Download - Synaptic Potentials
Synaptic Potentials
B.Sc 2002
5-30 ms
-70 mv
-65 mv
Time of arrival ofaction potential inafferent terminals
Motoneuronemembranepotential
EPSP
EPSP's from same stimulation can vary insize
EPSP's from stimulation of different agonistmuscles can vary in size and shape
Inward ionic current atsynapse generatesoutward capacitativecurrent at hillock; inwardcurrent causes start ofEPSP; outward currentcauses decay of EPSP
ionicinwardcurrent
ionicinwardcurrent
net voltage change
capacitativeoutwardcurrent
outwardcapacitativecurrent
Inward synaptic ioniccurrents sum to givelarger outward currents
ionicinwardcurrent
capacitativeoutwardcurrent
overallvoltage change
Summation of EPSPs
• Unless cell is already bombarded with other epsps, spatial or temporal summation is necessary to trigger an action potential
• Motoneurones may be constantly affected by subthreshold inputs…concept of a ‘resting potential’ for a central nerve cell may be erroneous
Electrotonus
• Synapses near or on cell body produce large EPSP’s with sharply rising front edges
• Synapses on distal dendrites produce smaller EPSP’s with slowly rising front edges
• Different input axons can form synapses at different distances on dendrites; giving different EPSP shapes
There are multiple synaptic boutons for each functional
synapse• There is evidence that not all boutons
release transmitter with each arriving action potential
• This could be reason for variability in EPSP’s from a single axon
Reliable connections from unreliable synapses?
Input
Multipleparallelsynaptic
connections
Output
Inhibitory synapses
• An inhibitory synapse does not have to produce an IPSP to be effective
• Reducing membrane resistance proximal to an excitatory synapse effectively shunts excitatory synaptic current out of cell and stops it reaching axon hillock
• Presynaptic inhibition does not produce IPSPs
Mixed pre-and post-synaptic inhibition
• GABA-B receptors may terminate glu transmission by blocking glu release & speeding up repolarisation of terminal
• GABA-A receptors hyperpolarise post-synaptic membrane net effect is to decrease ‘synaptic refractory period’
• This may increase maximum rate of transmission
Renshaw cell
motoneurone axonout ventral root
to muscle
Renshaw cell feedback inhibition
motoneurone cell body
ACh
Renshaw cells regulate rate of firing of motoneuronesde-tune them to the low frequency useful to drivemotoneurones