Neurons Flashcards
1
Q
NERVOUS SYSTEM
A
- neurons = cells in neuronal tissues (ie. neuropiles/nerves/chord/brain) where they form interconnected neural networks
- CNS = central nervous system (brain/spinal chord)
- PNS = peripheral nervous system (elsewhere)
- CNS/PNS neurons interact in many ways
2
Q
EVOLUTION IN NERVOUS SYSTEM
A
- metazoa (animals bar sponges) have neurons
- Cambrian explosion (540my) = species appeared w/^ complex body plans/elaborate movement/senses/reproductive systems
- cephalisation = central ganglia/brains formation in one end of animal body
- dorsal chord = in spine/vertebral column of vertebrates
- ventral chord = in invertebrates
3
Q
NERVOUS SYSTEM FUNCTIONS
A
- to monitor/regulate/coordinate inner systems/organs
- to release chemical messengers
- to change internal states (sleep/hunger/emotions)
- to mate/produce offspring
- to acquire/analyse info from environment/innards
- to make decisions (sensing/thinking/cog)
- to generate/coordinate/control movement (beh responses/motor patterns/muscle movement/vocalisations/organ motility)
4
Q
NEUROACTIVITY AS BIOELECTRICITY SOURCE
A
LUIGI GALVANI (1794)
- discovered bioelectricity in frog muscles
- electrodes (electrical conductors) make contact w/non-metallic parts of electric circuit in living specimen/cells
- recording electrode placed inside/outside neuron (microelectrodes) or further on body surface (ie. EEG/EMG electrodes)
5
Q
NEURONS AS TYPICAL ANIMAL CELLS
A
- each neuron has similar organelles as any other cell:
- nucleus = w/DNA (gene majority)
- mitochondria = powerhouse of cells; w/mtDNA
- cytoplasm
- BUT cell membrane = special
6
Q
WATER MOLECULES IN NEURONS
A
- human body = 60% (45%-70%) water; 2/3 inside cells (cytoplasm/intracellular fluid); rest is interstitial fluid/7% blood plasm/<1% cerebrospinal fluid
- only few soluble/uncharged molecules (ie. O2/CO2) can pass any cell membrane
- other molecules (nutrients/waste products/proteins/ions) need channels/pumps/transporters to move in/out
7
Q
ELECTRIC SIGNALS
A
- ion channels in the neuron distributed along neural membrane
- cell membrane = not only barrier BUT also polarised/selectively permeable
- movement of ions (flux) across neural membrane generates tiny localised bioelectric currents
- cation/positive ion (ie. Na+/K+/Ca2+) = protons > electrons
- anion/negative ion (ie. Cl-) = electrons > protons
8
Q
ION CHANNELS/PUMPS REGULATING ION FLUX
A
- most channels made from 4 proteins assembling themselves to make central pore
- have selectivity filter that only allows ions of particular type (ie. charge/size) to pass through
- each neurons membrane has dif ion channel classes
- nearly all ion channels open for v brief time periods
- only minority of channels always open (leaks)
- pumps always active to stabilise ion concentrations inside/outside neuron
9
Q
IONS IN CYTOPLASM/EXTRACELLULAR FLUID
A
- ion concentrations inside neuron differ from outside extracellular fluid (ECF)
- concentration of Na+/Cl- ions is lower; K+ is higher inside the neuron
10
Q
CHEMICAL CONCENTRATION GRADIENTS
A
- w/o a barrier, ions would move along a concentration gradient til equilibrium is reached
- a barrier ie. layers of cell membrane prevents exchange of ions between dif compartments
11
Q
SEMI-PERMEABLE NEURAL MEMBRANE
A
- a barrier w/some openings (semi-permeable) won’t let all particles through
- when ion channels in neural membrane are open, ions can diffuse for a short period of time
12
Q
ELECTROCHEMICAL DRIVING FORCES ACT ON THE IONS
A
- besides strong chemical concentration gradient, there was also electrostatic attraction/repulsion forces that pull ions towards membrane
- when channels are closed, membrane of neuron prevents exchange of ions which then tend to accumulate near membrane due to electro forces
- when channels open, ions cross membrane (in/out of neuron) at a rate/in direction that depends on both forces
13
Q
WHEN NEURON GENERATES SIGNAL…
A
- ion channels in membrane briefly open
- depending on type of channel that opens, respective ions are pushed into cell (Na+/Cl- ions) or leave the cell (K+ ions)
14
Q
THREE CLASSES OF ION CHANNELS
A
- channels differ in selectivity for certain ion types
ION CHANNELS - remain closed til activation for v brief time period either via electrical signal/voltage-gated or drugs/neurotransmitters/ligand-gated
ION PUMPS - actively transport ions (Na+/K+/Ca2+) from one side of membrane to other; carry them across to other side against concentration gradient; costly (most of brain’s energy consumption)
LEAK CHANNELS - allow specific ion type to freely diffuse (ie. they’re always open; let K+ through but not Na+)
15
Q
DIFFERENT POPULATIONS OF ION CHANNELS IN NEURON
A
- all neurons have:
INPUT ZONE = soma/dendrites
INTEGRATION ZONE = between soma/axon
CONDUCTION ZONE = axon
OUTPUT ZONE = axon terminals - neural signals always travel in 1 direction from input zone towards output zone
- ion channels in axon differ from in dendrites/soma and axonal endings
16
Q
NEURONS GENERATING ELECTRIC SIGNALS
A
HODGKIN & HUXLEY (1946-1952)
- discovered only 70y ago
- theory of action potential
- work marks start of computer-based quantitative modelling in neuroscience research
17
Q
RESTING POTENTIAL
A
- resting potential = membrane potential when no signals transmitted (around -70 mV)
- when ion channels are closed, they are always negative ions on inside, positive on outside
- accumulated along membrane due to electrochemical driving forces
18
Q
ELECTRIC SIGNAL MEASUREMENT
A
HODGKIN & HUXLEY (1952)
- measured electric signals directly via inserting sharp electrodes (microelectrode type) into squid’s giant nerve cell
- isolated axon (2cm) laid in sea water bath; recording microelectrode was placed inside of axon and a reference one outside
- recorded resting potential of neuronal membrane in inactive neuron
19
Q
ESM: 2ND PAIR OF ELECTRODES
A
- experimental manipulations to determine which ion movements contribute to generation of neural signal
- strength/direction of stimulus current (I, milliAmper) w/second electrode pair varied (tip of sharp electrodes filled w/conducting fluid (ie. KCI)
- simultaneously measured membrane potential w/first electrode pair (V, milliVolt)
- in other exps they manipulated ion concentration in saline bath; observed how membrane potential changed
20
Q
2 NEURAL SIGNALS: GRADED POTENTIAL/ACTION POTENTIAL
A
- graded potential hyperpolarised when membrane potential = ^ negative than resting potential
- graded potential depolarised = ^ positive membrane potential than resting potential
- action potential = spike following depolarisation that crosses voltage threshold
- the stronger the stimulating current, the stronger the graded potential
21
Q
ACTION POTENTIAL ALL/NOTHING RESPONSE
A
- the stronger the above threshold excitation, the higher the frequency of action potentials (measured in Hertz as number of action potentials p/s)
22
Q
ACTION POTENTIALS ONLY IN CONDUTCTION ZONE
A
- graded potentials generated in soma/dendrites in each neuron; travel towards integration zone
- action potentials in most neurons generated at integration zone; travel along axon to axonal terminals
- some neurons (non-spiking) never generate action potentials BUT only graded potentials which travel towards integration zone
23
Q
MICROELECTRODE RECORDINGS OF MEMBRANE POTENTIALS
A
- microelectrode either placed inside (intracellular)/outside (extracellular) recordings
- neuronal signals measured as dif in potentials between wired electrodes (units -> millivolt) via creating electrical circuit that connects electrodes w/intra/extracellular fluids; strength of small currents can be measured (units -> milliampere)
24
Q
DIFFERENCES IN RECORDED SIGNALS FROM NEURONS
A
- depending on electrode position (inside/ourside neuron soma/axon)/size/number, dif signals recorded
- intracellular inside = action potentials in neuron axon
- extracellular outside = placed v closely; action potentials from neuron axon
- extracellular (placed at some distance/large electrode surface) = records broad signal that captures activity of many neurons
25
Q
HODGKIN-HUXLEY MODEL
A
- resting potential = voltage-gated Na+; K+ channels closed
- rising phase = depolarisation caused via opening of voltage-gated Na+ ion channels
- overshoot = membrane potential becomes positive as ^ Na+ flow into cell (positive feedback loop)
- falling phase = Na+ ion channels become inactivated/close while K+ channels open leading to positive charge reduction inside cell
- undershoot = K+ ion flow out cell through open K+ channels (hyperpolarisation)
- recovery = refractory period during all channels are closed; membrane potential returns to resting value
26
Q
UNDIRECTIONAL TRANSMISSION OF ACTION POTENTIAL
A
- due to refractory period, voltage gated Na+ channels can open only on one side
- action potential travels along axon away towards output zone
27
Q
SUMMARY
A
- nervous system = CNS/PNS
- neural membrane = semi-permeable ion channels (Na+/K+/Cl-/Ca2+)/leak channels/ion pumps (Na+/K+ pump) regulate ion flux
- bioelectricity generation intracellular microelectrode recordings to measure membrane potential
- resting potential = membrane potential when neuron doesn’t receive signal
- 2 types of neural signal types = graded/action potential during which membrane potential changes from resting potential for brief time period
