BHAV 455/Module 2: Structural Anatomy and Behavioral Neurobiology
Course Introduction and Overview
- The module focuses on the structural anatomy of the brain within the context of "Neurology for Behavioral Health" (potentially to be renamed "Behavioral Neurobiology").
- The goal is to provide an exhaustive introductory landscape of the brain’s anatomy, connecting physical structures to human behavior, lived experience, and psychopathology.
- The lecture emphasizes massive complexity: the human brain contains approximately , , and of wiring.
Developmental Stages of the Brain
- In the first trimester, neurons and synapses begin to develop in the spinal cord.
- By approximately , the rudimentary brain separates into three distinct parts: front brain, midbrain, and hind brain.
- During the second trimester, the brain assumes control over bodily functions; movements and breathing begin.
- In the third trimester, the brain separates into right and left halves and continues growing rapidly until birth and beyond.
- Morphologically, the brain begins as a tubular structure that eventually folds into a sphere.
Anatomical Directional Terminology (The Brain's Compass)
- To navigate the brain, use the following positional terms:
- Dorsal: The top side (analogous to a shark's dorsal fin).
- Ventral: The bottom or "belly" side.
- Rostral (Anterior): Points toward the front (the nose or "beak").
- Caudal (Posterior): Points toward the back or the "tail."
- Lateral: Toward the sides.
- Medial: Toward the middle.
Major Regions and Lobes of the Cortex
- The brain is often divided by the Central Sulcus, a line that separates perception from action.
- The Frontal Lobe (Action): - Comprises approximately the front one-third of the brain.
- Acts as the "executive floor," managing planning, decision-making, and action selection.
- Focused on survival and thriving; weighs "what if" scenarios and predictions.
- Contains the Pre-central Gyrus, which is the primary motor cortex.
- In trauma (PTSD or ACEs), the frontal lobe can become skewed, prioritizing the avoidance of even a chance of threat repetition.
- The Back Sections (Perception): - Comprises the back two-thirds of the brain.
- Parietal Lobe: Determines spatial location (where things are).
- Occipital Lobe: Responsible for visual processing (seeing).
- Temporal Lobe: Responsible for object identification (what things are) and processing auditory information.
The Sensory Homunculus
- A "Homunculus" is a model representing the proportionality of brain resources dedicated to various body parts.
- Humans have massive cortical real estate devoted to the hands, lips, and tongue.
- This reflects evolutionary priorities: fine motor dexterity (specifically the opposable thumb) and complex social communication through facial expressions and speech.
Deep Brain Structures and Functional Systems
- Corpus Callosum: A massive bundle of nerves connecting the left and right hemispheres.
-Cingulate Gyrus: Wraps around the corpus callosum; acts as a master regulator of attention, impulses, and emotions.
- Hippocampus: - The "gateway to sustained memory"; it shifts short-term/working memory into long-term storage. - It processes context and is a critical link in understanding trauma and PTSD.
- Case Study: Patient HM (1950s): Doctors removed HM’s hippocampi to treat epilepsy. While seizures stopped, he lost the ability to form new long-term memories, though his existing long-term memories remained intact. This proved the hippocampus files memories rather than storing them permanently.
- Amygdala: - The primal fear center; manages affect and emotions.
- Strongly activated by faces (interpreting threats or anger) and primal signals (food, mating). - Fornix: An arching bundle of fibers that carries information out of the hippocampus.
- Mammillary Body: The terminus for information traveling via the fornix.
- Thalamus: An egg-shaped structure acting as "Grand Central Station" or a central switchboard, directing sensory and motor signals to the correct cortical areas.
- Basal Ganglia:- The "chief of operations" for selecting and initiating actions and habits.
- Composed of the Putamen and the Caudate (which together look like a tadpole, with the Putamen as the head and the Caudate as the tail), and the Globus Pallidus.
- Internal Capsule: A major highway of nerve fibers running through the basal ganglia, connecting the cortex to the rest of the body.
Neural Circuits and Behavioral Health
- The Papez Circuit: A cyclical pathway vital for memory and emotion.
- Path: Information enters from the Temporal Lobe/Environment -> Hippocampus (processing) -> Fornix -> Mammillary Bodies -> Anterior Thalamus (relay) -> Cingulate Gyrus (comparison) -> Hippocampus. - The Cingulate Gyrus compares current stimuli to past memories to assign meaning. Behavioral health issues often stem from a "mismatch" where safe stimuli are interpreted as traumatic threats.
- Limbic Circuit: Connects emotion centers to survival instincts; governs "wanting" to do something.
- Sensory Motor Circuit: Manages complex movements and habits; governs the "actual doing."
Habit and Reward (Basal Ganglia and Addiction)
- The Basal Ganglia manages raw survival behaviors and reward processing.
- Animal studies show that direct stimulation of the reward center (specifically the nucleus accumbens via dopamine or cocaine) will dominate the motivational system.
- Addicted organisms will choose reward over food, water, or mating, ignoring survival instincts. This explains why humans may continue self-harming behaviors (addiction) despite losing family, housing, or health.
The Neurochemistry of Movement and Action
- The human system does not naturally sit at a "resting state"; it is held in check by an "emergency brake."
- GABA Inhibition: The Substantia Nigra Reticulata continuously slow-drips GABA, which inhibits movement and keeps the body still.
- Dopamine Activation: To move, the brain must "inhibit the inhibition." The Substantia Nigra Compacta releases a shot of dopamine to lift the "inhibitory gate."
- Direct Pathway: High dopamine levels inhibit GABA, allowing a "Go" signal.
- Indirect Pathway: The inhibitory gate remains closed unless enough dopamine is present to displace the GABA.
- Clinical Implications: - Parkinson’s Disease: Dopamine is depleted in the Sustantia Nigra Compacta. The "key" to the gate is missing, so patients struggle to initiate smooth motor actions despite working muscles. - ADHD: Switching attention requires a similar dopamine "shot" to disengage from one focus and re-engage with another.
# Behavioral Health Application: Relaxation and Reframing - Because the brain is a "comparison machine," mental health care involves helping clients manage a hyperactive amygdala and a mismatched comparison process. - Relaxation activities (deep breathing, yoga) are suggested not just for comfort, but to physically shift the system out of the sympathetic nervous system's "fight or flight" state. - Once the sympathetic state is triggered, it takes time to turn off; therapists work to reinterpret and reframe stimuli to prevent the hippocampus and hypothalamus from flipping the survival switch unnecessarily.