BHAV 455 MODULE 3- Neurobiology and Neuronal Communication in Behavioral Health

The Relevance of Neurology to Behavioral Health Professionals

• Context of the Lecture: This module focuses on neurology for behavioral health, also known as behavioral neurobiology, emphasizing the transition from structural anatomy to functional communication between neurons.

• Applicability to Practice:

  • While structural anatomy (e.g., brain injuries) is critical for specialists in rehabilitation centers, helping professionals (case managers, counselors, psychotherapists, psychologists) are often more interested in neuronal communication.

  • Primary clinical presentations such as anxiety and depression are rarely attributed to structural visible damage like a "small hypothalamus" or a "tear in the amygdala."

  • Instead, these psychological phenotypes usually manifest from issues with neurotransmitters (brain chemistry) and electrical activity.

• Diagnostic Exceptions: Structural issues may be relevant in cases involving specific impairments in memory, expressive/receptive communication, or impulse control.

• The Subsurface Reality of Behavior: Every human action is underpinned by neurological processes:

  • Hunger: Neurons in the hypothalamus release messages indicating low energy.

  • Planning and Habit: The thought "I should eat breakfast for energy" involves the firing of neurons.

  • Motor Sequences: Reaching for coffee involves planning, intention, and action sequences mediated by neuronal communication.

  • Reward: The feeling of pleasure or happiness from a drink (e.g., coffee) is a result of neural reward pathways.

The Unit of Communication: The Neuron

• Definition: Neurons are specialized nerve cells that receive, integrate (interpret), and transmit information.

• Hardware vs. Software Analogy:

  • Human brains are not hardware running software; the hardware is the software.

  • The "human biocomputer" differs from standard computers because the biological structure (the brain) is dynamic and self-shaping.

• Total Count: There are approximately $100 \times 10^9$ (100 billion) neurons in the human brain (some estimates suggest $86 \times 10^9$).

• Primary Functional Types:

  • Sensory Neurons: Detect sensory and environmental stimuli.

  • Motor Neurons: Control, initiate, and manage movement.

  • Pyramidal Neurons: Located in the neocortex; essential for conscious thought, memory, and self-reflective thinking (metacognition).

  • Mirror Neurons: Specialized neurons that fire both when performing an action and when witnessing someone else perform that same action.

    • Social/Vicarious Learning: These allow individuals to learn from others' mistakes (e.g., a child seeing a sibling burn their hand on a stove).

    • Empathy and Compassion: They facilitate "attuning" to another's experience, creating a resonance where the observer feel what the other is feeling.

Neural Networks and Alliances

• Formation: Neurons interact and form networks through selective communication based on shared goals and functionality.

• Neural Alliances: These are like co-ops or alliances formed through experience and habit.

• Examples:

  • Skill Acquisition: Once a neural network for riding a bike is established, the neurons fire together automatically, eliminating the need to relearn the skill.

  • Addiction and Triggers: Strong neural alliances can be formed for survival or via maladaptation. In addiction, environmental cues (the "clink of a glass") can trigger a network that leads to automatic motor sequences (drinking) before conscious thought occurs.

  • Trauma: Adverse Childhood Experiences (ACEs) and trauma lead to rapidly formed, powerful alliances dedicated to fight-or-flight survival responses.

Anatomy of a Neuron

1. Dendrites: Short, branch-like structures that detect neurochemical signals from neighboring neurons.

2. Cell Body (Soma): The "microprocessor" of the cell; it collects, integrates, and interprets information received by the dendrites. It contains the nucleus, DNA, and genetic material.

3. Axon: A long fiber extending from the cell body that transmits electrical impulses (action potentials). They can range from millimeters to over a meter in length.

4. Terminal Buttons (Synaptic Knobs): Located at the ends of axons; they form the synapses and release chemical signals.

• Myelin Sheath: An insulatory material (fatty layer) that coats the axon.

  • Functions: Speeds up communication and prevents the signal from degrading as it travels.

  • Nodes of Ranvier: Microscopic gaps in the myelin sheath. Signals can "jump" across these gaps (saltatory conduction), further increasing speed.

  • Multiple Sclerosis (MS): A condition characterized by the breakdown of myelin, leading to inefficient or failed message transmission.

Glial Cells: The Support System

• Prevalence: Glial cells make up approximately $90\,\%$ of the brain's cells, while neurons make up only $10\,\%$.

• Historical View: They were once thought to be simple "packing peanuts" used to hold neurons in place.

• Current Understanding: They are active dynamic participants in the brain's environment.

  • Microglia: The brain's specialized immune system and "janitorial crew"; they repair damage and remove waste.

  • Oligodendrites: Specialized glia that produce the myelin sheath in the central nervous system.

  • Astrocytes: Star-shaped cells and the most abundant type of glia.

    • Regulate the chemical environment.

    • Form the blood-brain barrier.

    • Provide metabolic support (oxygen/energy) to neurons.

    • Signal Modulation: They wrap around synapses to "eavesdrop" on communication. They can amplify important signals ("pump up the volume") or mute others through calcium-based communication.

The Action Potential (Neural Firing)

• Definition: A rapid electrical signal and brief change in voltage across a neuron's membrane driven by ion movement.

• Resting State: The neuron maintains a charge of $-70\,mV$ (millivolts).

  • There is a high concentration of Sodium ($Na^+$) ions outside and Potassium ($K^+$) ions inside.

  • Sodium-Potassium Pump: Constantly works to maintain this equilibrium; requires a disproportionate amount of human energy (burning fuel) to keep the brain "ready."

• Threshold: If a stimulus is strong enough, the neuron hits a threshold of $-55\,mV$.

• Depolarization: Voltage-gated sodium channels open, allowing $Na^+$ to rush in, making the cell interior more positive.

• Peak: The membrane potential spikes to roughly $+30\,mV$.

• Repolarization: Sodium channels close and potassium channels open, letting $K^+$ flow out to restore the negative charge.

• Hyperpolarization: A brief period where the membrane becomes more negative than $-70\,mV$ due to excess potassium outflow.

• All-or-Nothing Principle: Once the threshold is met, the signal fires completely and then resets.

Synaptic Transmission

• The Synaptic Cleft: Neurons do not touch. There is a microscopic nanometer-scale gap (the cleft) between the pre-synaptic and post-synaptic cells.

• The Process:

  1. The electrical action potential reaches the terminal button.

  2. Neurotransmitters (chemicals) are released from vesicles (small storage sacs).

  3. Neurotransmitters float across the synaptic cleft.

  4. Lock and Key Analogy: Neurotransmitters bind to specific receptors on the post-synaptic dendrite. If they do not fit the specific "lock," no message is sent.

  5. The post-synaptic cell decides whether to pass the message on or terminate it.

• Termination of Signal: Once the message is delivered, the cleft must be cleared to allow for future clarity:

  • Re-uptake: The pre-synaptic neuron reabsorbs the neurotransmitter for recycling. This conserves energy and prevents over-excitation.

  • Enzymatic Breakdown: Enzymes dissolve the neurotransmitters in the cleft.

The Role of Calcium in Neurobiology

• The Second Messenger: While sodium and potassium create the electrical charge (the spark), calcium ($Ca^{2+}$) triggers the cell to take action.

• Mechanism: When the action potential hits the axon terminal, it opens Voltage-Gated Calcium Channels (VGCCs).

• Neurotransmitter Release: The influx of calcium causes the vesicles to fuse with the cell membrane, allowing neurotransmitters to be released into the cleft.

• Gene Expression: Calcium can activate CREB (cAMP response element binding proteins).

  • CREB acts as a master switch for long-term memory and neuronal survival.

  • It triggers the synthesis of new proteins to alter the brain's physical architecture.

Synaptic Plasticity and Learning

• Neuroplasticity: The brain's ability to strengthen or weaken connections based on experience.

• Long-Term Potentiation (LTP): The biological basis of memory.

  • When a synapse is frequently activated, a large amount of calcium enters.

  • This increases the number of receptors on the receiving neuron.

  • The connection becomes stronger and more efficient (e.g., forming a habit or a bias toward negative thinking).

Major Neurotransmitters

• Acetylcholine (ACh): Motor control, learning, memory, sleeping, and dreaming.

• Norepinephrine: Arousal, vigilance, and attention (linked to anxiety and trauma responses).

• Serotonin: Emotional states, impulse control, and dreaming.

• Dopamine: Reward, motivation, and motor control over voluntary movement.

• GABA: Primary inhibitory neurotransmitter; helps with anxiety reduction and calming.

• Glutamate: Primary excitatory neurotransmitter; crucial for memory formation.

• Endorphins: Pain reduction and neural reward.

Neuropharmacology in Behavioral Health

• Agonists: Substances that enhance a neurotransmitter's effect.

  • They can mimic the neurotransmitter (e.g., Heroin mimics natural pain-relieving chemicals).

  • They can increase the manufacturing or release of the chemical.

  • They can block re-uptake.

• Antagonists: Substances that inhibit a neurotransmitter's effect.

  • They can block receptors so the real neurotransmitter cannot lock in.

  • They can destroy neurotransmitters in the synapse or decrease their production.

  • Example: Botox is an antagonist that inhibits the release of Acetylcholine, preventing muscle contraction.

• SSRIs (Selective Serotonin Re-uptake Inhibitors):

  • Popular pharmacological interventions for depression/anxiety.

  • Rationale: They inhibit the re-uptake process, leaving more endogenous serotonin in the synaptic cleft for a longer period, increasing the chance of it binding to the post-synaptic receptors.

Questions & Discussion

• Question (Internal Reflection/Student Prep): Is the difference between $-70\,mV$ and $-55\,mV$ always the same for every neuron?

• Response: The lecture implies these are standard thresholds used in the textbook as generalized estimates for most neurons during rest and activation.

• Discussion on Evolution: The lecturer notes that humans may have higher levels of norepinephrine than currently needed because the body's "hardware" is still evolved for survival in primordial environments, leading to modern anxiety.

• Closing Remark: Students are encouraged to review the supplemental videos (Animation on Thought and Two-Minute Neurology series) in the provided order for better synthesis.