Week 5: The basics of brain anatomy, function and brain processes

In this week, we will look at the brain.  Our lesson will Include the structure of the brain (the basic components and their role in cognition), some of the vital systems (such as the reward system, the memory system, and the stress response system), some key players (neurons, neurotransmitters, and an explanation of plasticity) and finally how a healthy lifestyle impacts our brain health.  At the end of the lesson, you should have a working understanding of your brain and how it contributes to your physical and mental health.

Cerebrum:

The cerebrum is the largest part of the brain.  It is responsible for cognitive functions like thinking, learning, memory, and decision making.  The outer layer of the cerebrum (cerebral cortex) is comprised of folds which increase the surface area, allowing for more neurons to be included, which increases the processing and storage capacity in the brain.

It is divided into two hemispheres (left and right) and further divided into four lobes:

·         Frontal lobe:  Decision-making, problem solving, planning, and some aspects of your personality and behavior

·         Parietal lobe:  Processes sensory information (touch, temperature, and pain) and relationship of spatial awareness and coordinating movements to manipulate objects

·         Temporal lobe:  Involved in processing auditory information as well as memory and emotion

·         Occipital lobe: Responsible for visual processing

Cerebellum:

The cerebellum is sometimes referred to as the “little brain”  and is in the back of the brain, underneath the cerebrum.  It is critical in motor control, coordination, precision, and the timing of movements.  It fine-tunes motor activities, resulting in smooth and balanced muscular activity.  It also assists in maintaining posture and balance.  The cerebellum is essential for everyday activities that require coordination.

Brainstem:

The brainstem is in the base of the brain and bridges the various parts of the nervous system.  It is responsible for regulating autonomic functions like breathing, heart rate and blood pressure, relaying information between the brain and spinal cord, controlling sleep and consciousness and integrating reflexes to auditory and visual stimuli, which helps coordinate movements of the eyes and head in response to sensory input. 

The stress-response system:

The stress response system is also known as the hypothalamic-pituitary-adrenal axis (HPA) and provides a network of communication among glands, hormones and parts of the brain related to the stress response.  It regulates the production and release of cortisol and other stress hormones, which prepare the body for threat by enabling things like increasing the heart rate, blood pressure and energy supplies.  Although it enhances survival in short term responses, chronic activation can lead to health problems including anxiety, depression, and cardiovascular disease.

The memory system: 

The memory system enables the encoding, storage, and retrieval of information.  It allows us to learn from experience and adapt to the environment.  It includes several types of memory (sensory memory, short-term memory, long-term memory) and involves various brain regions including the hippocampus, amygdala, and areas of the cerebral cortex.  Each area plays a specific role in the processing and recall of memory.

The executive function system:  

This system uses a variety of cognitive processes that control and regulate thoughts, actions, and emotions.  It is crucial for goal-directed behavior.  It includes things like planning and organization, working memory, inhibitory control, cognitive flexibility (switching between tasks) and problem-solving.  The prefrontal cortex is the main area of the brain involved in executive function, but it can interact with other brain regions.  This system is important for daily activities, decision-making, adapting to new situations and is heavily involved in learning, social behavior, and self-regulation.

Limbic system:

The limbic system is a complex system of nerves and networks in the brain, which involves areas near the edge of the cortex related to instinct and mood.  It controls basic emotions (fear, pleasure, and anger) and drives (hunger, sex, dominance, and care of offspring).  Key components of the limbic system include:

·         Hippocampus:  Memory formation and spatial navigation

·         Amygdala:  Emotion processing, especially fear and pleasure

·         Thalamus:  A relay station in the brain, channeling sensory and motor signals to the cerebral cortex

·         Hypothalamus: Regulates vital bodily functions (temperature, hunger, thirst, and sleep-wake);  also controls pituitary gland, which influences hormone production and release

Ventricular system:

The ventricular system is a series of fluid-filled cavities (ventricles) in the brain.  The ventricular system functions to cushion and protect the brain (reducing trauma when there  is an impact to the brain), provide nutrients and waste (to maintain optimal conditions for neural signaling) and aids in the exchange of substances between the brain and the bloodstream. 

Default Mode Network

The default mode network (DMN) is a large-scale brain network primarily active when a person is not focused on the outside world and the brain is at wakeful rest, such as during daydreaming, mind-wandering, and envisioning the future. It's called "default" because it seems to become active by default when a person is not engaged in a task that requires attention to the external environment.  This system includes areas of the brain involved in self-talk, emotional regulation, the ability to understand others and consolidation of memories.  In some conditions, the DMN may display altered activity.  For example, with depression,  the DMN may show increased activity during rest periods, which is associated with excessive self-focus, rumination (repetitive, negative thoughts about oneself), and a negative bias in thinking about the past and future.  We have discussed methods such as mindfulness and meditation which can help restore the functions of the default mode network.  

Neurotransmitters:

Neurotransmitters are chemical messages used by neuron cells to communicate among other neurons as well as other cells.  There is a process of neurotransmission that includes several steps:

·         Synthesis:  They are first created in the  neuron (either the cell body or in the axon terminal)

·         Storage:  They are stored in small vesicles in the axon terminal of the neuron

·         Release:  When the electric signal reaches the axon terminal, it triggers the opening of “calcium channels”.  This influx of calcium ions causes the vesicles to fuse with the presynaptic membrane (the sending neuron) and release the neurotransmitter into the synaptic cleft (the gap between neurons).

·         Binding:  The released neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic membrane (the receiving neuron). 

·         Response:  The binding of the neurotransmitters to the receptors initiates a response in the postsynaptic neuron.  This response can be excitatory (making the neuron more likely to fire) or inhibitory (making the neuron less likely to fire).  The exact response depends on the neurotransmitter and the receptor it binds to.

·         Termination:  The action of the neurotransmitter must then be terminated to prevent continuous activation of the postsynaptic neuron.  Various mechanisms include reuptake (the neurotransmitter is taken back to the presynaptic neurons) degradation (the neurotransmitter is broken down by enzymes in the synaptic cleft) or diffusion (movement away from the synaptic cleft). 

Example:  Serotonin

Serotonin is a neurotransmitter involved in mood, appetite, sleep, memory, and learning.  This is how the process of serotonin would look:

·         Synthesis:  Serotonin is synthesized from an amino acid (tryptophan), which is converted into serotonin by an enzyme (aromatic amino acid decarboxylase)

·         Storage:  Serotonin is stored in vesicles within the presynaptic neuron

·         Release:  When a signal reaches the presynaptic terminal, it triggers the release of serotonin into the synaptic cleft

·         Binding and response:  Serotonin then binds to receptors on the postsynaptic neuron.  There are a variety of serotonin receptors, each triggering different responses when activated.  The response can either inhibit or excite the postsynaptic neuron, which can lead to different physiological and psychological effects.

·         Termination: Serotonin terminates using reuptake which returns the serotonin to the presynaptic neuron and recycles it.  Or, in some cases, it is broken down by the enzyme monoamine oxidate (MAO) which makes it inactive and ultimately excreted from the body

·         Regulation:  The serotonin cycle is regulated to maintain balance.  Dysregulation has been seen in various psychiatric disorders, such as depression and anxiety.

Physical exercise:   

Physical activity benefits the brain and influences the structure, function, and overall health.  This includes the stimulation of brain-derived  neurotrophic factor (BDNF) which promotes the growth of new neurons, the release of “feel-good” hormones and neurotransmitters which play roles in mood regulation and managing stress and depression, reducing inflammation in the brain while improving insulin sensitivity and releasing antioxidants in the brain (which ultimately reduce the risk of neurodegenerative diseases), improving sleep and reducing stress (by reducing the levels of stress hormones) and by increasing blood flow to the brain (sending more oxygen and nutrients to the brain).

Mental exercise: 

 One of the key components of our brain fitness class is BrainHQ, a brain training program.  The theory behind this is that challenging cognitive tasks improve brain function.  Regardless of the activities which challenge the brain, the mental exercises stimulate neuroplasticity (forming new neural connections).  By continually challenging the brain, the brain’s capacity to rewire and adapt improves, which also improves cognitive resilience.   When engaging in mental activities which require learning  new information or skills, the short-term and long-term memory systems are improved.  Likewise, the reward from mentally stimulating activities can provide a sense of accomplishment, boost self-esteem and serve as a distractor from stress.

Stress management:  

We have learned that short-term stress can be beneficial to the brain, but chronic stress can cause multiple problems in physical and mental functioning. Managing stress (mindfulness, meditation, exercise) can reduce inflammation to the brain.  Also, when your stress is managed, there is less cortisol in the blood which protects brain cells.  Reduced stress also supports neurogenesis (new cell growth) and can prevent and mitigate symptoms of mental health disorders like anxiety, depression and PTSD.  Finally, managing stress can encourage healthier lifestyle choices through healthy coping mechanisms like regular exercise, a balanced diet and socialization.

Sleep:  

Sleep is critical in promoting a healthy brain.  Some of the ways that it benefits the brain include memory consolidation (reorganizing the brain), clearing the toxins (possibly even beta-amyloid, associated with Alzheimer’s disease), supporting cognitive functions like attention, problem solving and decision-making, maintaining emotional balance and resilience, supporting neuroplasticity (which occurs during the reorganization and strengthening of connections as you sleep), promoting the release of growth hormones and regulating hormones that control appetite and metabolism.