Physiology of Skeletal Muscle Flashcards
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What is striation?
Alternating dark bands and light bands
What are dark bands and light bands?
Myosin and actin, respectively
Describe skeletal muscle
-Striated
-Voluntary control (somatic nervous system)
Describe cardiac muscle
-Striated
-Involuntary (autonomic nervous system)
Describe smooth muscle
-Non-striated
-Involuntary (autonomic nervous system)
Describe what neurogenic initiation of contraction, where it is, and how it happens
-The connection between a motor neuron axon terminal and a muscle fibre
-Occurs at a neuromuscular junction site
-A chemical synapse where a motor neuron transmits a signal to muscle fibre to initiate a muscle contraction.
Describe what and where myogenic initiation contraction, and how it happens
-A contraction initiated in the muscle itself and not dependent on neural stimulation.
-Can be modified by neural and hormonal stimulation.
Name 3 differences between skeletal and cardiac muscle initiation of contraction
-Neurogenic vs Myogenic initiation of contraction
-Motor units vs no motor units
-Neuromuscular junctions vs not
-No gap junctions vs present
Where does calcium in contraction come from in skeletal vs cardiac muscle?
-Sarcoplasmic reticulum
-ECF and sarcoplasmic reticulum (calcium induced calcium release)
Gradation of skeletal muscle contraction
By (1) motor unit recruitment (2) or summation of contractions
Gradation of cardiac muscle contraction
Frank-stirling mechanism: depends on the preload of the heart aka how much it fills up
Name 5 functions of skeletal muscle
- Maintenance of posture
- Purposeful movement in relation to external environment
- Respiratory movement
- Heat production
- Contribute to whole body metabolism
Which muscles have more or less muscle fibres per motor unit? Why?
-Muscles which serve fine movements have fewer fibres per motor unit, as precision is more important than power
-Muscles where power is more important than precision will have hundreds to thousands of fibres per motor unit
How are muscle fibres arranged in skeletal muscle?
Muscle fibres sit parallel bundled by connective tissue in skeletal muscle, and fibres usually extend throughout entire muscle
How are skeletal muscles usually attached to skeleton?
Tendons
How are the range of body movements brought about?
Bones, muscles and joints form lever systems that allow a range of body movements
What are muscle fibres made up of?
- Each muscle fibre contains many myofibrils which contain sarcomeres
What are myofibrils?
Alternating segments of myosin (dark) and actin (light) protein fibres
What are sarcomeres?
The functional units of the muscle
What is the structure of a sarcomere?
- The sarcomere is found between two Z lines (connect the thin filaments of 2 adjoining sarcomeres)
-The sarcomere has 4 zones
What is the sliding filaments theory?
Muscle tension is produced by sliding of actin filaments on myocin filaments
What is required for contraction and relaxation of muscles?
ATP required for contraction (to powers cross bridges) and relaxation (release of cross bridges and to pump Ca2+ back into SR)
- Ca2+ required for cross bridge formation
What is the excitation-contraction coupling?
The process where the surface action potential results in activation of the contractile structures of the muscle fibre by Ca2+
How does calcium couple excitation couple with contraction?
-Ca2+ is released from the lateral sacs of the sarcoplasmic reticulum when the surface action potential spreads down the T-tubules
What are T-tubules?
T-tubules are extensions of the surface membrane that dip into the muscle fibre
Describe the 7 events that lead from release of ACh to muscle excitation and relaxation
- ACh released by axon of motor neuron, crosses cleft and binds to receptors on motor end plate (no continuity of cytoplasm between nerve and skeletal muscle cells)
- Action potential generated in response to ACh is propagated across surface membrane and down T-tubules of muscle cell
- Action potential in T-tubules triggers Ca2+ release from sarcoplasmic reticulum
- Ca2+ ions released from lateral sacs bind to troponin on actin filaments - leads to tropomyosin being physically moved aside to uncover cross-bridge binding sites on actin
- Myosin across bridges attach to actin and bend, pulling actin filaments towards the centre of sacromere - energy is provided by ATP
- Ca2+ is actively taken up by SR when there is no longer action potential
- With Ca2+ no longer bound to troponin, tropomyosin slips back into its blocking position over binding sites on actin, contraction ends, actin passively slides back to resting position
What factors determine graduation of muscle tension?
-Number of muscle fibres stimulated within the muscle
-Tension developed by each contracting muscle fibre
How do the number of muscle fibres stimulated affect the strength of contraction?
-Motor units can stimulate a number of muscle fibres
-A stronger contraction could be achieved by stimulation of more motor units, aka “motor unit recruitment”
-Asynchronous (not at the same time) motor unit recruitment during submaximal contractions help prevent muscle fatigue
What 3 factors determine the tension developed by contracting muscle fibres?
- Frequency of stimulation and summation of contractions (see below)
- Length of muscle fibre at the onset of contraction
- Thickness of muscle fibre
What occurs if a skeletal muscle is stimulated once?
A single contraction called a twitch is produced
What is a twitch?
A single twitch produces little tension and is not useful in bringing about meaningful skeletal muscle activity
How can a twitch be amplified?
If the skeletal muscle recieves a second stimulation before it has had time to completely relax, the second response is added to the first and a greater muscle tension is developed
How does tetanus occur?
If a muscle fibre is stimulated so rapidly that it does not have an opportunity to relaxed at all between stimuli, tetanus occurs
What is tetanus?
A maximal sustained contraction
Why can cardiac muscle not be tetanised?
It is prevented by the long refractory period of cardiac muscle
When can maximal titanic tension be achieved?
Optimum length of muscle allows for optimum overlap between thick filament cross bridges and thin filament cross bridge binding sites
What is the optimum length of muscle?
The resting length of a skeletal muscle is approximately its optimum length
Name 2 types of skeletal muscle contractions
-Isotonic contraction
-Isometric contraction
What is isotonic contraction? What is it used for?
-Muscle tension remains constant as the muscle length changes
-Used for body movements and moving objects
What is isometric contraction? What is it used for?
-Muscle tension develops at constant muscle length
-Used for supporting objects in fixed positions and maintaining body posture
How is muscle tension transmitted to bone?
In both isotonic and isometric contractions muscle tension is transmitted to bone via the elastic components of muscle
What are the main differences between different types of skeletal muscle pathways?
-The enzymatic pathways for ATP synthesis
- The resistance to fatigue - muscle fibres with greater capacity to synthesise ATP are more resistant to fatigue
- The activity of myosin ATPase - determines the speed of contraction (cross bridge formation)
What are the metabolic pathways that supply ATP in muscle fibres?
- Transfer of a high-energy phosphate from creatine phosphate to ADP - immediate source of ATP
- Oxidative phosphorylation - main source when O2 present
- Glycolysis - main source when O2 not present
What are the three types of skeletal muscle fibres?
-Slow oxidative (type I) fibre (aka slow-twitch fibres)
-Fast oxidative (type IIa) fibres (aka intermediate-twitch fibres)
- Fast glycolytic (type IIx) fibres (aka fast-twitch fibres)
What are slow oxidative fibres used for?
Mainly for prolonged relatively low work aerobic activities e.g. maintenance of posture, walking
What are fast oxidative fibres used for?
Use both aerobic and anaerobic metabolism and are useful in prolonged relatively moderate work activities e.g. jogging
What are fast glycolytic fibres used for?
Use anaerobic metabolism and are mainly used for short-term high intensity activities e.g. jumping
Where do motor nerves receive inputs from?
The motor nerves receive inputs from the brain and inputs from a variety of receptors which can be inhibitory or excitatory
What is a reflex action?
-A stereotyped response to a specific stimulus
-The simplest form of coordinated movement
What are the pathway for reflexes used for in clinical context?
To identify lesions in the motor system e.g. knee jerk (stretch reflex)
What is the stretch reflex? What is its purpose? What does it help?
-The simplest monosynaptic spinal reflex
-It serves as a negative feedback mechanism that resists passive change in muscle length to maintain optimal resting length of muscle
-Helps to maintain posture e.g. while walking
How is stretch reflex activated and how does it bring about a contraction?
-The sensory receptor is the muscle spindle and is activated by muscle stretch (can be elicited by tapping the muscle tendon with a rubber hammer)
-Stretching the muscle spindle increases firing in the afferent neurons (quadriceps femoris in knee jerk) → contraction)
-The afferent neurons synapse in the spinal cord with the alpha motor neurons (efferent limb of the stretch reflex) that innervate the stretched muscle
- Activation of the reflex results in contraction of the stretched muscle
What does white matter contain and what is it responsible for?
-Myelinated and unmyelinated nerve fibres which conduct information either upwards (ascending/sensory fibres) or downwards (descending/motor fibres)
-Responsible for conduction of information
What does grey matter contain and what is it responsible for?
-Cell bodies of the neurons giving it a light grey appearance
-Responsible for receiving sensory information from the periphery and sending motor stimuli to the periphery
What is the grey matter divided into?
-The anterior wings are the ventral horns and contain motor neurons
-The posterior wings are dorsal horns and contain sensory
What are muscle spindles? What are they known as?
A collection of specialised muscle fibres which act as sensory receptors for the stretch reflex (aka intrafusal fibres)
What are ordinary muscle fibres known as?
Extrafusal fibres
Where are muscle fibres found?
Found within the belly of muscles and run parallel to ordinary muscle fibres
What are the sensory nerve endings of muscle spindles known as? What happens to them as muscle stretch increases?
-Annulospiral fibres
-Increases as the muscle is stretched
Do muscle spindles have their own efferent (motor) nerve supply?
True
What are the efferent neurons that supply muscle spindles are called? What is their role?
-𝛾-motor neurons**
-They adjust the level of tension in the muscle spindles to maintain their sensitivity when the muscle shortens during contraction
Does the contraction of intrafusal fibres contribute to the overall strength of muscle contraction?
False
