Synapse

Neuroethology - Biology 419/580

Bowling Green State University, Spring 2004

Neurons talk to each other

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).

last modified: 1/14/04