Scientific Understanding of Consciousness
Consciousness as an Emergent Property of Thalamocortical Activity

Brain Stem


Brainstem is a very small region of the central nervous system and is jam-packed with nuclei and circuitry involved in different functions. (Damasio; Looking for Spinoza, 63)

Some of the brainstem nuclei are tiny, and a minimal variation in the standard anatomy could lead to a significant rerouting of neural signals. (Damasio; Looking for Spinoza, 63)

Brainstem is a complicated place.  It is like the subbasements in skyscrapers, full of pipes, events, wires, and gauges, which are connected to the rest of the building.  They keep everything running smoothly. (Gazzaniga; Human, 280)

Two kinds of nervous system organization that are important to understanding how consciousness evolved:  (1) brainstem, together with the limbic (hedonic) system.  (2) thalamocortical system. Edelman; Bright Air, 117)

Limbic-brain stem system are often arranged in loops; they respond relatively slowly (seconds to months), and do not consist of detailed maps. (Edelman; Bright Air, 117)

Activity in brain stem afferents is associated with more alert behavioral states. (Shepherd; Synaptic Organization of the Brain, 323)


Link to — Brain Stem Nuclei Involved in Homeostasis


Complex human responses composed of simple, stereotyped motor actions

Basic behaviors are organized by the brain stem and consist of relatively simple stereotypic motor responses. (Kandel; Principles of Neural Science, 873)

Feeding involves coordination of chewing, licking, and swallowing, motor responses that are controlled by local ensembles of neurons in the brain stem. (Kandel; Principles of Neural Science, 873)

Infants can cry, smile, suckle, and move their eyes, face, arms, and legs. (Kandel; Principles of Neural Science, 873)

Brain stem can organize virtually the entire repertory of newborn's behavior. (Kandel; Principles of Neural Science, 873)

Simple motor responses can be assembled into more complex behaviors under voluntary control by the forebrain.  The precise patterns of motor response are organized locally in the brain stem reticular formation. (Kandel; Principles of Neural Science, 887)

Reticular Formation

The core of the brain stem is the reticular formation.  It is homologous to the intermediate gray matter of the spinal cord and is likewise complex. (Kandel; Principles of Neural Science, 873)

Reticular formation, a region of the brain stem so named because of its diffuse and relatively nonnuclear appearance. (Kandel; Principles of Neural Science, 332)

Like the spinal cord, the reticular formation of the brain stem contains ensembles of local circuit interneurons that generate motor patterns and coordinate reflexes and simple stereotyped behaviors. (Kandel; Principles of Neural Science, 873)

Intermediate gray matter of the spinal cord contains primarily interneurons that coordinate spinal reflexes and motor responses. (Kandel; Principles of Neural Science, 880)

Reticular formation has long projection axons that ascend to the forebrain or descend to the spinal cord. (Kandel; Principles of Neural Science, 889)

Reticular formation is composed of systems of neurons with specific neurotransmitters and connections. (Kandel; Principles of Neural Science, 889)


The reticular formation is a densely packed cluster of nerve cells containing mostly gray matter inter­laced with fibers of white matter. It is an anatomical region about the size of a little finger, extending through the central core of the brainstem, from the upper spinal cord, through the medulla oblongata and pons, into the midbrain. For the most part, the reticular formation is not distinctly identifiable as a discrete system of CNS nuclei. Rather, it communicates, via networks of interneurons with (1) afferent sensory pathways that course through this ana­tomical region, (2) sensory nuclei and motoneuronal cell groups located in the brainstem, and (3) neural pathways that operate through the autonomic nervous system. In addition, the reticular formation has elaborate connections with the thalamus and hypothalamus.

The interneurons of the reticular formation are very short, allowing messages to be relayed quickly from one nerve cell to the next. This allows it to function as sieve, continuously sifting through the mass of incoming data. Only those sensory inputs that the neural hardwiring construes to be essential, unusual, possibly dangerous, and/or in some sense "action-provoking" are passed along for further processing and forwarding to the higher levels of the brain.

In summary, the reticular formation is best viewed as a heterogeneous collection of distinct neuronal clusters in the brainstem tegmentum that either modulate the excitability of distant neurons in the forebrain and spinal cord, or coordinate the firing patterns of more local lower motor neuronal pools engaged in reflexive or stereotypical somatic motor and visceral motor behavior. (Purves; Neuroscience, 399)

Reticular formation of the brainstem constitutes the major input to intralaminar nuclei. (Afifi; Functional Neuroanatomy, 247)

Arousal is controlled by the reticular activating system, which connects the frontal lobes, limbic system, brainstem, and sense organs. (Ratey; User's Guide to Brain, 115)

Reticular formation is concerned with somatic and visceral motor functions as well as with consciousness, attention, and sleep. (Afifi; Functional Neuroanatomy, 123)



Return to — Brain Anatomy

Link to — Consciousness Subject Outline

Further discussion — Covington Theory of Consciousness