Skeletal Muscle Physiology 2 Flashcards
Gradation of skeletal muscle tension depends on which 2 primary factors?
No. of muscle fibres contracting within the muscle
Tension developed by each contracting muscle fibre
What do motor units allow?
Motor units allow simultaneous contraction of a no. of muscle fibres
What is motor unit recruitment?
Stronger contraction could be achieved by stimulation of more motor units
How is muscle fatigue prevented?
ASYNCHRONOUS motor unit recruitment during sub-maximal contractions helps prevent muscle fatigue
What does the tension developed by each contracting muscle fibre depend on?
Frequency of stimulation and summation of contractions
Length of muscle fibre at the onset of contraction
Thickness of muscle fibre
What can contractions be made stronger?
In skeletal muscle, duration of a.p is much shorter than duration of resulting muscle twitch
Thus, it is possible to SUMMATE TWITCHES to bring about a stronger contraction through repetitive fast stimulation of skeletal muscle
Describe what happens to stimulate consecutive single muscle twitches
If a muscle fibre is re-stimulated after it has completely relaxed, 2nd twitch is the same magnitude as the 1st
Single twitches produce little tension and are not useful for purposeful muscle activity
Describe what happens in twitch summation
If a muscle fibre is re-stimulated before it has completely relaxed, the 2nd twitch is added on the 1st
Results in summation
and the 2nd twitch causes a greater muscle tension to develop
Describe what happens in tetanus
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 occurs
This continues until either stimulation ceases or fatigue begins
Unique feature of cardiac muscle with relation to contractions?
CANNOT be tetanised - long refractory period prevents generation of tetanic contraction
How is tension in skeletal muscle increased?
Increase frequency of stimulation to modulate the force of contraction
How is maximal tetanic contraction achieved?
When the muscle is at its OPTIMAL LENGTH (lo) BEFORE the onset of contraction
i.e: developed tension depends on the initial length of skeletal muscle fibre
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What is the optimal length of a skeletal muscle fibre?
Its resting length is approx. the same as its optimal length
How can skeletal muscle length tension relationship be explained?
Sliding filament mechanism
How is skeletal muscle tension transmitted to bone?
Skeletal muscle tension - as a result of cross-bridge cycling (contractile component)- is transmitted to bone via stretching & tightening of muscle connective tissue and tendon (elastic component)
2 types of skeletal muscle contraction and what they are used for?
Isotonic contraction - muscle TENSION remains CONSTANT as muscle length changes; used for:
- Body movements
- Moving objects
Isometric contraction - muscle tension develops at CONSTANT MUSCLE LENGTH; used for:
- Supporting objects in fixed positions
- Maintaining body posture
Similarity between both isotonic and isometric contractions?
Muscle tension is transmitted to bone via the elastic components of muscle
Relationship between velocity of muscle shortening and load?
Velocity of muscle shortening decreases as the load increases
When there is maximum load, there is 0 velocity of shortening, i.e: isometric contraction
Differences between different types of skeletal muscle fibres?
- Enzymatic pathways for ATP synthesis
- Resistance to fatigue - muscle fibre with greater capacity to synthesise ATP are more resistant to fatigue
- Activity of myosin ATPase - determines the speed at which energy is made available for cross-bridge formation
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3 metabolic pathways that supply ATP in muscle fibres include?
- Transfer of high energy phosphate from creatine phosphate to ADP - immediate source for ATP
- Oxidative phosphorylation - main source when O2 is present
- Glycolysis - main source when O2 is not present
3 types of skeletal muscle fibres?
- Slow-oxidative (type I)
- Fast-oxidative (type IIa)
- Fast-glycolytic (type IIx)
Uses of slow-oxidative type I fibres?
AKA slow-twitch fibres; these mainly use oxidative phophorylation (lots of ATP) but have low myosin ATPase activity (speed is low)
Mainly for prolonged, relatively low work aerobic activities, e.g: maintenance of posture, walking
Uses of fast-oxidative type IIa fibres?
AKA intermediate-twitch fibres; produce lots of ATP and have high speed
Use BOTH AEROBIC and ANAEROBIC metabolism and are useful in prolonged, relatively moderate work activities, e.g: jogging
Uses of fast-glycolytic type IIx fibres?
AKA fast-twitch fibers; produce little ATP but speed is high
Use ANAEROBIC metabolism and are mainly used for short-term high intensity activities, e.g: jumping
What input do the motor nerves to skeletal muscles receive?
Excitatory or inhibitory input from the brain and also from a variety of receptors
What is a reflex action?
Stereotyped response to a specific stimulus; they are the simplest form of coordinated movement
What is the simplest monosynaptic spinal reflex?
Stretch reflex
What is the stretch reflex?
Negative feedback that resists passive change in muscle length to maintain optimal resting length of muscle; this helps to maintain posture, e.g: while walking
Steps in the stretch reflex?
- Sensory receptor is the muscle spindle and is activated by muscle stretch
- Stretching the muscle spindle increases firing in the afferent neurons
- Afferent neurons synapse in the spinal cord with the α-motor neurons (efferent limb of the stretch reflex) that innervate the stretched muscle
- Activation of the reflex results in contraction of stretched muscle
Coordination of the stretch reflex?
Simultaneous relaxation of antagonist muscle
Elicitation of stretch reflex?
Tapping the muscle tendon with a rubber hammer; this rapidly stretches the muscle,
E.: knee jerk reflex
(AKA patellar tension reflex) stretches the quadriceps femoris, causing its contraction
Spinal segment and peripheral nerve stimulated by knee jerk reflex?
Spinal segment - L3, L4
Peripheral nerve - femoral nerve
Spinal segment and peripheral nerve stimulated by ankle jerk reflex?
Spinal segment - S1, S2
Peripheral nerve - tibial nerve
Spinal segment and peripheral nerve stimulated by biceps jerk reflex?
Spinal segment - C5-C6
Peripheral nerve - musculocutaneous nerve
Spinal segment and peripheral nerve stimulated by biceps brachioradialis reflex?
Spinal segment - C5-C6
Peripheral nerve - radial nerve
Spinal segment and peripheral nerve stimulated by triceps jerk reflex?
Spinal segment - C6-C7
Peripheral nerve - radial nerve
What are ordinary muscle fibres also known as?
AKA extrafusal fibres
What are muscle spindles?
AKA intrafusal fibres (run parallel to extrafusal fibres)
Specialised muscle fibres that have a sensory function and nerve endings called annulospiral fibres
Discharge from the spindle sensory ending is increased as the muscle is stretched; contraction of these fibres does not contribute to overall strength of contraction
Motor supply to muscle spindles?
These neurons are called γ motor neurons and DO NOT stimulate contraction; rather, they adjust tension in the muscle spindles to maintain their sensitivity during muscle contraction
4 causes of impairment of skeletal muscle function?
- Intrinsic disease of muscle
- Disease of the neuromuscular junction
- Disease of lower motor neurons
- Disruption of input to motor nerves, e.g: upper MND
Genetically determined myopathies that can cause intrinsic muscle disease?
- Congenital myopathies - characteristic microscopic changes lead to reduced contractile ability of muscles
- Chronic degeneration of contractile elements, e.g: muscular dystrophy
- Abnormalities in muscle membrane ion channels, e.g: myotonia
Acquired myopathies that can cause intrinsic muscle disease?
- Inflammatory myopathies, e.g: polymyositis, inclusion body myositis
- Non-inflammatory myopathies, e.g: fibromyalgia
- Endocrine myopathies, e.g: Cushing syndrome, thyroid disease
- Toxic myopathies, e.g: alcohol, statins
Symptoms of muscle disease?
Muscle weakness/fatigue
Delayed relaxation after voluntary contraction (myotonia)
Muscle pain (myalgia)
Muscle stiffness
Ix for neuromuscular disease?
Electromyography - differentiate primary muscle disease from muscle weakness caused by neurological disease
Nerve conduction studies
Muscle enzyme tests, e.g: creatine kinase for muscle damage
Inflammatory markers, e.g: CRP, plasma viscosity
Muscle biopsy
