MYERS DEWALL MODULES 4-6: Biology of Mind: Neural and Hormonal Systems; Tools of Discovery, Older Brain Structures, and the Limbic System; The Cerebral Cortex

The Mind and Biology

• Psychology is deeply biological: every idea, mood, and urge is a biological process involving brain, nervous system, hormones, and body.

• Nurture works on nature: biology changes in response to experience; brain adapts across the lifespan.

• The brain is plastic: it can rewire itself with practice and experience.

The Power of Plasticity

• The brain is sculpted by genes and life experiences; plasticity strongest in childhood but continues through life.

• Examples of plasticity:

  • London taxi drivers: ~25,000 streets learned → enlarged hippocampus for spatial memory.

  • Pianists and other skilled performers: enlarged auditory cortex areas related to their skill.

  • Well-practiced dancers/jugglers show practice-related brain changes.

• Plasticity underlies learning, memory, and recovery after injury.

Neurons and Neural Communication

• Neurons: the basic building blocks of the nervous system; neuron parts include:

  • Dendrites (receive messages), cell body (life-support), axon (sends messages).

  • Myelin sheath speeds neural impulses; gaps between myelin segments speed transmission via saltatory conduction.

  • Glial cells (glia): support neurons, provide nutrients, insulate, guide connections, and may influence learning/memory.

• Neural impulse (action potential): brief electrical charge traveling down the axon.

  • Resting potential: outside is positive, inside negative.

  • Threshold: about $-55\,\text{mV}$; if exceeded, an action potential fires.

  • Speed varies by fiber type; typical range from $2\ \text{mph}$ to >$200\ \text{mph}$.

  • All-or-none: impulses are the same strength; intensity is coded by the number of neurons firing and the fire rate.

  • Refractory period: short pause after firing; no new spike during this time.

• Synaptic transmission:

  • Neurons communicate at synapses (junctions) via neurotransmitters.

  • Neurotransmitters cross the synaptic gap, bind to receptors on the receiving neuron, and influence whether that neuron fires.

  • Excess neurotransmitters are reabsorbed (reuptake), drift away, or are broken down by enzymes.

• Key neurotransmitters and their roles:

  • Acetylcholine (ACh): enables muscle action, learning, memory; motor neuron junctions.

  • Dopamine: movement, learning, attention, emotion; implicated in schizophrenia and Parkinson's disease when imbalanced.

  • Serotonin: mood, hunger, sleep, arousal.

  • Norepinephrine: alertness and arousal.

  • GABA (gamma-aminobutyric acid): major inhibitory neurotransmitter.

  • Glutamate: major excitatory neurotransmitter.

  • Endorphins: natural opioids; pain relief and pleasure.

• Drugs and neurotransmission:

  • Agonists increase neurotransmitter action (increase release or mimic the transmitter).

  • Antagonists block transmitter action (block receptors or release).

  • SSRIs (e.g., Prozac) block reuptake of serotonin to increase its synaptic presence.

  • Botulin blocks ACh release; Curare blocks ACh receptors causing paralysis.

The Nervous System: Central and Peripheral

• Central nervous system (CNS): brain and spinal cord; processing and decision-making.

• Peripheral nervous system (PNS): connects CNS to the body; two subsystems:

  • Somatic nervous system: voluntary control of skeletal muscles.

  • Autonomic nervous system (ANS): controls glands and internal organs; self-regulating.

  • Sympathetic division: arouses and energizes (fight-or-flight).

  • Parasympathetic division: calms and conserves energy.

• Neuron types and pathways:

  • Sensory (afferent) neurons: carry incoming information to CNS.

  • Motor (efferent) neurons: carry outgoing information to muscles/glands.

  • Interneurons: process information within CNS.

• Reflexes:

  • Simple reflex arcs involve sensory input → interneuron → motor output, often managed by the spinal cord (no brain involvement required).

The Endocrine System

• Endocrine system uses hormones as chemical messengers via the bloodstream.

• Speed: neural messages are fast; endocrine messages travel slower but last longer.

• Major glands and roles:

  • Hypothalamus: brain region that regulates the pituitary and maintains homeostasis.

  • Pituitary gland: the master gland; releases hormones that regulate other endocrine glands.

  • Adrenal glands: epinephrine and norepinephrine (fight-or-flight response).

  • Pancreas: regulates blood glucose.

  • Ovaries/Testes: sex hormones influencing brain and behavior.

• Interaction: brain (nervous system) controls endocrine secretions; hormones can influence brain function and behavior.

• Oxytocin: promotes bonding and social trust.

• Feedback: brain → pituitary → other glands → hormones → body/brain; hormones can outlast neural messages.

The Brain's Major Structures

• Brainstem (oldest part): automatic survival functions.

  • Medulla: heartbeat and breathing.

  • Pons: coordinate movements and sleep.

  • Brainstem crossover: most nerves connect to opposite body side.

• Thalamus: sensory relay station; routes senses (except smell) to cortex and transmits replies.

• Reticular formation: filters incoming stimuli; controls arousal.

• Cerebellum: coordinates movement, balance; enables nonverbal learning and memory; damaged affects coordination.

• Limbic system (border region between old and new brain): amygdala, hypothalamus, hippocampus; involved in emotion, motivation, and memory.

  • Reward centers located in hypothalamus and nucleus accumbens; dopamine-related.

  • Hippocampus: explicit memories (facts/events).

The Cerebral Cortex

• Four lobes (each hemisphere):

  • Frontal lobe: planning, judgment, speaking; motor cortex is at the rear of the frontal lobe.

  • Parietal lobe: somatosensory cortex at the front; processes touch and body position.

  • Occipital lobe: visual cortex at the rear; processes visual information.

  • Temporal lobe: auditory cortex; language and memory processing.

• Motor cortex and Somatosensory cortex:

  • Motor cortex controls voluntary movements; representation is disproportionately large for fine motor control (e.g., fingers, mouth).

  • Somatosensory cortex maps touch and body sensation; more sensitive areas have larger cortical representations.

• Association areas:

  • Regions outside primary motor/sensory areas involved in higher-order functions: thinking, planning, language, memory.

  • Prefrontal cortex: planning, inhibition, decision making.

• Hemispheric specialization and the split-brain findings:

  • Left hemisphere: language and logical processing.

  • Right hemisphere: inference, recognizing patterns, spatial abilities, emotion, face recognition.

  • Corpus callosum connects the two hemispheres; split-brain studies show how each hemisphere can operate independently on certain tasks.

• Localization vs. networks:

  • Early efforts attempted strict localization; modern view emphasizes distributed networks across multiple areas.

Brain Laterality and Split-Brain

• Split-brain: severing corpus callosum to treat severe epilepsy; leaves each hemisphere operating separately.

• Classic findings:

  • Left hemisphere typically controls language; right hemisphere processes nonverbal information.

  • When shown a word to the left visual field (right hemisphere), patients may not verbally report it but can identify it with the left hand (controlled by the right hemisphere).

  • Two hemispheres can appear to have two minds; the left hemisphere often provides post-hoc rationalizations for actions.

• Everyday implications:

  • In intact brains, hemispheres interact to produce integrated thoughts and actions.

Tools of Discovery, Older Brain Structures, and the Limbic System

• Neuroimaging and recording tools:

  • EEG: measures electrical activity; good temporal resolution.

  • MEG: measures magnetic fields from brain activity.

  • PET: tracks radioactive glucose to show metabolic activity.

  • MRI: structural images of brain anatomy.

  • fMRI: measures blood flow to infer brain activity; shows function plus structure.

• Brain mapping and connectivity:

  • Diffusion spectrum imaging (a type of MRI) maps long-distance connections.

  • Human Connectome Project maps neural pathways to understand brain networks.

• The limbic system and functions:

  • Amygdala: emotion, fear, aggression.

  • Hypothalamus: maintains homeostasis; regulates hunger, thirst, temperature; links to endocrine system.

  • Hippocampus: explicit memory formation and retrieval; affected by aging and injury.

• Plasticity after damage and neurogenesis:

  • Brain can reorganize after injury; children show greater plasticity.

  • Neurogenesis occurs in the hippocampus and other regions; can be promoted by exercise, sleep, etc.

• Brain self-regulation and rewards:

  • Reward centers drive survival-related behaviors; dopamine plays a central role in reward and motivation.

The Cerebral Cortex: Integration and the Mind-Body Problem

• The cortex is the brain’s thinking crown; two hemispheres work together to enable perception, thought, memory, and language.

• The mind is what the brain does, but consciousness is not reducible to neural activity alone; biology and experience shape mind and behavior as an integrated system.

Key Numbers and Concepts ( Essentials )

• Neurons: roughly $86\times 10^9$ in the human brain; trillions of synapses (~$3\times 10^{14}$ connections).

• Cortex: about $20\text{–}23\times 10^9$ neurons and $3\times 10^{14}$ synapses in the cortex (approximate).

• Plasticity: most pronounced in childhood but persists across life.

• Threshold for action potential: $-55\,\text{mV}$.

• Conduction speeds: roughly $2\ \text{mph}$ to over $200\ \text{mph}$ depending on fiber type.

• Balance of neural signaling: excitation vs inhibition; stronger stimuli recruit more neurons and higher firing rates, but do not increase the strength of a single action potential.

Master Concepts for Quick Recall

• Biological basis of psychology: mind and behavior arise from brain and biological processes.

• Plasticity and neurogenesis explain learning, memory, skill acquisition, and recovery from brain injury.

• The nervous and endocrine systems coordinate to regulate behavior, with the brain as the central controller.

• The cerebral cortex enables higher-order processes via its lobes, motor/somatosensory maps, and association areas.

• Lateralization and split-brain studies reveal specialized but integrated hemispheric functions.

• Modern brain imaging reveals structure and function, revealing networks and connectivity underlying mental life.

Quick Retrieval Practice (concepts to recall)

• What are the four lobes of the cerebral cortex and the primary functions of motor and somatosensory cortices?

• What does plasticity mean in the brain, and what are examples of it in experts (e.g., taxi drivers, musicians)?

• Which brain structures comprise the limbic system, and what are their primary roles?

• How do the sympathetic and parasympathetic divisions of the ANS differ in function?

• What is the role of the corpus callosum in hemispheric communication?