Neuromodulation and Neural Plasticity :: Biology Essays Research Papers

Neuromodulation and Neural Plasticity Neuromodulatory synaptic transmission differs from classical chemical synaptic transmission in both mechanism and function. The function of a classical synapse is to convey information rapidly from the presynaptic neuron to its target cell, producing a short-term effect. The neuromodulatory synapse may do the same initially, but its primary function is to transmit information that will have long-lasting effects on the postsynaptic neuron's metabolic activity, and on its response to subsequent input. These effects are fundamental to the development and adaptation of the nervous system, and are believed to be the basis of such higher functions as learning and memory. Neurotransmitters released from a classical presynaptic neuron bind to specific receptor proteins in the postsynaptic cell membrane, causing ion channels in the membrane to open or close. If the resulting flow of ions depolarizes the membrane relative to its resting potential, the probability that an action potential will be generated increases, and the synapse is considered excitatory. If the ion flow results in a net hyperpolarization of the membrane, the probability that an action potential will be generated decreases, and the synapse is considered inhibitory. Neuromodulatory synapses can be either excitatory or inhibitory. A neurotransmitter released from the presynaptic neuron may cause the postsynaptic membrane to depolarize or to hyperpolarize by the same mechanism used in classical synapses, but the resulting postsynaptic potential will be relatively weak and slow. Whereas a neurotransmitter in a classical synapse may induce postsynaptic effects lasting from ten to one hundre d milliseconds, a neuromodulator's postsynaptic effects may persist from several hundred milliseconds to several hours. Neuromodulation of the postsynaptic neuron depends not so much on the neurotransmitter as on the receptor to which it binds, called a metabotropic receptor. Whereas classical ionotropic receptors affect postsynaptic membrane permeability directly, metabotropic receptors effect changes in the postsynaptic neuron via intracellular molecules called a second messengers. When a neurotransmitter binds to a metabotropic receptor, a protein inside the postsynaptic cell initiates a cascade of biochemical events that influence the neuron's future response to stimuli. Although the neurotransmitter, or "first messenger," becomes inactivated rapidly, the effects of the second messenger may last several days. One way in which the second messenger induces prolonged effects is by initiating the synthesis of new proteins, which remain in the cytoplasm of the postsynaptic neuron, influencing its activity. Certain proteins can affect the genome of a postsynaptic cell, permanently altering the cell's ac tivities.