Comprehensive Biology and Behavior Study Notes

Organization of the Nervous System

• Definition of the Nervous System: The nervous system is the body's speedy, electrochemical communication system composed of specialized nerve cells called neurons. It is responsible for everything humans think, feel, and do.

• Primary Direction of Neural Signal Transmission: Information moves systematically from the Peripheral Nervous System ($\text{PNS}$) into the Central Nervous System ($\text{CNS}$) for processing, and then back out through the $\text{PNS}$ to generate action ($\text{PNS} \nrightarrow \text{CNS} \nrightarrow \text{PNS}$).

• Major Anatomical Divisions:

  • Central Nervous System (CNS): Composed strictly of the brain and the spinal cord.

  • Peripheral Nervous System (PNS): Composed of all other nerve tissue throughout the rest of the body outside the $\text{CNS}$.

• Functional Divisions of the Peripheral Nervous System:

  • Somatic Nervous System: Transmits sensory and motor signals back and forth between the $\text{CNS}$ and the body's skin, skeletal muscles, and joints. It controls voluntary bodily movement.

  • Autonomic Nervous System: Transmits sensory and motor signals between the $\text{CNS}$ and the body's internal glands and organs. It regulates automatic, involuntary internal processes.

• Subdivisions of the Autonomic Nervous System:

  • Sympathetic Nervous System: Prepares the body for action in response to stress or danger ("fight or flight" response).

  • Parasympathetic Nervous System: Returns the body to its resting state following stress or activity ("rest & digest" response).

• Organ-Specific Physiological Actions Across Autonomic Divisions:

  • Eyes: Sympathetic activity dilates pupils; Parasympathetic activity contracts pupils.

  • Lungs: Sympathetic activity relaxes bronchi; Parasympathetic activity constricts bronchi.

  • Heart: Sympathetic activity accelerates and strengthens the heartbeat; Parasympathetic activity slows the heartbeat.

  • Stomach and Intestines: Sympathetic activity inhibits digestive activity; Parasympathetic activity stimulates digestive activity.

  • Internal Blood Vessels: Sympathetic activity contracts blood vessels; Parasympathetic activity dilates blood vessels.

Cellular Structure and Neural Communication

• The Neuron: The basic functional, structural, and smallest unit of the nervous system.

• Three Main Functional Types of Neurons:

  • Sensory Neurons (Afferent Neurons): Detect physical information from the external world and internal physical state, receiving sensory input and transmitting these signals directly to the brain.

  • Motor Neurons (Efferent Neurons): Direct muscles to contract or relax, thereby producing voluntary and involuntary movement.

  • Interneurons: Communicate entirely within local or neural circuits. They enable complex internal processing by bridging communication between sensory neurons, motor neurons, and the $\text{CNS}$.

• Structural Components of a Neuron:

  • Dendrites: Branch-like extensions that receive chemical messages from other neurons.

  • Cell Body (Soma): The central hub of the neuron where incoming messages from dendrites are collected and integrated.

  • Axon: A narrow outgrowth down which electrical impulses are passed away from the cell body toward other neurons, muscles, or glands.

  • Myelin Sheath: A fatty layer covering the axon of many neurons, created by Schwann cells in the peripheral nervous system, which insulates the axon and dramatically speeds up the transmission of neural impulses.

  • Nodes of Ranvier: Periodic gaps along the myelin sheath where the axon membrane is exposed.

  • Terminal Buttons (Axon Terminals): Small nodules at the end of the axon branches that release chemical substances (neurotransmitters) into the synaptic gap.

  • Synapse: The microscopic fluid-filled space between the axon terminal buttons of a "sending" (presynaptic) neuron and the dendrites of a "receiving" (postsynaptic) neuron.

• Physiology of Neural Resting and Firing:

  • Resting Membrane Potential: When a neuron is not actively sending a signal, it is "at rest." During rest, there are relatively more negative electrical charges inside the cell membrane than outside, leaving the neuron polarized and prepared to fire.

  • Selective Permeability: The cell plasma membrane regulates the flow of specific ions ($\text{Na}^+$, $\text{K}^+$) in and out of the cell via specialized channel proteins and the sodium-potassium ($\text{Na}^+ \text{--} \text{K}^+$) pump.

• Chemical Input Processing:

  • Inhibitory Signals: Instruct the cell to STOP firing and maintain or increase polarization.

  • Excitatory Signals: Instruct the cell to GO by depolarizing the cell membrane.

• Action Potential Threshold: If the neuron receives sufficient excitatory input to reach its threshold of $-55\,\text{mV}$, an action potential is triggered.

• Action Potential: An electrical pulse or ripple that travels rapidly down the axon away from the cell body, driven by depolarization.

• All-or-None Principle: A neuron either fires completely or does not fire at all (similar to a pass/fail outcome). There are no larger or smaller action potentials; every action potential produced by a given neuron is of identical strength and magnitude.

Neurotransmitters and Neuropharmacology

• Synaptic Transmission:

  • Neurotransmitters: Chemical messengers synthesized within neurons that cross the synaptic gap to transmit signals across cells.

  • Receptors: Specialized protein molecules located on the postsynaptic membrane. Neurotransmitters selectively bind to these receptors using a lock-and-key mechanism (a given neurotransmitter binds only to its specific receptor site).

  • The binding of a neurotransmitter to a receptor produces either an excitatory or inhibitory post-synaptic signal.

• Termination Mechanisms of Synaptic Transmission: Once released into the synapse, a neurotransmitter continues to exert influence until one of three primary termination events occurs:

  • Reuptake: The neurotransmitter is drawn back into the presynaptic terminal buttons for recycling or repackaging.

  • Enzyme Deactivation: Specific enzymes in the synaptic cleft break down and destroy the neurotransmitter molecule.

  • Auto-reception: Neurotransmitters bind to autoreceptors on the presynaptic membrane, signaling the sending cell to cease further neurotransmitter release.

• Major Neurotransmitters and Their Psychological Functions:

  • Acetylcholine: Controls motor function over muscles; regulates processes of learning, memory, sleeping, and dreaming.

  • Norepinephrine: Regulates states of arousal, vigilance, and attention.

  • Serotonin: Regulates emotional states, impulsiveness, and dreaming.

  • Dopamine: Primary mediator of reward and motivation; controls voluntary motor movement.

  • GABA (gamma-aminobutyric acid): Major inhibitory neurotransmitter responsible for inhibiting action potentials and reducing anxiety.

  • Glutamate: Major excitatory neurotransmitter responsible for enhancing action potentials, facilitating long-term learning and memory.

  • Endorphins: Endogenous opioids responsible for pain reduction and reward.

• Neuropharmacological Classifications:

  • Agonists: Drugs or exogenous toxins that enhance or amplify the actions of specific neurotransmitters. Mechanisms include:

  • Introducing precursor substances that increase neurotransmitter synthesis.

  • Blocking presynaptic reuptake receptors.

  • Directly binding to and mimicking the action of neurotransmitters on postsynaptic receptors.

  • Antagonists: Drugs or exogenous toxins that inhibit, block, or decrease the actions of neurotransmitters. Mechanisms include:

  • Introducing substances that decrease neurotransmitter synthesis.

  • Facilitating the enzymatic destruction of neurotransmitters in the synapse.

  • Blocking postsynaptic receptor sites to prevent endogenous neurotransmitters from binding.

• Clinical Applications and Examples of Pharmacological Agents:

  • L-DOPA: A dopamine agonist that increases dopamine production; used clinically to treat Parkinson's disease.

  • Selective Serotonin Reuptake Inhibitors (SSRIs): Agonists that bind with presynaptic reuptake transporters to block serotonin reuptake, increasing serotonin availability in the synapse.

  • Heroin: An agonist that mimics natural endorphins at postsynaptic receptor sites.

  • Dopamine Antagonists: Drugs that block dopamine from binding to postsynaptic receptors; used clinically in managing Schizophrenia.

Localization of Brain Function and Historical Perspectives

• The Localization Question: Historically, neuroscientists debated whether distinct functional regions exist within the brain or whether the entire brain acts as an undifferentiated whole in cognitive functions.

• Phrenology: Promoted by German physician Franz Joseph Gall ($1758\text{--}1828$), phrenology posited that specific mental faculties and personality traits were localized to individual cortical regions, and that the size of these regions correlated directly with observable bumps on the human skull.

• Empirical Evidence for Localization of Function:

  • Broca's Aphasia: Named after Paul Broca. Resulting from damage to Broca's area in the left frontal lobe, this condition causes severe impairment in the motor production of spoken and written language, despite the patient retaining full language comprehension. Speech therapy can lead to functional recovery over time.

• Wernicke's Aphasia: Documented by Carl Wernicke ($1848\text{--}1905$). Resulting from damage to Wernicke's area in the left temporal lobe, affected individuals can fluently produce spoken language, but lose the ability to comprehend spoken or written language from others.

Structural and Functional Brain Imaging Techniques

• Neuroimaging Methodologies:

  • Electroencephalography (EEG): A device that measures electrical activity across the scalp produced by neural firing.

  • Resolution Characteristics: Great temporal resolution (exact timing); bad spatial resolution (location imprecise).

  • Positron Emission Tomography (PET): Tracks brain metabolism by injecting a harmless radioactive substance into the bloodstream to measure metabolic activity.

  • Resolution Characteristics: Okay temporal resolution; bad spatial resolution.

  • Magnetic Resonance Imaging (MRI): Uses a powerful magnetic field to produce high-resolution structural images of brain tissue.

  • Resolution Characteristics: Bad temporal resolution; fantastic spatial resolution.

  • Functional Magnetic Resonance Imaging (fMRI): Measures changes in blood oxygen levels (blood-oxygen-level-dependent signal) to infer localized brain activity.

  • Resolution Characteristics: Okay temporal resolution; fantastic spatial resolution.

  • Transcranial Magnetic Stimulation (TMS): Uses intense magnetic fields to temporarily disrupt normal neural firing in targeted cortical areas, allowing researchers to determine if a specific brain region is necessary for a given function.

Cerebral Cortex, Hemispheric Specialization, and Subcortical Structures

• Cerebral Cortex Anatomy: The wrinkled outer layer of the brain responsible for high-level thought, perception, and complex voluntary behaviors.

• Tissue Types:

  • Gray Matter: Outer cortical tissue composed primarily of neuron cell bodies, dendrites, and unmyelinated axons.

  • White Matter: Deeper tissue dominated by long-range axons covered in fatty myelin sheaths.

• Interhemispheric Communication:

  • Corpus Callosum: A massive bridge of myelinated axon fibers connecting the left and right cerebral hemispheres, allowing continuous exchange of information between them.

  • Split-Brain Condition: Occurs when the corpus callosum is surgically severed to treat severe epilepsy, preventing the two hemispheres from communicating directly.

  • Left Hemisphere Specialization: Dominant for language production, speech, logic, and analytical processing. Processes information from the right visual field and controls the right side of the body.

  • Right Hemisphere Specialization: Superior for spatial relationships, facial recognition, and artistic processing. Processes information from the left visual field and controls the left side of the body.

• Neuromyth vs. Fact: Popular culture claims that people are either strictly "left-brained" or "right-brained." Scientific evidence shows that while localized functional specialization exists, both hemispheres work together, and individuals do not show disproportionate reliance on one hemisphere over the other.

  • The Four Anatomical Lobes of the Cortex:

• Frontal Lobe: Responsible for thought, planning, movement, and executive control. Separated from the parietal lobe by the central fissure and from the temporal lobe by the lateral fissure.

• Parietal Lobe: Responsible for processing touch stimuli, spatial relations, and directing attention.

• Temporal Lobe: Responsible for auditory processing, speech comprehension, and complex memory storage.

• Occipital Lobe: Situated at the back of the brain; dedicated exclusively to vision.

• Cortical Mapping and the Homunculus:

  • Primary Motor Cortex (Frontal Lobe) & Primary Somatosensory Cortex (Parietal Lobe): Mapped topographically to body parts.

  • Cortical Representation: Body parts requiring fine motor control or high sensory sensitivity (such as the hands, fingers, lips, face, and tongue) occupy disproportionately large areas of both cortical strips.

• Prefrontal Cortex and Historical Case Studies:

  • Phineas Gage: A railroad foreman who survived an accident in which a large iron rod pierced his skull, destroying much of his prefrontal cortex. He retained basic motor and intellectual abilities, but suffered profound personality changes, becoming vulgar, fitful, and unable to maintain social relationships.

  • Lobotomy: A surgical procedure involving deliberate damage or severing of connections to the prefrontal cortex. Pioneered by Portuguese neurologist António Egas Moniz (who won the Nobel Prize in $1949$), it was widely practiced in the late $1940\text{s}$ and early $1950\text{s}$ to treat psychiatric conditions.

• Subcortical Structures:

  • Insula Cortex: Located deep inside the lateral fissure; critical for taste perception, feeling disgust, bodily awareness of nervousness, pain processing, and empathy.

  • Thalamus: The sensory gateway or switchboard of the brain; receives and relays all incoming sensory information (except smell) to the relevant cortical regions.

  • Hypothalamus: The master regulatory center; controls internal body temperature, hunger, thirst, circadian rhythms, and endocrine output.

  • Hippocampus: Essential for the formation and consolidation of new long-term declarative memories.

  • Amygdala: Essential for processing emotional stimuli, learning emotional responses, and detecting fear or threat.

  • Basal Ganglia: Subcortical system critical for planning, coordinating motor movements, and processing reward.

Brain Stem, Cerebellum, and Neuroplasticity

• The Brain Stem: The region where the spinal cord widens as it enters the brain.

  • Components: Composed of the Medulla Oblongata, Pons, Midbrain, and the Reticular Formation.

  • Reticular Formation: A diffuse network of neurons responsible for regulating sleep, alertness, and arousal.

  • Primary Function: Controls basic physiological survival mechanisms, including heart rate, respiration, and swallowing.

• The Cerebellum:

  • Derived from Latin for "little brain."

  • Critical for motor control, precise timing, coordinated movement, and physical balance.

  • Involved in motor learning and motor memory storage.

  • Highly vulnerable to disruption from alcohol consumption.

• Neuroplasticity:

  • Definition: The brain's structural and functional capacity to alter its organization in response to environmental experience or physical injury.

  • Primary connections are established chemically, but fine-scale wiring is shaped by experience and patterns of neural activity.

  • Neurogenesis: The biological production of entirely new neurons.

  • Remapping: The functional reorganization of the brain wherein undamaged regions assume functions previously performed by damaged areas (demonstrated in cases like Michelle Mack, who was born without a left hemisphere yet developed full language abilities in her right hemisphere).

Principles of Genetics, Heredity, and Environmental Interactions

• Gene-Environment Interaction:

  • Fundamental principle: "The genome provides the option, and the environment determines which option is taken."

  • A gene's influence on behavior depends directly on environmental context and individual experience.

  • Genetic predispositions lead individuals to select specific environments, while environmental factors directly influence gene expression.

• Structural Hierarchy of Genetic Material:

  • Human Body $\rightarrow$ Cell $\rightarrow$ Cell Nucleus $\rightarrow$ Chromosomes $\rightarrow$ DNA $\rightarrow$ Genes.

  • Genome: The master genetic blueprint for an entire organism.

  • Chromosomes: Thread-like structures located inside the nucleus consisting of tightly coiled strands of $\text{DNA}$. Humans possess $23$ pairs of chromosomes per cell (one chromosome in each pair inherited from the mother, and one from the father).

  • DNA (Deoxyribonucleic Acid): A double-helix molecule that serves as the molecular building block of genes.

  • Gene: A specific functional sequence or unit of $\text{DNA}$ encoded to produce specific proteins.

• Human Genome Project: The international effort that mapped the complete sequence of human $\text{DNA}$. The primary discovery revealed that human beings possess fewer than $30{,}000$ individual genes.

• Mendelian Genetics:

  • Established by Gregor Mendel in $1866$ through selective breeding experiments on pea plants (tracking traits such as seed shape, seed color, flower color, flower position, pod shape, pod color, and stem height).

  • Dominant Gene: A gene that is expressed in the phenotype whenever it is present in the genotype.

  • Recessive Gene: A gene that is expressed in the phenotype only when paired with an identical recessive gene from the other parent.

  • Genotype: The complete underlying genetic constitution of an organism, determined at the moment of conception.

  • Phenotype: The observable physical and behavioral characteristics displayed by an organism, resulting from interactions between its genotype and its environment.

• Trait Complexity:

  • Monogenic Traits: Physical characteristics determined entirely by a single gene.

  • Polygenic Traits: Traits produced by the combined action of multiple distinct genes (e.g., human height, intelligence).

• Sexual Reproduction and Genetic Diversity:

  • Sexual reproduction yields over $8\,\text{million}$ possible unique combinations of the $23$ chromosome pairs from any single pair of biological parents.

  • Sex Chromosomes: The $23\text{rd}$ pair of chromosomes determines biological sex.

  • Females possess two $X$ chromosomes ($XX$).

  • Males possess one $X$ chromosome and one $Y$ chromosome ($XY$).

• Mutations and Evolutionary Adaptation:

  • Mutations are spontaneous errors that occur during cell division and $\text{DNA}$ replication. While mostly neutral, mutations can confer adaptive advantages or disadvantages.

  • Industrial Melanism: An evolutionary change where darker moth phenotypes became more adaptive than lighter ones following soot deposition during the Industrial Revolution.

  • Sickle-Cell Disease: A recessive disorder caused by a mutation that alters hemoglobin and oxygen transport in blood.

  • Sickle-Cell Trait: The heterozygous condition where individuals carry one sickle-cell gene; while experiencing mild symptoms under extreme stress, carriers gain evolutionary protection against malaria.

• Behavioral Genetics and Heritability:

  • Heredity: The biological transmission of genetic traits from parents to offspring (offspring inherit $50\%$ of their genetic material from each parent).

  • Heritability: A statistical estimate of the proportion of phenotypic variation within a specific population that is attributable to genetic differences among individuals.

  • Heritability applies strictly to populations, not to individuals.

  • Shared environmental habits and behaviors can run in families without being genetically determined.

• Methodologies in Behavioral Genetics:

  • Twin Studies:

  • Monozygotic (Identical) Twins: Result from a single fertilized egg (zygote) splitting into two; share $100\%$ of their genes.

  • Dizygotic (Fraternal) Twins: Result from two separately fertilized eggs; share $50\%$ of their genes (genetically no more alike than standard siblings).

  • Adoption Studies: Compare similarities between adopted children and their biological versus adoptive parents. Greater resemblance to biological parents implies strong genetic influence, whereas greater resemblance to adoptive parents implies strong environmental influence.

• Epigenetics:

  • Meaning "on top of genetics," epigenetics is the study of how environmental exposures alter gene expression without modifying the underlying $\text{DNA}$ sequence.

  • Factors such as chronic stress, smoking, alcohol use, and poor diet alter chemical markers on $\text{DNA}$, determining whether specific genes are switched on or off.