Neurobiology For Dummies. Frank Amthor

nervous system

      The spinal cord is like a subcontractor of the brain that executes the brain’s instructions and reports on their progress. The spinal cord is part of the central nervous system, contiguous with it as it merges with the medulla of the brainstem. Chapter 6 discusses its basic organization.

      

The spinal cord is the transition below the neck between the central and peripheral nervous systems. The peripheral nervous system includes the motor neuron axons that originate within the spinal cord gray area and project from the cord to synapse on muscle cells. Sensory neurons — whose cell bodies are located in dorsal root ganglia just outside the spinal cord — send one axon collateral to the periphery to elaborate into a sensory receptor, while the other collateral makes conventional synapses in the spinal cord gray area for both local circuit spinal feedback control and for relaying sensory information to the brain.

The autonomic and enteric nervous systems are involved in body homeostasis (keeping the body’s major systems stable and functioning) through controlling glandular secretions, heart rate, respiration, and smooth muscle function. The autonomic nervous system has major subdivisions into sympathetic and parasympathetic branches that tend to oppose each other’s actions. The sympathetic system prepares us for action in the fight-or-flight mode, while the parasympathetic system organizes resources for digestion, and the maintenance and conservation of energy.

      The brainstem, limbic system, hypothalamus, and reticular formation

      When we look at a human brain from above, almost all that we see is neocortex. Students beginning to study the brain often mistakenly think that the neocortex is the real, important part of the brain that has largely superseded phylogenetically older structures that are now almost vestigial and unnecessary (like the appendix).

      This is an understandable mistake. However, non-mammalian vertebrates like lizards, frogs, and crocodiles execute complex behavior without any neocortex. Some mammals have very little neocortex as well. The relationship between the neocortex and “lower” brain areas is as much their servant as master, an idea Chapter 7 explores.

      The brainstem is not only a transition region between the spinal cord and higher brain centers, but an essential integration and control center by itself. The brainstem includes the medulla at the intersection with the spinal cord, the pons just above the medulla, and the midbrain above that. The cerebellum hangs off the back of the brain behind the pons. The brainstem nuclei convey information between the senses and the spinal cord and higher brain centers. Brainstem nuclei also control essential aspects of homeostasis such as the regulation of heart rate, respiration, and temperature.

      

Limbic system is an archaic term for a diverse set of subcortical brain areas that are thought to control instinctive behaviors. Areas included in the limbic system’s original formation include the hippocampus, amygdala, and cingulate cortex. Chapter 7 discusses how these areas interact with neocortex and other parts of the brain, not as a modular system, but as a set of crucial brain areas each with distinct functions.

      The hypothalamus sits just above the pituitary gland and receives sensory input from the autonomic nervous system. It controls many homeostatic processes (such as circadian rhythms — the body’s internal “clock”) by secretions of hormones into the bloodstream and by projecting to the pituitary, which itself secretes hormones.

The reticular formation is a diffuse network of neurons and axon tracts that runs through the brainstem up into portions of the thalamus. This area controls body state through processes such as controlling wakefulness versus sleep, alertness, and homeostatic mechanisms such as heart rate.

      Basal ganglia, cerebellum, motor and premotor cortex, and thalamus

      Chapter 8 takes up the basal ganglia, major controllers of behavior. The major basal ganglia nuclei include the caudate and putamen, which together make up the input region called the striatum. The globus pallidus is the major basal ganglia output to the motor portion of the thalamus, which projects and receives input from motor areas in the frontal lobe. The basal ganglia nuclei interact extensively with the substantia nigra in the midbrain, and the subthalamic nucleus.

      The cerebellum is a motor learning and coordination center. It receives sensory input from spinal sensory neurons and cranial nerves of the vestibular and visual systems. Its major output is to motor thalamus that projects to frontal motor cortex. The cerebellum is necessary for learning coordinated, well-timed movements. It operates as a feed-forward controller that generates error signals used to reprogram motor areas such as premotor cortex to generate appropriate limb movements.

      Two frontal areas just anterior to primary motor cortex, the supplementary motor area (SMA) and premotor cortex (PMC), contain motor programs that command and coordinate multi-limb movements to accomplish goals. One main difference between these two areas is that motor programs in SMA tend to be those that we can learn to do with little sensory feedback, such as typing. PMC control tends to occur when sequences are being learned, and depends more on peripheral feedback and cerebellar error signals.

      The thalamus is often called the gateway to the neocortex, since all senses — except for some of olfaction (the sense of smell) — relay through it. But the neocortex projects back extensively to the thalamus. These back projections come from “higher” cortical areas, as well as from the primary areas that receive inputs from that area of the thalamus.

The gateway metaphor for the thalamus implicitly makes the neocortex the real seat of neural control and computation. The thalamus is the modulator of transmission to the cortex, emphasizing some pathways at the expense of others as a mediator of attention. A different metaphor for thalamic-cortical interactions is that the thalamus is running the “main” program, which makes “subroutine” calls to the neocortex for some detailed neural computations. This makes processing in the thalamus the primary controller of brain activity, including consciousness (see Chapter 14). Activity in the neocortex becomes the content of that consciousness. It’s too early to say whether this subroutine metaphor will be as useful as the gateway metaphor has been.

      The neocortex

      The neocortex is one of the most important “inventions” of mammals. It dominates the mammalian brain in volume, particularly in primates. One of the most remarkable properties of the neocortex is that it has the same six-layered structure virtually everywhere, with the same cell types in what appears to be the same general minicolumn circuit. This is in stark contrast to the rest of the brain, where each area tends to have its own distinct set of cell types and neural circuits.

      

Mammals became the dominant land animals on earth after the demise of the dinosaurs about 65 million years ago. Some neurobiologists conjecture that mammals were able to rapidly diversify into all the niches abandoned by the extinction of the dinosaurs, as well as many new ones, by expanding the standard neocortex circuit for processing whatever visual, auditory, or fine motor acuity that niche demanded.

      Neocortical processing power is primarily a function of area. Increased area in neocortex has two main uses:

       Increasing “acuity,” whereby, for example, a larger area can support a higher density of peripheral receptors, such as retinal ganglion