Muscle physiology Flashcards
Define skeletal muscle
Movement of bones - muscle attached to skeleton to provide support, frame and protection
Define cardiac muscle
Allow the heart to pump blood throughout the body
Define smooth muscle
Muscles of the internal organs which support the activities of the systems e.g respiratory - air flow
what are the cell shapes of: skeletal, cardiac and smooth muscle
Skeletal - elongated
Cardiac- often branched
Smooth- spindle
What are the striations on the 3 types of muscle
skeletal - visible striations
Cardiac- visible striations
smooth- no visible striations
what is the control of each muscle type
skeletal - voluntary, Calcium & troponin
Cardiac - involuntary, calcium and troponin
smooth - involuntary, Calcium & calmodulin
What are the functions of skeletal muscle
control of movement by contraction, and control of respiration.
- Homeostatic- bone stores Calcium ions, skeletal muscle stores potassium
- role in metabolism and temperature regulation
What are the ways in which skeletal muscle contracts
1- tissue organisation
2- excitation-contraction coupling
(excitation = nervous control of the NMJ, contraction = sliding filament model)
how is tissue organised in skeletal muscle
2 ends - attached to bones via tendons.
- Each muscle is surrounded by connective tissue- epimysium
- Within epimysium- tissue is organised into fascicles- bundles of muscle cells
- Connective tissue called perimysium separates individual fascicles
- Individual muscle cells (myofiber) in fascile are surrounded by connective tissue - endomysium
what are the key features of skeletal muscle (5)
- sarcolemma – plasma membrane of skeletal
- T-tubule - invagination of sarcolemma, extends deep into muscle fibre
- Sacroplasmic reticulum - surrounds each myofibril, ends near the T tubule region -called terminal cisternae
- Triad - region where 1 t tubule is flanked by 2 terminal cisternae
- myofibril - bundle of contractile filaments within muscle fibre
what are the 2 myofibril proteins?
Actin (thin filaments)
Myosin ( thick filaments)
- which give muscles striated appearance
what is A band in skeletal
quite dark, length of thick myosin filaments
What is I band in skeletal
V light region with thin actin. Length decreases during contraction
what is Z line in skeletal
bisects the I band - protein disc onto which thin filaments attach
what is H zone in skeletal
length decreases during contraction
What is M line in skeletal
Middle of the A band - region where thick filaments attach
What is the sarcomere? (skeletal)
Fundamental contractile unit region between 2 Z lines
what is the innervation of skeletal muscle and how does an AP arrive at the muscle?
It is voluntary so thus requires neurogenic innervation.
The AP travels from the spinal cord to muscle along 1 alpha-motor neuron
what are small and large motor units for?
Small - muscles requiring fine control e.g eyes
Large- muscles requiring stronger contraction e.g quadriceps
what needs to happen for skeletal muscle to contract?
Each cell must be stimulated by motor neurone process - the motor unit is the functional element of muscle contraction
what does the sliding filament theory require (muscle contraction)? (6)
ACT-4
- ATP
- calcium ions
- Thin filaments
- Thick filaments
- Tropomyosin
- Troponin
what is the sequence of events that happen at the NMJ? Skeletal
- APs from alpha motor neurone depolarise terminal - this opens voltage dependent calcium channels and Ca dependant vesicle fusion
- ACh is released from nerve terminal = exocytosis. Ach diffuse across synaptic cleft to activate the nicotinic ACH receptors. - opening receptor ionophore and leading to Na influx and depolarise membrane
- Na+ entry causes an excitatory end plate potential - these are summate and if obtain threshold they generate an AP.
- Muscle AP spreads along sarcolemma to T tubules triggering calcium release from terminal cisternae - this triggers excitation contraction coupling.
- Ca binds to troponin on actin fibres leading to cross bridge formation and the muscle contracts according to the sliding filament model. (ATP)
- Ach is continuously degraded into acetate and choline by the enzyme acetylcolinesterase at the NMJ. There is a decrease sarcoplasmic [Ca] and actin-myosin interactions are inhibited an the muscles relax.
What is excitation-contraction coupling? skeletal
AP travels along sarcoplasmic T tubules from the motor end plate.
- AP activates DHP, DHP activates RR by changing its conformation. Activated RR pumps calcium ions from SR into the cytosol to initiate muscle contraction
what are the 2 important physically associated proteins in excitation contraction coupling
- Dihydropyridine receptor DHP - (l type voltage gated Ca channel) - on triads at t tubules
- Ryanodine receptor on the SR
what does each actin monomer contain?
a single myosin binding site on the external surface (MBS)
what are the 2 types of skeletal muscle fibres?
Type I, slow twitch, red
Type II, fast twitch, white
Red skeletal fibres: where are they?
What features does it have?
What is energy production?
In postural muscle (surrounded by adipocytes)
- half diameter of white fibre
- dense with capillaries
- rich in mitochondria and myoglobin (red)
- low glycogen
Energy: it maintains contraction for long to sustain energy but the contraction rate is SLOW due to use of oxidative phosphorylation for energy production - hence the name ‘slow’ or oxidative’
white skeletal fibres: where are they?
What features does it have?
What is energy production?
In fat muscle- for quick action and large power production.
- White (low myoglobin)
- few capillaries
- low mitochondria
- High glycogen
Energy; Glycolysis - so they’re fast but are fatiguable.
(fast or glycolytic muscle)
What types of contractions normal muscle activity controlled by?
Isometric and isotonic contractions
what are isometric and isotonic contractions?
Isometric - contraction involves development of tension without any change in length
isotonic - contraction involves a change in length, no change in tension
when does isotonic contraction occur?
occurs when force of muscle contraction is greater than the load. - tension constant while the length of muscle shortens to fit the load
when does isometric contraction occur?
when load is greater than force of contraction so muscle length same but tension increases
Name some acquired skeletal muscle myopathies (diseases)
- inflammatory - polymyositis
- NMJ - myasthenia gravis, lambert eaton
- Endocrine - Cushings disease
Name some generic skeletal muscle diseases
muscle dystrophy e.g Duchenne MS
Myotonic dystrophy
Ion channel diseases
what happens when muscle contraction is ending? (i.e how do we end muscle contraction)
the muscle will keep contracting as long as it has ATP and Ca ions to expose the myosin binding site on actin.
- Contraction is terminated by Ca being removed from cysotol and back into SR - involves the Ca pump - SERCA
- Ca pumped against its [gradient] so needs energy
- The AP stops, no more Ca released but Ca is high, so Ca binds to SERCA, which induces binding of ATP to SERCA- ATP gets hydrolysed and Ca transported.
how are thick and thin filaments kept in order (i.e linear)?
Thin (actin) - by nebulin proteins
Thick - by titin proteins
what is the length tension relationship in skeletal?
when all myosin heads overlap with actin filaments - this is maximum tension at optimal sarcomere length
(tension is related to the number of cross bridges that form)
Describe excitation contraction coupling in cardiac muscle
Phase 2 cardiac AP, myocardial depolarisation permits Ca entry via L-type Ca channels in T tubules of sarcolemma.
- Ca entry is sensed by Rr to trigger Ca release from SR - triggers ECC
- Ca interacts with troponin-C (TN-C) causing TN-1 to uncover myosin binding site on actin to produce the force required for muscle contraction (sliding filament)
- After contraction, Ca reabsorbed into SR via SERCA pump and removed from cell via the Na+/Ca2+ exchanger or ATP dependant pump
- Ca decreases, dissociates from TN-c and the MBS on actin is inhibited.
ATP is required to unbind myosin from actin and reset sarcomere to normal length
what is preload in cardiac muscle?
How does this affect force of contraction?
The pressure exerted by the left ventricle during diastole
- Determined by the amount of blood returning to the heart (venous return).
- Increase in preload means an increase in actin/myosin overlap, leading to increased force of contraction
what is afterload in cardiac muscle
The pressure exerted by the aorta.
It determines how much force is used to generate pressure / muscle shortening
what is contractility? how does It affect force of contraction?
Change in length / change in time
Provides a preload indépendant mechanism to increase force of contraction
(frank starling forces)
What does the force of cardiac muscle contraction depend on?
- Initial muscle fibre length (set by frank starling)
2. Control of ANS - sympathetic nerves cause fibres to develop increased force from same initial fibre length
how are smooth muscle adapted for different functions (2)?
single unit - coupled by gap junctions
multi unit - under neural control
describe excitation of smooth muscle (more complex than cardiac/skeletal)
- SM APs are longer and involve influx of BOTH Na and Ca ions
- driven by Ca/2nd messenger pathways - NTs stimulate Ca release from SR (receptors linked to IP3) to allow direct ca influx
- Hormones increase CaM to increase Ca release from SR also
- 2nd messengers e.g NO release Ca
whats different between smooth muscle and cardiac/skeletal actin & myosin?
Smooth uses actin and myosin but they aren’t arranged as regular sacromeres
how is contraction of smooth muscle different from skeletal? (4)
it uses tropomyosin but lacks troponin C
no T tubule system
Contraction develops slower in Smooth but lasts longer
ATP usage is less and slower for a similar contraction in skeletal
How does smooth muscle contract?
Ca acts through calmodulin to activate MLCK to phosphorylate myosin heads.
This increases myosin ATPase activity to provide energy for myosin to form cross bridges with actin (sliding filament)
NO T TUBULE SYSTEM
how much longer are smooth muscle action potentials compared to skeletal/cardiac?
10-50ms
what is the nervous control of skeletal, cardiac and smooth?
Skeletal - alpha motor neuron
Cardiac - autonomic neurons
Smooth - autonomic neurons
how are each muscle activated? (Ca2+ sensor)
Skeletal and cardiac - troponin
Smooth - calmodulin and MLCK
compare contraction speeds of the 3 muscles
Skeletal fastest
Cardiac intermediate
Smooth slowest
How is each muscle contraction terminated?
Skeletal - breakdown of Ach by acetylcholinesterase
Cardiac - action potential depolarisation
Smooth - myosin light chain phosphatase
