(i) Neural systems: Activation refers to increased activity within neural systems and can be used at the level of the individual NEURON, in terms of activation of a POSTSYNAPTIC membrane, or it could be at the level of entire neural systems. Activation at the level of the individual neuron occurs when an impulse arrives at a PRESYNAPTIC membrane. NEUROTRANSMITTERS are released into the SYNAPTIC CLEFT and bind to RECEPTORS on the postsynaptic membrane, leading to the opening of ion channels (see ION CHANNEL) and postsy naptic membrane activation. Each synaptic terminal produces a slow, weak and graded synaptic potential. This could be an EXCITATORY POSTSYNAPTIC POTENTIAL (EPSP) or INHIBITORY POSTSYNAPTIC POTENTIAL (IPSP), depending on the neurotransmitter and ionic channels involved. In the central nervous system, postsynaptic neurons can be activated with EPSPs and IPSPs from adjacent synaptic terminals. The activation caused by a single synapse is usually not sufficient for the post-synaptic neuron to fire (see FIRING). A stronger activation signal requires SUMMATION of activity, either by addition of a number of post-synaptic potentials (spatial summation), or by increasing the frequency of discharge of individual synapses (temporal summation).
Activation by a GLUTAMATE-containing neuron always results in excitation of the postsynaptic membrane; GABA release leads to inhibition. In all but the simplest organisms, activation of individual neurons is not sufficient to perform the integrative and regulatory actions of the nervous system; in the vertebrate nervous system this occurs through the activation of millions of synapses grouped into specific networks. For instance, when groups of long-axoned CHOLINERGIC and MONOAMINERGIC neurons located in the brainstem—and belonging to the ASCENDING RETICULAR ACTIVATING SYSTEM (ARAS)—fire, one result is the activation of networks of neurons in the THALAMUS and CEREBRAL CORTEX. The type of activation seen in the cortex depends on the pattern of activity in component networks of the ARAS, and the resulting balance of excitation and inhibition in target neural systems. In the cortex, activation can be measured using an ELECTROENCEPHALOGRAM (EEG). Levels of activation in different neural systems, during performance of particular tasks, can be compared by using imaging techniques, such as positron emission tomography (see PET) or magnetic resonance imaging. Just as activation at the postsynaptic membrane can result in excitation or inhibition, high levels of activation in a particular neural system measured by imaging techniques may be excitatory or inhibitory. An additional way of measuring activation in neural systems is through the use of EVENT-RELATED POTENTIALS.
(ii) Behaviour: Behavioural activation refers to heightened motor output. As behavioural activation may be a sign of increased AROUSAL, it is often used synonymously with this, though such use should be avoided; see also ALERTNESS; MOTOR READINESS.
WENDY L.INGLIS AND VERITY J.BROWN
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