for perception, action potentials must be generated with precise timing. for precise timing, a precise stimulus isn’t enough. for precise timing, the stimulus must be biphasic. positive - negative. depolarising - hyperpolarising The message
Important article on synchronous clapping After a performance, audiences clap at first randomly and then synchronously. Can we compare synchronous clapping to neuronal synchronisation… Self organising processes: the sound of many hands clapping. Neda et al Nature (2000) Global sound intensity Local sound intensity Index of synchrony Time sec
Gamma frequency oscillations synchronous oscillations at 30 – 70 Hz cortex, hippocampus perception, attention, and sensorimotor coordination the binding problem – how to link the activity of different neurones engaged in the same cognitive task. Gamma oscillation evoked by visual stimulation in area 17 of the visual cortex Of the awake cat (Gray, 1994)
Synchronous clapping I cells in gamma oscillations Must be excited Interaction between members Synaptic interactions between of audience interneurones Must not clap at different rhythms Interneurones must have similar properties Clapping and interneurons: common mechanisms for synchrony
Excitatory drive to interneurones in gamma oscillations In vivo – glutamatergic excitation and liberation of ACh. In vitro « models » of gamma oscillations tetanic stimulation – glutamate plus? agonists at muscarinic AChR agonists at kainate receptors agonists at mGluR 20 mV 0.2 s Control mAChR blocked Stim Slow, excitation of layer 5 cortical P cell Initiated by stim of cholinergic pedunculopontine tegmental nucleus cat - ketamine / xylazine (Steriade & Amzica, PNAS, 1996) Power Freq Hz Intra Spikes Field Time ms Mouse somatosensory cortex. 0.3 µM kainate + 20 µM carbachol Buhl, Tamas & Fisahn J Physiol, 1998
Interneurone connectivity a)Interneurons that inhibit any target cell b)Interneurons that inhibit exclusively interneurons c)Interneurons that excite interneurons What molecular and developmental mechanisms underly the formation of distinct connectivities?
Interneurons that inhibit any target cell In a network, these interneurons generate an inverse synchrony in principal cells – they tell them when not to clap… Popln Activity Sp / s Time ms I-cell intra extra 20 mV 20 µV 20 ms (Cohen & Miles, 2000)
Interneurons that inhibit interneurons These cells generate a rhythm by synchronising IPSPs within the population of I-cells. They synchronise and spread an anti-clapping message. Gulyas, Hajos & Freund, 1996 Interneurons containing Calretinin contact selectively other I-cells Reconstruction: Red – axons Black – soma dendrites Black – CR+ I-cell axon Brown –CB+ I cell soma 10 µm 200 µm 20 mV 2 mV 20 ms 20 mV I cell IPSP Random firing Synchrony Synchrony occurs when IPSPs cohere with intrinsic cellular AHP Lytton & Sejnowski, 1991 Wang & Rinzel, 1993
Interneurons that excite interneurons. Gap junctions occur between subsets of cortical and hippocampal interneurons. Mediate electrotrotonic coupling Formed by the connexin family of proteins Transmit electrical signals rapidly between coupled cells 200µm 20 µm 0.1 µm Tamas, Buhl, Lorincz & Somogyi, Nat Neurosci, 2000 Gibson Bierlein & Connors, 1999 Galaretta & Hestrin, 1999
Gap junctional coupling between cortical interneurons Gap junctions transmit signals rapidly They act as low pass electrical filters. Slow events (AHP) are better transmitted than fast events such as action potentials. A presynaptic action potential induces a postsynaptic « spikelet » of 0.5 – 2 mV. Gap junctions and GABAergic synapses may exist between two interneurones Current Cell 1 Cell 2 Galaretta & Hestrin, Nature 1999 Gibson, Beierlein & Connors, Nature nA 30 mV 0.5 mV 25 ms Freq Hz CR 10 mV 0.5 mV 5 ms 0.5 mV
Interneurons should have similar properties… Interneurones form ~10% of cortical nerve cells. Diversity in co-transmitter peptides – VIP, CCK, SS Ca-binding proteins – PV, CB, CR. Dendritic arborisation – which fibres can excite? Axonal arborisation – site of inhibition: somatic, dendritic, axonal. Parra, Gulyas & Miles, Neuron, 1998 Morphological diversity
Interneurons should have similar properties … Diversity in firing patterns – fast firing cells – Kv3 channel (Lien & Jonas 2003) Diversity in expression of AMPA, NMDA and mGluRs. Receptors for modulating transmitters. Musc mGluR NA 5HT 20 mV 2 m Musc ACPD NA 5HT SLM SR SO Parra et al 1998 Cluster analysis - Cortical I cells Cauli et al 2000 Cell number Linkage GluR Markers Physiol Tout
Differences in expression of proteins associated with gamma Group 1 mGluRsConnexins Cx36 Cx32 Expression of two distinct connexins in 6 interneurons from RT- PCR Venance et al PNAS, mV 4 mV 100 ms Electrical coupling 10 s 100 pA tACPD Type I Type II Type III Type IV Single cell RT-PCR for mGluR1 & mGluR5 % Of Cells mGluR Van Hooft et al, J Neurosci 2000
Recieve a slow excitation Interact between themselves Have quite similar properties Interactions between inhibitory cells Synaptic excitation of inhibitory cells Excitation of pyramidal cells So, as for synchronous clapping, inhibitory cells…. But to understand how the rhythm emerges, must think about interactions involving I cells …
Chemical inhibition + gap junction Three types of interaction between inhibitory cells GABA-mediated inhibition Excitation mediated by gap junction 0.5 mV 50 ms 200 ms 1 mV Biphasic signals generated by gap and GABAergic interactions give the best spike transmission at gamma frequencies Tamas et al, Nat Neurosci, 2002
Synaptic excitation of inhibitory cells I cells have active dendrites Ca transients evoked by somatic firing 50 µm hot spots Active post-synaptic properties modify EPSP shape -75 mV -55 mV 5 ms 0.25 mV Galarretta & Hestrin, Science, 2001 Voltage-dependent activation of both Inward – peak of EPSP enhanced and Outward – decay of EPSP accelerated Kaiser et al J. Physiol, 2001
Temporal precision of EPSP – spike coupling in hippocampal neurones (Fricker & Miles Neuron 2000) Inhibitory cellPyramidal cell EPSP Firing Temporal precision 2 mV 20 ms 20 mV ms
Mechanism of precise EPSP – spike coupling in I-cells So precise spike timing depends on a biphasic signal EPSP initiates inward - outward current EPSP voltage dependance Integral Peak Vm mV mV 20 ms 5 mV V-clamp response to EPSP waveform Command Inward plus outward Outward (TTX) Inward (4AP+TEA) 10 ms 40 pA 200 pA
But is pyramidal cell spike generation precise or imprecise? Fricker & Miles say isolated events initiate spikes with variable timing but Mainen and Sejnowski say noisy stimuli initiate firing with millisecond precision Mainen & Sejnowski, Science, 1995 noisy stimulus square pulse 30 mV 200 pA 200 ms Or maybe the precision of the timing depends on the variance / amplitude of the noisy stimulus? High variance Low variance Axmacher & Miles, soumis 200 ms 20 mV 200 pA
Noise amplitude – cellular currents – precision in P-cells Noise Low variance High variance 20 mV 20 pA Cellular current Firing HP PP mV HP PP mV 20 ms Low variance noise elicits purely inward currents and give low temporal precision High variance noise elicit inward – outward currents and give precise firing (maybe voltage dependent inactivation of K current and enhanced persistant Na current) Axmacher & Miles
Precision in timing of Pcell firing: EPSP – IPSP sequence Voltage clamp Current clamp Afferent EPSC sum 2 ms Di-synaptic IPSC EPSP Di-synaptic EPSP Inhibition functional…blocked Two puise – Inhibition functional - Inhibition blocked 2.5 ms5 ms 10 ms Precise spike timing depends on a biphasic EPSP – IPSP sequence Pouille & Scanziani, Science, 2001
Somatic vs Dendritic summation - precision Pouille & Scanziani, Science, 2001 Dual somatic –dendritic records Dendrite Soma afferent EPSP sum di-synaptic IPSP sum Soma Dendrite Double pulse summation Summation Interval ms Feedforward inhibition terminates on the soma Restricted somatic window for summation.
Biphasic signals generate precisely timed spikes Gap junctional excitation – chemical IPSP EPSP initiates inward - outward current Monosynaptic EPSP - di-synaptic IPSP 1 ms 2.5 ms 10 ms
But why do we need precise spike timing? Oscillations - binding problem LTP LTD depend crucially on pre- and post spike timing Binaural sound discrimination depend on resolving timing differences in 100µs. Olfactory discrimination depend on resolving signals from oscillations And is imprecise, or delayed, firing ever useful? Late firing can maintain persistent activity in a network (XJ Wang) Reverberating circuits.