Flashcards in Physiology Deck (114):
What causes the appearance of striated muscle? (Fibres and their appearance)
Dark bands of thick myosin and light bands of thin actin.
What arm of the nervous system innervates smooth and cardiac muscle?
What arm of the immune system innervates skeletal muscle?
What type of nerves innervate skeletal muscle?
What is the name for a motor neurone and all the muscle fibres it supplies?
A motor unit
Describe a motor unit.
A myelinated motor neurone splits into unmyelinated branches, near the muscle. Each branch innervates a single muscle fibre. Individual branches further divide and end in a terminal bouton which synapses with the muscle membrane at the NMJ.
What is the neurotransmitter at the NMJ?
What is the relationship between the amount of muscle fibres a motor neurone innervates and the type of movements it produces?
Muscles which have fine movements have fewer fibres per motor unit.
E.g. The eye muscles have a 1:1 relationship, 1 neurone per muscle fibre. The intrinsic hand muscle also have few fibres per neurone. Whereas the thigh has lots of fibres per neurone to produce power.
Where is the body of an alpha motor neurone?
In the ventral horn of the spinal cord or brain stem.
What are the three anatomical components of the synapse at the NMJ?
Terminal bouton of motor neurone.
Motor end plate of muscle.
What surrounds a terminal bouton?
A Schwann cell.
What comprises the motor end plate and how is it arranged?
Sarcolemma of the muscle fibre arranged into junctional folds/waves. Nicotinic ACh receptors are on the muscle sarcolemma folds closest to the terminal bouton.
Where do synaptic vesicles congregate in the terminal bouton before they are discharged?
What five steps happen to ACh at the NMJ?
What is the name of the process by which the surface action potential triggers contraction of muscle?
Excitation contraction coupling
What are the five steps which happen in the terminal bouton to release ACh?
Choline is brought into the terminal
It is made into ACh
ACh is concentrated in the vesicles
Action potential causes influx of calcium into the bouton
Calcium causes vesicles docked at the active zone to discharge into the synaptic cleft.
How is choline transported into the bouton and what happens at the same time?
Through a choline transporter, in symport with Na+
What is ACh synthesised from and what enzyme does this?
Choline from outside of cell and acetyl CoA from mitochondria.
Choline acetyltransferase (CAT or ChAT) is the enzyme.
What is the structure of a nicotinic ACh receptor?
A pentamer of glycoprotein subunits surrounding a cation selective pore.
What causes a nicotinic ACh receptor to open?
Binding of two molecules of ACh
What happens when an ACh receptor opens?
Na influx and K efflux.
Na has a greater driving force and so depolarisation happens.
What is the depolarisation that occurs at the motor end plate called?
End plate potential.
What is the name for the amount of ACh in a vesicle?
What is the name of the response to one quantum activating an AaCh receptor?
Miniature end plate potential
What is the sum of all the miniature end plate potentials called?
The end plate potential.
How does an end plate potential trigger action potential?
When it exceeds a certain threshold it triggers an all or none action potential.
What is one to one coupling?
The normal situation that one action potential in the nerve triggers one action potential in the muscle causing a twitch of the muscle.
What muscles are striated?
Cardiac and skeletal
How are voltage gated Na channels arranged on the muscle fibre?
Lots surrounding the ACh receptor and then at intervals down the muscle fibre.
Describe the arrangement of transverse tubules and Sarcoplasmic reticulum on muscle fibres.
A net of sarcoplasmic reticulum surrounds muscle fibres (more than 1) and t tubules cross between muscle fibres. Lateral sacs of SR are on either side of the T tubules.
How does a wave of action potential cause release of calcium for the sarcoplasmic reticulum?
Action potential propagates across the sarcolemma and enters T tubules. This causes release of calcium from the lateral sacs of the SR.
What are T Tubules?
Invaginations of sarcolemma that dip into the muscle cell.
In simple terms what happens to muscle when calcium is released from sarcoplasmic reticulum?
What is the A band of a sarcomere?
Thick myosin filaments in the centre and thin actin filaments coming in from the Z lines.
What is the H zone in a sarcomere?
Lighter area within the middle of the A band where the thin filaments don't reach in.
What is the M line in a sarcomere?
A line that extends vertically down the middle of the A band myosin filaments.
What is the I band is a sarcomere?
The remaining portion of thin filaments that do not extend into the A band.
What are the three periods in muscle contraction after an action potential has occurred?
The latent period, contraction period and then the relaxation period.
What covers actin when a muscle fibre is relaxed that prevents myosin binding to it?
A troponin tropomyosin complex.
What part of actin binds to myosin. And what is the name for the area of myosin that it binds to?
The cross bridge binding site on actin and the myosin cross bridge with actin binding site.
What happens to the troponin tropomyosin complex when a muscle is excited? What is the result of this?
The muscle fibre releases Calcium, which binds to troponin, pulling the complex aside, exposing the cross bridge binding site on the actin.
What happens when actin binds to the myosin cross bridge?
It triggers a power stroke which pulls the thin filament inward.
What happens to calcium in muscle fibres when there is no longer an action potential?
It is re taken up by the sarcoplasmic reticulum.
What does acetylcholinesterase do to ACh?
It hydrolysis it into choline and acetate.
How efficient is AChE?
Extremely is hydolyses virtually all ACh molecules in a few milliseconds terminating the e.p.p
What are the symptoms of neuromyotonia?
Cramps, stiffness, slow relaxation (myotonia) and fasiculations.
What do the antibodies found in neuromyotonia attack? What result does this have?
Voltage gated K channels in the motor neurone.
Results in hyper excitability from repetitive firing.
What are the symptoms of lambert Eaton myasthenic syndrome?
Muscle weakness in the limbs.
What is LEMS associated with?
Small cell carcinoma of the lung.
What antibodies are made in LEMS and what does this result in?
Antibodies to calcium channels in the motor neurone resulting in reduced calcium entry and release of ACh.
What are the symptoms of myasthenia gravis?
Progressive muscle weakness during periods of activity. Weakness of eye and eyelid muscles.
What antibodies are made in myasthenia gravis and what does this result in?
Antibodies to nicotinic ACh receptors in the end plate. Resulting in a reduction in the amplitude of the e.p.p
What does botulinum toxin do?
Modifies enzymes involved in the docking of vesicles in the terminal bouton. Stops exocytosis and therefore release of ACh.
What do curare like compounds do and what are they used for?
Act as competitive antagonists of ACh at post synaptic nicotinic receptors. This reduces the amplitude of the end plate potential to below threshold. They are used as surgical muscle relaxants.
What two factors affect gradation of muscle tension?
The number or muscle fibres contracting within the muscle and the tension developed by each contracting muscle fibre.
What is motor unit recruitment?
When a stronger contraction is achieved by recruiting more motor units meaning many more fibres contract at the same time.
What prevents muscle fatigue during sub maximal contraction?
Asynchronous recruitment of motor units.
What four factors affect the tension developed by each muscle fibre during contraction?
Frequency of stimulation.
Length of muscle fibre at the onset of contraction
Thickness of the muscle fibre
Summation of the contractions.
What is the duration of action potential compared to the resulting twitch duration?
The action potential duration is much shorter.
What happens during summation of muscle twitches?
A stronger contraction is brought about by repetitive fast stimulations of skeletal muscle.
What happens to the muscle tension if a skeletal muscle is allowed to completely relax before a second twitch happens?
The second twitch will have the same tension as the first.
What happens to the tension in a muscle fibre if it is allowed to only partially relax before a second twitch happens?
The second twitch is added to the first twitch resulting in summation.
What happens if a muscle fibre is stimulated so rapidly it doesn't have time to relax at all in between twitches?
Can a single twitch bring about a meaningful muscle contraction?
No it produces too little tension.
What is the optimal length of muscle in relation to tetanic contraction?
The point of optimal overlap of the actin and myosin filaments so that tetanic contraction can be achieved.
When are muscles at the optimal length for tetanic contraction?
When they are resting.
What are the two types of skeletal muscle contraction?
Isotonic and isometric
What is isotonic muscle contraction? What types of movement is it used for?
Muscle tension remains constant as the muscle length changes.
It is used for body movements and moving objects
What is isometric muscle contraction and what is it used for?
Muscle tension develops as muscle stay at a constant length. Used for supporting objects in fixed positions and for maintaining body posture.
What happens to the velocity of muscle shortening as load increases?
What is a reflex?
A stereotyped response to a specific stimulus
What reflex is the simplest mono synaptic spinal reflex?
The stretch reflex.
What does the stretch reflex resist and why?
It resists passive change in muscle length to maintain optimal resting length. It helps maintain posture while walking for example.
What is the sensory receptor in the stretch reflex?
The muscle spindle.
What happens when the muscle spindle is stretched during the stretch reflex?
It increases firing in afferent neurones. These synapse with alpha motor neurone in the spinal cord, which cause the stretched muscle to contract.
What nerve is the knee jerk reflex testing?
What nerve is the ankle jerk reflex testing?
What nerve is the biceps jerk reflex testing?
What nerve is the brachioradialis reflex testing?
What nerve is the triceps jerk reflex testing?
What are muscle spindles and what is another name for them?
Collections of specialised muscle fibres. Also called intrafusal fibres.
What is another name for ordinal muscle fibres?
Where are muscle spindle fibres found and what do they run parallel to?
In the belly of muscles, parallel to ordinary muscle fibres.
What nerve endings do muscle spindles have?
Sensory called annulospiral fibres.
What efferent nerves supply muscle spindles?
Gamma motor neurones.
Does contraction of intrafusal muscle fibres contribute to the strength of a muscle?
What do gamma motor neurone do to intrafusal fibres as a muscle shortens/contracts and why?
They adjust the tension levels of the spindles to maintain their sensitivity.
What makes some muscle fibres more resistant to fatigue?
They have a greater capacity to synthesise ATP
What three metabolic pathways supply ATP in muscle fibre?
Transfer of high energy phosphate from creatinine phosphate to ADP.
What type of movements are type 1 skeletal fibres used for?
Prolonged low aerobic activities e.g. Posture maintenance and walking.
What type of movements are type 2a skeletal fibres used for?
Prolonged relatively moderate work requiring both aerobic and anaerobic metabolism e.g. Jogging.
What type of movements are type 2b skeletal fibres used for?
Work requiring anaerobic metabolism like short term high intensity activities e.g. Jumping
What three things provide joint lubrication?
Cartilage interstitial fluid
Synovium derived lubricin
What kind of molecule is lubricin?
What gives synovial fluid its high viscosity?
What kind of molecule is it?
Mucin (derived hyaluronic acid) which is a disaccharide polymer.
What does the synovial membrane do to synovial fluid?
It constantly replenishes and absorbs it.
Is the viscosity of synovial fluid always the same?
No it changes with movement. Rapid movement is associated with decreased viscosity and increased elasticity.
Where is synovial fluid Uric acid obtained from?
Dialysis of blood plasma.
What kind of cells do we find in synovial fluid?
What can turn synovial fluid red?
Traumatic synovial tap and haemorrhagic arthritis.
What is the colour and clarity of normal synovial fluid?
Colourless and transparent.
What is the colour and clarity of inflammatory synovial fluid?
Straw to yellow and translucent
What is the colour and clarity of septic synovial fluid?
Variable but opaque
What and the 4 different zones in hyaline cartilage?
Calcified (made of subchondral bone)
Where is the water distributed in hyaline cartilage?
Highest percentage is near the articular surface
What happens to hyaline water content with age?
What is the main type of collagen in cartilage?
Where are the highest concentrations of proteoglycan found in hyaline cartilage?
The middle and deep zones
What are proteoglycans mainly composed of?
Glycosaminoglycan e.g. Chondroitin sulphate
What are proteoglycans in hyaline cartilage mainly responsible for?
Compressive properties associated with weight bearing
What do catabolic factors in bone do?
give two example factors
Stimulate proteolytic enzymes and inhibit proteoglycan synthesis.
TNF alpha and IL1
What do anabolic factors in bone do?
give two example factors
Stimulate proteoglycan stimulus and counteract effects of IL1.
Tumour growth factor - TGF beta
Insulin like growth factor IGF 1
What are two markers of cartilage degradation?
Serum and synovial keratin sulphate
Type 2 collagen in synovial fluid.