Physiology Flashcards
What are the three types of muscle?
Voluntary or involuntary?
Striated or non-striated?
Skeletal - striated and voluntary
Cardiac - striated and involuntary
Smooth - non-striated and involuntary
What nervous systems are the muscle types innervated by?
Somatic - skeletal
Autonomic - cardiac + smooth
What is a motor unit?
“A single alpha motor neuron and all the skeletal muscle fibres it innervates”
How does the number of muscle fibres per motor unit depend on the functions served by that muscle?
muscles which serve fine movements (e.g. external eye muscles, muscles of facial expression; and intrinsic hand muscles) have fewer fibres per motor unit.
Vice versa
Summarize the levels of organization of skeletal muscle
Whole muscle (whole organ) --> Muscle fibre (one cell) --> Myofibril (a specialized cell organelle) --> Sacromere (functional unit)
Describe the differences in initiation and propagation of contraction in skeletal muscle vs cardiac muscle
Skeletal - Neurogenic initiation of contraction - Motor units - Neuromuscular junction present - No gap junctions Cardiac - Myogenic (pacemaker potential) initiation of contraction - No neuromuscular junction - Gap junctions present
Describe the differences in excitation contraction coupling between skeletal and cardiac muscle
Skeletal - Ca++ entirely from sacroplasmic reticulum
Cardiac - Ca++ from ECF and sacroplasmic reticulum
Describe the differences in graduation of contraction between skeletal and cardiac muscle
Skeletal - by 1. Motor unit recruitment and 2. summation of contractions
Cardiac - depends on the extent of heart filling with blood (preload) - Frank Starling mechanism
How do action potentials transmitted in alpha motor neurons cause muscle contraction?
Excitation contraction coupling = the process whereby the surface action potential results in activation of the contractile mechanism of the muscle fibre.
In skeletal muscle fibres Ca2+ is released from the lateral sacs of the sarcoplasmic reticulum when the surface action potential spreads down the transverse (T)-tubules (T-tubules are extensions of the surface membrane that dip into the muscle fibre).
Acetylcholine is the transmitter at neuromuscular junction.
Spread of action potential down the T-tubules triggers the release of Ca2+ from lateral sacs of sarcoplasmic reticulum.
What is the predominant structure of skeletal muscle fibers?
What are they?
What do they contain?
Each muscle fibre (cell) contains many MYOFIBRILS. These are specialised contractile intracellular organelles. The myofibrils have alternating segments of thick and thin protein filaments. The ACTIN (thin filaments) causes the lighter appearance in myofibrils and fibers. The MYOCYIN (thick filaments) causes the darker appearance. Within each myofibril: actin and myocin are arranged into SARCOMERES - these are the functional units of muscle.
What is a sarcomere?
Where is it found?
The sarcomere is the functional unit of skeletal muscle.
The sarcomere is found between two Z-lines - connect the thin filaments of 2 adjoining sarcomeres.
What are the four zones of the sarcomere?
A-band: Made up of thick filaments along with portions of thin filaments that overlap in both ends of thick filaments
H-Zone: Lighter area within middle of A-band where thin filaments don’t reach
M-Line: Extends vertically down middle of A-band within the centre of H-zone
I-Band: Consists of remaining portion of thin filaments that do not project in A-band
What are ATP and Ca++ required for?
ATP is required for:
- Contraction - to power cross bridges
- Relaxation - to release cross bridges + pump Ca++ back into the sarcoplasmic reticulum
Ca++ is required to:
- Switch on cross bridge formation
- Ca++ is the link between excitation and contraction
- Ca++ is entirely derived from sarcoplasmic reticulum in skeletal muscle
What is the transmitter at the neuromuscular junction?
Acetylcholine
What does graduation of skeletal muscle tension depend on?
- Number of muscle fibres contracting within the muscle
- Tension developed by each individual contracting muscle fibre- depends on
- Frequency of stimulation and summation of contractions
- Length of muscle fibre at the onset of contraction
- Thickness of muscle fibre
In skeletal muscle, how does the duration of the AP relate to that of the muscle twitch?
Why is this relevant?
In skeletal muscle: the duration of action potential is much shorter than the duration of resulting twitch.
It is therefore possible to summate twitches to bring about a stronger contraction through repetitive fast stimulation of skeletal muscle.
Describe twitch summation and tetanus in skeletal muscle
If a muscle fiber is restimulated after it has completely relaxed, the second twitch is the same magnitude as the first twitch
If a muscle fiber is restimulated before it has completely relaxed, the second twitch is added to the first twitch, resulting in summation
If a muscle fiber is stimulated o rapidly that it does not have an opportunity to relax at all between stimuli, a maximal sustained contraction known as tetanus occurs
How does maximal tetanic contraction relate to muscle length?
Maximal tetanic contraction can be achieved if the muscle is at its optimal length (lo) before the onset of contraction
–> the resting length of a skeletal muscle is approximately its optimal length
What are the two types of skeletal muscle contraction and what do they depend on?
This depends on whether or not the muscle changes length.
Isotonic contraction: used for (1) body movements and for (2) moving objects. Muscle tension remains constant as the muscle length changes.
Isometric contraction: used for (1) supporting objects in fixed positions and for (2) maintaining body posture. Muscle tension develops at constant muscle length.
In both isotonic and isometric contractions muscle tension is transmitted to bone via the elastic components of muscle.
How does the velocity of skeletal muscle shortening change with the load?
Inverse relationship
What are the main differences between different types of skeletal muscle fiber?
- The enzymatic pathways for ATP synthesis
- The resistance to fatigue - muscle fibres with greater capacity to synthesise ATP are more resistant to fatigue; and
- The activity of myosin ATPase - this determines the speed at which energy is made available for cross bridge cycling i.e. the speed of contraction
What are the three types of skeletal muscle fiber?
Slow oxidative type I fibres (also known as slow-twitch fibres) are used mainly for prolonged relatively low work aerobic activities e.g. maintenance of posture, walking. Fast oxidative (Type IIa) fibres (also known as intermediate-twitch fibres) use both aerobic and anaerobic metabolism and are useful in prolonged relatively moderate work activities e.g. jogging. Fast glycolytic (Type IIx) fibres (also known as fast-twitch fibers) use anaerobic metabolism and are mainly used for short-term high intensity activities e.g. jumping. LOOK AT TABLE
What are muscle spindles?
Where are they found?
What happens when the muscle is stretched?
Muscle spindles are the sensory receptors for stretch reflex; they are a collection of specialised muscle fibres.
These are specific types of muscle fibre (intrafusal) which act as receptors – not for contraction.
Muscle spindles are found within the belly of muscles and run parallel to ordinary muscle fibres (extrafusal fibres).
Muscle spindles have sensory nerve endings known as annulospiral fibres.
The discharge from the muscle spindles sensory endings increases as the muscle (and hence the spindles) is stretched.
What is special about the nerve supply to muscle spindles?
Muscle spindles have their own efferent (motor) nerve supply – mainly there to adjust their sensitivity so that they remain sensitive to stretch.
The efferent neurons that supply muscle spindles are called gamma (g) motor neurons.
The g-motor neurons adjust the level of tension in the muscle spindles to maintain their sensitivity when the muscle shorten during muscle contraction.
The contraction of intrafusal fibres does not contribute to the overall strength of muscle contraction.
