1. (in anatomy) Either of a pair of structures in the vertebrate forebrain that lie above the hypothalamus and form part of the limbic system. Each relays sensory information to the cerebral cortex on the same side of the brain, such as inputs from the retina of the opposite eye via the optic nerves. The thalamus acts as a ‘gatekeeper’ for the neuromodulatory inputs of neurons in the brainstem, and thus regulates the wakefulness of neurons in the cortex. Thalamocortical neurons project from the thalamus to all parts of the cortex and maintain wakefulness by releasing excitatory neurotransmitters, such as glutamate and aspartate, and maintaining a characteristic spontaneous rhythmic electrical activity. At the onset of sleep, inputs from the brainstem hyperpolarize the thalamocortical neurons, which switch to an oscillating pattern of electrical activity that synchronizes cortical neurons to the same pattern—one characteristic of sleep when monitored by an electroencephalogram. This pattern essentially disconnects the cortex from the outside environment. Parts of the thalamus, along with other structures of the limbic system, are involved in the generation and expression of emotions.

2. (in botany) See receptacle.