Excitable cells Flashcards
Claudius Galen
Brain as hegemonikon (ruler). Described anatomy + aspects of brain w/o dissection.
Used lesioning to map spinal chord function, supported humoral theory.
Vesalius
1st systematic work on anatomy corrected Galen’s mistakes .
Defined nerves as sensory + motor fibres arising from brain, nerves not hollow.
Challenged humoral theory.
Hook
c. 1670 developed double lens microscope allowed study in far greater detail.
Cajal first detailed drawing of retina.
AlphaFold2
CASP14:2020 can predict tertiary structure of protein using base sequence only
Cannot predict consequence of post translational processing + others mods.
Golgi & reticular theory
Invented silver stain - individual cells in great detail, believed neurites fused together to form reticulum network
Cajal & neuron doctrine
Each neuron discrete, dynamic polarisation (directionality), connectional specificity (connections made in ordered way).
Brain imaging advances
- Electron microscope proved Cajal’s theory, cell ultrastructure, resolution 0.1 nm
- Immunofluorescent labelling methods
- Confocal microscope combines lasers w/ high sensitivity cameras -> 3D images, can look at live cells, modest resolution 1 um
- Brainbow genetically modifies animal so its cells produce rand combinations of 4 different dyes
- Clarity makes brain transparent so permeable to certain tags + light
Glial cells
Can divide unlike neurons
Astrocytes - majority of glia, star shaped, fill space, regulate fluid composition, direct proliferation + differentiation neural stem cells
ODs myelinate many axons in CNS. Schwann cells myelinate single axon in PNS.
Microglia - phagocytic immune function, can migrate
Ependymal cells - line ventricles + direct cell migration in development, produce CSF.
Huntingtons disease
Symptoms abnormal movements + cognitive problems.
Autosomal dominant , huntingtin gene mutation codes glutamine. Poly Q region has > 40 repeats.
-> fragments of protein accumulate in neurons as inclusion bodies
Basal ganglia sensitive + vital for movement control
Astrocytes + microglia can be activated -> neuroinflammation
Alzheimers disease
Protein accumulation:
- beta amyloid plaque deposits (insoluble) around neurons, amyloid precursor cleaved to beta amyloid
- hyperphosphorylation of tau proteins clumps to form neurofibrillary tangles inside neurons disrupting cargo movement
glial activation + astrocytes can become ‘reactive’ -> neurotoxicity
Neuron structure
Dendrites - rough ER, ribosomes, golgi.
Axons - synaptic vesicles
Cannot divide but can trigger APs.
Polarity manifested as axons have many more Na+ & K+
Dendrites never myelinated
Neuronal cytoskeleton
Get proteins to correct place as no ribosomes in axons.
MTs involved w. structure + transport, can (de)polymerise - 20nm wide
Neurofilaments used for mechanical strength - 10nm wide
Microfilaments mediate shape change, made of actin polymers tethered to membrane - 5nm wide
Classification of neurons
Number of processes:
Bipolar - interneurons
Unipolar - sensory/afferent
Multipolar - motor/efferent
Sectional planes in the brain
Axial/transverse - horizontal
Sagittal - longitudinal
Coronal - frontal (slice of bread)
Mammalian embryological development
endoderm (organs, viscera), mesoderm (bones, muscles), ectoderm (NS + skin)
ectoderm specialises into neural plate (neural tube) + epidermis.
- CNS develops from walls of neural tube
- PNS from neural crest
notochord derived from mesoderm (signalling for development)
Anterior + posterior neuropores ate end of neural tube should close
Anencephaly
Failure of anterior neural tube to close so brain does not develop, death
Spina bifida
posterior neural tube does not close -> gap in spinal column + open defect causing paralysis
can be prevented - folic acid supplements
some anti-epilepsy/bipolar drugs increase risk
Spinal chord
Protected by spinal column, surrounded by meninges + CSF
Ventral roots have motor neurons. dorsal roots have sensory neurons
Grey matter (middle) - neuron cell bodies
White matter - myelinated axons
Decussation
Contralateral sensory/motor pathways
Right side of brain controls + receives signals to and from left half of body, vice versa
- decussate at medulla
Cerebral palsy
Most common in children, 90% congenital, 10% acquired, root cause unknown
Spastic - damage to white matter of motor cortex, hypertonia -> plegias (legs, one side or all 4 limbs affected), can affect other body parts
Dyskinetic - damage to basal ganglia, athetosis, chorea + dystonia (repetitive twisted movements)
Ataxic - damage to cerebellum, problems w/ balance/coordination
Williams syndrome
deletions of 27 genes on chromosome 7
- abnormalities frontal/cortex + cerebellum (motor tasks)
- abnormalities in parietal cortex + amygdala (no fear of social interactions, exaggerated fear responses)
Angelman syndrome
Paternal imprinting of UBE3A on chrom 15 - maternal mutated/ deleted
Seizures, ataxia, learning difficulties, uncontrolled laughter
Hippocampus + cerebellum affected.
Prader Willi syndrome
Maternal imprinting UBE3A on chrom 15 - paternal mutated/deleted.
Mild cognitive deficits, good at visual organisation, insatiable appetite.
Underdevelopment in many brin regions (hypothalamus)
Electrophoresis
Movement of charged substance in an electric field .
Total electrochemical gradient = gradient caused by diffusion +- gradient caused by electrophoretic movement
Ohms law
Rate of movement of ions across membrane depends on:
- size of electrochemical gradient
- nature of ion
- number of ion channels
- properties of ion channels
Ions in water
Physiologically useful (Na+, K+, Cl-) & biochemically useful (trace metals), Ca2+ both
Hydration shell of ion affects mobility, interactions w/ proteins + is effective size of the ion.
Co-transporters & pumps
Sodium-calcium exchanger is antiporter - 2Na+ in, Ca2+ out
2000Ca+/sec vs calcium pump 30Ca+/sec
Ion channels
Faster than transporters, only allow passive transport
Selective permeability on basis of charge+ size controlled by selectivity filter.
-> surrounded by charged amino acid rings
Gate located at bottom to let ion pass through after selection.
Ligand gated ion channels
Pore let ions through, ligand binding site, coupling mechanism, desensitization mechanisms
e.g. cys loop receptors like nAChR is pentamer of 5 subunits, 50% outside cell where bind sites are
Characterised due to
-> Taiwanese banded krait has irreversible antagonist that can purifiy nAChR
-> Torpedo ray has electron organ w/ extremely high number of nAChRs
Voltage gated K+ channels
6 TM domains (a-helices), between 5th + 6th is mem. dipping domain lining channels pore.
Ca, Na & TPC arose from gene duplication - consist of Kv in multiples of 2 or 4.
Sodium and calcium voltage gated ion channels
Pore forming unit is a-subunit.
- consists of 4 copies of Kv like structure joined together as single peptide (4 pseudo-subunits)
small accessory proteins associate w/ 10 Ca subunits/9Na subunits
LGICs in prokaryotes
ELIC in E. chrysanthemi cation channel gated by amine
GLIC in G. violaceus cation channel gated by protons
- both are homopentamers
Acetylcholine binding protein (AChBP)
Water soluble protein extracted from snails that seemed to be N terminus extracellular domain of nAChR
Role is to be released by glial cells into synapse + act as molecular sponge reducing ACh levels . Not found in vertebrates.
- can act as template for homology modelling
Anatomy of action potentials
Rising phase where Na+ channels open.
At peak Na+ close, K+ open.
Neuron - Ap lasts 2ms
Heart - AP lasts 200ms
Skeletal muscle - AP lasts 5ms
Na channels open rapidly, then inactivated after 1ms, inactivation gate swings up blocking channel.
K channels open slower + inactivate slowly.
Feedback loops in APs
Positive feedback loop in Na+, generates rapid rise in membrane potential, - controlled by inactivation
Negative feedback in K+ generates membrane repolarisation which self terminates
Refractory periods
Absolute - cannot produce another AP as one is occurring
Relative - cell less excitable de to hyperpolarisation (raised threshold)
- ensure unidirectionality, ion concs do not change during an AP
Dravet syndrome
Rare from of epilepsy, worsens w/ age
Mutation in SCN1A genes codes for NaV 1.1. Inhibitory neurons affected so some regions overractive.
- cannabinoids potential effective treatment (CBD over THC)
Capacitors
Device for storing energy via separation of charge
In cells, Na+ ions line up outer mem. anions line up on inner face of mem - separated by insulating membrane (capacitor function)
Goldman Hodgkin Katz equation
Can be used to calculate RMP, acts as ‘weighted’ Nernst equation as it accounts for differing ion permeabilities
Electrical synapses
Gap junction forms between neurons by connexons (6 connexins per neurons).
- direct ion transfer
- non rectifying
- fast transmission
- signal often attenuated
- highly reliable, specialised situations
Rapid response (reflex) or for coordination over large tissue area, not common in vertebrates.
e.g. giant fibre in drosophila, mutant neurons use innexin not connexin which is non-functional so junctions not formed
Chemical synapses (soup)
Converts electrical signal -> chemical, very flexible so more common.
Acetylcholine in surrounding fluid able to transfer impulses.
- Loewi experiment on vagus (parasympathetic) nerve to show transmission via a bathing fluid
Vesicular release
Lots of mt needed for vesicular release.
Involves SNARE proteins:
SNAP-25 + syntaxin on pre-synaptic mem.
Synaptobrevin on vesicles.
SNARE proteins bind each other drawing vesicle close to membrane.
Increase in Ca2+ detected by synaptotagmin causing vesicle to fuse w/ mem releasing its contents
Quantal release
Spontaneous NT release w/o calcium produces miniature endplate potentials (MEPPS)
- stepwise variation in MEPP amplitude
NT release is quantal, 1 quantum = amount per vesicle
Neurons release up to 200 quanta per AP in evoked release.
Vesicle recycling & why is it needed?
After vesicle fuse w. membrane, clathrin binds it to form clathrin coat (truncated icosahedron).
Vesicle dragged away from membrane.
- constant vesicle size
- constant vesicle number
- constant size of nerve terminal
Tetanus
Cause by anaerobic bacterium - C tetani which has toxin that cleaves SNARE proteins
Enters via NMJ, travels to CNS where its released by dendrites into inhibitory neurons (GABA, glycine) - destroys SNARE proteins
Transmission in dendrites
Current is attenuated as it leaks out via cell membrane.
Dendrites short + have many different inputs so attenuation not a problem.
Transmission is passive, no wave of APs.
Transmission in axons
No attenuation + fast transmission speed.
- very high sodium channel density (~100-fold higher), especially in axon hillock
- increased azonal diameter improves conduction velocity (squid)
Myelination
Nodes of ranvier have very high Na+ channel density (1200/um) rise time of AP decreased
Internodes 1mm long, low Na+ channel (20/um), spaced at regular intervals + have myelin sheath decreases membrane capacitance.
Saltatory conduction means current is extremely rapid through internode (passive), x 15 faster than via AP
How is myelin sheath formed?
Extension of cytoplasm of ODs or Schwann cells.
It is a spiral structured fatty substance around a neuron
-> rotary sheath migration
Multiple sclerosis (MS)
Most common autoimmune disease in N. Europe.
Caused by demyelination of CNS neurons, immune attack on ODs.
Characterised by relapses + remissions. Eyes often effected first.
- Na+ channel distribution unevenly spread due to previous myelination, neurons cant send signals.
Linked to low vitamin D during development.
How to diagnose MS
Visual evoked potential test (VEP) electrode placed on scalp + patient shown checkerboard pattern (delayed response in MS)
MRI scan can reveal sclerotic plaques
Guillan Barre syndrome (GBS)
Schwann cells attacked, PNS neurons unmyelinated.
-> progressive weakness
Walking + respiratory functions impaired.
Exact aetiology unknown but linked w/ infection (campylobacter food poisoning, cytomegalovirus, CV-19)
Cholinergic terminal
Choline acetyl transferase converts acetyl CoA + choline -> ACh + CoA, ACh packaged into vesicles.
ACh hydrolysed by acetylcholinesterase in cleft -> choline + acetate
Cholinergic transmission system
If 1 impulse, 1 ACh released, -> fast response (nAChRs) then slow response (mAChRs), NO AP as threshold not met.
If 2 impulses rapidly, 2 ACh released -> 1st gives fast + slow response, 2nd gives fast nAChR which coincides w/ 1st mAChR , AP generated.
- multiple types gives flexibility
Metabotropic GluRs
Family C, always operate as dimers, agonist binding site in N terminus domain.
Ionotropic GluRs
3 types named after synthetic agonists - NMDA, AMPA, Kainate.
NMDA receptor is special - highly permeable to Ca+, blocked by Mg+ at resting potential, needs glycine (D-serine) as co-agonist
Glutamatergic transmission (main excitor)
1st current flow induced by AMPA -> local brief depolarisation. NMDA activated (Mg+ unblocks it at -30mV) -> increased depolarisation, Ca+ & Na+ influx.
mGLuR activated, long slow depolarisation helps unblock NMDA by Mg2+
Synapse can be strengthened by LTP
GABA(B) receptor
Family C GPCR, slower
B1 subunit binds GABA, B2 interacts w/ G protein
Inhibits VG Ca+ channels & opens K+ channels
-> adenylyl cyclase inhibited
GABA(A) receptor
LG chloride channel, faster, pentamer.
Target of target of many drugs, many allosteric sites.
Positive allosteric modulators damp down NS as they increase effects of GABA
Autoreceptors
When activated, regulate release of same NT they activated by (usually inhibitory so negative feedback but can be positive)
e.g. noradrenaline release form cardiac sympathetic neurons
a2 adrenoreceptors have Gi protein - decreases cAMP, less noradrenaline released
Can be countered by antidepressant mirtazapine (antagonist)
Heteroreceptors
Responds to NT release from different neuron, activated by NT from complex extracellular ‘soup’.
e.g. acetylcholine regulates dopamine release in striatum
Dopamine release increased by nAChR activation on presynaptic membrane , due to greater Ca+ permeability
Regulators of nAChRs
Series of peptides produced in brain interact w/ nAChRs + change their function
e.g. Lynx1 is soluble regulator during brain development, member of protein superfamily Ly6
V. similar structure to a-neurotoxins (krait + cone snails)
Spatial summation
PSPs from multiple synapses influence neuron behaviour + how close to axon hillock.
Current attenuated in dendrites - can be related to distance, if close to hillock has more influence.
Temporal summation
Multiple PSPs from single synapse.
- must be additive
- if new PSP arrives before PSP decays, it will be compounded
Shunting inhibition
Single inhibitory synapse close to hillock/cell body can shut off all excitatory inputs from that dendrite.
Cl- ions go from GABAa receptors to inhibitory synapse, into dendrite cancelling out EPSP
How is information encoded?
Amplitude of stimulus size converted to frequency modulation (FM)
-> frequency + pattern of APS
Rate is largely determined by relative refractory period - can be bypassed if stimulus large enough (increased AP frequency)
e.g. reward pathway, dopaminergic neurons have cell body in ventral tegmental area + axons in nucleus accumbens.
Tonic is normal frequency rate, drugs can mimic natural reward system
Extracellular recording
Electrode placed outside but very close to cell - easy but not specific, gives info on group of cells
e.g. EEG & ECG
Intracellular recording
Electrode inserted into cell - hard to carry out, gives info on specific neuron, useful for large cells (>50um)
Patch clamp
Developed Sakmann & Neher early 1980s
4-5 different configurations of single channels (cell attached, inside-out, outside-out) or many channels (whole cell, perforated patch)
Uses micromanipulator to convert large hand movements into small microscopic ones.
Whole cell & perforated patch techniques
Whole cell: suctions disrupt membrane so inside of electrode continuous w/ inside of cell.
Perforated patch: antibiotic added to create pore in membrane -> continuity.
Single channel patch techniques
Cell attached: gentle suction at 1-2 channels
Inside-out: seal formed on membrane + channel pulled away, electrode removed from bath so stick membrane ends seal onto electrode
Outside-out: piece of membrane ripped off allowing ends to seal onto each other - outer face of membrane + channel facing bath.
Single channel recordings
k-1 determines stability of opening state, big value means brief openings
k+1 determines stability of closed state, big value means brief closings.
Agonist/voltage can influence rate constants -> length of openings/closings can vary
Conductance in channels
Ability of channel to movie ions through membrane, if high then large current blip.
Does not affect length of opening.
Applications of patch clamps
Slices of brain tissue - jet solution washes away debris as electrode inserted, usually done blind + changes in electrical resistance used to know when cell under tip
Reverse transcription polymerase chain reaction - measuring of individual mRNA molecules in cell, can infer expression levels of proteins
Use of fluorescent indicator dyes (FURA-2)
e.g. Calcium fluoride absorbs UV then re-emits in visible spectrum
FURA-2 is small organic molecules - low affinity calcium indicator
- has 4 carboxyl groups which attract divalent Ca2+
- distortion of dye portion changes wavelength of light that FURA-2 will absorb
Can excite at 340nm (Ca bound) & 380nm, ratio taken of the 2
BUT it is polar, must be esterified by AM ester which covers -COOH groups so it can enter
-> can look at real time ion fluxes, determine spatial distribution
Role of GINAs
Proteins so genetically encoded (GCaMP)
Targeted to individual tissues/cells + can have temporal control.
GCaMP is GFP + calmodulin + M13
- calmodulin undergoes the conformational change
Fluorescence resonance energy transfer (FRET)
Uses 2 fluorophores, donor + acceptor which are excited + emit at different wavelengths.
- only occurs when at very close proximity i.e. ligand binding receptor
Optogenetics
Opsins are light sensitive proteins in our eyes (GPCRs w/ retinal)
Chlamydomonas algae has channelrhodopsin2activated by blue light (cation channel). Halobacterium has halorhodopsin activated by yellow light (chloride pump)
How is light delivered for optogenetics?
LED + collimator for small sample
Fibre optic probe for animals (mouse) - delivers light to brain
Optogenetics to treat disease
Mouse w/ kainic acid injected into hippocampus (induced epilepsy activity)
It is transgenic , has channelrhodopsin directed to GABAergic neurons + fibre optic cable.
Blue light sent down probe + activates GABergic neurons.
Advantages + disadvantages of radioligand binding experiments
A: simple set up, easily scaled up to look at large number of drugs
D: does not tell us drug function, poor time resolution, uses hazardous materials + hazardous waste
Smell/olfaction
Receptors are GPCRs, APs fired to glomeruli.
When activated: GTP replaces GDP, adenylate cyclase converts ATP -> cAMP, Na+ & Ca+ channels opened (depolarisation)
Volatile chemicals recognised by combinatorial coding.
Olfactory cortex in temporal lobe.
Taste (gustation)
Bitter, sweet, umami, salty, sour + (fat), all have different GPCR structures.
- sour + salty respond to mineral compounds
Gustatory neurons found on papillae on tongue: circumvallate (back), foliate (sides), fungiform (front)
Gustatory cortex in insular and frontal lobes
Vomeronasal organ (Jacobsons)
Can provide info about prey or pick up pheromones from same species.
-> can produce profound behavioural changes in animal.
Hearing (audition): auditory
We detect variation sin air pressure vs insects which detect speed of moving particles (6 inch radius)
Auditory system: focuses soundwaves onto tympanic membrane (eardrum)
Cochlea is spiral shaped + fluid filled. Soundwaves converted from changes to air pressure -> fluid movement.
This moves basilar membrane up + down, connected to hair cells in fixed tectorial membrane (relative position changes)
- converted to electrical signal
Variations in stiffness + width of basilar membrane allow it to resonate at different frequencies
Stereocilium is on apical side of hair cells, staircase arrangement + it connects hair cells to fluid filled tectorial membrane.
Auditory cortex in temporal lobe
How do hair cells convert movement to electrical signal?
Mechano-transduction
Mechanical force pulls on cannel causing physical opening, these neurons have low [K+] so K+ moves in (depolarisation).
AP generated so Ca+ moves in, vesicular glutamate released, spinal ganglion sends sign to brain.
Very quick as no signal cascade.
Hearing (audition): vestibular
Detects gravity acceleration + head rotation.
Otoliths detects gravity using calcium carbonate crystals.
Semicircular canals use fluid movement to detect head rotation - can have a lag (dizziness, sea sickness)
Vestibular cortex in parietal lobe.
Touch
Merkel cells (SAI) + Ruffini endings (SAII) detect steady pressure: skin indentation, texture discrimination (IIIIII)
Meissner corpuscle (RAI) + Pacinian corpuscle (RAII) detect vibration (II II II II)
Structure of vertebrate eye
Adjustable lens crystalline protein structure - focuses light to retina w/ help of stronger fixed cornea.
Cornea has greater refractory power but lens is adjustable (focus)
-> lens accommodation
Retina is part of CNS /brain. Image on retina is inverted
Optic nerve takes visual info to lateral geniculate nucleus in thalamus.
Imperfections of vertebrate eye
Myopia - short sightedness, focal point comes too soon, fixed w/ concave lens
Hyperopia - far sightedness, focal point occurs too late, fixed w/ convex lens
Retina structure
Rods + cones in outer nuclear layer + photoreceptor layer. Link retinal pigment epithelium (RPE) to bipolar cells in inner nuclear layer.
Rods + cones unevenly distributed
-> many cones at fovea (max visual acuity), colour sensitive
-> rods provide night + peripheral vision (x1000 more sensitive)
Sclera (black) has RPE - recycling of retinaldehyde, helps rods + cones neutralise oxidative stress.
- tapetum lucidum gives reflective shine (night vision)
Photoreceptors
Rods x2 size of cones, far more membranous discs w/ photopigments.
Opsin: GPCR 7 TM domains, 5 types, bound to rhodopsin.
Retinal: vitamin A derivative, absorbs light + changes conformation, induces signal.
Light causes hyperpolarisation SO photoreceptors detect darkness
-> in dark cGMP activates Na+ channels, depolarisation causes glutamate release
-> in light, cGMP degraded by enzymes to GMP, Na+ close so hyperpolarisation
Phototransduction in rods + cones
- 11 cis retinal absorbs light -> all-trans retinal which activates rhodopsin
- G protein transducin has GDP, expelled + replaced by GTP
- GTP binding causes dissociation of G protein complex from rhodopsin, phsoshodiesterase released inhibits cGMP, GMP accumulates
- Na+ channels close
Colour blindness
Humans are trichomats. R and G opsins on X chromosome, B on chrom 7.
Men far more likely to have colour blindness .
Some women tetrachromats due to 2 red alleles
Horizontal cells
light intensity adaptation, spatial + colour processing
Amacrine cells
Directional motion, modulate light adaptation + circadian rhythm, night vision.
Retinal ganglion cells (RGCs)
Further process colour, motion + shapes. Only output cells + fire APs
Some (ipRGCs) detect light via melanopsin.
- signals via Gq rather than transducin in rhodopsin
Skeletal muscle fibre structure
Muscle fibres multinucleate, formed by fusion of cells.
Sarcoplasmic reticulum wrapped around myofibrils.
Terminal cisternae interact w/ T tubules to form triad (coupes excitation w/ contraction)
Titin largest protein in human genome - spring like function to allow muscle to return to rest state.
In sarcomere during contraction:
H zone (only myosin) - decreases
I band (only actin) - decreases
A band (overlap) - stays same
Myosin & actin filaments
Myosin: hexamer of 2 heavy chains w/ large head group. 4 light chains (regulatory + essential). 8 types in cardiac/skeletal, 1 smooth.
Head has ATPase activity.
Actin: globular + filamentous form.
Associated w/ tropomyosin, 3 troponins (T associates tropomyosin, I associates w/ actin + inhibits binding, C binds calcium)
How is contraction regulated?
Each tropomyosin associated w/ trimeric troponin.
TnI inhibits formation of cross bridges by binding to actin, covering up myosin binding sites .
Ca+ binds TnC -> conformational change uncovers myosin binding site
Excitation-contraction coupling
Depolarisation transferred by sarcolemma to SR by T tubule at termina cisternae: 2 cisternae for each triad.
SR has v high [Ca+] (100nM) but sarcoplasm does not -> caused by SERCA active transporter.
Ca+ enter sarcoplasm from SR, controlled by ryanodine receptor w/ physical linkage to DHPs on muscle membrane.
-> VSCC DHP changes conformation, transmitted to ryanodine
-> Ca+ influx
There are 4 DHPs to 1 ryanodine receptor.
SERCA restores calcium imbalance
Myasthenia Gravis
Autoimmune attack on skeletal muscle -> muscle weakness + fatigue.
Common in women 20-40 yrs old.
Thymus gland thought to play role. Attacks target nAChRs, compromises contraction.
Can be diagnosed by pattern of muscle weakness, presence of antibodies, electromyography (EMG), tensilon/edrophonium test (short acting acetylcholinesterase inhibitor)
Can be modelled using purified nAChRs from Torpedo ray - provoke immune response.
Pathological mechanism of MG
Antibodies in MG target main immunogenic region (MIR), extracellular part of a-subunits of nAChR.
- Receptors become internalised, then hydrolysed into a.acids by cells
- Destruction/simplification of endplate, folds + nAChRs lost, synapses widened
- Block of acetylcholine binding sites, competitive antagonism
Treatment of MG
Reversible acetylcholinesterase inhibitors - ACh concs increased
Immunosuppressants can suppress immune response but increased risk of infection/cancer
Plasma therapy - if very severe, plasmapheresis or plasma exchange can be used to remove autoantibodies
Sarin
Irreversible acetylcholinesterase inhibitor.
nAChRs enter desensitised state so channels remain closed despite binding -> no new APs fired
Attaches to serine residue forming strong bond so AChE cannot be recycled (serine cannot accept acetate from ACh, choline not released)
Antidotes include atropine (muscarinic antagonist) & pralidoxime (can recycle AChE back to active form)
Skeletal muscle control mechanisms
Single AP - twitch, some force absorbed by titin
Summation + unfused tetanus - repeated stimulations -> larger Ca+ conc, proportional to force generated by muscle.
Fused tetanus - higher rates, maximal stimulation
Henneman’s size principle
Motor unit is smallest contractile unit, controlled by single motor neuron.
As muscle is stimulated to contract, motor units recruited in order of size (smallest first).
Slow twitch oxidative fibres
Type 1, contain lots of myoglobin, many mitochondria + rich blood supply.
Slow, sustained contractions+ resistant to fatigue.
Fat twitch fibres
Fast myosin isoforms, produce fast Ca+ transient (high SERCA pump), rapid shortening but high energy cost, ATP hydrolysed quickly.
IIA/oxidative - lots of mt, good blood supply, high glycogen stores, resists fatigue.
IIB, IIX/glycolytic - fatigue v. quick as lactate accumulates + acidosis can limit concentration, high glycogen, fewer mt, lower blood supply.
Duchenne muscular dystrophy
X linked disorder, mutation in dystrophin gene, 1:3500 male births
- Skeletal muscle fibres not linked properly to ECM
- Excess Ca+ enters, muscle fibres die+ replaced by fat/connective tissue
No treatment, avg. life expectancy 25-30 yrs.
Cardiac muscle structure
Striated. Has branched syncytium w/ incompletely fused cells joined by intercalated discs.
-> cells drawn together by desmosomes + gap junctions couple cells electrically
APs have plateau effect -> VSCC that inactivate slowly, lasts 200 ms.
Cardiac excitation coupling
L type Ca+ channels (DHP) not physically linked to ryanodine receptors.
Instead rely on Ca+ influx through channel which induces Ca+ release from SR - 80~90% Ca+ from SR via CICR
Initiation of contraction myogenic, cells in SAN generate I(f) which is carried by HCN cation channel
Cardiac contraction
Pacemaker potential has characteristic slope influenced by (para)sympathetic activation.
Force is determined by degree of stretch of cardiac muscle + conc. of cytoplasmic Ca+ (modulated by ANS)
-> uses oxidative metabolism as must beat continuously
Smooth muscle structure
No striations or t-tubules, small spindle shaped cells.
Often electrically couple by gap junctions but can also be independent.
Controlled by ANS, functions to propel contents.
Actin-myosin cross-bridges but contracts very slowly, more energy efficient, large range of tensions.
Excitation contraction coupling in smooth muscle
Troponin not involved.
- Can be CICR via L type channels like in cardiac.
- M1, M3, M5 GPCRs cause phospholipase C to produce IP3 which binds intracellular LGIC releasing Ca+ from SR (no AP needed)
- Store operated Ca+ channels open allowing influx if SR depleted
ATPase activity in smooth muscle myosin heads
Much lower - but can be increased by Ca+
Calmodulin binds Ca+, converts MLCK to active form.
MLCK phosphorylates regulatory light chains, turning on ATPase activity.
Cross bridges form, contraction occurs.
- enzymes make process slow
Can remove Ca+ to stop contraction
Brain areas associated w/ memory
Hippocampus - explicit memory, has place cells
Cerebellum + basal ganglia - procedural memory
Amygdala - emotional response
Cortex - short + long term explicit memory
How are short + long term memories formed?
Short - second/hours, limited capacity, labile so sensitive to disruption, does NOT require new RNA/protein synthesis, stored + retrieved sequentially.
Long - days/years, unlimited capacity, consolidated, requires new RNA/protein synthesis, stored by association.
- working memory used to hold info ‘in mind’, requires rehearsal
Long term potentiation (LTP)
Persistent strengthening of synapses following high frequency stimulation.
-> long lasting increase in signa transmission
1 of many mechanisms of synaptic plasticity
LTP mechanisms
Presynaptic changes: increased NT vesicles + increased NT release
Postsynaptic changes: increased dendritic area + spines (sensitivity) + increased AMPA receptors
*AMPA require glutamate, allow Na+ influx
*NMDA blocked by Mg+, requires glutamate, glycine + depolarisation, allows Ca+ & Na+ influx
- strong depolarisation leads to high Ca+ influx
Long term depression
Long lasting decrease in efficiency of synaptic transmission - caused by transmission at same time as weak/modest depolarisation at post synaptic neuron.
- weak depolarisation leads to little Ca+ influx
Amnesia
Loss of memories often due to trauma, transient or permanent.
Retrograde - difficulty remembering past info.
Anterograde - difficulty retaining new info.
Dementia + its causes
Group of symptoms affecting memory, thinking + social abilities, different causes
- progressive
AD - >80% cases in elderly, intracellular neurofibrillary tangle + extracellular beta amyloid plaque accumulation
Vascular - after stroke or damage to blood vessels (reduced circulation), symptoms can in memory, reasoning. planning or judgement.
- treatments involve managing risk factors
Non-associative learning in aplysia
Habituation - loss of response due to repeated stimulus, reduced Ca+ influx so reduced NT release
Dishabituation - recover of innate response (shown through stimulation of siphon + gill movement, stronger electrical shock -> dishabituation)
Sensitisation - response stronger than normal, caused by electric tail shock
Associative learning in aplysia
Classical conditioning
Tail shock used as unconditioned stimulus, tactile stimulus to siphon is conditioned response.
- paired stimulus prompted larger response
- no learning w/ backward pairing so CS needs to be before US (0.5s)
Long term memory in aplysia
Show LT memory for habituation, sensitisation + classical conditioning.
LT sensitisation increases neuron branching, mores synapses
LT habituation decreases branching, fewer synapses made.
Its NS has ganglia which communicate through connectives arranged in bilaterally symmetrical pairs.
- abdominal ganglion unpaired
-> 20 siphon sensory neurons
Mechanistic analyses of learning (sensitisation)
Synaptic - input markedly increases Ca+ influx (serotonin involved)
Biophysical - serotonin closes K+ channel temporarily, repolarisation slowed so more Ca+
Molecular - serotonin increases cAMP so more NT released, PKA causes spike broadening + increased excitability (substrate proteins phosphorylated)
Circadian rhythm
Principal pacemaker located in suprachiasmatic nucleus (SCN) in hypothalamus.
Pair of nuclei side by side (~10,00 neurons)
Core SCN processes info + adjusts rhythm - sent to shell SCN which sends projections to other parts of brain to synchronise body
-> core + shell SCN express different peptides
Molecular clock
Collection of clock genes which are expressed in rhythmic fashion
- melatonin secreted by pineal gland (sleep hormone)
Chronopharmacology
Kinetics + dynamics of medication are affected by endogenous rhythms & dosing time affects biological rhythms
e.g. cancer chronotherapy found time of day drug treatments given affects survival + toxicity
-> oxaliplatin
e.g. lithium treats BP disorder, affects suppression numerous circadian genes -> period lengthening
Stages of sleep
REM (25%) + 4 NREM stages
REM is dreaming stage
NREM 3+4 is slow wave ‘deep’ sleep
Cycling of REM & NREM lasts 70-90 mins
Brain control of sleep
E. lethargica epidemic post WW1 studied by von Economo
‘flip-flop’ switch model:
When awake, arousal areas of brain (LHA, LDT, Raphe) most active.
When asleep, sleep areas (VLPO) most active.
- controlled by orexin
Iocus coeruleus is small nucleus in brainstem v important for arousal
Circadian control of sleep
SCN projects to DMH which projects to LHA (awake) & VLPO (asleep)
- lesions of DMH attenuate or eliminate circadian sleep rhythm
At ‘sleep gate’ melatonin increases, core temp decreases.
Narcolepsy
Profound daytime sleepiness caused by disruption to brains orexin pathways.
Orexin is a neuropeptide produced in hypothalamus that acts on Hcrtr2 (GPCR) influencing ‘flip-flop’ model .
Narcolepsy is autoimmune attack on orexin producing neurons.
Cause in dogs due to premature atop codon in Hcrtr2 receptor.
Brain surgeries
Cranial trepanation in ancient Egypt
Experimental ablation:
1) electrode produces heat killing cells (not specific)
2) cannula a.acids (kainic acid) stimulate neurons to death, spares nearby axons
Shame lesion is a placebo procedure
Histological methods used to observe location of lesion.
Accidents in brain function
Phineas Gage railwork accident, tamping iron through head -> personality change
Wilder Penfield electrical brain stimulation on awake patients for epilepsy treatments -> used info to create functional maps of brain cortex
Deep brain stimulation
electrode implanted in brain connected to pacemaker under skin of chest
Impulses regulate abnormal impulses or affect certain cells/chemicals within brain
Can treat: dystonia, Parkinson’s, OCD, epilepsy, offers relief for dyskinesia symptoms (caused by anti-PD drugs)
Computerised tomography
X ray images for different angles combined to form cross -sectional images
-> internal injuries
PET, MRI & fMRI
PET - radioactive tracer shows brain activity, useful for tumours, can be combined w/ CT + MRI
MRI uses magnetic field + radiowaves to create detailed images of brain (powerful diagnostic tool)
fMRI measure metabolic changes in brain, demonstrates areas of stimulation, can be used for those being considered for surgery
