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Parts of the Brain
Physiologists have divided the brain into sections depending on
the apparent function of that part of the brain.
The frontal lobe, containing the prefrontal cortex which controls
higher-order cognitive functions including planning and decision making, problem solving, abstract rule learning, cognitive flexibility, and spatial working memory. The
parietal lobe is associated with perceiving tactile sensory information such as pressure,
touch, and pain. The occipital lobe interprets visual information,
and the temporal lobe interprets sounds and language. The temporal lobe is linked to memory since it includes the hippocampus.
The cerebellum is located at the rear of the brain and is critical
for fine motor control.
Memory is one of the important functional aspects of the central nervous system (CNS) and is categorized as sensory, short term, and long-term. Sensory memory is dependent upon the parietal and temporal lobes. Short-term memory is dependent on the function of the prefrontal and parietal lobes, while long-term memory depends on the function of larger areas of the brain.
Prefrontal Cortex
The prefrontal cortex (PFC) is located at the front of the frontal lobe. It is implicated in a variety of complex behaviors, including planning, and greatly contributes to personality development. Studied under stress, it has provided important clues to age-related loss of resilience and impaired memory as well as effects of circadian disruption and extinction of fear memory. Within the prefrontal cortex chronic stress causes some neurons (medial PFC neurons) to lose branches and shrink dendrites. However, some types of neurons (orbitofrontal cortical neurons) experience expanding dendrites which may be related to increased vigilance.
The Temporal Lobe
The temporal lobe is involved in primary auditory perception, such as hearing, and holds the primary auditory cortex. It is home to a number of glands and clusters of neurons with specific functions. The amygdala is linked to emotions. The striatum, in addition to its role in control of motivated movement, is also involved in working memory, abstract rule learning, and attention control. The hippocampus is responsible for memory storage and is critical for the formation and consolidation of declarative (factual) memories. Cognition means reception and perception of perceived stimuli and its interpretation, which includes learning, decision making, attention, and judgment, which is mainly formed in the hippocampus, amygdala, and temporal lobe.
Hippocampus
Total function of memory and the conversion of short-term memory to long-term memory are dependent on the hippocampus, an area of the brain with the highest density of glucocorticosteroid receptors and also represents the highest level of response to stress. Glucocorticoids are a class of corticosteroid hormones more commonly known as glucocorticoids. They bind to glucocorticoid receptors in the hippocampus and are necessary to improve learning and memory. Studies have shown that stress can cause functional and structural changes in the hippocampus including atrophy and neurogenesis disorders. Chronic stress and, consequently, an increase in plasma cortisol, leads to a reduction in the number of dendritic branches, and neurons, structural changes in synaptic terminals, and decreased neurogenesis in hippocampus tissue. Glucocorticoids induce these changes by effecting the cellular metabolism of neurons, increasing the sensitivity of hippocampus cells to stimulatory amino acids, and/or increasing the level of extracellular glutamate. Excitatory amino acids, particularly glutamate, play a key role in structural as well as functional changes in the brain since glutamate is the major excitatory transmitter. At the same time, excess causes damage and inflammation.
Amygdala
The amygdala, thought to be part of the limbic system, produces the emotional experiences of memory. Within the amygdala are two hormones involved in the memory process that responds to daily stress. There is a mutual balance between them for creating a response in the memory process. First, noradrenaline, a hormone and neurotransmitter, creates emotional aspects of memories stored in the basolateral amygdala area. Second, corticosteroids facilitate the memory process. However, if high levels of chronic stress cause excessive release of corticosteroids, noradrenaline effectiveness is suppressed. This can cause a negative effect on memory formation in the amygdala.
Basal Ganglia
The basal ganglia are involved in a wide range of processes such as emotion, reward processing, habit formation, movement, and learning. This part of the brain is particularly involved in coordinating sequences of motor activity, as would be needed when playing a musical instrument, dancing, or playing basketball. It is the section of the brain along with the substantia nigra pars compacta and locus coeruleus are most effected by Parkinson’s disease.
Parietal Lobe
The parietal lobes have two roles:
- processing sensation and perception from the outside world (especially touch, taste and temperature), as with perceiving pain, and
- integrating sensory input, primarily with the visual system, but including our sense of the position of our limbs.
Damage to the parietal lobe may lead to dysfunction in the senses.
Cerebellum
The cerebellum is a separate structure located at the rear of the brain and it is critical for fine motor control. It controls and coordinates fine voluntary movement such as balance, posture, coordination, as well as speech and learning.
Brain Cells
The neuron is the brain cell at the core of our ability to process information. The adult brain contains an estimated 100 billion neurons which communicate with each other through junction points called synapses in order for us to think and to send messages to other cells in the body. Neurons have dendrites that branch out like tree branches into secondary and tertiary dendrites. Each of these dendrites creates thousands of synapse connections with other neurons. Neurons fire impulses at a rate of approximately 10-100 times per second, depending on the type of neuron. Each neuron is connected to an average of 7,000 – 10,000 other neurons (and some more).
The human brain is capable of forming new connections between neurons. When we take in new information, an electro-chemical signal is sent across the space between neurons (called the synaptic space). This ability of the brain to form new connections or neural pathways to communicate with each other is often referred to as brain plasticity. Brain plasticity is now understood to be the very foundation of learning and memory. Through the mid-1990’s, it was thought that the brain was not capable of generating new neurons and neural passages. This theory has been now totally debunked. Neurogenesis, and the growth of nerve cells is now part of our understanding of how the brain regenerates parts of itself and maintains its plasticity.
Microglia
Microglia are the resident macrophages (large cells) and primary immune cells of the brain, and they have a multitude of functions, including attacking and consuming bacteria (phagocytosis), removing waste, providing neuroprotection, and contributing to the growth of new neurons. They interact with a number of cell types, including astrocytes, neurons, and endothelial cells.
Glial Cells
Glial cells maintain homeostasis, form the myelin sheath that protects nerve cells, and provide support and protection for neurons. In addition, they support synaptic contacts and the signaling abilities of neurons. Glia are more numerous than nerve cells in the brain, outnumbering them by a ratio of perhaps 3 to 1. They include star-shaped astrocytes which maintain brain homeostasis and nerve metabolism, oligodendroglia cells which maintain the myelin covering of nerve cells, and satellite cells that help regulate the chemical environment.
