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Unformatted text preview: Small Circuit Page 11-1 How to tell whether a postsynaptic potential is depolarizing or hyperpolarizing You will need to know (or be able to calculate): (a) the resting membrane potential of the cell, Em Em could be obtained by (i) measuring the the voltage difference between electrodes inserted inside and outside of the cell, or (ii) using GHK equation to calculate the potential. (b) the equilibrium potential of the ion(s) which flows through the ligand-gated channel, Eeq To measure Eeq: (i) using patch clamp experiments by applying ligands to open channels and finding out the voltage at which no current flows, or (ii) using Nernst equation to calculate the potential.
(A) Eeq > Em (B) Em > Eeq Em Eeq Em (c) Em = Eeq Eeq Em If the channel allows more than one type of ion to flow through it, use reversal potential (Erev): (a) Erev could be obtained by using GHK equation. (b) Erev is different from Em, because only the ions that go through the channel count and the permeability of the ion changes. How to tell whether a postsynaptic potential is excitatory or inhibitory Excitatory potentials make it MORE likely a cell will fire an action potential. Inhibitory potentials make it LESS likely a cell will fire an action potential. You will need to know (or be able to calculate): (a) the resting membrane potential of the cell, Em (b) the equilibrium potential of the ion(s) which flows through the ligand-gated channel, Eeq or Erev (c) the action potential threshold (A) Eeq > Threshold > Em Eeq Threshold Em (B) Threshold > Em > Eeq Threshold (C) Threshold > Eeq > Em Threshold Eeq Page 11-2 Em Eeq Em When the channel opens, the potential is: When the channel opens, the potential is: When the channel opens, the potential is: The transmitter released by a pre-synaptic cell and the receptor on the postsynaptic cell combined with the ionic environment inside and outside of the cell determine whether a connection is excitatory or inhibitory. Voltage Connections between neurons Properties that determine circuit behavior (a) Connectivity: what neurons are connected to each other Excitatory neuron and synapse (b) Synaptic properties: types of neurotransmitters and their receptors, neurotransmitter limits, postsynaptic potentials, and second messenger effects. Inhibitory neuron and synapse (c) Intrinsic cellular properties: receptors and channels, endogenous spontaneous activities. Action potentials Time Different responses resulting from monosynaptic excitation (1) Excitation, B has no spontaneous activity Page 11-3 A A B B A B A B A B A B A B A B (2) Excitation with amplification (3) Excitation, B has high spontaneous activity A B (4) Excitation, B habituates A B Different responses resulting from monosynaptic inhibition (1) Inhibition A A B B (2) Inhibition, B has high spontaneous activity (3) Inhibition, B has post-inhibitory rebound Page 11-4 A B A B (4) Inhibition, A is a pacemaker, B has high spontaneous activity A B A B Mechanisms Circuit behavior is determined by connectivity, synaptic properties, and intrinsic cellular properties: (a) Different interconnection can give you same firing pattern (b) Same interconnection can give you different firing pattern (2) Excitation with amplification A A A B B B A B A B A B (2) Inhibition, B has high spontaneous activity (3) Inhibition, B has post-inhibitory rebound Page 11-5 (1) Amplification of a signal at a synapse can be caused by the post-synaptic cell being very close to threshold. It can also be caused by different types of post-synaptic receptors causing a prolonged response to transmitter release. Examples: (a)Glutamate ionotropic receptors, the NMDA and AMPA receptors; (b) metabotropic receptors, through second messenger. (2) Habituation can be caused by a large number of mechanisms. One simple mechanism is the pre-synaptic terminal running out of available neurotransmitter after firing a number of times. Another is the pre-synaptic terminal calcium levels becoming depleted due to inactivation of voltage gated calcium channels. (3) Post-inhibitory rebound is caused by inhibition lowering the AP threshold to below the cells resting membrane potential. When the inhibition is removed, the membrane potential exceeds the threshold and generates action potentials. Generation of coordinated movement (1) The motor control is arranged hierarchically. Increasingly complex motor tasks are organized in successively higher centers. (a) Simple reflex: sensory neurons synapse with motoneurons in the spinal cord. (b) Central pattern generators: networks of interneurons in the spinal cord or brainstem to coordinate the interplay of multiple antagonistic motor groups. (c) Higher levels of control in motor cortex, cerebellum, or basal ganglia.
Most cells: Cells show post-inhibitory rebound: (2) Central pattern generators There are two mechanisms: A is a pacemaker (a) Rhythmic alternations generated in a single neuron; a B has high spontaneous activity pacemaker cell (i) Pacemaker cells: The cell is spontaneously active. A B (ii) Depolarization opens voltage-gated calcium channels. Increased intracellular calcium opens calcium-dependant potassium channels. Potassium flows out of cell. The cell A hyperpolarizes below action potential threshold and stops firing. (iii) Calcium channels then inactivate. Calcium dependant potassium channels close, depolarizing cell membrane potential. B (iv) The cycle then begins again. Page 11-6 (b) Rhythmic alternations generated by synaptic interactions between members of a neuronal network Four examples: (I = Interneuron, F = Flexor, E = Extensor) (i) Both the extensor and the flexor have strong post-inhibitory rebound (PIR); Excitatory input I F E (ii) Both the extensor and the flexor habituate, and it takes longer for the interneuron to excite the extensor than the flexor
D I I F E F E I F E (iii) Both the extensor and the flexor have strong post-inhibitory rebound (PIR); Inhibitory input I Page 11-7 F E I F E I F E
(iv) Two pacemaker cells (1 and 2) excite F and E, respectively. The pacemaker cells discharge with a spontaneous rhythm and are coupled by inhibitory interneurons (blue). ...
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- Summer '08