Cellular neurones

Question 1

explain the concept of excitable cell

Answer 1

potential difference across plasma membrane
this can be through passive movement through the permeability or driving force or active transport against conc/elec gradient, requires expenditure of metabolic energy by the cell

Question 2

describe the permeability of a plasma membrane of a typical excitable cell

Answer 2

impermeable or slightly permeable if there is a large driving force or readily permeable (small driving force required)

readily permeable to:
K+ and Cl-
poorly permeable to Na+
impermeable to various large inorganic anions

Question 3

state the typical intracellular and extracellular concentration of K+, Na+ and Cl-

Answer 3

Na+ inside: 15mM
Na+ outside: 145mM
K+ inside: 150mM
K+ outside: 5mM
Cl- inside: 5mM
Cl- outside: 100mM

Question 4

explain the concept of electrochemical equilibrium

Answer 4

ions moving down the electrical gradient
high to low
causing potential difference

sometimes electrical and concentration gradients can form at the same time

Question 5

use the nernst equation to calculate equilibrium potentials

Answer 5

x = ion
Ex = Equilibrium potential for x
R = universal gas constant
T = temperature (in degrees absolute)
z = the valence of the ion
(e.g. +1 for K+; -1 for Cl-)
F = faraday’s constant
[x]o = concentration of X outside the cell
[x]i = concentration of X inside the cell

At 37oC (body temperature) Ex = 61 log [x]o / [x]i millivolts

tells the magnitude of the electrical gradient that would exactly balance a given concentration gradient of a given ion
gives equilibrium potential for that ion

Question 6

describe the origin of the resting membrane potential (RMP)

Answer 6

two properties:
1 - unequal distribution of ions across the membrane (Na+/K+ pump maintains this)
2 - selective permeability of the cell membrane (more permeable to K+ than Na+)

Question 7

use the GHK equation to calculate the resting membrane potential

Answer 7

P = membrane permeability to each particular ion based on the number of ion
channels open, closed etc.

PK = 1
PNa = 0.04
PCl = 0.45 these as an example

Question 8

describe the responses of an excitable cell to depolarisation

Answer 8

threshold is met
depolarisation of membrane can occur
causes production of AP

Question 9

explain the concept of threshold

Answer 9

degree of polarisation and is based on ion channels in the membrane
varies between neurones and different parts of the same neurone
thicker fibres have lower thresholds (diameter provides less resistance to the flow of ions)
-50mV

Question 10

describe the ionic basis of an action potential (including the roles of ligand and voltage gated ion channels, activation and inactivation gates)

Answer 10

basement conc Na+ inside is -70mV
threshold potential of -50mV is met
Na+ influx as rising phase (depolarisation
-30mV
falling phase where Na+ conc decreases and K+ conc increases (K+ efflux) (repolarisation)
hyperpolarisation then resting potential

triggered event
depolarisation
opening of some voltage-gated Na+ channels
influx of Na+
positive feedback cycle

Question 11

describe the cellular mechanisms underlying absolute and relative refractory periods

Answer 11

Na+ is fast acting
K+ is slower acting
absolute refractory period, all or nothing
prevents depolarisation too soon by causing hyperpolarisation which is relative refractory period

Question 12

describe the mechanism of contraction in skeletal muscle

Answer 12

excitation contraction coupling: propagation of AP down T-tubules by ACh as transmitter and AChR at motor end plate
activation of dihydropyridine receptors (DHPR) (T-tubules; conformational coupling with ryanodine receptors (RyR))
release of calcium from sarcoplasmic reticulum (SR)
binding of Ca2+ to troponin (conformational change tropomyosin)
cross bridge formation (actin and myosin and ATP)
cross bridge cycling (power stroke and release of ADP and Pi)
Ca2+ removed from troponin restoring tropomyosin and Ca2+ taken back up by SR

Question 13

explain the sliding filament theory of muscle contraction, including the detailed sequence of events that occurs in a cross-bridge style

Answer 13

thin - actin
thick - myosin
z disc will move in
cross bridge cycling:
binding of myosin to actin
releases inorganic phosphate
power stroke
movement of actin pulled to middle of sarcomere
rigor (myosin in low energy form)
unbinding of myosin and actin (this requires ATP because it is an active process, broken down to ADP and Pi)
then cocking of myosin head in high energy form ready to bind again

Question 14

describe the role of calcium in muscle contraction, including triggering of contraction, release and re-uptake

Answer 14

Ca2+ release from terminal cisternae of SR
contractile machinery activated
Ca2+ needs to be removed to end muscle contraction
Ca2+ pumps on SR to pump it back into SR
requires ATP
diffuses to terminal cisternae of SR ready to be released again

Question 15

plot a classical experimental length-tension curve for a single muscle fibre, and interpret it in terms of the sliding filament theory

Answer 15

x axis - striation spacing in micrometers
y axis - tension (% maximum)
the striation spacing indicates how much space is in between the z disc and then the tension created at this striation

Question 16

what does a myogram measure?

Answer 16

twitch tension development in muscle

morphological differences:
fast = white (lower myoglobin and capillary content)
slow = red (high myoglobin and capillary content)

Question 17

what is tetanus?

Answer 17

the prolonged contraction of a muscle caused by rapidly repeated stimuli

Question 18

describe the ultrastructure of cardiac muscle

Answer 18

10 micrometres diameter
100 micrometres in length

intercalated disks:
mechanical junctions - fascia adherens and desmosomes
note: desmosomes are mechanical junctions between adjacent muscle fibres
electrical connections - gap junctions
gap junctions are electrical connectivity between adjacent muscle fibres
cardiac muscle:
highly organised contraction
refilling of heart requires synchronised relaxation

Question 19

describe the ionic basis of cardiac action potentials

Answer 19

two types of responses:
slow response (pacemaker cells)
- unstable resting membrane potential allows spontaneous depolarisation
- no early repolarisation (phase 1 and 2)
- phase 0 due to slow inward current of Na+ and Ca2+ influx causing depolarisation
- phase 1 and 2 are not present
- phase 3 repolarisation due to closing of calcium channels and efflux of K+
- phase 4 is slightly less negative, gradual depolarisation

fast response (cardiac AP)
- phase 0 due to Na+ entry
- phase 1 initial repolarisation due to K+ efflux
- phase 2 due to Ca2+ entry (different in cardiac muscle) and sodium-calcium exchanger
- phase 3 more K+ efflux
- phase 4 RMP slightly more negative than RMP at the beginning

SAN - slow
atrial and ventricular myocytes - fast
purkinje fibres - fast
AVN - slow

Question 20

explain the detailed mechanisms underlying contraction and relaxation of cardiac muscle

Answer 20

absolute refractory period (ARP)

fibrillation occurs when duration of ARP is decreased

• AP through adjacent cell
• VGIC (calcium) open and Ca2+ enters the cell, main source is extracellular
• calcium ions induce Ca2+ release from SR
• Ca2+ ions bind to troponin enabling filament sliding theory
• muscle relaxes when Ca2+ unbinds
• Ca2+ pumped into SR for storage
• Ca2+ is exchanged with Na+ at sarcolemma
• Na+-K+ ATPase restores Na+ gradient
  CICR and RyR enables the release of more Ca2+
  Ca2+ enters through L-type calcium channels

relaxation uses SERCA or SR pump
NCX (sodium-calcium exchanger) 3Na for 1 Ca
and sarcolemma CA++ ATPase

Question 21

explain how the force of contraction is regulated in cardiac muscle cells

Answer 21

light I bands, dark A bands
elastic protein ‘titin’ prevents overstretching and may act as signalling role as a stretch receptor

cardiac muscle has high resistance compared to skeletal muscle due to high abundance of connective tissue to prevent muscle rupture and overstretching

regulated by starlings law - helps heart pump whatever volume of blood it receives - can be enhanced by sympathetic stimulation

Question 22

ionic basis of pacemaker potential

Answer 22

funny channels (unusual behaviour: open at hyperpolarised potentials) - inwards Na+ current

allowing positive positive charge in at rest causing unstable RMP

Question 23

positive chronotropy
positive inotropy
positive lusitropy

Answer 23

increase rate of contraction
increased force of contraction
increased rate of relaxation

Question 24

give some examples of the location and functions of different types of SM

Answer 24

location:
internal organs
walls of blood vessels
around hollow organs
function:
move food, urine and reproductive tract secretions
control diameter of respiratory passageways
regulate diameter of blood vessels

non straited, single nucleus

Question 25

compare single and multi-unit smooth muscle

Answer 25

single: gap junctions cause propagation and the whole muscle functions together to contract e.g: GI tract and bladder multiunit: different parts of the muscle can function independently e.g: iris, vasculature, airways

Question 26

describe tonic and phasic contraction

Answer 26

tonic: slow contraction with long durability )resistant to fatigue) phasic: quick contraction with short durability (easily fatigued)

Question 27

explain the roles of calcium-calmodulin, myosin light chain kinase (MLCK) and myosin phosphatase (MP) in SM contraction

Answer 27

smooth muscle contraction is thick filament regulated (myosin): increase in cytosolic Ca2+ Ca2+ binds to calmodulin in cytosol Ca2+ calmodulin complex binds to myosin light-chain kinase myosin light-chain kinase uses ATP to phosphorylate myosin cross bridges phosphorylated cross bridges bind to actin filaments cross bridge cycle produces tension and shortening the binding of myosin to actin depends on phosphorylation of the cross-bridge by CA++ - calmodulin-dependent myosin light chain kinase (MLCK) cross-bridges cycle until myosin is dephosphorylated by myosin phosphatase removal of Ca2+ from the cell promotes relaxation of SM

Question 28

describe the 'latch state' of SM

Answer 28

latch state = an adaptation of SM which allows sustained muscle tone - enables sustained SM tone + low rate cross-bridge cycling - occurs when some of the cross-bridges attached to thin filaments become dephosphorylated (by myosin phosphatase) - slows rate of cross-bridge detachment to maintain tone - filaments tend to remain locked together

Question 29

compare and contrast skeletal, cardiac and SM structure, function, anatomy and physiology

Answer 29

similarities to skeletal and cardiac muscle: sliding filament theory and cross-bridge cycling, calcium plays role in contraction differences (skeletal and cardiac): SM contraction is thick filament regulated, contractions can be slow and sustained muscle tone due to the 'latch state' skeletal is long cylindric fibres (striated), cardiac is branched (striated) cylindrical and smooth is spindle shaped (non-striated)

Question 30

state the key factors that determine the movement of solutes by diffusion, including driving forces (elec and chem gradients) and membrane permeability

Answer 30

membrane permeability electrical gradient intra and extracellular Na+, K+ and Cl- concs nernst equation goldman hodgkin equation

Question 31

describe different types of ion channels and explain the term 'channel gating'

Answer 31

selective single channel conductance ranges from 0.1-100pS gating = fluctuation between open and closed states ionotropic: fast, on off, made of several subunits, triggers AP metabotropic: GPCR, slower and prolonged responses, Gs or Gi to amplify signals, monomer receptor proteins

Question 32

outline the general properties of carrier mediated transport systems

Answer 32

uniporters: transports single molecule across membrane, GLUT2 (glucose into cell), mutations can cause diabetes symporters: movement of two or more molecules across the membrane, co-transport for movement in the same direction, NKCC2 in kidneys antiporters: movement of two or more molecules across membrane in opposite direction, exchanger or counter transporters, Na-H exchanger

Question 33

describe the difference between facilitated diffusion and AT

Answer 33

facilitated diffusion passive process down the conc gradient AT is an active process moving against the conc gradient

Question 34

explain primary and secondary AT mechanisms with examples

Answer 34

primary: transport is directly coupled to ATP hydrolysis (move against conc gradient) e.g: Na+ -K+ pump secondary: energy for the transport comes from the electrochemical gradient. the energy from one molecule is used to move another molecule against the electrochem gradient e.g: 3Na+ - Ca2+ antiporter

Question 35

factors that control gating of ion channels

Answer 35

membrane voltage extracellular agonists or antagonists intracellular messengers mech stretch of plasma membrane

Question 36

describe osmosis

Answer 36

diffusion of water across a semi-permeable membrane water moves from high water conc to low water conc

Question 37

describe hydrostatic pressure

Answer 37

the pressure exerted by a stationary fluid on an object - 'pushing force'

Question 38

describe osmotic pressure

Answer 38

measure of tendency for water to move into that solution because of its relative conc of non-penetrating solutes and water - 'pulling force' net movement of water continues until the opposing hydrostatic pressure exactly counterbalances the osmotic pressure interstitial fluid is an example of these - fluid around cells

Question 39

compare molarity and osmolarity

Answer 39

molarity - number of molecules in a solution (mol/L) osmolarity - number of particles in a solution (Osm/L)

Question 40

compare and contrast osmolarity and osmolality

Answer 40

osmolarity: This measures the solute concentration in a volume of solution. osmolality: This measures the solute concentration in a mass of solvent.

Question 41

define the term tonicity

Answer 41

the effect the osmotic pressure gradient has on cell volume

Question 42

explain what would happen to a cell if it was placed in the following situations: - isotonic - hypotonic - hypertonic

Answer 42

isotonic - no net water movement hypotonic - cell will swell hypertonic - cell will shrink

Question 43

describe the function of afferent and efferent neurones

Answer 43

afferent - sensory (towards) efferent - motor (away)

Question 44

explain cellular metabolisms involved in synaptic transmission

Answer 44

two types of synapses: chemical - prevent direct electrical propagation of AP from pre- post synaptic neuron 0.5ms delay synapse is 20-30nm removal of enzymes, reuptake and uptake by glial cells electrical - rare in CNS (gap junctions as an example)

Question 45

describe cellular pathways involved in vesicle release

Answer 45

opening of Ca2+ channels and actin fusion protein macromolecules (FPM) separate to allow fusion vesicle membrane incorporated into presynaptic membrane clathrin molecules assist inward movement if the vesicle membrane, dynamin assists in FPM pairs and pinching the neck of the emerging vesicle vesicle is now free for recycling

Question 46

describe the mechanisms involved in removing neurotransmitters from the synaptic cleft

Answer 46

enzymatic breakdown active reuptake (rapid) - pumped back into pre-synaptic terminal active uptake (rapid) - pumped into glial cells

Question 47

describe the ionic basis of excitatory postsynaptic potential (EPSPs) and inhibitory postsynaptic potential (IPSPs)

Answer 47

EPSPs - using glutamic acid and ACh (Na+) IPSPs - glycine and GABA (Cl- and K+)

Question 48

explain temporal and spatial summation

Answer 48

temporal - adding the signals on the same neurone spatial - adding signals received at different locations

Question 49

define convergence and divergence and state their significance in synaptic physiology

Answer 49

convergence - one cell influenced by many others divergence - one cell influences many others

Question 50

describe pre-synaptic inhibition

Answer 50

inhibition before the synapse you are recording at

Question 51

outline the principles of an intracellular recording

Answer 51

recording electrical activity across a membrane of one single cell (one electrode inside the cell and one is outside) e.g: slices of brain tissue - kept alive in solution that mimics the extracellular fluid lipid membranes have capacitance meaning: - lipid membrane stores charge - voltage is produced across the membrane charge = current x time voltage = charge stored / capacitance (ability of the membrane to store charge)

Question 52

compare action potential propagation in non-myelinated and myelinated cells

Answer 52

conduction velocity - the speed at which propagation of the AP occurs measured in m/s non-myelinated: influx of sodium causing depolarisation resulting in AP to neighbouring cells, moves along neurone, K+ efflux to repolarise the cell myelinated: saltatory conduction from one node of Ranvier to the next (100m/s)

Question 53

state the factors that affect conduction velocity and explain underlying mechanisms

Answer 53

myelination of the axon - good insulation and increases the speed of AP propagation diameter of the axon - the finer the fibre, the slower it conducts, fine fibres are invariably unmyelinated

Question 54

describing demyelinating condition of the CNS and PNS (+central and peripheral neuropathies)

Answer 54

decreased conduction velocity in affected axons can eventually result in block of conduction will cause axonal death in the long term central neuropathies: multiple sclerosis: - disruption of myelin sheaths of CNS neurones symptoms: - numbness or tingling - progressive muscle weakness - mobility problems peripheral neuropathies: guillain barre syndrome - disruption of myelin sheaths in PNS neurones symptoms: - pins and needles in hands and feet - limb weakness - uncoordinated movement

Question 55

outline the principles of an extracellular recording

Answer 55

extracellular electrodes record from populations of neurones two electrodes measure the charge in relation to each other stimulates the whole nerve to measure bulk flow charge across two electrodes sum of potential changes as AP propagate down axons a biphasic compound action potential thick fibres = lower thresholds (easier to stim APs)

Question 56

state how TTX, TEA+ and novichok impact neural transmission

Answer 56

TTX: - puffer fish - potent and selective - blocks Na+ channels - respiratory paralysis TEA+: - non-specific K+ channel blocker novichok: - nerve agents - inhibits acetylcholinesterase - prevents muscle relaxation - cardiac arrest - fast acting