When studying the many powerful entheogens that are found the world over, it is very common that one comes across information about how particular plants and chemicals effect the brain at a cellular level. In order to fully study entheology, it is extremely important to have at least a very basic amount of knowledge regarding the structure of the nervous system. Understanding the structures of the nervous system will allow the amateur enthnobotanist much greater ability to comprehend both the mechanisms of particular plants and the ways in which these plants can be worked with safely and effectively.
The Nervous System
The human nervous system is broadly divided in to two sections, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is made up of the brain and the spinal cord. The PNS is made up of the nerves which radiate out from the spinal cord to the rest of the body.
The PNS is further divided in to the somatic nervous system, which is responsible for voluntary control of body movements, and the autonomous nervous system (ANS), which is responsible for the unconscious regulation of the internal organs, glands and hormones. Finally, the ANS is divided in to the sympathetic and parasympathetic nervous systems. The sympathetic nervous system is generally responsible for holding the body together in times of stress, and is well known for mediating the stress response known as the fight-or-flight response. The parasympathetic nervous system, on the other hand, is responsible for the stimulation of activities that occur when the body is at rest and in low stress states, including sexual arousal, digestion, excretion, and so forth (Blakemore & Frith 2005).
The main mass of the brain is the cerebrum, which is divided in to the left and right hemispheres. The right hemisphere controls the left side of the body, and vice versa. The right side of the brain seems to be primarily involved in spatial orientation, abstract thoughts, creativity, music and rhythm and so forth. The left side of the brain seems to primarily process verbal language and rational thoughts. In men, both lobes are of approximately equal size, while in women the left hemisphere is slightly bigger than the right (Boeree 2011).
The hindbrain is thought to be the oldest part of the nervous system structure, and is made up of the cerebellum, the pons, and the medulla oblongata. The cerebellum is located underneath the cerebrum, and is primarily a center of movement control, regulation and coordination, and balance and posture.
The brain stem lies just underneath the cerebellum, and is connected to both the cerebellum and the cerebrum. It is through the brain stem that messages are sent from the brain to the rest of the body, and that messages from the rest of the body are received by the brain.
The medulla oblongata is another ancient part of the brain, which is involved in a wide range of sensory and motor functions.
Finally, the pons is a small area of the hindbrain which relays signals from the forebrain to the cerebellum. It also seems to be involved in sleep, respiration, swallowing, bladder control, hearing, and facial expressions (Voogelbreinder 2009).
The midbrain is located just above the hindbrain and below the forebrain, right around the center of the brain. It is primarily associated with vision, hearing, motor control, sleep/wake cycles, arousal and temperature regulation (Boeree 2011).
The forebrain is the largest part of the brain, and is primarily composed of the cerebrum (made up of the temporal lobe, frontal lobe, parietal lobe and occipital lobe). The forebrain also contains the thalamus, the hypothalamus, the pituitary gland, the amygdala, the corpus callosum, the hippocampus and the basal ganglia (Voogelbreinder 2009).
The function of the thalamus includes relaying sensation, spatial sense, and motor signals to the cerebral cortex, as well as the regulation of the sleep/wake cycle and alertness. All sensory systems, other than the olfactory system, relay information to the thalamus, which is believed to process the sensory information and relay it to other parts of the brain (Boeree 2011).
The primary function of the hypothalamus is to link the nervous system to the endocrine system by way of the pituitary gland. It creates and secretes neurohormones which stimulate or inhibit the secretion of pituitary hormones. Thus, the hypothalamus controls body temperature, appetite, fatigue, sleep and circadian rhythms (Boeree 2011).
The Pituitary Gland
The pituitary gland is an endocrine gland which is connected to the hypothalamus. It secretes hormones which help to control growth, blood pressure, uterine contractions, sex organ function, thyroid gland function, metabolism, and temperature regulation, among other things (Boeree 2011).
The amygdala are small groups of nuclei located within the temporal lobes of the brain. The amygdala perform a primary role in the processing and memory of emotional reactions. It is thought that the amygdala are essential in the formation and storage of memories associated with emotional events, as in fear conditioning (Voogelbreinder 2009).
The Corpus Callosum
The corpus callosum is a bundle of neural fibers that is located beneath the cortex and which connects the right and left cerebral hemispheres, allowing for inter-hemispheric communication. It is the largest white matter structure in the brain (Boeree 2011).
The hippocampus plays an important role in the consolidation of information from short-term to long-term memory. It is also an essential part of spatial navigation. The hippocampus is one of the first parts of the brain to suffer damage in cases of Alzheimer’s disease, and damage to the hippocampus leads to memory problems and disorientation. Individuals suffering from serious hippocampal damage sometimes experience anterograde amnesia, that is, the inability to form or retain new memories (Boeree 2011).
The Basal Ganglia
The basal ganglia are a group of nuclei located at the base of the forebrain. They are associated with a variety of functions, including procedural learning (habit formation), eye movements, emotions, cognition and voluntary motor control. Research suggests that the basal ganglia is the part of the brain that is used when one has to make a decision as to which of several possible actions should be executed at a particular time. It seems that the basal ganglia inhibit a number of different motor systems, and when the basal ganglia stops that inhibition, those systems are allowed to become active. This mechanism is influenced by signals from the prefrontal cortex, which plays a key role in executive functioning (Voogelbreinder 2009).
The cerebrum is the largest portion of the brain, and also the newest. The surface of the cerebrum is known as the cerebral cortex, and it is made up of six layers of neurons. It is heavily folded and convoluted, and if it were spread out it would take up about 2.5 square feet of space. It contains around 10 billion neurons and about 50 trillion synapses. The folds of the cortex have ridges, called gyri, and valleys, called sulci. Some of the sulci are very long and pronounced, and these serve as boundaries between the four divisions of the cortex that are known as lobes (Blakemore & Frith 2005).
The Frontal Lobe
The frontal lobe is located at the front of the brain, and is responsible for voluntary movement and planning. It contains the most dopamine-sensitive neurons in the cerebral cortex. Since the dopamine system is associated with reward, attention, short-term memory, and planning, it is not surprising that this portion of the brain is the most heavily associated with decision making. At the back portion of the frontal lobe is an area called the motor cortex. This portion of the brain is though to control voluntary muscle movement (Boeree 2011).
The Prefrontal Lobe
The very foremost portion of the frontal lobe is known as the prefrontal lobe. This portion of the brain seems to be involved in planning cognitive behaviors, decision making, personality expression, and moderating correct social behavior. In other words, it is thought that the prefrontal lobe coordinates thoughts and actions with internal short and long term goals. It is also thought to be the portion of the brain which carries out executive functions, which allow the individual to recognize future consequences of present actions, to choose between good and bad actions, to suppress unacceptable responses, and to see differences and similarities between things (Boeree 2011).
In 1936, neurologists Freeman and Watts created a method of lobotomy known as the Freeman-Watts procedure, which separated the prefrontal lobe from the rest of the brain. This process was thought to be helpful to individuals with serious mental disorders, as it seemed to calm them down. However, it also completely removed the ability of these individuals to make their own decisions in any way, making them generally docile but also extremely mentally disabled in most cases (Shorter 1997).
The Parietal Lobe
The parietal lobe is located behind the frontal lobe. The foremost portion is known as the somatosensory cortex, and this portion of the brain seems to be responsible for receiving sensory information from all over the body. The parietal lobe integrates sensory information, particularly in relation to spatial sense and navigation. It also maps objects perceived visually into body coordinate positions (Boeree 2011).
The Temporal Lobes
The temporal lobe is located on both sides of the brain, and is involved in auditory perception. It is also very important in the processing of language in speech and vision. The temporal lobe also contains the hippocampus which, as noted above, is essential in the formation of long-term memory. In the left temporal lobe there is a portion of the cortex known as Wernicke’s area which is essential for the understanding of written and spoken language (Boeree 2011).
The Occipital Lobe
The occipital lobe is located at the rear portion of the brain and is the brain’s visual processing center. It is also the part of the brain from which dreams seem to originate (Boeree 2011).
Hopefully, this article will help to clarify any questions you may have regarding the structure of the brain and the rest of the nervous system. You may also be interested in information on neurotransmitter systems, or on the alkaloids found in entheogens that produce psychoactive responses in those systems.
Boeree, C.G. “The Cerebrum”, June 28, 2011. http://webspace.ship.edu/cgboer/genpsycerebrum.html.
Shorter, E. A History of Psychiatry. Hoboken: Wiley, 1997.
Voogelbreinder, Snu. Garden of Eden: The Shamanic Use of Psychoactive Flora and Fauna, and the Study of Consciousness. Snu Voogelbreinder, 2009.