ABCs of psychobiology, The

Perspectives in Psychiatric Care, Jul-Sep 2003 by Keltner, Norman L

This column may be more conversational in tone than previous columns because I want to outline a few basic concepts that are needed if you are to be conversant in today's changed world of psychiatric nursing.

First, calling an article the ABCs of anything might suggest a certain level of arrogance on the part of the author. If the title provokes that image for you, discard it immediately. This writer is very clear about the mountains of information he has yet to climb. Second, there is only so much that can be covered in a few pages, but over the years I have learned that psychiatric nursing practice improves significantly when nurses understand the basics of brain biology. Such knowledge informs our understanding of mental disorders, improves treatment with psychotropic drugs, and helps shift our thinking in an empathic direction. Thus a brief review of the ABCs of brain biology is presented: Anatomy, Biochemistry, and Connections.

Anatomy

The adult brain weighs about 3 to 4 lb, but weighs only a quarter of that at birth. Neuronal numbers change little, if at all, over the years between birth and adulthood, but brain size and weight change dramatically. A significant portion of the size and weight is related to axonal and dendritic growth patterns that result in a quadrillion synaptic connections. These connections occur over time in a development sequence orchestrated in a manner some have likened to a hound dog following a trail. Somehow, in healthy individuals, axons find the right targets, covering "microscopic miles" to do so. As noted, these connections or pathways are programmed before birth to occur in a relative stable sequence, the last major connections occurring in late adolescence to early adulthood. It is thought that an insult (of many origins) can occur to a particular pathway and remain relatively "silent" for years.

Knowledge of certain brain nuclei is important for psychiatric nurses. Just as it is often dangerous to start thanking people at certain functions due to the risk of leaving someone unmentioned, I also run the risk of omitting brain sites you deem crucial. Nonetheless, the basics, in my opinion, include a discussion of the major divisions of the brain, cortex, basal ganglia, motor strip, hypothalamus, hippocampus, and nucleus accumbens.

Major divisions of the brain. The brain is divided into the cerebrum, brainstem, and cerebellum. The cerebrum is further divided into four lobes: frontal, temporal, parietal, and occipital. Although perhaps an oversimplification, the frontal lobe is associated with cognition, movement, emotions, and inhibition. The temporal lobe is associated with auditory functions, the occipital lobe with vision, and the parietal lobe with sensations from the periphery. The brainstem is composed from the top (rostrally) down (caudally) into the midbrain, pons, and medulla. As is noted later, three important neurotransmitters are synthesized in the brainstem. The cerebellum is a poorly understood structure, yet it contains more neurons than the rest of the brain combined. It is known that cerebellar lesions cause impaired movement.

The cortex. The cortex is the outer layer of gray matter (i.e., neuronal cell bodies and dendrites) and is described as the workhorse of the brain. It is about one-eighth of an inch thick. Our brain surface dips in and out of grooves (sulci), permitting more space for its work. The fact that I write this article and you can read it testifies to functioning cortices.

The basal ganglia. The basal ganglia are subcortical nuclei near the center of the brain. They are subcortical because they are located beneath the cortex. They are nuclei because they are composed of gray matter. Both idiopathic Parkinson's disease and extrapyramidal side effects (EPSEs) result from impaired basal ganglia function.

The motor strip (aka precentral gyrus). The motor strip lies on the surface of the brain and is the most dorsal frontal gyrus. It is located just in front of the central sukus, the distinct fissure that separates the frontal and parietal lobes. The motor strip drives voluntary movement. Lesions affecting the motor strip neurons or their proximal extensions cause opposite side (contralateral) movement disruption, including paralysis.

The hypothalamus. The hypothalamus controls many essential functions and attempts to maintain homeostasis. Temperature regulation, satiety centers for thirst and hunger, and release and inhibition of pituitary hormones are some of its critical functions.

The hippocampus. The hippocampus is most commonly associated with memory and learning. Injury to the hippocampus affects these essential activities. Individuals suffering from Alzheimer's disease experience hippocampal atrophy. On magnetic resonance imaging, this finding is readily detected by noting enlargement of the inferior horn of the lateral ventricles.

The nucleus accumbens. The nucleus accumbens is a small subcortical nucleus that becomes the final common pathway for pleasurable sensations. It is perhaps the most important anatomical point on the so-called reward or pleasure pathway. It has been said that pleasurable experiences from snorting cocaine, getting promoted, and enjoying sex can be reduced to a burst of dopamine in this nucleus. But, due to the stronger impact of drugs and alcohol on this system, substance abusers quickly reach a point where normal highs (e.g., a job well done) no longer trigger the system effectively.