What are the properties of muscle?
Muscle- 40% of body mass. Highly metabolically active tissue. Adapts to the loads we put on it.
Key properties:
Extensibility- can be stretched
Elasticity- returns to original length
Force production, what makes muscle unique
Generates movement by:
Changing shape and pressure
Changing length and pulling on levers
What is cardiac muscle?
Actin and myosin form cross-bridges- same as skeletal muscle.
Sliding filament mechanism.
Node cells (pacemaker cells) produce spontaneous action potentials- authorhythmicity.
Electrical coupling between cells- action potentials pass along each cardiac muscle cell.
Refractory period (period of not being reactivated after a contraction) of 250ms to prevent tetanic contraction.
What is smooth muscle?
Makes up walls of gastrointestinal tract, used for digestion.
Surround hollow structures
Change in length changes shape of cells.
Actin and myosin form cross-bridges.
Sliding filament mechanism.
Controlled by amount of calcium in cell.
Calcium release controlled by autonomic nervous system- in response to hormonal changes.
Spontaneous action potentials or drifting of polarity in some smooth muscle cells.
What is skeletal muscle?
Changes in length of skeletal muscle moves skeleton.
Actin and myosin form cross-bridges.
Sliding filament mechanism.
Controlled by motor neurons- voluntary contractions and reflex contractions.
Skeletal muscle generates force to act on the skeleton. It changes length whilst generating force to move and control the skeleton. It’s voluntarily and involuntarily controlled.
What is the skeletal muscle-tendon unit (MTU)?
Muscle- force generating element of muscular-skeletal system, requires metabolic energy to develop force.
Tendon- attach muscle to bone, all muscle fibres connect to tendonous structure running through the muscle- this is aponeurosis. They transmit force from muscle to skeleton and vice versa. Primarily collagen. Passive force (no metabolic energy required).
What is the architecture of the MTU?
It refers to the arrangement of the muscle fibres and the tendon, usually related to position in the body and primary function. Muscles by the core are used for stability, short muscle fibres. Muscles that control shoulders and hips have big ROM, so need long muscle fibres. Muscles at ankles and feet have shorter muscles fibres and longer tendons.
How is force transmitted to create movement?
Force is generated within the fibres of the muscle belly; it’s transmitted from the muscle fibres to the connective tissue (aponeurosis) with the muscle belly. The sheets of aponeurosis form the tendons of the muscle; force is transmitted through these to the skeleton. If the muscle changes length in the contraction, this will be translated to the skeleton, and the bone will move.
How is force transmitted to resist or control movement?
Sometimes, an external load is applied to the body and muscles must resist the effect of the force. 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. The stretch and recoil of tendon within certain MTUs is vital for economical locomotion.
What are the characteristics of skeletal muscle fibres?
Fibres have some defining characteristics- multinucleated (lots of nuclei, muscle cells are very big), contains many mitochondria, transverse tubules (T tubules), myofibrils and sarcomeres, specific terms for some intracellular structures:
Sarcolemma=plasma membrane
Sarcoplasm=cytoplasm
Sarcoplasmic reticulum=smooth endoplasmic reticulum
What are myofibrils?
The structures that give skeletal and cardiac muscle their characteristic striated appearance. They’re arrangements of thick (myosin) and thin (actin) protein filaments.
What is the sliding filament theory?
Muscle force and length change is generated by the overlapping and interaction of actin and myosin filaments. Cross-bridges are formed by ‘heads’ of the myosin filament.
What is actin?
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?
Regulatory protein
Overlaps binding sites on actin for myosin and inhibits interaction when in the relaxed state, allows a muscle to relax
What is troponin?
Regulatory protein
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
What is the sarcoplasmic reticulum?
Is similar to the endoplasmic reticulum found in most cells. Ca2+ is stored and released following membrane excitation. The T-tubules and SR are connected with junctions which involve two integral membrane proteins- 1 in the T-tubule membrane and 1 in the membrane of the SR. The T-tubule protein is a modified voltage-sensitive Ca2+ channel (dihydropyridine receptor), which acts as a voltage sensor.
What is 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-bridges 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+ is taken back into sarcoplasmic reticulum.
Ca2+ removal from troponin restores tropomyosin which blocks myosin binding sites on actin.
How does neural input reach skeletal muscle?
Stimulation of the nerve fibres to a skeletal muscle is the only mechanism by which action potentials are initiated in skeletal muscle. Motor neurons innervate skeletal muscle, and their cell bodies are located in either the brainstem or the spinal cord. Large diameter myelinated axons propagate action potentials at high velocities, allowing signals form the CNS to travel to skeletal muscle fibres with minimal delay.
What is a motor unit?
A motor unit is 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. There are many motor units in a whole muscle. There is more than one way to turn the muscle on.
How is communication managed with muscle fibres?
The axon terminals of a motor neuron contain vesicles similar to the vesicles found at synaptic junctions between two neurons.
The vesicles contain the neurotransmitter acetylcholine.
The region of the muscle fibre plasma membrane that lies directly under the terminal portion of the axon is the motor end plate
The junction of an axon terminal with the motor end plate is the neuromuscular junction.
What communication process happens at the 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
What enzyme does the synaptic junction contain?
All neuromuscular junctions are excitatory (a motor neuron can’t turn off the action potential in a cell). There is an ‘all or nothing’ response. In addition to receptors for ACh, the synaptic junction contains the enzyme acetylcholinesterase- breaks down ACh, just as it does at ACh-mediated synapses in the nervous system.
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Atoms, Molecules and Cells65
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Cell membranes and signalling64
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Nervous system60
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Muscles- structure, function and control21
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Muscles- skeletal muscle fibre types and motor unit recruitment21
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Cardiovascular system31
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Respiratory system17
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Urinary system28
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Lab sessions- Blood sampling and Respiratory physiology10
