Physiology 1A - Further Cell Notes Flashcards
Membrane permeability?
Determined by number of channels
Continuous K+ efflux (high permeability, low force) = continuous Na+ influx (low permeability, high force)
Passive voltage spread vs. active propagation
Passive
- Local current flows between different potentials
- Decreases over distance
- All membranes
Active
- action potential regenerated along axon
- Voltage-gated channels, Na+ influx/K+ efflux
- Only for depolarising stimuli (requires threshold)
- Same amplitude across a nerve axon
Electrical vs chemical signalling
Electrical
- Adjacent cells only
- ‘Gap junctions’ made up of connexin channels
- Ions (and hence Vm) in connected cells is the same
- Adjacent cells are coupled and synchronised
E.g. peristalsis, heart contraction
Chemical
- Specific chemical and its target receptor
- Synthesis, transport and release of a chemical, getting to target receptor, - - Response and termination of the response
Types of chemical signalling
Paracrine (adjacent cells)
Autocrine (same cells)
Endocrine (via the blood system)
Neurotransmission (typically paracrine)
Metatrophobic receptors
Metatrophobic receptors
- indirectly modulate ion channels via G-proteins and second messengers (i.e. G-proteins, cAMP, and Ca2+)
- Slower, longer-lasting effects
- Final ‘effector’ is often an ion channel or enzyme
Ionotropic receptors
- Ligand-gated ion channels
- Directly open to allow ion flow upon neurotransmitter (ligand) binding
- Fast signalling and ligand binding trigger ionic flux
- The response depends on what ion is allowed to pass through
Step of synaptic transmission at the neuromuscular junction
- A.P in presynaptic motor nerve terminal –> depolarisation
- Activation of presynaptic voltage-dependent Ca2+ channels causes influx of Ca2+ into the nerve terminal
- Ca2+ –> exocytosis of ACh from synaptic vesicles and ACh molecules diffuse across the synaptic cleft
- ACh binds to gates, activating nicotinic ACh receptors at endplate
- Open receptor channels –> influx of Na+ (and a little bit of K+ efflux) - Net influx of Na+ depolarises the post-synaptic membrane, causing the end-plate potential, EPP
- Local depolarisation spreads beyond the endplate to depolarise the muscle membrane potential past the voltage threshold and cause a muscle action potential
- Choline is recycled into the nerve terminal and used to re-synthesise ACh
Excitatory post-synaptic potentials (EPSP)
- Depolarisation so the neuron is more likely to fire AP (excitatory)
- Ligand-gated channel –> Na+ influx
- Depolarisations can also be from Cl- efflux or from closing K+ channels
- Major excitatory system is glutamate
- Two major ionotropic receptor types, AMPA and NMDA
Too much:
- seizures
- cell death
- excitotoxicity (e.g. anxiety, strokes, trauma, some neurodegenerative diseases)
Inhibitory post-synaptic potentials (IPSPs)
- Hyperpolarisation –> the neuron is less likely to fire AP
- Ligand-gated channel –> Cl- influx
- Hyperpolarisations can also be from K+ efflux
- ‘Shunting’ inhibition - just by opening the channel can cancel out an excitatory response
- Major inhibitory system is GABA
Too much:
- sleep
- coma
- sedation
- analgesia
- anaesthesia
Neuronal integration
Temporal summation (A+A)
- repeated activation of same synapse at brief intervals can produce a larger synaptic response
Spatial summation (A+B)
- activation of different synapses at around the same time can produce a larger synaptic response (for EPSP)
- IPSP can cancel EPSP if they occur around the same time
A rapid and brief response to neurotransmitters by…
Uptake transporters
- presynaptic, postsynaptic
- secondary active transporters
- specific for different neurotransmitters
Enzyme breakdown
- enzymes for different transmitters
membrane bound or soluble
- ACh broken down by AChE
- byproducts can be recycled
Desensitisation/internalisation
- may become less sensitive or change its concentration or location in the membrane
- acute (desensitisation) or chronic (internalisation) effects
Diffusion or flow
- chemical leaves receptor
- hormone carried away by blood
transmitter diffuses away from synapse
Simplified neuromuscular junction process
- A.P –> depolarisation
- Ca2+ channels causes influx of Ca2+ into the nerve terminal
- Ca2+ –> exocytosis of ACh from vesicles and ACh molecules diffuse across
- ACh binds to gates, activating nicotinic ACh receptors at endplate
- Open receptor channels –> influx of Na+ - Net influx of Na+ depolarises, causing the end-plate potential
- Depolarisation of muscle membrane potential past the voltage threshold, causing a muscle action potential
- Choline is recycled
Describe the first process of excitation-contraction coupling in the axon terminal
Axon terminal
1. action potential fires
- membrane depolarises, ACh is released and diffused down the synaptic cleft
- ACh binds to ACh receptors in motor end plates
- When it binds, it depolarises and fires another action potential
What happens at the neuromuscular junction when excitation-contraction coupling?
- at the motor end plate, the nerve membrane depolarises, opening Ca2+ channels
- Ca2+ flows into the cell (Ca2+ influx)
- Ca2+ binds on surface of vesicles which fuse to the cell membrane and stored ACh into a cleft
- diffuse randomly along the cleft and bind to ACh receptors and acetylcholinesterase
- ACh that encounter a nicotinic AChR will bind, opening the channel to allow Na+ and K+ to flow through, which causes depolarisation
- when depolarisation reaches a certain threshold, an action potential is generated and travels along the sarcolemma
Excitation-contraction coupling in the triad
- muscle action potential travels along membrane
- follows the membrane structures down the sarcolemma but also into and across the t-tubule
- Through t-tubules, the skeletal triad is met
- When action potential enters t-tubule, it depolarises the membrane
- DHPR (Ca2+ channel) voltage sensor changes conformation, also changing RyR receptor conformation to allow Ca2+ to pass through into the cytosol
What happens after? Cross bridge cycle
At the triad, upping Ca2+ concentration is necessary to free the binding site on the actin molecule
- Ca2+ binds to TnC through calcium binding, allowing troponin conformational change
- moves to allow myosin binding site to touch myosin head) - when Ca2+ concentration rises, tropomyosin molecule moves away from myosin binding site and changes conformation
- myosin head (acts as an enzyme, ATPase) binds to binding site on actin
- binds ATP molecule
- hydrolysis to split ATP into ADP and P - phosphate dissociates off myosin head, leading to a power stroke
- displaces actin molecule
- makes actin filament slide along myosin molecule - all the forces generated by each myosin head adds up
- hydrolysis of ATP generates energy - myosin also loses ADP but only mino movement
- now there is a pure binding of myosin and actin without phosphate or ATP (rigor state)
- ATP required to soften the muscle and re-initiate cycle
Muscle relaxation?
- free Ca2+ bind to cytosolic Ca2+ buffers
- troponin C (TnC) can been seen as Ca2+ buffer protein - Ca2+ and TnC dissociate, ions are bound to parvalbumin, removing Ca2+ from cross bridge cycle
- on a slower time scale, Ca2+ ions are pumped back into SR by a sarco-/endoplasmic reticlumlum Ca2+ - ATPase (SERCA)
- presence of ATP and low Ca2+, causes muscle fibre to relax
- hydrolysis of ACh at motor endplate by cholinesterase prevents ACh from eliciting further muscle action potentials
First mechanism of force regulation
- firing rate of motor nerve
- electrical activity of different motor units with increasing force
- observation of firing rate increases
- nerves fire more rapidly
temporal summation of stimuli
- induce excitation-contraction coupling by electric stimulus
Types of movements
- single twitch
- single contraction of fibre
- tension will go down, fibre will relax - summation (adding)
- before next fibre can relax, another stimulus sent
- force is resultantly high than the first
- leads to an increase in force - summation leading to unfused tetanus
- incomplete, titanic
- in increasing steps, a ramp until maximum (unfused tetanus)
- still small relaxations present - summation leading to complete tetanus
- no bouts of relaxation
- cannot distinguish single twitches
- reaches a plateau (complete tetanus)
Second mechanism of force regulation?
- motor neuron recruitment
small fibres - precise movement
large fibres - mass movement
Henneman’s size principle
- contractions are activated from smallest to largest
Third mechanism of force regulation
- fibre/sarcomere length.
see graph.
- optimum length 2-2.3 micrometers
- no overlap = no contraction
- myosin and actin must interact to produce force
If not stretched enough?
- crumpling, z-disk collision
Fourth mechanism of force regulation?
- fibre diameter
- muscle fibre adds more myofibrils and sarcomeres in diameter in each cell
- increase in fibre diameter → more myofibrils per fibre
- all connected to tendon - forces are added
Types of contraction
- isometric contractions
- muscle length stays constant
- muscle produces tension
- no net shortening or lengthening occurs
- no movement/chang in muscle length
e.g. pushing against a wall - isotonic contractions
- muscle tension remains constant
- length of muscle changers
e.g. bicep curl, push-up or squat
Two types of isotonic contractions:
1. concentric contraction
- muscle shortens while it develops tension
e.g. lifting a weight
- eccentric contraction
- muscle lengthens, although the cross-bridge cycle is running
e.g. lowering a weight
Skeletal muscle fibre types
- slow oxidative fibres (type I)
- produce low forces
contract slowly but for a long duration (fatigue resistant)
- regenerate ATP for long time periods via oxidative metabolic pathways
- need many mitochondria and a high density of capillaries (blood vessels)
- have a high content of the oxygen-binding molecule myoglobin which gives them red colour - fast oxidative fibres (type IIA)
- fast glycolytic fibres (type IIX)
- produce large forces
- have higher speed of shortening, but cannot contract for long times (non-fatigue-resistant)
- regenerate ATP via anaerobic pathways (glycolysis), producing lactate
- have less mitochondria and fewer capillaries
- low myoglobin and appear white
All have different ATP hydrolysis rates, as per their specific metabolic demand
Starting process of cardiac excitation-contraction coupling from the pacemaker cells
- pacemakers generate action potentials
- once threshold is reached, an action potential is generated
- Na+ current which activates K+ - excitation spreads throughout the heart and will excite the cardiac myosins
Cardiac muscle excitation
- pacemaker cells and cardiac myocytes connected via gap junctions (tunnel proteins)
- action potential travels through the syncytium and through the conduction system of the heart
- excitation reaches contractile myocytes
- depolarisation and calcium rise involves all fibres
- sequential activating of myocytes
Cardiac action potential?
- cardiac AP has sustained Ca2+ current, generating plataeu phase
- time scale: 200-300 ms
- no tentanisation of cardiac muscle (cardiac arrest)
Cardiac AP propagation
- cardiac myocyte AP moves along sarcolemma
- AP entres t-tubule
- junction: dyad
At the dyad?
- depolarisation activates cardiac DHPR Ca2+ channel
- Ca2+ influx from extra to intracellular space
- Ca2+ activates RyR isoform, releases Ca2+ from SR
- calcium-induced calcium release - calcium wave
Skeletal vs. cardiac coupling
Cardiac
- DHPR conducts large Ca2+
current into cell
- this opens RyR to create additional Ca2+ influx into the cytoplasmic reticulum
- therefore, two sources of Ca2+ (extracellular and from cytoplasmic reticulum)
- positive feed
back loop
Skeletal
- DHPR and RyR are linked - just conformational change opens the Ca2+ channel
Relaxation
- calcium binds to buffer proteins in cytosol after binding to troponin and activating contraction
Two ways:
1. pumped back into CR via ATPase that hydrolyses ATP and uses the energy to transport Ca2+ against its gradient into the SR Ca2+ store (cardiac SERCA isoform)
- it can be moved out of the cell
Important mechanisms in myocytes
- cytosolic Ca2+ buffering (proteins)
- transport of Ca2+ into SR by Ca2+-ATPase (the cardiac SERCA isoform)
- cardiac myocytes have a plasma membrane Na+/Ca2+ exchanger (NCX) that extrudes 1 Ca2+ ion from the intra- to the extracellular space, while 3 Na+ ions enter the cell
SERCA - primary active (pump, ATPase)
NCX - secondary active, dependent on Na+ gradient
Smooth muscle activation
- when smooth muscle depolarises (activated by autonomic nerve neurons), it can depolarise and mediate a Ca2+ current
- Ca2+ that enters the cellular space can activate a smooth muscle isoform of the RyR receptor (calcium wave -> positive feedback loop)
OR/
- via metabotropic receptors
- common agonist is ACh released from ANS
- when it binds to the receptor, it starts a molecular signalling cascade that produces a small molecule called IP3 which contains receptors that are large calcium channels similar to RYR
Single-unit vs. multi-unit
Single-unit
- all individual spindle-like smooth muscle cells are connected
- coupled via gap junctions (like cardiac muscles) (currents, small molecules)
- contracts as one
Multi-unit
Smooth muscle excitation-contraction coupling
- extracellular Ca2+ influx and/or
- intracellular Ca2+ release (from the SR)
Calcium sensor: calmodulin (CaM)
- activates myosin light-chain kinase (MLCK)
MLCK phosphorylates myosin light-chains (long-lasting modification)
- phosphorylated myosin has high ATPase activity and an accessible actin binding site -> contraction
Smooth muscle relaxtion
- Ca2+ removal via Ca/ATPases
- plasma membrane - extracellular
- SERCA - SR
- sec. active Na+/Ca2+ exchanger (extraceullar)
- CaM-Ca dissociates (CaM + Ca2+)
- myosin light chain phosphatase: dephosphorylates and inactivates myosin
- ATP metabolism + contraction stops
Mechanisms:
- myosin light chain (MLC) dephosphorylated by MLC-phosphatase
- Ca2+ buffered by proteins
- Ca2+ reuptake into the SR via pumps (Ca2+ ATPases)
- Ca2+ extrusion into the extracellular space via pumps (Ca2+ ATPases)
- Ca2+ extrusion into the extracellular space via plasma membrane Na+/Ca2+ exchange
Why is lower Ca2+ in smooth muscle not enough to relax it?
- Ca2+ controls the myosin light chain via phosphorylation, but lowering it only stops new activarion
- it requires myosin light chain phosphatase to actively turn myosin off
Contraction in smooth muscle
- Ca2+ rises
- binds to calmodulin, activiating myosin light chain kinase (MLCK)
- MLCK phosphorylates myosin, allowing it to bind actin and contract the muscle
- lowering Ca2+ turns of MLCK so new phosphorlysation happes, but myosin that is already phosphorylated stays on and keeps contracting unless phosphate is removed
- removing the phosphate requires myosin light chain phosphatase (MLCP)
Features of cardiac muscle
cardiac myocytes = cardiac muscle cells
syncytium = network of cardiac myocytes
intercalated disks = darker lines
gap junctions = tunnel proteins (ions can diffuse)
- makes myocytes electrically coupled!
dyad = 1 SR cisterna, 1 t-tubule
sarcoplasmic reticulum
- less extensive than smooth muscle SR
mitochondria
- higher than skeletal or smooth (rely on oxidative metabolism)
individual myocytes
- branched
- one central nucleus
