Muscle physiology Flashcards
what are physiological functions of skeletal muscles
- maintain posture
- purposeful movement
- respiratory movement
- heat production
- whole body metabolism
what type of muscle is skeletal
striated - alternating dark bands (caused by myosin thick filaments) and light bands ( caused by actin thin filaments)
what innervates skeletal muscle
somatic nervous system and are subject to voluntary control
what initiates and propagates contraction in skeletal muscle
Neurogenic initiation of contraction
Motor units
Neuromuscular junction present
No gap junctions
what is the gradation of contraction by skeletal muscle
- motor unit recruitment
2. summation of contractions
what is the neurotransmitter between the nerve and skeletal muscle
acetylcholine
what is a motor unit
single alpha motor neurone and all the skeletal muscle fibres it innervates
what structures does a muscle fibre contain
myofibril and sarcomere that contain myosin and actin
myosin (thick filament) and actin (thin filament)
what causes the light appliance in myofibrils and fibers and what causes the dark appearance
light - actin
dark - myosin
what are sarcomeres
actin and myosin are arranged into sarcomeres - these are the functional units of muscle
what is the functional unit of any organ
smallest component capable of performing all the functions of that organ
what is the functional unit of the skeletal muscle
sarcomere
what is the function of the Z line
connect the thin filaments of 2 adding sarcomeres
what is A band
made up of thick filaments along with portions of thin filaments that overlap in both ends of thick filaments
what is the H zone
lighter area within middle of A band where the thin filaments dont reach
what is the M line
extends vertically down the middle of A band with the centre of H zone
what is the I band
consists of remaining portion of thin filaments that do not project in A band
how is muscle tension produced
by sliding actin filament of myosin filament = sliding filament theory
when is Ca2+ released from the lateral sacs in the SR
when the surface action potential spreads down the transverse (T)-tubules
what is the process of muscle contraction
- ACh released by axon of motor neurone crosses clear and bind to receptors on motor end plate
- Action potential generated in response to binding of ACh and subsequent end-plate potential is propagated across surface membrane and down T tubules of muscle cell
- Action potential in T tubules triggers Ca2+ release from the sarcoplasmic reticulum
- Ca2+ released from lateral sacs, bind to troponin on actin filaments and leads to tropomyosin being physically moved alongside to uncover cross bridge binding sites on actin
- myosin cross bridges attach to actin and bend, pulling actin filaments towards centre of sarcomere; powered bu energy provided by ATP
- Ca2+ actively taken up by SR when there is no longer an action potential
- with Ca2+ no longer bound to troponin, tropomyosin slips back to its blocking position over binding sites on actin, contraction ends, actin passively slide snack to its original resting position
what is Ca2+ needed for in regards to skeletal muscle
- switch on cross bridge formation
- Ca2+ is the link between excitation and contraction
- Ca2+ is entirely derived from SR in skeletal muscle
what is ATP needed for when it comes to skeletal muscle
During muscle contraction
- to power cross bridges
During relaxation
- to release cross bridges
- to pump Ca2+ back into the SR
what does the graduation of skeletal muscle tension depend on
- number of muscle fibres contracting within the muscle
2. tension developed by each contracting muscle fibre
what is produced when skeletal muscle is stimulated once
twitch
how does the tension of skeletal muscle increase
increase frequency of stimulation
when can maximal sustained contraction be achieved
when the muscle is at its optimal length (lo) before the onset of contraction
what are the 2 types of skeletal muscle contraction
- Isotonic contraction: used for body movements and moving object. Muscle tension remains constant as the muscle length changes
- Isometric contraction: used for supporting objects in fixed positions and for maintaining body posture. Muscle tension develops at constant muscle length
how is muscle tension transmitted to the bone
via elastic components of the muscle
does the velocity of muscle shortening increase or decrease as the load increases
decreases
what are the main differences between different types of skeletal muscle fibres
- the enzymatic pathway 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 - this determines the speed at which energy is made available for cross bridge cycling ie the speed of contraction
what are the 3 metabolic pathways that supply ATP in muscle fibres
- transfer of high energy phosphate from creatine phosphate to ADP - immediate source of ATP
- oxidative phosporylation: main source when O2 is present
- glycolysis: main source when O2 is not present
what are slow oxidative type 1 fibres
- slow twitch fibres
- prolonged relatively low work aerobic activities ie maintain posture
what are fast oxidative type IIa fibres
- immediate twitch fibres
- use both aerobic and anaerobic metabolism and are useful in prolonged relatively moderate work activities eg jogging
what are fast glycolytic type IIx fibres
- fast twitch fibers
- use anaerobic metabolism
- mainly used for short term high intensity activities eg jumping
what is the stretch reflex
serves as a negative feedback that resists passive change in muscle length to maintain optimal resting length of muscle
what does activation of the stretch reflex cause
contraction of stretched muscle
what coordinates the stretch reflex
simultaneous relaxation of antagonist muscle
when can the stretch reflex be elicited
by tapping the muscle tendon with a rubber hammer which rapidly stretches the muscle resulting in contraction
what spinal segment and peripheral nerve are in relation to the knee jerk
L3,L4
femoral nerve
what spinal segment and peripheral nerve are in relation to the ankle jerk
S1,S2
tibial nerve
what spinal segment and peripheral nerve are in relation to the biceps jerk
C5,C6
musculocutaneous nerve
what spinal segment and peripheral nerve are in relation to the brachioradialis
C5,C6
radial nerve
what spinal segment and peripheral nerve are in relation to the triceps jerk
C6,C7
radial nerve
what are muscle spindles
- sensory receptors for stretch reflex
- collection of specialised muscle fibres
- intramural fibres
- found within the belly of muscles and run parallel to ordinary muscle fibres (extramural fibres)
- muscle spindles have sensory nerve endings known as annulospiral fibres
what are the efferent neurones that supply to muscle spindles are called
gamma (g) motor neurones
what may impair skeletal muscle function
- intrinsic disease of muscle
- disease of NMJ
- disease of lower motor neurones which supply the muscle
- disruption of input to motor nerves (eg upper motor neurone disease)
what are features of congenital myopathies
characteristic microscopic changes leading to reduced contractile ability of muscles
chronic degeneration of contractile elements - muscular dystrophy
abnormalities in muscle membrane ion channels eg myotonia
what are types of acquired myopathies
- inflammatory myopathies eg polymyositis
- non inflammatory myopathies eg fibromyalgia
- endocrine myopathies eg cushings syndrome
- toxic myopathies ef alhohol
what are symptoms of muscle disease
- muscle weakness/tiredness
- delayed relaxation after voluntary contraction (myotonia)
- muscle pain (myalgia)
- muscle stiffness
what are useful investigations in neuromuscular disease
- electromyography
- nerve conduction studies
- muscle enzymes
- inflammatory markers
- muscle biopsy
