Structure and Functions (Brain I) — Comprehensive Study Notes Summary & Study Notes

🧠 Overview

The brain produces both mental contents (knowledge, beliefs, desires, feelings) and mental processes (attention, perception, memory). These arise from the activity of interconnected neurons organized into brain circuits, with supporting roles played by glial cells and hormonal and immune systems.

⚡ Brain Circuits and the Neuron

The basic computational unit of the nervous system is the neuron. Neurons receive inputs, integrate them, and generate outputs. There are three functional neuron types: sensory neurons (carry signals from sense organs to the CNS), motor neurons (send commands to muscles and organs), and interneurons (connect neurons within the brain and spinal cord).

🔬 Structure of a Neuron

Key anatomical parts: the cell body (metabolic center), dendrites (receive inputs), a single axon (transmits outputs; may branch), and terminal buttons (release chemicals). The neuron is enclosed by a cell membrane that controls ionic flow.

⚙️ Neural Impulses and Action Potentials

Neurons maintain a negative resting charge (resting potential) due to ionic distributions. When stimulated, ion channels open: Na+ rushes in, depolarizing the cell; subsequently K+ flows out and ion pumps restore resting conditions. The traveling shift in charge is the action potential, which obeys an all-or-none law. Many axons are insulated by myelin to speed conduction; demyelination (e.g., in multiple sclerosis) slows impulses.

🔗 Synapses and Neurotransmitters

Communication between neurons occurs at the synapse across the synaptic cleft. An action potential triggers vesicles to release neurotransmitters, chemical messengers that bind to receptors on the receiving neuron. Major transmitters include acetylcholine, dopamine, noradrenaline, serotonin, GABA, and endogenous cannabinoids.

🧩 Receptors, Excitation, and Inhibition

Receptors on dendrites are selective—each binds specific neurotransmitters. Binding produces excitatory or inhibitory effects. A receiving neuron's fate (to fire or not) depends on the summed excitatory and inhibitory inputs. Reuptake clears neurotransmitters from the cleft to terminate signals.

💊 Agonists, Antagonists, and Drugs

An agonist mimics a neurotransmitter at its receptor or increases transmitter levels (e.g., SSRIs block serotonin reuptake). An antagonist blocks neurotransmitter action. Dysregulation of transmitters contributes to disorders (e.g., dopamine loss in Parkinson’s disease), and drugs can ameliorate symptoms but may lose efficacy with continued use.

🧶 Glial Cells

Glial cells outnumber neurons and provide structural and metabolic support: cushioning, cleanup of debris and excess neurotransmitters, nutrient delivery, and guiding synapse formation. Glia communicate chemically and can influence neuronal communication, plasticity, and circuit coordination.

🌐 Peripheral Nervous System (PNS)

The PNS includes the sensory-somatic system (sensory inputs and voluntary motor outputs) and the autonomic nervous system (ANS), which regulates involuntary functions. The ANS splits into the sympathetic branch (fight-or-flight responses) and the parasympathetic branch (rest-and-digest), which generally counterbalance each other.

🧭 Central Nervous System (CNS) and Spinal Cord

The CNS consists of the brain and spinal cord. The spinal cord contains ascending tracts (sensory to brain) and descending tracts (motor commands), with reflex circuits that can produce immediate responses independent of conscious control.

🧩 Cerebral Cortex and Lobes

The cerebral cortex (gray matter) is folded into sulci and gyri. Each hemisphere controls the opposite side of the body and contains four lobes:

• Occipital lobe: primary visual processing.
• Temporal lobe: auditory processing, memory encoding, aspects of language.
• Parietal lobe: spatial processing, somatosensory representation (somatosensory strip).
• Frontal lobe: planning, motor control (motor strip), reasoning, emotion, and speech control. Damage to frontal regions can disrupt personality and self-control (classic example: Phineas Gage).

↔️ Hemispheric Specialization and Split-Brain

The two hemispheres communicate via the corpus callosum. Split-brain patients (corpus callosum severed) reveal lateralized processing: each hemisphere can process different information independently, showing specialization for particular narrow tasks, but generalizations like “left = logical, right = creative” are oversimplifications.

🧠 Subcortical Structures

Important inner-brain structures include:

• Thalamus: relay and switching station for sensory and motor information; involved in attention and sleep.
• Hypothalamus: homeostasis (temperature, hunger, thirst, sexual behavior) and major regulator of hormones through the pituitary.
• Hippocampus: essential for forming new declarative memories and for memory consolidation; does not permanently store memories but helps drive storage elsewhere.
• Amygdala: emotion processing, especially fear and anger, and interpreting emotional expressions.
• Basal ganglia: movement planning and habit learning; heavily dependent on dopamine. The nucleus accumbens (part of basal ganglia) mediates reward and is implicated in addiction.

🌉 Brainstem and Cerebellum

The brainstem (including medulla and pons) controls vital autonomic functions (breathing, circulation, swallowing) and arousal via the reticular activating system. The cerebellum contributes to coordination, timing, and attention and has a large surface area comparable to the cortex.

🧪 Neuroendocrine and Neuroimmune Systems

The brain interacts with the body via hormones. The pituitary gland (master gland) is regulated by the hypothalamus and controls other endocrine glands. The HPA axis (hypothalamic-pituitary-adrenal) is activated by stress and infection, releasing cortisol, which modulates immunity and can impair hippocampal-dependent memory when chronically elevated.

🧾 Methods for Probing the Brain

Lesion studies (strokes, injuries) link damaged regions to lost functions. Recording techniques include EEG (electrical activity), MEG (magnetic fields), and single-cell recordings. Structural imaging: CT/CAT and MRI. Functional imaging: PET (radioactive tracer) and fMRI (blood oxygenation). Brain stimulation methods include direct cortical stimulation and TMS (noninvasive magnetic stimulation) to test causal roles of regions. Each technique has strengths and limitations; imaging shows correlations, while stimulation and lesions are better for causal inference.

🧬 Genes, Development, and Plasticity

Genes provide blueprints (genotype) while the phenotype emerges from gene-environment interaction. Development involves pruning (eliminating unused connections) and experience-dependent synapse formation. Plasticity—the brain’s capacity to change—peaks in childhood but persists in adulthood, enabling learning and recovery after injury.

🔁 Gene-Environment Interactions

Genes and environment form a single interactive system: environmental inputs can turn genes on/off; behaviors shape environments in passive, evocative, and active ways. Behavioral genetics uses twin and adoption studies to estimate heritability—the proportion of variance in a trait due to genetic differences in a given environment.

🌱 Evolutionary Considerations

Evolution by natural selection shapes traits that improve reproductive success. Not all traits are adaptive; caution is needed when inferring current functions from evolutionary history. Brain features result from complex interplay of variation, environmental pressures, and chance.

📝 Key Takeaways

The brain’s functions emerge from integrated networks of neurons and glia, chemical signaling, hormonal modulation, and lifelong interactions with the environment. Understanding neural structure, communication, systems-level organization, and methods for studying the brain provides the foundation for linking brain activity to behavior and cognition.