In a few sentences, describe Ionotropic receptors and Metabotropic receptors and

An ionotropic receptor, when activated, directly affects the activity of a cell by directly opening ion channels.

A metabotropic receptor influences the activity of a cell indirectly by first initiating a metabolic change in the cell. This metabolic change may ultimately affect the opening or closing of an ion channel or may alter some other activity of the cell such as protein transcription.

The ionotropic receptor is actually a class of chemically-gated ion channel. The open or closed state of this class of chemically-gated channel is regulated by the binding of a neurotransmitter to an external domain on the ion channel. For ionotropic receptors, the receptor and primary effector are actually parts of the same macromolecule since the ion channel itself is the primary effector (the entity that initiates the change in the functional status of the neuron). Eg. the nicotinic acetylcholine receptor, the acetylcholine molecule binds to the receptor portion of the channel. Upon binding, the channel opens, changing the ionic permeability and hence changing the membrane potential.

In contrast to the ion channel that comprises the ionotropic receptor, metabotropic receptors are comprised of a single membrane-spanning protein. An extracellular region of this protein has a high affinity for a neurotransmitter and functions as the binding site. When a neurotransmitter binds to the binding site of a metabotropic receptor, the receptor undergoes a configurational change that either directly or indirectly activates an enzyme. This enzyme, representing the primary effector, commonly catalyzes a change in the metabolism of the neuron by converting some substrate into an intracellular bioactive metabolite known as a second messenger. The extracellular first messenger, which is the neurotransmitter, has now led to production of an intracellular second messenger. The second messenger may in turn activate a secondary effector within the neuron that can carry out subsequent metabolic changes. Thus, the production of the second messenger is often the initial link in a chain of physiological alterations that ultimately lead to the biological response of the neuron.

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