Excitable cell Flashcards
3 mains roles of neurons
Collect
Integrate
Output
Role of the nervous system
System of communication that allows an organism to react rapidly and modifiably to changes in its environment
What are the 2 mechanisms of the nervous system?
Electrical activity for rapid speed
Chemical messengers for flexibility
What are 2 subsections of electrical signals?
Grading potential
Action potentials
Define an action potential
A fixed size, all or nothing signal that propagates an axon
Define a grading potential
A variable size, local signal which can pass both ways along neuronal membranes
How are APs coded?
By frequency
How are GPs coded?
By size
When can neurons transmit action potentials?
When they reach a depolarisation threshold
What causes neurons to have a resting potential?
Selectively permeable membrane
Unequal distribution of charged molecules/ions
Physical forces
What happens to the membrane potential if the membrane becomes non-selective?
Membrane potential becomes 0
What is the difference between a membrane channel and a membrane pump?
A channel confers selectivity
A pump assists unequal charge distribution
What 2 forces control the flow of ions in an aqueous solution?
Diffusion
Electrical field
Why does a membrane have to have a potential difference and channels?
Bilayers provide a barrier to ion movement
So, if no channel/potential difference, conductance is 0
What is current flow dependent on?
Electrical potential (voltage)
Electrical conductance
Electrical resistance
What would not occur without ion pumps?
Resting membrane potential
In which direction does the Na+/K+ ATPase work?
Against the conc gradient
Internal sodium for extracellular potassium
(High Na+ outside membrane, high K+ inside membrane)
Why is a low intracellular Ca+ important?
High Ca+ is toxic and can kill neurons
Ca is a signalling ion
When does Equilibrium potential occur?
When electrostatic force exactly counteracts the diffusional force
What determines the rate at which ions are driven across the membrane ?
Rate proportional to the diff between membrane and equilibrium potential
What is the Nernst equation?
Eion = 2.303 x (RT/zF) x log (ion o/ion i)
R = gas constant
T = temperature in K
z = charge of ion
F = faraday constant
i = intracellular
o = extracellular
Which ion is the neuronal membrane permeable to at rest?
K+
What does increasing the extracellular K+ do to membrane potential?
The membrane potential will increase and the membrane will become depolarised
What is the goldman equation used for?
To estimate the real Vm (membrane potential) when the membrane is permeable to other ions
What is the goldman equation?
Vm = 61.54 mV x log Pk(K+)o + PNa(Na+)o /
PK(K+)i + PNa(Na+)i
How do ionic gradients influence membrane potentials?
They determine the membrane potential that would have to be achieved in a neurone if the membrane was selectively permeable to the ion
What is ion movement rate proportional to ?
IDF proportional to Vm -Eion
What are the 4 features of an action potential?
Rising phase
Overshoot
Falling phase
Undershoot
Describe the rising phase
Rapid depolarisation of the membrane
Describe the overshoot
Membrane potential is above 0
Describe the falling phase
Rapid repolarisation of the membrane (more -ve than the starting resting membrane potential)
Describe the undershoot
After hyperpolarisation
- gradual decline so the membrane potential comes back to resting level
Does amplitude change if you change the holding membrane potential?
No it doesn’t change
When does a neurone depolarise?
When the permeability to Na is increased
Describe the change in sodium channels with an action potential
At rest - most sodium channels ar closed
Depolarisation - sodium channels open, selective permeability increases for sodium
Repolarisation - sodium channels close, a decrease in membrane potential causes the channels to close again fully at rest
What is the structure of the voltage-gated Na+ channels?
Several transmembrane domains,
Domain 4 has lots of positively charged amino acids
Describe the conformational change of a voltage-gated Na+ channel
Due to small ion movements
Lipid part of the protein tries to move from one side of channel to the other
Minimal opening of the pore allows selectivity
How fast is Na+ channel inactivation?
1ms
When does the relative refractory period occur?
Whilst voltage gated K channels remain open
When does the undershoot ware off?
As voltage gated K+ channels close
How does an action potential conduct down the axon?
Spread of charged particles (Na+)
Why does an action potential only travel in one direction?
As the ap travels down the axon, Na channels behind the ap close, therefore Na+ cannot move retrospectively.
What happens to membrane potential after ap?
The membrane potential rises due to depolarisation
What factors affect conduction velocity?
Diameter
Myelination
Permeability of the membrane
How does diameter affect conduction velocity?
Resistance to current flow increases as the cross sectional diameter decreases
How does myelination affect convection velocity?
Prevents membrane from leaking
Allows ap to ‘jump’ from node to node (saltatory conduction)
Why may some small axons not be myelinated?
Myelination costs lots of energy
Benefit of high membrane resistance is reduced by the high internal resistance
Why does a myelin sheath not wrap the whole axon?
Space is needed for the voltage gated channels
What is the difference between an axon and a dendrite?
Dendrites have voltage sensitive channels but rarely produce action potentials
How do action potentials carry electrical signals?
In a non-decremental manner (amplitude remains the same)
What is action potential frequency dependent on?
The size of the depolarising stimulus
What is the absolute refractory period?
1 ms
- the neuron is incapable of generating another ap
What is the relative refractory period?
A few ms
- you can fire another ap but a stronger stimulus is required due to threshold being raised
What is stimulus intensity controlled by?
The action potential frequency in the nervous system
What are the 2 types of graded potential?
Inhibitory (hyperpolarizing)
Excitatory (depolarizing)
What are graded potentials caused by?
Opening of neurotransmitter gated ion channels or
Opening/closing of potassium channels
What is an EPSP?
Excitatory post synaptic potential
What is an IPSP?
Inhibitory post synaptic potential
What is summation?
The act of integrating multiple neuronal inputs together to reach a threshold
What is spatial summation?
When progressively larger numbers of primary afferent neurons are activated simultaneously
What is shunting?
Making the membrane leaky to dissipate the shunting effect caused by the opening of non-selective cation channels in axon membrane
How do ions travel through the membrane?
Through GAP junctions
