Physiology of neurons Flashcards
State 6 differences between electrical and chemical synapses
- Faster
- Require no learning/plasticity
- bidirectional
- Smaller gap (3.5nm compared to 30nm)
- No amplification
- Couple via connexon- the pore of a gap junction (rather than neurotransmitter)
What does the fact that electrical synapses lack amplification mean for the signal?
The signal is always weakened (successively) due to electrical resistance
Signal will not transmit if postsynaptic cell is much bigger than presynaptic cell
Excitatory presynaptic signal CANNOT inhibit the post-synaptic cell
Where would electrical synapses be used?
For defensive reflexes in invertebrates, brain, retina
What is spatial summation?
Addition off all the tiny signals (both inhibitory and excitatory) synapsing on neuron
Allows small depolarisations to reach threshold
What is temporal summation?
Input neuron is firing fast enough so that the receiving neuron adds the frequent tiny signals –> threshold reached
Happens where the ability to recover from tiny depolarisations (refractory period) is diminished –> next signal arrives whilst neuron is still depolarised
Consider ion channels.
- What does the fact they are proteins mean for their function?
- How do voltage gated channels work?
- How is an inactivated channel different to a closed channel?
- Proteins have varied conformational states. CHANGE SHAPE –> CHANGE FUNCTION
- Voltage gated channel states based on transmembrane voltage. Open when positive inside, stop conducting when inside of membrane is negative
- Both mean that the channel is non-conducting. Inactivated channels stop conducting when inside is POSITIVE. Closed channels stop conducting when inside of membrane is NEGATIVE. Different conformational shape same functional effect
How is an electric field created across a channel and why is this significant to its function?
Electrical field created by mismatch of positive and negative ions via gating - that is that the channels allow flow of only one type of ion
How is cell membrane voltage determined?
The voltage of the cell membrane is determined by inter-related feedback loops
…increased permeability to K+ makes membrane more negative (AS ITS COMING OUT)
Opposite for sodium
Define the two forces that act on an ion
Chemical (diffusional force)- based upon the difference in concentration across the membrane.
e.g. if 10x the concentration of sodium outside than inside, the chemical force of Na+ is 60mv directed INTO cell)
Electrical (voltage force)- based upon membrane potential.
State the definition of an equilibrium potential and the values for Na, K, Ca and Cl.
The membrane potential at which the electrical force is equal in magnitude but opposite in direction to the chemical force is called the equilibrium potential for that ion
Na +60mv
K. -90mv
Ca +123mv
Cl -45 (-65 in neurons)
Describe the forces acting on …..at equilbrium:
- K+
- Na+
- Ca2+
- Cl-
- Electrical force is -90mv inwards and a chemical force of +90mv out (More K+ ions inside than out)
- Electrical force is +60mv inwards a chemical force of -60mv INWARDS (More Na+ ions outside cell)
- Electrical force is +123mv OUTWARDS a chemical force of -123mv INWARDS (More Ca2+ ions outside cell)
- Electrical force is -45mv OUTWARDS a chemical force of +65mv INWARDS (More Cl- ions outside cell)
A force is said to be positive if it drives a positive ion OUT of cell/ Negative ion IN
How is Ex calculated?
Using the Nernst equation
The more permeable a cell is to a particular ion, the more Vm tends to Ex for that ion.
- What would happen if both K+ and Na+ channels were open and permeability was equal?
- What would happen if extracellular K+ increased?
- Vm would tend towards the average of their Ex… so -15mV
- Vm would be LESS NEGATIVE As Ek is the major determinant of resting membrane potential. Paradoxically leads to decreased excitability as long term depolarisation inactivates Na+ channels so harder to reach threshold
What are graded potentials?
Where do they occur?
A graded potential is an ordinary change in Vm that can vary in voltage, amplitude and time duration
- Changes in Vm do not propagate far cia passive electric forces.
- The signal diminishes from as distance from source increases as axon has a finite resistance.- NOT AMPLIFIED
Electrically localised
Last a long time
Much flatter in shape
Conducted almost instantly
Whereas APs occur along length of neurons, GPs happen at receptors (rods/cones)
Describe electronic conduction in an alpha neuron axon
Myelin: high resitant- shields electric field
Cytoplasm: low resistant- instantly transmits electric field
Saltatory conduction is faster as its between nodes.
Initiating an AP at each node is slower as conformational change of ion channels
LARGER DIAMETER–> LOWER RESISTANCE –> FASTER CONDUCTION
