Ohm's Law

I = g * V

The voltage present across the cell's membrane can be measured between a reference electrode and a conducting glass capillary tube drawn out to a very fine point, and inserted into the cell. At rest the neuronal interior is negative relative to the outside at between -60 to -80mV, while most non-neuronal cells feature a potential of about -30 mV. The current is carried by ions moving across the membrane at ion conductances when specific channels allow them to pass. Ignoring its active properties, the axon can in electrical terms be viewed as an insulated cable. An electric potential is able to spread passively along any stretch of membrane. In the process its strength decays and the slope of on- and offsets becomes less steep with distance due to the passive cable properties of the membrane. The latter include resistance along its length and both a resistance and a capacitance component across it: Signals will spread fastest when longitudinal resistance is low (e.g., via increased axonal diameter) and they will spread furthest when resistance across the membrane is high (e.g., with layers of myelin for added electrical insulation).		a. b. c. d. The electric properties of the neuronal membrane can be expressed as a circuit diagram. © 2004 lobsterman. a. Golgi-stained neuron with long axonal process. b. Circuit diagram of neuronal membrane combining membrane resistance (Rm), membrane capacitance (Cm) and internal or axial resistance (Ri). c. Decay in signal size over distance with Space Constant. d. Change along the axon in Voltage response to the injection of a square current pulse due to membrane capacitance

Ion Concentrations in- and outside of Neurons

The Resting Membrane Potential

The resting membrane potential arises from an unequal distribution of ions across the cell's membrane. An excess of K⁺ and of various Anions^- exists on the inside and of Na⁺, Cl^-on the outside. The membrane is dotted with proteins (i.e., ion channels) that control the flow of specific ions across it. In a cell at rest only potassium ions (K⁺) are able to flow freely between the two compartments. Initially we observe a net flow of positively charged potassium ions from the inside where they are more numerous to the outside where their concentration is much lower. With a net flow of positively charged ions from the inside across the membrane, the inside becomes increasingly negative relative to the outside. The resting potential settles into an equilibrium when as many potassium ions are pushed out of the cell along their concentration gradient, as will enter the cell along the accompanying electrical gradient. This equilibrium potential for potassium can be calculated using the Nernst potential for known inside and outside concentrations of any given ion

The Nernst potential for potassium is thus:

The cell's resting membrane potential combines all relevant ion currents with K⁺ ions figuring most prominently due to their high resting conductance. In addition a small number of Na⁺ ions leak into the cell. The resulting resting membrane potential is thus slightly lower compared to the E_K+ at around -65mV. The concentration gradient across the membrane is maintained by the Na⁺/K⁺ ATPase (ion pump).

The Action Potential

• Action potential is generated when the membrane potential rises above threshold either spontaneously or because of depolarizing input. During the action potential, the membrane potential briefly rises to about +10 mV and then repolarizes to the resting membrane potential.
• Voltage-gated Na⁺ Channel: Sodium channels are opened when the potential rises (i.e., depolarizes) from -65mV to -55mV. They close again when it further rises to +10mV. Action potentials depend on a transient inward current of cations (Na⁺, Ca²⁺) followed by an outward current (K⁺).
• Axonal Propagation of Action Potentials: passive spread of depolarization opens Na⁺ channels in neighboring areas of membrane
• Repolarization: return to resting potential
• Refractory period follows the action potential during which time the threshold for firing is higher than normal.

The Duration of an action potential is 1-2ms in Vertebrates and 1-100ms in Invertebrates. Frequency of firing ranges from <1 to about 100/sec (100Hz). The Amplitude ranges between 70-80mV when recorded intracellularly and 5-200µV when recorded extracellularly.

Electrotonic Junctions between Neurons

Two electrically excitable cells, such as neurons or muscle cells, may be electrically coupled where an action potential in one cell moves directly into the other via arrays of gap junctions. Electrical synapses are fast but cannot be modulated. They are mostly used in neuronal circuits for escape behaviors where speed of conduction is essential. Electrical synapses (gap junctions, electrotonic junctions) allow current to flow between separate neurons when ions pass through gap junctions. Connexons (where 6 connexin proteins form a hemi-channel) are the actual pores that allow ions to flow past the two membranes. A connexon in the presynaptic membrane lines up precisely with its respective equivalent in the postsynaptic membrane, forming a continuous channel from one neuron to another. With a pore diameter of about 1.5m^-9 many small molecules can pass through efficiently. Intracellular Ca²⁺ concentration, pH, or phosphorylation of connexins can profoundly alter the easy with which ions annd proteins may pass through the pore. As there is no synaptic delay in transmission of current from cell to another, the conduction of potential changes is considerably faster than through chemical synapses. Although electrical synapses are often bi-directional, some synapses pass current better in one direction than the other (i.e., rectifying synapse) Electrical synapses are commonly used in time-critical processes (escape behaviors), when rapid synchronization of many cells is needed (e.g., vertebrate cardiac muscle), between glial cells, or early in development.

Chemical Junctions between Neurons

Chemical synapses: A one-directional connection made between nerve or muscle cells where the signal leads to the release of a neurotransmitter from the presynaptic terminal, diffusion across the synaptic cleft, and binding to receptors in the postsynaptic membrane. Receptors: ionotropic or metabotropic. Binding alters ion conductances at the postsynaptic cell membrane (e.g., increase in Na⁺ conductance is excitatory, CL^- is inhibitory). Excitatory, the postsynaptic cell is depolarized with an excitatory post-synaptic potential (EPSP) and an action potential is elicited if the threshold is reached; inhibitory, the postsynaptic cell is hyperpolarized with an inhibitory post-synaptic potential (IPSP) and it will thus be harder for other inputs to drive the cell towards an action potential. A single input is rarely sufficient to lead to an action potential in the post-synaptic cell. Multiple EPSPs may add and reach the threshold when a series of action potentials arrive at high rate . Chemical synapses are capable of integrating a complex scenario of inputs:

Spatial and Temporal Summation of EPSPs, e.g. excitation from several neurons has to arrive concurrently for an activation of the crayfish lateral giant interneuron

<Neurotransmitter> refers to a compound that is released at a synapse and diffuses across the synaptic cleft to act on a receptor located on the membrane of a postsynaptic cell, which may be another neurone, a muscle cell or a specialized gland cell. It is released from nerve endings by nerve impulse activity at morphologically distinguishable synaptic junctions producing suitable changes in the excitability of the postsynaptic membrane. Ca²⁺ influx at the axon terminus is required for synaptic release.

<Neuromodulator> refers to a compound that is released within a localized region of CNS, the receptor for which is not necessarily sited on an anatomically apposed postsynaptic cell. Thus a neuromodulator may affect several postsynaptic cells with specificity conferred mainly by the distribution of receptors. Main action is on second messenger systems, eg. cAMP or inositole triphosphate, presumably affecting protein phosphorylation

Sometimes the same neurochemical may have rapid transmitter type effects, followed by longer modulatory influences. This suggests that neurotransmitter and neuromodulator effects may be most effectively classified at the receptor level. Activation of receptors on a protein structure directly incorporating an ion channel (a ionophore) are defined as neurotransmission while activation of receptors coupled indirectly to ion channels (eg. via second messanger systems) are defined as neuromodulation (Hasselmo, 1995).

Sensory Processing

<Receptive Field>: In sensory systems, the specific region of a sensory surface (e.g., specific area on the retina) that when stimulated causes a change in activity of a neuron

Neural design features

Coincidence Detection:

• Spatial Summation: if a neuron receives inputs from multiple sources it will respond best if the input from these sources matches in time. Activity in either presynaptic neuron alone is not sufficient to produce and action potential in the postsynaptic neuron, but an action potential is produced if signals arrive at the same time in both presynaptic inputs.

Contrast Enhancement:

• Lateral Inhibition: Multiple units with similar characteristics are wired to inhibit each other's activity. The unit that fires first/strongest will prevent all others from firing. (e.g., Discrimination of left vs. right in auditory signals using a pair of Omega neurons in crickets).

Measure Time-Delay between two inputs

• Delay Lines: Action potentials (AP) travel along axons at a defined speed. They thus take longer to arrive at the target the further the AP needs to travel. Multiple neurons with similar characteristics are layed out in a longitudinal array. They receive input from two sources one fed in from one side of the array, the other from the other side. Acting as coincidence detectors they respond best when the signals on both inputs match. Each member of the array is most sensitive to a particular time difference. (e.g., Spatial localization of auditory signals in the laminaro nucleus of Owls).

Acoustic processing

When localizing a sound source, systems for the discrimination of left vs. right are often based on two sub-systems. These are often mirror-images of each other and located to the left and the right of the midline (i.e., Omega neurons in crickets). They are tightly coupled through lateral inhibition, where activation of one side automatically shuts off its contralateral (i.e., opposite side) opponent. Such a design is uniquely able to allow resolution of extremely small time differences in when a sound signal arrives at the ear facing the source than in the one facing away.

Chemical Synapse © 2000 lobsterman

Reading Assignment

• Chapter 7: The Nervous System and Behavior. pp 80-85

Links of Interest

• Histology of the Nervous System

WikiBooks

• Proximate Mechanisms in Behavior

Food for Thought

• Pop-Out effect: Why do we have an ability to detect the odd object? What do you think causes this perceptual difference?

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last modified: 10/06/03

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