Week 3: Neurobiological bases of behavior — Comprehensive notes

Neurons and neural communication

  • Neuroanatomy refers to the study of the parts and functions of neurons. Neurons are the individual nerve cells that make up our entire nervous system, from the brain to the nerves that fire when you stub your toe.
  • Structure of a neuron (Figure 1): major parts include dendrites, soma (cell body), axon, myelin sheath, terminal buttons, and synapses.
  • Dendrites: root-like extensions that stretch from the cell body; grow to make synaptic connections with other neurons; receive messages and conduct impulses toward the cell body.
  • Soma (cell body): contains the nucleus and other parts needed to sustain life of the cell.
  • Axon: a wire-like structure that extends from the cell body; carries information away to other neurons, muscles, or glands.
  • Myelin sheath: a fatty covering around the axon that speeds neural impulses.
  • Terminal buttons (end buttons, terminal branches of the axon, synaptic knobs): the branched ends of the axon that contain neurotransmitters.
  • Neurotransmitters: chemicals in terminal buttons that enable neurons to communicate; fit into receptor sites on the dendrites of the next neuron like a key fits a lock.
  • Synapse: the space between the terminal buttons of one neuron and the dendrites of the next neuron.
  • Neuron types (in the neural pathway):
    • Afferent (Sensory) Neurons: carry information from senses to the brain.
    • Interneurons: relay information within the brain/spinal cord, sending messages elsewhere in the brain or to efferent neurons.
    • Efferent (Motor) Neurons: carry information from the brain to the rest of the body.
  • The Nerve Impulse and synaptic transmission (Figure 2: How Neurons communicate)
    • Resting state: a neuron has a slightly negative charge inside due to more negative ions inside and more positive ions outside.
    • Transmission begins when terminal buttons release neurotransmitters into the synapse.
    • Neurotransmitters bind to receptor sites on the dendrites of the next neuron.
    • Threshold: if enough neurotransmitters are received, the second neuron becomes permeable and positive ions rush into the cell.
    • Action potential: the electrical message that travels down the neuron; speed is about 120 \ \mathrm{m/s}.
    • When the charge reaches the terminal buttons of the second neuron, they release neurotransmitters into the synapse; the process may begin again if enough neurotransmitters bind to the next neuron to reach threshold.
    • All-or-none principle: a neuron either fires completely or not at all.
    • Restatement: if dendrites receive enough neurotransmitters to push past threshold, the neuron fires completely every time.
  • Neurotransmitters: functional roles and notable malfunctions
    • Acetylcholine (ACh): enables muscle action, learning, memory; influences movement, learning, attention, and emotion. Associated with Alzheimer's disease when disrupted.
    • Dopamine: influences movement, learning, attention, and emotion.
    • Excess dopamine receptor activity linked to schizophrenia.
    • Dopamine deficiency linked to tremors and reduced mobility in Parkinson’s disease.
    • Serotonin: affects mood, hunger, sleep, and arousal.
    • Undersupply linked to depression; SSRIs (e.g., Prozac) raise serotonin levels.
    • Norepinephrine: helps control alertness and arousal.
    • Undersupply can depress mood.
    • GABA (gamma-aminobutyric acid): major inhibitory neurotransmitter.
    • Undersupply linked to seizures, tremors, and insomnia.
    • Glutamate: major excitatory neurotransmitter; involved in memory.
    • Oversupply can overstimulate brain, producing migraines or seizures; some people avoid MSG (monosodium glutamate) in food.
  • The Nervous System: organization into central and peripheral components
    • Nervous system key divisions:
    • Central nervous system (CNS): brain and spinal cord.
    • Peripheral nervous system (PNS): all nerves outside the bones; connects CNS to rest of the body.
    • Subdivisions within PNS:
    • Somatic nervous system: controls voluntary muscle movements; motor cortex sends impulses to the somatic nervous system.
    • Autonomic nervous system: controls automatic functions of the body (heart, lungs, internal organs, glands); divided into:
      • Sympathetic nervous system: mobilizes the body to respond to stress (the alert system) – increases heart rate, blood pressure, and respiration; slows digestion.
      • Parasympathetic nervous system: calms the body after a stress response (the brake pedal).
  • Tools of discovery: methods to study, monitor, and examine the brain
    • Lesioning: the removal or destruction of part of the brain; historically used but not for experimental purposes; used to infer function when tissue is damaged (e.g., brain tumor removal); frontal lobotomy is a famous historical example; drug treatments have largely replaced lobotomies.
    • EEG (electroencephalogram): detects brain waves; amplified recording of electrical activity across the brain’s surface; electrodes placed on the scalp; widely used in sleep research to identify sleep stages and dreaming.
    • CAT/CT scan (Computerized Axial Tomography): sophisticated X-ray imaging; rotating X-ray cameras produce a detailed 3D picture of brain structure; shows structure but not brain activity.
    • MRI (Magnetic Resonance Imaging): uses magnetic fields to measure density and location of brain material; provides structural images, not function.
    • PET scan (Positron Emission Tomography): shows brain areas most active during tasks by injecting a small amount of a radioactive sugar; detectors pick up gamma rays; computer translates data into a functional map.
    • fMRI (Functional MRI): combines MRI and PET features; shows brain structure with information about blood flow to indicate activity during cognitive tasks.
  • Brain: major sections and functional organization
    • The brain is often considered in three broad categories: hindbrain, midbrain, and forebrain.
    • An evolutionary framing is the “old brain” (hindbrain + midbrain) and the “new brain” (forebrain).
  • Hindbrain
    • The hindbrain is the life-support system and controls basic biological functions.
    • Brainstem: located at the base of the skull; oldest and innermost region; automatic survival functions.
    • Medulla: regulates breathing and heart rate (automatic functions).
    • Pons: coordinates movements; involved in sleeping, walking, and dreaming.
    • Reticular formation: network of neurons extending upward to higher brain areas; screens information and is responsible for arousal and alertness.
    • Cerebellum: processes sensory input and coordinates movement output and balance.
  • Midbrain
    • Acts as an information superhighway connecting the forebrain and hindbrain.
    • Coordinates with eyes and ears to integrate sensory information with muscle movements; supports fine motor adjustments (e.g., turning the head and keeping eyes focused on text).
  • Forebrain
    • Forebrain comprises two major regions:
      1) Limbic system (the emotional brain): hippocampus, amygdala, hypothalamus, and thalamus.
      2) Cerebrum (cerebral cortex) with four lobes: frontal, parietal, occipital, and temporal.
  • Limbic System details
    • Thalamus: responsible for receiving sensory signals from the spinal cord and routing them to appropriate areas in the forebrain; also regulates sleep, alertness, and wakefulness.
    • Hippocampus: located near the ends of each hippocampal arm; essential for memory encoding; memories are not permanently stored here but are processed and then stored in cortex; damage impairs the ability to retain new information.
    • Amygdala: vital to experiences of emotion.
    • Hypothalamus: key link in bodily maintenance; regulates temperature, sexual arousal (libido), hunger, thirst, and the endocrine system.
  • Cerebral Cortex Lobes
    • Frontal Lobe: located at the front of the brain, behind the eyes; contains:
    • Prefrontal cortex (anterior frontal): often described as the brain’s central executive; important for directing thought processes, foreseeing consequences, pursuing goals, and maintaining emotional control; involved in abstract thought.
    • Broca’s area: in the frontal lobe; controls muscles involved in producing speech; damage can impair speech production.
    • Motor cortex: a thin vertical strip at the back of the frontal lobe; sends signals to muscles, controlling voluntary movements.
    • Parietal Lobe: located behind the frontal lobe on the top of the brain; contains the somatosensory cortex, which receives information about pressure, pain, touch, and temperature from the body.
    • Occipital Lobe: located at the back of the brain; processes visual information; contains the visual cortex.
    • Temporal Lobe: processes sounds sensed by the ears; auditory processing occurs here; Wernicke’s area (language comprehension) is located here; damage can produce fluent speech that lacks proper meaning or comprehension.
  • Genetics and biology of behavior
    • Genetics: a major biological factor in thoughts and behavior; gene-by-genome contributions to traits.
    • Basic genetic concepts:
    • Every human cell contains 46 chromosomes in two sets of 23 chromosomes (one set from each parent).
    • Genetic material is DNA (deoxyribonucleic acid).
    • Certain segments of DNA control the production of proteins that influence traits; these segments are genes.
    • Genes can be dominant or recessive. If two recessive genes for a trait are inherited, that trait will be expressed; otherwise the dominant trait is expressed.
    • Researchers study how different gene combinations contribute to physical and behavioral tendencies.
    • Chromosomal abnormalities
    • Gender is determined by the twenty-third pair of chromosomes.
      • Men have an X and a Y chromosome (XY); women have two X chromosomes (XX).
      • A father contributes either an X or a Y chromosome to a child, determining sex.
    • Turner’s syndrome: XO; individuals have a single X chromosome in the 23rd pair; characteristics include short stature, webbed neck, and differences in sexual development.
    • Klinefelter’s syndrome: XXY; clinical presentation varies but can include minimal sexual development and tendencies toward introversion.
    • Down syndrome: trisomy 21; an extra chromosome on the 21st pair; physical features include rounded face, shorter fingers and toes, and slanted eyes; typically associated with some degree of intellectual disability.
  • Connections to broader themes and implications
    • The history of phrenology (Franz Gall) illustrated early attempts to link skull bumps to brain function; while bumps on the skull do not reveal brain function, the module’s emphasis on distinct brain regions controlling different behaviors aligns with modern neuroanatomy.
    • Understanding lesioning and its ethical considerations highlights how neuroscience informs clinical practice and raises ongoing questions about brain manipulation and treatment, with modern pharmacological approaches largely replacing invasive procedures like frontal lobotomy.
    • The interplay of genetics and environment (nature vs nurture) is central to psychology and informs how we study traits and behaviors.
    • The correlation of neurotransmitter function with behavior and disorders underscores the neurochemical basis of mental health and the rationale behind pharmacotherapy.
  • Quick references to the material
    • Basic building block of the nervous system: neuron; communication via synapses and neurotransmitters.
    • Key concepts: threshold, action potential, all-or-none firing, synapse, receptor sites, synaptic cleft.
    • Major brain divisions: hindbrain, midbrain, forebrain; old vs new brain framework.
    • Four lobes of the Cerebral Cortex: Frontal, Parietal, Occipital, Temporal; primary regions include Broca’s area, Wernicke’s area, motor cortex, and somatosensory cortex.
    • Primary autonomic control: sympathetic vs parasympathetic divisions.
  • References and supplementary materials (as listed in the transcript)
    • Myers, David G., & DeWall, C. Nathan (2017). Psychology (Twelfth Edition). Worth Publishers.
    • Brain atlas entries and neuroscience resources on hindbrain and genetics.
    • Online supplementary readings on memory, brain regions, and online videos for visualizing brain structure and function.