A Quick Tour through the Brain
Begin Small: Neurons
The brain is made up of neurons. Neurons are the functional units in the brain and throughout the nervous system. There are more than 180 billion neurons with at least 80 billion involved in cognitive processes. Each neuron connects with hundreds of other neurons, and so we have vast potentials for an enormous amount of interactions occurring simultaneously.
Neurons have two main functions: They process certain chemicals within them, and they communicate with other neurons. This communication process of inputs, integration, and outputs occurs across the synaptic gap between neurons. When the gap is close enough, the electrical signal can simply leap across and keep going. But more often, the gap is too large for this to happen. With the larger gaps, the electrical signal is converted into a chemical, known as a neurotransmitter, and the neurotransmitters swim across the gap to then be converted back into an electrical impulse when they reach the other side
We have a number of different kinds of neurotransmitters. Glutamate is an excitatory neurotransmitter that is found throughout the nervous system. GABA (gamma aminobutyric acid) is inhibitory and, like glutamate, is found everywhere in the nervous system. Certain neurotransmitters are more specific in what they communicate. For example, dopamine is related to pleasure and reward. Serotonin involves emotionality and sleep patterns, norepinephrine influences alertness, and endorphins alleviate pain. The neurotransmitter system usually has everything it needs already built in. But when the neurotransmitters are out of balance, drug therapy and psychological treatments such as meditation, hypnosis, and psychotherapy can stimulate or inhibit processes to stimulate a better balance. Medications for psychological problems such as depression and anxiety are just acting to stimulate the neurons to produce more of certain neurotransmitters or to block the action of other neurotransmitters.
The transmission across the synapse either activates the neuron to fire or deactivates it from firing. When neurons fire together repeatedly, these neurons tend to become wired together, known as HebbÕs Rule, forming a stronger synaptic connection. This firing and wiring strengthening, known as LTP (long term potentiation), explains, at a neuronal level, how learning and memory occur. It also helps to account for neuroplasticity.
The Central Nervous System and
Peripheral Nervous System
All the neurons combined make up the nervous system, consisting of the central nervous system (brain and spinal cord) and the peripheral nervous system (autonomic nervous system, cranial nerves, and spinal nerves). The peripheral nervous system extends through the whole body and communicates information to and from the central nervous system. The autonomic nervous system interacts closely with the central nervous system, often automatically and unconsciously.
The neurons of the autonomic nervous system include two key systems: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system prepares the body for vigorous action. The parasympathetic nervous system acts as an opposite to the sympathetic nervous systemÕs activations. So, when the sympathetic activation constricts blood vessels or inhibits digestion during exercise, the parasympathetic system relaxes vessel walls and stimulates digestion when the workout is over. Both systems work together to help foster appropriate responses. These systems of activation and deactivation are involved in emotions such as fear and anger, as well as participating in responses to stress and feelings of enjoyment. Together, these two systems maintain the control that keeps the mind, brain, and body in balance. Yoga breathing, postures, and meditations can shift the balance in the autonomic nervous system.
Brain Structures and Functions
The brain orchestrates the nervous system. It is often described in terms of its structures and functions. Unconscious processing tends to travel a short, subcortical path through the lower brain areas, known as bottom-up processing that does not engage the higher level-processing cortex. For awareness of emotions, sensations, and cognitions, the information usually travels a long path, sometimes called top-down processing, involving higher parts of the cortex.
Lower Brain Areas: Brainstem and Cerebellum
At the base of the brain is the brainstem, the transition between the spinal cord and the brain. This area is important in regulating vital body functions such as breathing, heart rate and other automatic functions. These lower brain areas coordinate their action with many other regions of the brain.
The cerebellum (Latin for little brain), located at the back of the neck, has two hemispheres with functional sections in each, known as lobes. The cerebellum interacts closely with other parts of the brain through loops of interaction. It serves a variety of functions including the regulation of higher cerebral processes in motor planning, cognition, involuntary functions, and problem solving. It also regulates posture and the command of movement. We have all experienced the effort required to learn new movements, such as playing a sport or mastering a dance pattern. During the learning period, the cerebellum is active. Once movement control is mastered, the cerebellum becomes less active and other parts of the brain get involved.
Interior Brain Areas: Basal Ganglia and Limbic System
The region that spans the area from the brainstem to below the cortex in the interior of the brain is the limbic system for emotions and the basal ganglia for voluntary movement and coordination.
The basal ganglia form a C-shape of four interconnected structures: the substantia nigra, the caudate nucleus, the putamen, and the globus pallidus. These structures are also involved in planning movement, performing movements in sequence, and maintaining learning. This area is also part of predictive control, attention, and working memory.
The limbic system has been given much attention for therapy since it is intimately involved in regulating emotion, fear conditioning, fight or flight and stress responses as well as learning, and memory. Many structures play a central role in the limbic system. The most central ones include the amygdala for emotions, the hippocampus for learning and memory, the hypothalamus for regulating many autonomic functions including biological rhythms and stress, and the thalamus as a gateway for sensory information. Several other structures are considered important for some aspects of emotion, and thus are the olfactory cortex, involved in the sense of smell, the pituitary gland regulating hormones, and the nucleus accumbens, important for reward, laughter, pleasure, addiction, and the placebo effect. Two cortical areas are also strongly linked to the limbic system: the cingulate gyrus for monitoring conflicts and the orbitofrontal cortex (part of the pre-frontal cortex). All of these structures interconnect and interact together, although some contribute more to one function than to another. With so many varied brain structures all closely interacting functionally with each other as well as with higher cortical functions, it makes sense as to why emotions play such an important role in every aspect of living.
Higher Brain Areas: The Cerebral Cortex
The cerebral cortex is the outer layer of the hemispheres with many convolutions, gyri, and folds, sulci. Folding increases the surface area of the cortex, so that more than two thirds of the surface is hidden from view. The cortex is sometimes referred to as the higher part of the brain. Each hemisphere is divided into four lobes: the frontal, parietal, temporal, and occipital. The lobes monitor different functions, although they are all interrelated, interacting together. The fibers that connect the two hemispheres are called the corpus callosum, which helps the two hemispheres to be able to communicate with each other.
The frontal lobe accounts for nearly one-third of the cerebral cortex. The prefrontal cortex is located at the front of the frontal lobe behind the forehead. The fact that the prefrontal areas have extensive links throughout the brain shows how interrelated brain functions really are. Areas of the prefrontal cortex are involved in executive functions that include planning, higher-level decision-making, sequencing, and goal directed behavior. Independent thinking is processed in this area as well. Another part of the prefrontal cortex processes personality characteristics such as the experience of empathy, socially appropriate behavior, and emotional control.
The primary motor area of the frontal cortex is located in the back (posterior) area of the frontal lobe. It is important for control of movement. This area has a map of the body on it. Larger portions of the cortical map are devoted to areas that are used more, such as the hands and face. The non-primary motor cortex, located in front of the primary motor cortex, includes a pre-motor area and a supplementary motor area. These areas are involved with movement and coordination in general, such as stance, gait, and initiation of voluntary movement sequences. Mirror neurons, are located in the motor area of the frontal lobe as well. These neurons are involved in understanding and empathizing with the intentions and actions of others and help with social understanding.
One other important area located in the frontal lobe is the cingulate gyrus, also called the cingulate cortex. This area is involved in motivated behavior, spontaneity, and creativity. Complex behavior and attention or conflict monitoring, are also processed in the cingulate gyrus. This area is primary during the emotional reaction to pain and in the regulation of aggressive behavior. It has also been found to play an important role in maternal attachment as evident in behaviors like nursing and nest building in animals.
The Parietal Lobe
Behind the frontal lobe and close to the cingulate gyrus is the parietal lobe. This lobe is involved in sensation and perception of touch, pressure, temperature, and pain. Sensory information from the body is correlated there during perception or cognition of a sensation. The parietal lobe is activated when locating objects in space and mapping the relationship of the body to the world. The back (anterior) portion of the parietal lobe is the sensory strip. The body is mapped on the sensory strip for sensations, similar to how the primary motor cortex is where movement is mapped for the body.
The Temporal Lobe
The temporal lobe houses the primary auditory cortex. It is located near the temples and moderates auditory information. WernickeÕs area, on the left hemisphere side, plays a larger role in understanding spoken language. Although most of the visual processing occurs in the primary visual areas in the occipital lobe, some visual processing is performed in the temporal lobes, involving perception of movements and face recognition.
The Occipital Lobe
The occipital lobes are located in the posterior region of the brain. Axons coming from visual input from the eyes pass through the thalamus and are directed to the primary visual cortex. The visual cortex is also sometimes called the striate cortex because of its striped appearance. Human beings rely on their vision quite heavily, and this is revealed in the complexity of this region of the brain. There are more than thirty-two zones for visual processing differentiating different aspects of seeing such as color, texture, and movement. All are located in the occipital lobes.
How the Brain Areas Work Together
So, how do all of these brain areas function? Senses provide a window to the world. First, the receptor organs (such as the eyes, fingertips, nose, tongue or ears) detect a stimulus. Each sensory system has its own pathway that sends the signal to the cortex. The signal registers on receptor fields for the particular sensory modality in a cortical map, located on the cortex. Maps can change depending on what stimuli are experienced. Maps that are used more often tend to grow larger. Attention, regulated in the frontal lobe, is what helps to notice important stimuli and ignore others, such as attending to reading while ignoring rain sounds on the roof.
The central nervous system is a complex collection of structures and functions that are organized in pathways. Thoughts, feelings, and behaviors are intimately involved in the flow of these pathways, dynamic systems of interactions between brain structures and the flow of energy and neurotransmitters.
A number of pathways through the nervous system help to coordinate the mind-brain-body balance.
One pathway processes sensory input and has a special pathway to process painful stimuli.
A reward pathway regulates positive emotions and drives toward fulfillment, satisfaction, and enjoyment.
The fear pathway, also called the HPA pathway, provides the capacity to respond to threat and then return to homeostatic balance. (When over-activated, the fear pathway becomes a stress pathway. Regulatory systems control the appetite and the sleep-wake cycle as well. When any of these systems are out of balance, disorders and problems tend to develop. All of these systems can be altered using the therapeutic use of meditation, hypnosis, as well as psychotherapy.