The HumanNervous System

Let’s see. Where did we leave off. We are looking for the biological and anatomic bases for thinking, awareness, and consciousness, the drivers of our behaviors. We’ve looked at the level of the atoms and molecules and at the level of the cell – the neuron. Now let’s turn to the level of gross anatomy and talk about the brain. We are building a vocabulary to discuss consciousness, a bridge between what we know and what we are really interested in.

The left and right hemispheres of the brain can be divided into functional lobes that are somewhat anatomically distinct, with names that describe their location. Each lobe has a twin in the other hemisphere. The frontal lobes are responsible for speech and motor behavior.  They lie behind your forehead and above your eyes. The temporal lobes, which are behind your temples (temporal bone of the skull), process hearing and the understanding of language, as well as important aspects of memory as the hippocampus lies deep inside. The parietal lobes (parietal means wall) which are above the temporal lobes and behind the frontal lobes, process sensory information from the body. The occipital lobes (Latin for back of the head), process vision (at least a major aspect of vision to be discussed later). The insular lobes (insular = island) are deep inside the lateral fissure and are involved in visceral processing from the gut. The cingulate lobes are on the inside surfaces of the hemispheres, down inside the longitudinal fissure that divides the left and right hemispheres and are involved in emotion and memory. I’ll break down each lobe in subsequent blogs, but first, let’s look at the brain from the outside-in.

The surface of the brain, called the cortex (Latin for bark), is a coating of cells, six layers deep. You can see this gray matter sheet in dissection of the brain. Each layer has a specific function, for example, input or output. The cell layers are also organized into columns, rows, and hypercolumns (a chunk of columns and rows) that have specific input or function. We will look more closely at how these hypercolumns function in the vision blog. Remember that we are talking about the organizational structure of cells. The patterns of connections among these cells create neural circuits that perform very specific functions, as we will see.

Deep inside the brain are dense clusters of cells that have distinctive dedicated functions, for example, the hippocampus in the temporal lobe, which has important memory functions, as I’m sure you know. We are our memories, the stories that stitch together our lives. However, memory impaired patients are not unconscious. Like the patient with blindsight, they have lost access to a part of their consciousness. They are not aware of the result of certain internal processes.

The hippocampus is a part of a circuit, called the Circuit of Papez, that connects the hippocampus with the midbrain, the thalamus, which we will discuss below, and the cingulate cortex (emotional cortex) and back to the hippocampus. Memory and emotion are neurologically wired together. Perhaps the “I” we are looking for is not in one of these structures, but rises from the activity of this circuit, this loop. Consciousness could be a property that emerges from activity at this local level. In blog 5 we are going to get a better definition of consciousness, and the science of consciousness. So far, I have used it in the folk sense, to mean your awake awareness of your internal processing.

Let’s dig a bit deeper. The word “Hippocampus” is derived from Greek and means seahorse. Having searched for and finding hundreds of these structures in the human, mouse, and sheep brain, I can tell you it looks more like a question mark. The entryway, called the subiculum, is the stalk of the question mark. Information about what you are currently doing, for example, visual, tactic, auditory, and emotional information, enters the subiculum and goes up around the curve of the question mark. Some of the neurons in the curve synapse back onto the neurons in the subiculum, creating a feedback loop. Tangles of proteins in the subiculum can hinder this feedback and is a cause of Alzheimer’s Disease. These patients can lose their social connection to the world, especially their past.

The firing patterns of neurons repeat around the hippocampal circuit and are eventually sent to the mammillary bodies of the midbrain. We won’t get into why they are called mammillary bodies, but since almost all early brain anatomists were male, you can figure it out. Damage to these structures, for example from chronic alcoholism, results in an inability to create short-term memories. These patients can remember the deep past but have no idea what just happened to them. They live in seven second exposures, repeatedly. The mammillary bodies and their connections would normally encode short term memory and send it around the circuit for more processing. The patient with Korsakoff syndrome, as this damage from alcohol is called, has no access to the results of this internal processing. Still, they are conscious, aware of the results of other processes, for example visual, auditory, long-term memory.

That’s a lot of neuroanatomy squeezed into a few paragraphs, but we need the appropriate lens to focus on our subject. Let’s continue our search. Moving closer to the center of the brain are the basal nuclei which look like dark triangles in a horizontal slice through the center of the brain. These pools of neurons are involved in at least five identified circuits or loops, including motivational and emotional loops. The motor loop smooths out our movements, preventing jerky and shaky movement. Parkinson’s Disease affects these structures and their connection to the substantia nigra (substance dark) of the midbrain, which is part of our dopamine circuit. As a result of damage to this loop, the Parkinson’s patient has constant tremors in the hands, called resting tremors. Our motor behavior is produced through the interaction of the motor cortices in the frontal lobe, the basal nuclei, and the cerebellum. Our behavior emerges from the interaction, the connectivity, of these brain structures.

Finally at the very center of the brain, is the thalamus, usually a two lobed structure, surrounded in a cocoon of axons. The thalamus is the hub for sensory processing. Sensory neurons from the spinal cord synapse there, before going to the somatosensory cortex in the parietal lobe of the brain. The thalamus performs other functions related to sleep, consciousness, and motor behavior. Damage to the entire thalamus usually results in coma – a loss of consciousness. These deep brain structures are good candidates for the locus of the ghost. Sensation, movement, and memory are the result of neural activity in several connected structures. Consciousness could be the threads, the axonal connections, stitching our experiences, or phenomenology together.

As we have discussed, the cortex and these deep structures look darker than their surroundings, both in dissection and imaging, and for that reason, are referred to as gray matter. The cortex, along with the internal gray matter, and the gray matter in the spinal cord are richly connected by bundles of axons that are coated with myelin, a fatty insulating sheath. This gives the axonal tracts a whitish appearance, and so are referred to as white matter. We can organize these connections into systems, such as the body sensory system (somatosensory, “soma” meaning on body), the motor system, the visual system, the auditory system, the vestibular system, and the autonomic system. Behavior, injury, and disease can be understood from this organizational approach. Another blog will cover these systems, which will give us more information to help understand consciousness, the brain, and our everyday behavior.

To end this discussion, let me introduce you to the idea of the connectome. I encourage you to search for the term, as some of the photographs you will see are amazing. The human connectome is an attempt to create a complete map of the connections in the human brain, similar to a genome. The connectome has been successfully mapped in the nematode C. Elegans, which is a small worm with sensory neurons, interneurons, and motor neurons. The human map is in progress with multiple research projects, and current maps have been used in characterizing mental disorders, as an example. While mapping the entire human connectome is an ambitious task, it gives us a framework for thinking about the causes of our behavior and the seat of our consciousness. The ghost is not in the machine. It arises out of the neural activity among those parts. Research with the connectome, using computer modeling of functional MRIs, will certainly inform our understanding of consciousness. Science is very close to linking brain activity, behavior, and cognition. We understand much about each of these pieces, but putting them together has eluded us.

We need to finish this story about the brain in the next blog, then we need to turn to thinking about the sets of connections we can see from a functional, organizational approach. Then we can move to the actual behaviors we want to understand.