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