Neurons Flashcards
where are neurons co-located
in tissues (neuropiles, nerves, chord, brain)
how are neurons organised by the NS?
in interconnected neural networks (sensory organs, information-processing pathways, motor pathways, regulatory networks, central brain areas)
Morphological division
CNS
central nervous system (brain and spinal chord)
PNS
peripheral nervous system (everywhere else in the body)
major changes in the NS during evolution
- During evolution the CNS appeared in animals that had evolved more complex body plans
- Cephalisation
- Dorsal chord in the vertebrate line of the animal kingdom
- Ventral chord in the invertebrate line
main functions of the NS
- To monitor, regulate and coordinate inner organs, release chemical messengers, change internal states (sleep, hunger, emotions)
- To acquire and analyse information from the environment and from inside the body (senses, thoughts, cognition)
- To generate and control motor patterns (movement, behavioural responses, signals, vocalisations and language, organ activity)
organelles in neurons
- a nucleus containing the DNA (with the majority of genes)
- mitochondria (the powerhouses of the cells, contain mtDNA with few genes)
- Cytoplasm
- Cell membrane
molecules in water and outside neuron
- Human body approx. 70% water. Inside the neuron - cytoplasm contains many molecules and proteins
- Outside the neuron - fluids in tissue contains molecules (nutrients, waste products, transporter molecules, chemical messengers)
- Only few soluble and uncharged molecules (e.g. O2, CO2) can pass any cell membrane.
neurons generate bioelectricity
- Like in any other cell, in the neuron the cell membrane acts a barrier, but the neural membrane is polarised (different electrical charge between inside and outside of cell).
- Ion channels distributed along the neural membrane.
- Their distribution and properties enable the neuron to generate tiny localised bioelectric currents.
- These are turned into neural signals if they are transmitted within the neuron.
electric current
flow of charged particles
- In metals electrons can travel between atoms which generates electric currents.
- Ions move because they are charged particles (unequal number of electrons and protons).
- This also generates electric currents.
positive ion
cation - more protons - Na+
negative ion
anion - more electrons - Cl-
what do all neurons have?
- Input zone (soma, dendrites)
- Integration zone (between soma and axon)
- Conduction zone (axon)
- Output zone (axon terminals)
- Neural signals travel from input zone towards the output zone
input zone
Where neurons collect and integrate info, either from env/other cells
integration zone
Where decision to produce neural signal made
conduction zone
Where info can be transmitted over great distances
output zone
Where neuron transfers info to other cells
what are most ion channels made of?
four proteins that assemble themselves to produce a central ‘pore’.
what do ion channels have?
a ‘selectivity filter’ that only allows ions of a particular charge and size to pass through
ion channels
- There are different classes of ion channels that are located in the membrane and in specific zones of the neuron
- The electrical and biochemical properties of certain channels have been characterised
- these channels are protein structures that span the membrane from the extracellular space to the cytoplasm
- They are thought to be cylindrical, with a hollow, water-filled pore wider than the ion passing through it except at one region called the selectivity filter
- This filter makes each channel specific to one type of ion.
voltage-dependent ion channels
activated by changes in the membrane potential
neurotransmitter sensitive channels
activated by neurotransmitter substances
diffusion
Particles move from areas of high to low concentration - down conc grad
semi-permeable membrane
When ion channels in the neural membrane are open, ions can diffuse.
non-permeable membrane prevents diffusion
When ion channels are closed, ions can diffuse inside the cell and along the membrane but not beyond the membrane and out of the cell.
electrostatic attraction and repulsion between ions
- Like charges repel
- Opposites attract
accumulation of ions at neuronal membrane
- Ion channels closed: Non-permeable state of the neural membrane
- Ion distribution differs inside and outside of cell which forms an electrochemical gradient
- Neural cell membrane accumulates charges on both sides acting as capacitor (similar to a battery)
when ion channels open
Semipermeable state of neuronal membrane: Ions start to move being pushed or pulled back depending how the electrochemical gradient changes as ions cross the membrane.
what do all ion channels together do for neuron?
- Channels make the neural cell membrane semi-permeable
- They allow particular ions to cross the membrane at defined instances and for defined periods of time - how a neuron works
- Their coordinated activity generates, sustains or ends neural signals
electrochemical driving forces act on all ions simultaneously and continuously example
- Diffusion through leak channel allows K+ out of the cell
- but then there is slightly more positive charge outside, and slightly more negative charge inside
- so electrostatic forces rapidly start to pull K+ back in
- Diffusion through voltage-gated Na+channels of Na+ into the cell along the concentration gradient
- but the more enters the more positive becomes the intracellular cytoplasm
- so electrostatic forces start pushing Na+ back out if the channels do not close or unless equal amounts of K+ flow out at the same time (for example through leak and open voltage-gated K+ channels)
3 classes of ion channels
- Gated ion channels remain closed until activation for a very brief period of time, either by electrical signals (voltage-gated) or by drugs or messenger molecules (ligand-gated).
- Ion pumps actively (against gradient) transport ions in and out of the neuron
- Leak channels allow a specific ion type to freely diffuse (e.g. they are always open and let K+ through but not Na+)
ion pumps
- Capture small molecules (e.g. Na+, K+ or Ca2+) from one side of the membrane and carry them across to the other side against concentration gradient
what are ion pumps powered through?
Powered through temporary phosphorylation of pump’s catalytic subunit which breaks off one phosphate group of ATP (Adenosine triphosphate) which becomes ADP (Adenosine diphosphate).
After some time the phosphate group is uncoupled to free up the binding site for the next transport action.
leak channels
There are potassium channels in the membrane that are always open and let K+ through but not Na+
what do pumps do for the neuron?
- Ion pumps help to restore and maintain the difference in ion concentrations inside and outside the neuron
- Most important pumps are Sodium-Potassium and Calcium
- Na/K pump - takes 3 Na+ out and 2 K+ in
- Ca pump - takes Ca+ out of cytoplasm
EEG
EEG electrodes record the summed potentials of signals that originate in active neurons but also of spontaneous bioelectricity from inactive neurons across many neuropiles and brain areas
microelectrode recordings - listening to single neurons and specific neural networks
- Microelectrodes are either placed inside (intracellular recordings) and outside (extracellular recordings) of the neuron.
- Neuronal signals are measured as difference in potentials on each side of the membrane (units - Volt) by creating an electrical circuit that connects wired electrodes with the fluids of the neural tissue.
how do we measure neural currents?
Voltage (V)
Strength of current (I)
Resistance (R)
Multimeter
Ohm’s law
how do microelectrode recordings work?
When a metal electrode is inserted in an aqueous ionic solution (electrolyte), ions can react with the electrode. The distribution of charges can be compared with that of a reference electrode
how do neurons generate electric signals?
- only discovered 60 years ago
- Squid giant axons can be up to 1 mm in diameter (100-1000 times larger than mammalian). They innervate the mantle muscle.
- Sir Alan Hodgkin and Andrew Huxley
resting potential
- 0 P diff when 2 electrodes in bath
- But when electrode enters axon, records neg P inside - inside axon more neg than outside
resting potential
membrane potential of a nerve cell at rest, e.g. neuronal membrane is polarised (typically -70 mV)
neural signal
change of resting potential to more negative or positive potential when ions move across the membrane
Hodgkin and Huxley (1952)
- They measured the electric signals directly by inserting sharp electrodes (type of microelectrode) into the squid’s giant nerve cell.
- The isolated axon (approx. 2cm) was laid in a bath of sea water. A recording microelectrode was placed inside of the axon and a reference one outside.
- They recorded the resting potential of the neuronal membrane in the inactive neuron
- A second pair of electrodes added generating electric currents (I) to excite the neuron.
- Experimental manipulations to determine which ion movements contribute to the generation of the neural signal:
- They varied strength and direction of stimulus current (I) and measure membrane potential with the first pair of electrodes (V)
hyperpolarisation
a more negative membrane potential than the resting potential
graded potential
- Hyperpolarisation
- The stronger the stimulating current, the stronger is the graded potential.
- Depolarisation
depolarisation
= a more positive membrane potential than the resting potential
what is hyper- or depolarisation of a GP measured in?
millivolts
where do GPs occur?
dendrites, cell bodies or axon terminals and refer to postsynaptic electrical impulses
why are they called GPs?
because their size or amplitude is directly proportional to the strength of the triggering event.
what determines whether GPs are depolarising or hyperpolarising?
the stimulus
what do APs lead to?
depolarisation of membrane and reversal of the membrane potential
what is amplitude proportional to?
the strength of the stimulus.
AP
- An all-or-nothing response when the depolarisation increases above a neuron-specific threshold (action potentials never occur during hyperpolarisation)
- The stronger the above-threshold excitation, the higher the frequency of action potentials generated (measured in Hertz, number of action potentials per second).
further experiments
- They positioned the recording electrode along the axon at different distances from the stimulus electrodes to determine how far neural signals travel
- They changed the ion concentrations in the bath to determine which ions contributed most to the generation of neural signals - By measuring how much current would flow (I) when a set value of change in membrane potential (V) relative to the resting potential was reached, Hodgkin and Huxley determined how Na+ and K+ ion flux changed the neuronal signal (membrane potential) (Hodgkin-Huxley model, 1952). - They postulated the existence of voltage-gated channels and predicted the mechanism that opens and closes them - These were confirmed only in later years when more advanced experimental techniques became available.
Action potential stages
resting P
rising phase
overshoot
falling phase
undershoot
recovery
resting P
voltage-gated channels are closed
rising phase
shooting up in the depolarisation caused by the opening of voltage-gated Na+ ion channels
overshoot
the membrane potential becomes positive as more and more Na+ flow into the cell (positive feedback loop)
falling phase
Na+ ion channels become inactivated and close, while K+ channels open leading to a reduction of positive charge inside of the cell
undershoot
K+ ion flow into the cell through the open K+ channels - reason why transmission only goes in one direction
recovery
refractory period during which all channels are closed and membrane potential returns to resting value
cephalisation
the formation of central ganglia and brains in one end of the animal body
voltage
difference in potential, measured in volts
strength of current
electrostatic force moving charge per second, measured in amperes
resistance
difficulty of passing a current along a conductor
multi meter
instrument to measure voltage, current, resistance
Ohms law
V = I x R
