
What are some key properties of muscle
Extensibility – it can be stretched
Elasticity – it will return to its original length
How does muscle generate force
Changes in shape and pressure
Changes in length and pulling on levers
What is autorhythmicity
Node cells (pacemaker) produce spontaneous action potentials
What is the refractory period in cardiac muscle and why
250ms to prevent tetanic contraction
What mechanism does cardiac muscle operate
sliding filament
Where is smooth muscle located
Surrounding hollow structures
Cardiac muscle extras
Electrical coupling between cells
Actin and myosin cross bridges form
How is smooth muscle controlled
Amount of calcium in cell - released by autonomic nervous system
Smooth muscle extras
Spontaneous action potentials or drifting of polarity in some smooth muscle cells
Change in length changes shape of cells
Change in length changes shape of cells
How are skeletal muscles controlled
Motor neurons - reflex and voluntary
Skeletal muscle extras
Change in length of skeletal muscle moves skeleton
Actin and myosin form cross-bridges
Sliding filament mechanism
How does skeletal muscle create and control movement?
Skeletal muscle generates force to act on the skeleton
Skeletal muscle changes length whilst generating force to move and control the skeleton
Skeletal muscle is voluntarily and involuntarily controlled
What is the job of tendons
Attach muscle to bone, containing a aponeurosis
What are tendons made of
Primarily collagen
What is the architecture of a muscle
the arrangement of the muscle fibres and the tendon
How does transmission of muscle force to create force occur
Force is generated within the fibres of the muscle belly (more on how this happens shortly….)
Force is transmitted from the muscle fibres to the connective tissue (aponeurosis) with the muscle belly
The sheets of aponeurosis come together to form the tendons of the muscle
Force is transmitted through these to the skeletal
If the muscle changes length in the contraction this will be translated to the skeleton and the bone (lever) will move
How does transmission of muscle force to resist or control occur
This process also happens in reverse
Situations where an external load is applied to a region or the body and muscles resist the effect that force would have
In these situations the tendons will stretch to allow joints to flex and the muscles will generate force to absorb the energy of the impact
There are less extreme examples of this in everyday locomotion
The stretch and recoil of tendon within certain muscle-tendon units is vital for economical locomotion
What are the defining characteristics of skeletal muscle cells
- multinucleated
- contains many mitochondria
- Transverse tubules (T tubules)
- myofibrils and sarcomeres
- specific terms for some of the intracellular structures:
Sarcolemma = Plasma membrane
Sarcoplasm = Cytoplasm
Sarcoplasmic reticulum = Smooth ER
What are the myofibrils and what are they arranged as
Structures that give skeletal and cardiac muscle their characteristic striated appearance.
Actin (thin)
Myosin (thick)
Refer to A level notes and sliding filament theory
What is actin and what does it do
Contractile protein
Structure is like pearls strung together on a string and then the strands of pearls are twisted together
Each actin has a binding site for myosin
What is tropomyosin and what does it do
Regulatory protein
Overlaps binding sites on actin for myosin and inhibits interaction when in the relaxed state
What is Troponin and what does it do
Regulatory protein
Troponin binds Ca2+ reversibly and once bound changes conformation to pull tropomyosin away from the myosin interaction sites
Ca2+ binding to troponin regulates skeletal muscle contraction because it moves the tropomyosin away and allows myosin to interact with the actin.
How does sarcoplasmic reticulum aid in propagating a voltage potential
The sarcoplasmic reticulum (SR) in muscle is homologous to the endoplasmic reticulum found in most cells.
Ca2+ is stored and is released following membrane excitation.
The T-tubules and SR are connected with junctions.
These junctions involve two integral membrane proteins, one in the T-tubule membrane, and the other in the membrane of the sarcoplasmic reticulum.
The T-tubule protein is a modified voltage-sensitive Ca2+ channel known as the dihydropyridine (DHP) receptor, which acts as a voltage sensor.
Describe the process of Excitation-contraction coupling
Muscle action potential propagated into T Tubules
Ca2+ released from lateral sac of sarcoplasmic reticulum
Ca2+ binding to troponin removes blocking action of tropomyosin
Cross-bridges form between actin and myosin and generate force
Cross-bridge moves through power stroke sliding the actin passed the myosin
ATP causes release of myosin head and its return to original state
If Ca2+ and ATP still present myosin head will attach again to new actin binding site
When action potentials cease Ca2+ taken back into sarcoplasmic reticulum
Ca2+ removal from troponin restores tropomyosin which blocks myosin binding sites on actin
What is the only mechanism by which action potentials are initiated in skeletal muscle
Stimulation of nerve fibres
Where are motor neurons that innervate skeletal muscle and their cell bodies located
Brainstem or spinal cord
How is a charge propagated from neural input to skeletal muscle
Large diameter myelinated axons propagate action potentials at high velocities, allowing signals from the central nervous system to travel to skeletal muscle fibres with minimal delay
What is a motor unit
the motor neuron and the skeletal muscle fibres it innervates
(One motor neuron innervates many muscle fibres, but one muscle fibre is innervated by only one motor neuron.
Within a whole muscle there are many motor units.) (slide 30)
Describe the process of an action potential reaching a neuromuscular junction
Motor neuron action potential
Ca2+ enters voltage-gated channels
Acetylcholine released
Acetylcholine binding opens ion channels in motor end plate of muscle fibre
Na2+ entry across motor end plate resulting in depolarisation
Current spreads from depolarised end plate to adjacent sarcolemma
Muscle fibre action potential initiated
Action potential propagates along sarcolemma
All or nothing response a level notes
What do synaptic junctions contain
Neurotransmitter acetylcholine
