T1L5 physiology of neurons

Question 1

electrical synapses

Answer 1

• faster
• bidirectional
• smaller gap junction
• no plasticity or amplification
• used in retina and brain

the signal is always weakened as it is transmitted between cells

• the signal will not transmit if the post-synaptic cell is much physically bigger than the presynaptic cell

Question 2

2 types of summation

Answer 2

1. spatial summation:
   - a neuron determines whether to fire based on the cumulative excitation (eg lots of tiny signals from other neurones synapsing to it can reach threshold
2. temporal summation:
   - when the input neuron is firing at a high enough frequency, the postsynaptic cell can reach threshold

Question 3

coding of intensity in neurons

Answer 3

• action potentials are either on or off
• cannot carry info on suze of stimulus
• code the intensity by frequency
• INCREASING FIRING THRESHOLD LOWERS FREQUENCY
• ## increasing excitatory synaptic activity increases frequency

Question 4

threshold and excitability

Answer 4

excitability = how easy it is to start nervous signalling:

• ‘sensitivity’ in sensory cells
• ‘irritability’ in muscle or effector cells
• risk of seizure or spasms

increased threshold lowers excitability

excitability changes are the basis of psychotropic pharmacology

Question 5

open/closed/inactivated channels

Answer 5

channels are proteins and so can have different conformational states

open = increased permeability to ions

closed = when membrane is negative, channel tends to stop conducting due to conformational change

inactivated = when membrane is positive

Question 6

voltage and channels

Answer 6

• when Na+ channels open, the membrane tends to become positive inside (Na is higher outside of cell)
• when K+ channels open, the membrane tends to become more negative inside as k+ leaves cell due to chemical gradient
• when Ca2+ channels open, membrane becomes more positive inside as it enters

Question 7

lidocaine

Answer 7

• local anaesthetic
• raises threshold
• lowers excitability
• stops action potentials
• blocks inactivated Na channels

Question 8

carbamazepine

Answer 8

• anticonvulsant
• carbamazepine inactivates sodium channels
• raises threshold so decrease excitability

Question 9

Vm

Ek

Answer 9

membrane potential- just the electrical force across the membrane

equilibrium potential- where electrochemical forces balance and the amount of k entering the cell by electrical force is equal to the amount of k leaving down chemical gradient

• the more permeable the membrane, the closer the membrane potential is to Ek
• if the membrane was only permeable to potassium, the membrane potential = Ek = -90mV

if membrane was equally permeable to k and na, the vm would meet at the average (-15mV)

Question 10

Ek, Ena, Eca, Ecl

Answer 10

Na= +60mV
K= -90mV
Ca= +123mV
Cl= -40mV

Question 11

graded potentials

Answer 11

• decrease as they move along
• electrically localised
• last a long time
• flatter in shape
• conducted almost instantly
  eg in rods and cones
• variable duration and voltage

graded potentials transmit action potentials; action potentials ares used to amplify graded potentials

Question 12

saltatory conduction

Answer 12

• ap jumps from node to node
• faster conduction velocity
• the electrotonic jumps between nodes are very fast
• initiating and action potential at each node is slower

Question 13

conduction velocity

Answer 13

faster:

• myelinated
• larger diameter

100m/s motor fibres
1m/s pain fibres