Consciousness
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Introduction
Consciousness has two main components:
Wakefullness
Awareness
Wakefulness
Wakefulness
Awareness
Difficult to assess.
Our assessment of awareness depends on the patient's response.
Video of Teri Shiavo to put her: she is awake, but not apparently aware.
The Reticular Formation
The reticular formation is a widespread network of tracts and nuclei spread across the brainstem that regulates consciousness, arousal, sensation. Single cells can have connections in the spinal cord, thalamus, and hypothalamus at the same time. Some can even synapse directly in the cortex, bypassing the usualy thalamic relay station. This arrangement allows for much convergence of sensory information and divergence of systemic effects.
It is a well-organized collection of white and gray matter, like a net, with multiple nuclei. It contains ensembles of local-circuit interneurons, with noradrenergic, serotonergic, cholinergic, and histaminergic pathways. It can almost act as a CNS almost unto itself, generating motor patterns and coordinating reflexes.
The reticulospinal tracts carry information from reticular nuclei to lower motor neurons in the spinal cord and brainstem. There are both excitatory and inhibitory components, the latter which prevent flexor muscle contraction during reflexes. Meduallry and pontine reticular nuclei appear involved in coordination and control of locomotion, as well as visceromotor control of heart rate, blood pressure, and respiration respiratory rate.
The raphe (ridge) nuclei are flattened nuclei that extend through brainstem parallel to midline. They use serotonin as a neurotransmitter. The midbrain periaqueductal gray, though the nucleus raphe magnus, signals to dorsal horn of spinal cord, producing the spinal gate. The raphe nuclei project to virtually all structures of the CNS, including basal ganglia and spinal cord, to mediate sleep-wake cycles and as well as, potentially, mood and emotional behaviour.
The midbrain reticular formation is involved in arousal and sleep-wake cycles. Spinal cord input about sensory info/activity projects to the thalamus, which has widespread effects on the cortex.
The locus ceruleus is involved in regulation of attention, arousal, sleep-wake cycles, learning, memory, anxiety, pain, mood, and brain metabolism. It is a small nucleus, of only approximately 12,000 cells, located near the floor of the fourth ventricle in the pons and medulla. However, each neuron can have 250,000 synapses, and one cell can send axon branches across the whole brain. As a result, it has tremendously widespread connections to the cortex, thalamus, hypothalamus, olfactory bulb, cerebellum, midbrian, spinal cord. The locus ceruleus uses norepinephrine as an neurotransmitter; likely the primary site of its use in the CNS.
Resources and References
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