In the center of the brain is a little structure called the thalamus. It seems to be a center of activity and one of the places where four systems cross: sensory, activation, motor, limbic. Taber, Wen, Khan and Hurley start their paper, The Limbic Thalamus, with the following statement.
“The thalamus has been referred to as the “Grand Central Station” of the brain because virtually all incoming information relays through it en route to the cortex. In turn, virtually all areas of the cortex project to divisions of the thalamus. Thus, knowledge of thalamic anatomy and connections is critical in understanding thalamic influence on cortical function and in the interpretation of functional brain imaging studies.”
The first system is the sensory one. All the sensory information, such as that carried by the optic nerves, enters the thalamus (with the exception of smells). In the thalamus is a map of the retinas that receives the sight information. There is a map of the Corti membranes that receives the hearing information and a map of the body to receive touch information. Pain, visceral feelings and taste come to the thalamus. The sensory information that the cortex receives and processes comes via the thalamus. The axons that run from the thalamus to the cortex are matched by axons running in the opposite direction. Each small area of cortex appears to receive input from the thalamus and also to send its output to the thalamus. This is also true of the associative areas where two senses mix. The associative areas of the cortex are in two way communication with the associative areas of the thalamus. The thalamus also appears to have control over how parts of the cortex communicate with other parts of the cortex.
The second system is the activating one. The nervous system as a whole is the spinal cord and its extension into what is called the brain stem which has a number of structures attached to it, that we call the brain. In the brain stem is a structure called the activating reticular formation. This is an extremely ancient part of the brain. It seems to control the level of alertness: sleep, dreaming, wakefulness, alertness, fatigue, motivation. Waves of activation from the reticular formation seem to keep consciousness (or dreaming) going. When it is quiet, there is a deep sleep. If it is damaged so that it cannot maintain activation, a deep permanent coma results. This structure ends where it merges into the thalamus.
The third system is the motor one. Planning, initiating and controlling action is done by a system that includes the frontal cortex especially the pre-frontal, pre-motor and motor cortex, the basal ganglia and the cerebellum. The thalamus has a motor portion in communication with areas of the frontal cortex. This is two-way traffic as with sensory communication. The cortex can directly send signals to muscles but without modification these result in jerky movements. The cortex works with the basal ganglia and the cerebellum for fine control to give smooth movements. Cortex signals can go directly to the basal ganglia and the cerebellum but the return path goes through the thalamus. The thalamus is therefore one of the important elements of motor control.
Finally there is the limbic system which is primarily concerned with emotion, smell and memory. It is not easy to list its components because different people have different borders to the area, but it at least contains the amygdala (chiefly associated with fear and anger), the hippocampus (associated by memory), the mammillary bodies (associated with memory), the hypothalamus (associated by drives and regulation of the internal body), the entorhinal cortex (associated with smell) and the limbic areas of the thalamus.
Input to the limbic thalamus comes from the amygdale, entorhinal cortex, septal nuclei and mammillary bodies. It is in mutually reciprocal communication with the parietal cortex, prefrontal cortex and cingulated cortex. The thalamus is also important to sensing and responding to pain.
It look very much to me, in fanciful moments, that the thalamus runs the cortex. I think of it as an ancient part of the brain that once was state-of-the-art in sensory perception and cooperated with other ancient parts of the brain to insure that responses were appropriate to situations. Then the thalamus got a brand new PC to use called the neo-cortex. The interaction between the thalamus and the cortex is probably the seat of consciousness.