Dargan Muscle Physiology Lectures

Question	Answer
Are smooth muscle cells smaller or larger than skeletal muscle cells?	They are smaller - around 100-300 micrometres in length and 2-5 micrometers width
True or false: similarly to skeletal muscle, smooth muscle cells can only hypertrophy - they cannot divide.	False. Smooth muscle cells can divide during normal growth and they can hypertrophy.
Name 3 places you might find smooth muscle.	Any from: internal organs, blood vessels, hollow organs (bladder), GI tract, RT tract
What are three of the functions of smooth muscle in the body?	moves food, moves urine, moves RT secretions, controls diameter of RT, regulates diameter of blood vessels
True or false: unlike skeletal muscle smooth muscle cells only have one nucleus.	True.
Where does innervation for skeletal muscle come from?	It comes from the autonomic nervous system.
True or false: similarly to skeletal muscle smooth muscle only responds to excitatory stimuli.	False - smooth muscle responds to both excitatory and inhibitory stimuli
How are neurotransmitters passed to smooth muscle due to the lack of post synaptic membrane?	Neurotransmitters are released from varicosities which are swellings in the nerve fibres close to the smooth muscle cells.
What is the difference in spread of excitation in single-unit smooth muscle cells and multi-unit smooth muscle cells?	Single-unit smooth muscle cells - electrical impulses spread via gap junctions between cells. Multi-unit smooth muscle cells - each muscle cell must be stimulated by an axon.
Where does the majority of the calcium come from during an AP in smooth muscle?	It comes from the extracellular fluid (unlike skeletal muscle where most of it comes from the SR).
What is the purpose of latch state of smooth muscle cross bridges?	It enables sustained smooth muscle tone by dephosphorylating the cross bridges and locking them together.
Between what values does tetanic fusion occur?	25-50 Hz
What are the two factors that have a variable effect on TFF?	1. which muscle it is occuring in - ranges from 15-200/sec in humans 2. the temperature - TFF decreases as temperature decreases
What are the four levels of muscle contraction?	single muscle twitches, temporal summation, unfused tetanus and fused tetanus
What does fused tetanus result from?	Repeated stimuli and summation of responses.
What method can be used to measure motor unit potentials?	Electromyography
In EMG what does the size of the peak relate to?	The number of muscle fibres firing simultaneously.
What increases on an EMG when the strength of contraction in a subject increases?	The frequency of potentials.
What is the result of acute denervation?	The nerve can no longer conduct APs and this results in paralysis.
What are fibrilation potentials?	They result in people who have suffered from acute denervation. They occur 2-3 weeks post trauma. The muscle fibres become hyper-excitable and contract spontaneously.
When does atrophy of muscles occur?	When nerves going to the muscles do not regenerate so the muscles are not functional.
Where does regeneration occur in the neurone?	Growth occurs down the endoneurial tube.
What is sprouting (of neurones)?	Sprouting occurs in partial denervation. Surviving motor neurons grow to reinnervate muscle fibres that have lost their nervous supply.
What are the resulting problems of upper motor neurone lesions?	Impairment of voluntary activity, exaggerated spinal reflexes, no muscle atrophy.
What is myasthenia gravis?	Myasthenia gravis is an autoimmune disease which results in decreased ACh receptors. This causes failure of neuromuscular transmission and therefore muscle weakness and fatigue.
What happens in dystrophin?	When the gene is abnormal membranes become leaky to Ca2+. This allows proteolytic enzymes to activate and contractile machinery is destroyed.
What is a twitch a response to and how long do they tend to last?	A twitch is the mechanical response of a single muscle fibre to a single action potential. On average they last 7-100 ms.
What is the delay called between an action potential reaching a muscle and the start of contraction.	Latent period. It reflects the time take for excitation contraction coupling.
What is the function of a myogram?	To measure twitch tension development.
Is a muscle twitch slow or fast compared to an action potential?	Muscle twitches are slow compared to action potentials.
Define temporal summation.	Temporal summation occurs due to lots of AP twitches bunching up which results in a force (contraction) that is greater than a single twitch.
Describe how tetanic contraction occurs.	Muscle twitches are slow compared to AP therefore many APs arrive before twitch is completed. The twitches bund up resulting in temporal summation. When freq of stimulation is so high that ca2+ peaks plateau occurs called tetanus.
What is physiological classification of muscle fibres dependent on?	The speed of contraction and their resistance to fatigue.
Image: images-1 (image/jpeg)	Fused tetanus.
Define contraction time. How can we determine contraction time?	Contraction time if the beginning of the peak to a twitch. It is determined by the rate of ATP splitting (as this relates to the amount of cross bridge cycling that is occurring).
If the frequency is 50 Hz what does this mean for the number of AP per second?	It means that 50 AP are occurring per second.
What are the three types of muscle fibres?	1 - slow twitch, type 1 2 - fast twitch fatigue resistant, type 2a 3 - fast twitch fatigable, type 2b
Give an example of isometric contraction.	When trying to lift an item is too heavy. The arm muscle contracts to try and lift it but no movement occurs.
Is the rate of ATP splitting and usage higher in fast twitch or slow twitch fibres?	Fast twitch.
What is the average speed of contraction in slow twitch fibres?	100ms
True or false: all muscle fibres of the same type are supplied by several motor units.	False. All muscle fibres supplied by 1 motor unit are of the same type.
When bench pressing weights what contraction is used: isotonic or isometric?	Both are used. Isotonic contraction is used to lift the weight and isometric contraction is used to hold it up.
Do postural muscles or muscles involving fine movements have more motor units?	Postural muscles have more motor units - some over 1500.
What would be the predominant type of muscle fibre found in the biceps? Why?	Fast twitch fatigable fibres. The biceps are used to produce large amounts of tension rapidly.
What factor can influence the proportion of fibre types in a muscle?	Neuronal input. (Cross innervating muscles can change their metabolic and contractile characteristics).
How may muscle fibres change during endurance training?	Fast twitch fatigable fibres often become fast twitch fatigue resistant fibres.
When do voluntary contractions occur?	When motor units are activated out of phase.
When there are strong contractions due to increased voluntary effort are there more or less motor units activated? What would their frequency be like?	In strong voluntary contractions more motor units would be activated at a higher frequency.
What is fatigue in muscles?	When muscles become incapable of contraction. It is awareness that increasing amount of effort is needed to maintain performance.
What factor may increase the extent to which muscles can be activated in individuals?	Motivation.
What is contraction in muscles dependent on?	The concentration of calcium.
What happens to calcium in the muscles following contraction? Where does it go?	The calcium returns in to the terminal cisternae of the sarcoplasmic reticulum via ATPase channels.
What is the rate of movement of calcium into the sarcoplasmic reticulum dependent on?	It is dependent on temperature as enzymes are responsible for moving calcium back to the SR.
What happens to the calcium concentration when muscles contract?	It increases.
What is DOMS?	DOMS is delayed onset muscle soreness. It is damage to muscle fibres such as bleeding and inflammation. It also results in high blood CPK levels.
When do giant motor unit potentials occur?	1. When there is chronic partial denervation. 2. When a nerve is acutely denervated but regenerates the nerves (as it often results in large motor units)
When 100% of the contractile machinery is activated what is the value for the concentration of calcium available?	Calcium concentration is greater than 10^-5 M when 100% of the contractile machinery is activated.
True or false: there are large stores of ATP in muscles?	False. There are only small stores in muscles. ATP must be synthesised to keep pace with utilisation.
When none of the contractile machinery is activated what is the calcium concentration in the cell?	The calcium concentration is less than 10^-7 M
What is creatine phosphate used for in muscle cells?	Creatine phosphate binds to ADP to make ATP + creatine (CP + ADP + H = ATP + creatine).
Is there more or less CP in cells than ATP?	There is around six times more.
How is ATP produced during anaerobic respiration? What other products build up?	It is produced via glycolysis. Large amounts of lactic acid build up.
What happens to the rate of calcium returning to the SR if there is a 10 degree fall in temperature?	It decreases by 2 times.
What is peripheral fatigue a result of?	Peripheral fatigue occurs due to biochemical changes in muscles for example accumulation of metabolites: H, ADP, AMP.
Is peripheral fatigue more likely in maintained contraction or quick bursts of contraction? Why?	Maintained contraction. Blood supply is decreased due to compression of vessels meaning metabolites are not washed away.
What is tetanic contraction?	Tetanic contraction is when a motor unit has been maximally stimulated by its motor neuron.
Define a fused tetanus.	A fused tetanus, also know as complete, is when the frequency of stimulus is so high that calcium levels peak and cause the maximum number of cross bridges to form therefore a tension plateau forms.
Define an unfused tetanus.	An unfused tetanus, also known as an incomplete tetanus, is when the relaxation time between successive twitches gets shorter as the strength of contraction increases.
What is the function of the sarcotubular system?	Conducts depolarisation in muscle fibres and allows contraction to occur.
What Ca2+ receptors are activated in the sarcotubular system upon arrive of an AP from tubules/sarcolemma?	DHP/L type Ca2+ receptors
Where is Ca2+ released from in the sarcotubular system?	The terminal cisternae of the sarcoplasmic reticulum.
Does Ca2+ activate troponin or tropomyosin?	Ca2+ activates troponin and causes it to change shape which in turn removes the blocking action of tropomyosin.
What are the five major steps in cross-bridge cycling?	1. active site exposure. 2. cross bridges formed. 3. myosin head pivots and pulls actin along. 4. cross bridges detach. 5. myosin head springs back by using ATP and is reactivated.
How does an overcontracted muscle affect the tension in the muscle?	If it is over contracted then this stops the ability of myosin to bind to actin and form cross bridges.
What are transverse tubules a result of?	The sarcolemma folding in and out/ invaginations of the plasma membrane (sarcolemma).
What is excitation contraction coupling?	Excitation contraction coupling is the path by which an action potential in a motor neuron results in muscle contraction.
What is a motor unit in muscles?	A motor unit is a single a-motor neuron and all of the corresponding muscle fibres it innervates. (It contributes to a motor neuron pool which is a collection of motor units that innervate a single skeletal muscle).
What is a motor neuron pool and where does it occur?	A motor neuron pool occurs at neuromuscular junctions. It is a collection of motor neurones or motor units that innervates a single skeletal muscle. There are normally 1-3 spinal cord segments/motor units in a motor neuron pool.
What is the delay between an action potential in the motor neurone and an action potential in a muscle?	around 0.2-0.5 miliseconds. VERY FAST.
Describe the process of events that occurs in neuromuscular transmission.	Action potential --> release of ACh from synapse --> ACh diffuses across synaptic cleft --> ACh binds to micotinic receptors on muscle cell membrane --> Activates entry of Na into cell causing depolarisation and action potential.
What happens to the acetylcholine involved in neuromuscular transmission once the action potential has occurred?	It is broken down by acetylcholinesterase and inactivated.
What is the enzyme that is used to synthesise acetylcholine (and CoA) from choline and acetyl CoA?	Choline acetyl transferase.
True or false: under normal conditions only 1 muscle fibre AP is triggered for each nerve AP due to the rapid breakdown of ACh.	True.
To which subunit in the ACh nicotinic receptor does ACh bind?	ACh binds to the N termini of each alpha subunit in nicotinic ACh receptors.
In muscle cells what are the names given to the cell membrane and the cytoplasm?	Cell membrane --> sarcolemma Cytoplasm --> Sarcoplasm
What forms the sarcotubular system?	The sarcolemma and the sarcoplasmic reticulum.
Which is the thick filament and which is the thin filament in the myofibrils?	Actin - thin Myosin - thick
What is the A band made up of?	The A band is dark as it is made of dark, thick myosin filaments.
What would you call the distance between two Z lines?	A sarcomere.
What are the dimensions of thin actin filaments?	1 micrometre x 5 micrometres
What regulatory proteins cover actin/myosin binding sites on actin?	Tropomyosin covers binding sites.
What are the three parts of thick myosin filaments?	Tail, hinge regions, and head
What is the displacement of myosin heads?	120 degrees
What is the sliding filament theory?	The sliding filament theory is how thick and thin filaments (actin and myosin) interact during contraction.
When myosin heads are in cocked/normal/upright position what do they contain: ATP or ADP + Pi?	They have ADP + Pi. ATP is hyrolysed when the myosin heads need energy to return into cocked position once sliding of actin filaments has occured.
When/where are cross bridges formed?	Cross bridges are formed between actin binding sites and myosin heads during muscle contraction.
What is the 'power stroke' in muscle contraction?	The power stroke occurs when myosin heads are bound to actin via cross bridges and they bend from their natural cocked position to pull the actin filament along.
What are the stages in cross bridge cycling (starting from cross bridge attachment)?	1 - cross bridge attachment 2 - power stroke occurs. Myosin head bends as it pulls the actin filament. 3 - cross bridge detachment occurs as ATP attaches to myosin head. 4 - Cocking of the myosin head occurs as ATP splits into ADP + Pi.
Below what level does calcium concentration need be for contraction to end?	below 1 micromol/dm3
What are T tubules?	T tubules are areas in the muscle fibres where the action potential can spread down to read the sarcolemma and stimulate release of Ca2+ to begin cross bridge cycling.
Describe the difference between the distance of Z lines in an uncontracted sarcomere and a fully contracted sarcomere?	Uncontracted sarcomere - the Z lines are maximum distance apart Fully contracted sarcomere - Z lines are as close together as they can get - overlap is maximised.
During contraction which bands of the myofibrils stay constant and which bands vary?	A bands remain constant. I bands and H bands vary.
Does the muscle cell get darker or lighter during contraction? Why?	Muscle cells get darker as contraction occurs as A bands get closer together and I bands dissapear. (REMEMBER: A bands - dark/myosin, I bands - light/actin)
What part of the nervous system generally supplies skeletal muscle? What neurotransmitters/receptors are used in this part of the nervous system?	Somatic nervous system. Neurotransmitter is acetylcholine and receptors are nicotinic cholinergic.
What are the two types of motor neurone?	Upper and lower.
What is the clinical relevance of the two types of motor neurones?	They can be used to differentiate between lower and upper motor neuron disorders and used in neurological testing.
What are hyperactive reflexes?	Exaggerated reflexes of the muscles.
What happens during isometric contraction of a muscle?	The length of the muscle stays the same but the tension increases.
Why do the neurones at neuromuscular junctions have lots of myelin?	To allow for rapid conduction of the nerve impulses.
What happens during isotonic contraction of a muscle?	The length of the muscle changes but the tension stays the same.
What does the length-tension relationship state? What is it due to?	The muscle exerts maximum tension when the muscle is within 10% of its resting length. It is due to the maximum resting overlap of myofilaments producing the maximum number of cross bridges and max. tension.

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Dargan Muscle Physiology Lectures

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