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L05.  INTRODUCTION TO NEURAL CIRCUITS

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© Carl D. Hopkins Aug. 31, 2011

AnnouncementsDISCUSSION SECTIONS

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OutlineL05. INTRODUCTION TO NEURAL CIRCUITS1. Spatial and temporal integration of excitatory and

inhibitory potentials.2. Neural coding of information in spike trains.3. Branch patterns and the neural control of effectors.4. Branching divergence and synaptic convergence in

sensory pathways.5. Elementary neural circuits and their properties

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Nerve cell at rest has a voltage across its membrane = resting potential

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Action PotentialsThe Action Potential

(spike): – Transient

(1 millisecond duration)–“de-polarization”

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p–peak voltage = +55 mV

inside(mainly due to influx of Na+ ions)

–All or None (threshold)–propagates along axon

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Some Ion Channels in Nerve Membranes are Voltage-Dependent• Ion selective AND Voltage dependent (opening controlled by voltage)

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outsidecell

cell at rest (-70 mV) cell partly depolarized (-30 mV)

cell depolarized(+55 mV)

The Sodium Channel

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Potassium Channels

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Structure worked out in 1998 (Doyle et al. –Roderick MaKinnon lab)

How the action potential propagates

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Propagation Velocity

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• Measuring velocity with two electrodes• velocity = distance / time• myelinated nerve: 10 to 100 m/sec

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Neuron as Rectifiers

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SYNAPSESExcitatoryInflux of cations (+ charge)

cause membrane depolarization

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Influx of chloride ions causes hyperpolarization

Neuron as a TransducerHow do signals get started in

neurons?sensory receptor neuron

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Crayfish Stretch Receptor Neural code for stretch intensity is the frequency of nerve impulses

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Adaptation

SA

RA

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Membrane channelsMSC mechanosensitiveNa – sodiumK – potassiumCl - chloride

Bo Rydqvist, Jia-Hui Lin, Peter Sand, Christer Swerup (2007) Mechanotransduction and the crayfish stretch receptor. Physiol. & Behavior. 92: 21-8.

Pharmachological blockers can sort out the various ion channels responsible for the receptor potential, adaptation, and spikes.

17Bo Rydqvist, Jia-Hui Lin, Peter Sand, Christer Swerup (2007) Mechanotransduction and the crayfish stretch receptor. Physiol. & Behavior. 92: 21-8. 18Wine, J. (1984). J. Experimental Biology. 112:283-319.

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The Neuron as an IntegratorNeuron as a filter.

Combinations of ion channesl give different neurons differing filter properties:

SensitivityAdaptationReboundFacilitationFatigueConduction velocityLabeled Line

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SummationSynapses: synaptic potentials sum on the

post-synaptic cell, providing for:-- spatial summation:-- temporal summation: -- subtraction:

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Fine Tuning the Neuron as Integrator (summing network)

Excitatory synapse generates a depolarizing potential.

Membrane voltage decreases exponentially with distance

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with distance.

Rise time increases with distance.

Consider a synapse on a dendrite.

Neuron as ConnectorSome neurons serve to

relay signals from one cell to another.’

Signal is relayed from input to the output

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Neuron LogicAND logic

Output spike only if 1 + 2 are active at same time.

OR logicOutput spike if either 1 or 2

ti2

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are active

NOT logicOutput spike if 1 OR 2, but NOT 3

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2

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Neuron MemorySynapses retain a

memory of recent events.

Depression: recent activity leads to decrease in response

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decrease in response.

Facilitation: recent activity leads to increase.

Pre-synaptic inhibition.

Pre-synaptic facilitation.

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Changing the Strength of a Synapse

• Fatigue– depletion of synaptic transmitter

• Habituation– decrease in amount of transmitter released, but not due to fatigue

• Sensitization

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– increase in amount of transmitter released

Changing the Strength of Synapses

• Pre-synaptic excitation– a synapse on a synapse

(primes synapse to be stronger).

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• Hebbian Learning– NMDA receptor for

glutamate: synapse is made stronger if activated when cell already depolarized

Neural Circuitsand Behavior

cell body

4. motorneuron

3. synapseTracing circuitry of neural connections leads to understanding of how behavior is influenced by neuronal action.

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stimulus

1. sensory transduction

2. sensory dendrite

neuron

muscle

Neural Circuits and Behavior

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Perception correlates with characteristics of neural circuit.

Neuronal Activity is both Necessary and Sufficient

A) Correlation between behavior and activity of a particular neuron (LGI)

B) Sufficient: artificial

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stimulation of the neuron causes both a spike, and the behavior.

C) Necessary: if the neuron spike is blocked, the natural behavior is blocked, even though stimulus is OK.

Complex Behavior, Complex CircuitsStomatogastric Ganglion of

Lobster.A restricted neural network (30

cells).Controls muscles of gastric mill

and the pylorus (movements involved in griding of food

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involved in griding of food and of digestion).

The PYLORIC muscles and patterns of contraction.

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The End

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43-52.Hodgkin, A. L. and Huxley, A. F. (1952e). A quantitative description of membrane current and its application to conduction and excitation in nerve. J. Physiol. (London) 117, 500-544.Hodgkin, A. L., Huxley, A. F. and Katz, B. (1952). Measurement of current-voltage relations in the membrane of the giant axon of Loligo. J Physiol116, 424-48.Keynes, R. (2005). J. Z. and the discovery of squid giant nerve fibers. Journal of Experimental Biology 208, 179-180.Krogh, A. (1929). The progress of physiology. American Journal of Physiology American Journal of Physiology 90, 243-251.MacKinnon, R., Cohen, S. L., Kuo, A., Lee, A. and Chait, B. T. (1998). Structural conservation in prokaryotic and eukaryotic potassium channels. Science 280, 106-9.Palade, G. and Palay, S. L. (1954). Electron microscope observations of interneuronal and neuromuscular synapses. Anatomical Record 118, 335-336.Palay, S. L. and Palade, G. E. (1955). The fine structure of neurons. J Biophys Biochem Cytol 1, 69-88.Ramon y Cajal, S. (1906). The structure and connexions of neurons. Amsterdam: Elsevier Publishing Company 1967.Ramón y Cajal, S. (1909-1911). Histologie du système nerveux de l'homme et des vertébrés. Paris: A. Maloine.Ramón y Cajal, S. (1995). Histology of the Nervous System. New York: Oxford University Press.Rydqvist, B., Lin, J. H., Sand, P. and Swerup, C. (2007). Mechanotransduction and the crayfish stretch receptor. Physiol Behav 92, 21-8.Wine, J. (1984). The structural basis of an innate behavioural pattern. Journal of Experimental Biology 112, 283-319.Young, J. Z. (1936). The structure of nerve fibers in cephalapods and crustacea. Proceedings of the Royal Society of London (B) Biological Sciences121, 319-337.Young, J. Z. (1938). The functioning of the giant nerve fibres of the squid. Journal of Experimental Biology 15, 170-185.