Neurophysiology: From Cells to Networks Flashcards
Neurones
- The basic unit of structure and function in the nervous system
- Cells that conduct impulses
- Process information
- Sense environmental changes
- Communicate changes to other neurons
- Command body response
- High energy usage, constant need for glucose and oxygen
Cell Body
• Contain nucleus and cellular
activity
Axons
• Single extension of the
neurones providing output.
Dendrites
• Branch like extensions that
receive messages from other neurones.
Neuronal types:
• Motor
– carry impulses away from the brain and spinal cord
• Sensory
– carry impulses from inside / outside the
body to brain / spinal cord.
• Relay
– process incoming impulses and pass them on to motor neurons
Neuronal Networks:
Neurones exist within neural tissue, where multiple neurones synapse with each other to produce an active network.
Activity is a measure of summation of inhibitory and excitatory action.
Can be recorded through an electroencephalogram (EEG).
Neuronal Networks: EEG
Sub-dermal electrodes placed at specific locations on the dog skull, corresponding to regions of the canine cortical lobes. • Pre-frontal (Fp) • Frontal (F) • Parietal (P) • Occipital (O) • Temporal (T)
EEG rhythm is a combination of local field potentials (LFP).
LFP is a measure of the underlying ionic environment, and therefore activity.
action potential
a rapid reversal of the resting membrane potential
how is an action potential generated
different ions with different electric charges are dissolved in the brain
movement of these ions across the membrane through specialised proteins is how an action potential is generated
sphere of hydration
ions are atoms with a net electric charge
water molecules “stick” electrostatically to ions and form a sphere of hydration
this sphere of hydration increases the relative size of the atom
an ion surrounded by a sphere of hydration is…
Much too large to pass through the membrane
main ions dissolved in the brain
calcium
sodium
potassium
chloride
ion channels
allow ions to pass in and out of a neuron
made form multiple subunits
a subunit is a protein that has been shaped into a tertiary structure
acts as a door to allow ions to pass through the membrane
come in a wide range of shapes and sizes
can open and close
highly selective
a portion of the channel may have an electric charge.
only ions that are…
small enough to fit through the pore and carry an opposite charge may pass
resting state
a neuron is integrating incoming signals and not generating an action potential. it is at rest
action state
a neuron has been excited past threshold and fires an action potential
negative charge inside of a neuron is
an absolute requirement for a functioning nervous system
membrane potential
the voltage across the membrane at iny moment (in millivolts)
represented by Vm
at rest, Vm = -65 mV
potential arises becuse of differences in electrical charge across the membrane
the inside of a cell is more negative to the outside
the resting membrane potential is determined by two forces
concentration
electrostatic pressure
a balance between these two forces creates the resting membrane potential
ohms law
I = gV
where:
I= the movement of ions across the membrane
G= whether there are channels open for the ions to pass
V= whether there is a difference across the membrane to move them
equilibrium potential
a balance of forces
the electrical potential that exactly balances an ionic concentration gradient if the membrane were permeable only to that ion
the point where diffusion and electrostatic pressure are exactly equal and there is no net movement of the ion across the membrane
resting membrane potential is largely determined by
the equilibrium potential
to balence all the K+ and A- inside the cell…
there needs to be an ion outside that cant cross the membrane
there is a high concentration of Na+ outside of a neuron that cannot get in
lots of Na+ lowers the concentration outside the cell creating an osmotic balance
at rest there are open channels for
K+ but not na+
describe the sodium potassium pump
binds three Na+ ions and ATP inside the cell
this changes the conformation of the protein
it releases the Na+ outside and picks up 2K+ ions
binding of K+ into the neuron
describe how the action potential is mediated by the movement of ions
resting state rising phase overshoot falling phase undershoot
resting state
membrane is at RMP (-65mV)
rising phase
a rapid depolarisation of Vm
positive interior repels voltage sensor thereby opening the voltage gated Na+ channel
overshoot
inside of the neuron is now positive relative to the outside
inactivation of the Na+ channels
falling phase
rapid repolarisation of Vm
voltage gated K+ channels activate at threshold too but take 1ms longer, known as delayed rectifiers
K+ flows out of the neuron down both its concentration and voltage gradients
undershoot
inside the neuron is now more negative than at rest (hyperpolarised)
K+ slow to close as well
the permiability of the membrane to K+ is even higher than at rest causing Vm to reach Ek
depolarisation
change in Vm positive to rest
repolarisation
change in Vm back to rest
Hyperpolarisation
change in Vm Negative to rest
injection of postitive current into a neuron will…
depolarise the membrane potential (vm)
unless the membrane potentia can depolarise sufficintly to the threshold…
no action potential will be generated
if the stimulus is strong enough the neuron will
fire action potentials
the stronger the stimulus the more action potentials a neuron will generate
absolute refractory period
Na+ channels still inactivated therefore making it impossible to generate another action potential
relative refractory period
Na+ channels have deinactivated and it takes more positive current to bring the neuron to firing threshold
since Vm = Ek
Voltage gated channels at rest
Negative interior attracts positive voltage charge of sensors on Na+ and K+ channels
Background k+ channels still open to maintain Rmp
If the neuron is depolarises to threshold then…
Both voltage gated Na+ and voltage. Gated K+ channels are activated
Increased positive charge on the neuron repels the voltage gated sensors. On both the channels
The channels pop open and Na+ immediately flows into the neuron bringing Vm close to Ena. K+ channels slowly start to open
Voltage gated NA+ channels inactive before Vm reaches Ena due to
the pore becoming blocked from the inside
VMware nearly reaches Ena but voltage gated Na+ channels inactivate. K+ channels have yet to fully open
Na+ channels still open but are inactivated due the block by the globular portion of the channel
Voltage gated K+ channels are slowly opening
Describe what happens when Voltage-Gated K’ channels finally pop open and leaves the neuron and repolarises Vm to a more negative value
During repolarisation, Na channels are open, but inactivated, and K+ is pushed out of the neuron down its concentration and voltage gradients
