Biology and Psychology — Comprehensive Neuroscience Notes

Phineas Gage (1848) — Famous Case of Brain Injury

  • Background: Railroad foreman who survived an accident when an iron rod pierced his skull.

  • Trajectory of injury: Rod entered below his left cheek and exited through the top of his skull, damaging the frontal lobes.

  • Significance: One of the earliest and most famous cases linking frontal lobe damage to changes in personality and behavior, contributing to early understanding of brain–behavior relationships.

The Nervous System: On Being Wired

  • Neurons are specialized cells of the nervous system that conduct impulses.

  • Neurons receive messages from multiple sources, including light, other neurons, and skin pressure.

  • The human brain contains more than

    • 100 billion neurons. Most are located in the brain.

  • Glial Cells

    • Remove dead neurons and waste products from the nervous system.

    • Nourish and insulate neurons; form the myelin sheath.

    • Myelin sheaths play a critical role in the neural transmission of messages.

Parts of a Neuron with Functions

  • Dendrites: Receive signals from other cells.

  • Cell body: Organizes and maintains cell function.

  • Cell membrane: Protects the cell.

  • Axon hillock: Generates impulse in the neuron.

  • Nucleus: Controls the entire neuron.

  • Node of Ranvier: Facilitates diffusion of ions along the axon.

  • Schwann cell: Produces the myelin sheath in the peripheral nervous system.

  • Axon: Transmits signals to other cells and organs.

  • Myelin sheath: Increases the speed of the signal along the axon.

  • Axon terminal: Forms junctions with other cells (synapses).

Myelination and Neural Maturity

  • As a child matures, axons lengthen and dendrites/terminals proliferate, creating vast interconnected networks for transmitting complex messages.

  • The myelin sheath minimizes leakage of electrical current along the axon, allowing messages to be conducted more efficiently.

  • Myelination is part of maturation that enables crawling and walking during the first year of life.

Development of Synapses

  • 36 weeks gestation to 6 years (and beyond): Synapse formation followed by synapse pruning as part of maturation.

  • Key idea: Neural networks become more efficient through selective pruning and strengthening of synaptic connections.

Afferent and Efferent Neurons

  • Afferent neurons: Carry information from sensory receptors to the central nervous system (CNS).

    • Also called sensory neurons.

  • Efferent neurons: Carry motor information away from the CNS to muscles and glands.

    • Also called motor neurons.

  • Location of cell bodies:

    • Afferent: Outside the spinal cord (dorsal root ganglion).

    • Efferent: Located in the ventral horn of the spinal cord.

  • Pathways:

    • Peripheral nervous system (PNS) includes autonomic and somatic divisions.

    • Autonomic division regulates involuntary body responses (e.g., blood vessels).

    • Somatic division governs voluntary movement by skeletal muscles.

The Neural Impulse: The Body Electric

  • Neural impulses travel within neurons at speeds ranging from roughly
    v ∈ [2,225] miles/hourv \,\in \,[2, 225] \text{ miles/hour}
    (2 mph in nonmyelinated neurons to 225 mph in myelinated neurons).

  • Distances in the body are short; a message can travel from a toe to the brain in about
    t≈150s=0.02st \approx \tfrac{1}{50} \text{s} = 0.02 \text{s}

  • Key concept: Neurons transmit electrical signals (action potentials) and chemical signals (neurotransmitters) across synapses.

An Electrochemical Voyage: Resting Potential and Action Potential

  • Ions in solution (positive and negative charges) underlie neural signaling.

  • Resting membrane area becomes permeable to positively charged sodium ions (Na+) upon adequate stimulation, leading to depolarization.

  • The action potential is the electrical impulse that propagates along the axon.

  • Resting potential: Vrest≈−70 mVV_{rest} \approx -70\ \text{mV}

  • Threshold: Vth≈−55 mVV_{th} \approx -55\ \text{mV}

  • Peak depolarization may reach around +40 mV, followed by repolarization and hyperpolarization.

Firing: How Messages Voyage from Neuron to Neuron

  • Firing: conduction of the neural impulse along the length of a neuron.

  • Transmission occurs along the axon and via neurotransmitter release at synapses.

  • All-or-none principle: A neuron either fires with the same magnitude or does not fire at all.

    • Mathematical representation (conceptual): if stimulus strength (S) exceeds threshold (V_{th}), an action potential of fixed amplitude is generated.

  • Refractory period: After firing, there is a brief period (a few thousandths of a second) when a neuron is insensitive to further stimulation and will not fire.

The Synapse: Being Well-Connected

  • Synapse: A junction between the axon terminals of one neuron and the dendrites or cell body of another neuron.

  • Synaptic transmission involves the release of neurotransmitters from synaptic vesicles into the synaptic cleft, binding to receptors on the postsynaptic neuron.

Neurotransmitters: The Chemical Keys to Communication

  • Neurotransmitters: Chemical substances involved in the transmission of neural impulses from one neuron to another.

  • Receptor site: A location on a dendrite of a receiving neuron tailored to receive a neurotransmitter.

  • Acetylcholine (ACh): Involved in memory, learning, attention, arousal, and involuntary muscle movement; prevalent in the hippocampus. Decreased ACh production is linked to Alzheimer's disease.

  • Dopamine: Affects pleasure, voluntary movement, learning, and memory. Linked to schizophrenia when overutilized.

  • Norepinephrine (noradrenaline): Similar in action to epinephrine; may play a role in depression.

  • Serotonin: Involved in emotional arousal and sleep.

  • Gamma-aminobutyric acid (GABA): An inhibitory neurotransmitter that helps calm anxiety.

  • Endorphins: Inhibitory neurotransmitters produced in the brain and bloodstream; structurally similar to morphine in function.

  • Dopaminergic system (illustrated concept): Dopamine-producing neurons release dopamine into synapses; dopamine receptors on receiving cells detect dopamine; metabolites of dopamine are present in the synaptic environment.

The Brain: Central and Peripheral Nervous Systems

  • CNS (Central Nervous System): Brain and spinal cord; responsible for perception, processing, and integration of information; execution of voluntary motor responses; regulation of homeostasis.

  • PNS (Peripheral Nervous System): Nerves and ganglia outside the CNS; subdivided into somatic (sensorimotor, voluntary) and autonomic (involuntary) divisions.

  • Somatic Nervous System: Connects CNS with sensory receptors, skeletal muscles, and the body surface; governs voluntary movement.

  • Autonomic Nervous System: Regulates glands and involuntary bodily functions (heartbeat, respiration, digestion, pupil dilation).

  • Autonomic divisions:

    • Sympathetic: Fight or flight responses.

    • Parasympathetic: Rest and digest.

  • Enteric Nervous System (ENS): Located in the digestive tract; capable of autonomous operation and can function independently of the brain and spinal cord.

The Central Nervous System: The Brain and Spinal Cord

  • Spinal cord: Information superhighway; transmits messages between sensory receptors and brain; also mediates spinal reflexes (unlearned responses, sometimes requiring only 2 neurons).

  • Gray matter: Consists of unmyelinated neurons.

  • Brainstem components:

    • Medulla: Regulates basic functions such as heart rate, blood pressure, and respiration.

    • Pons: Transmits information about body movement; involved in attention, sleep/arousal, and respiration.

    • Cerebellum: "Little brain"; maintains balance and controls voluntary motor behavior.

    • Reticular formation: Involved in attention, sleep, and arousal.

  • Thalamus: Relays sensory information to the cortex; involved in sleep and attention.

  • Hypothalamus: Regulates body temperature, motivation, and emotion.

  • Limbic system: Involved in memory, motivation, and emotion.

  • Amygdala: Facilitates stereotypical aggressive behavior.

  • Cerebrum: Large mass of the forebrain consisting of two hemispheres.

  • Cerebral cortex: The wrinkled surface area (gray matter) of the cerebrum; divided into lobes.

  • Corpus callosum: Thick bundle that connects the two hemispheres of the cortex.

  • Lobes of the cortex:

    • Frontal

    • Parietal

    • Temporal

    • Occipital

  • Cerebral cortex anatomy (descriptive):

    • Grey matter on the outer surface; white matter lies beneath.

    • Front view and side view representations (landmarks: sulci and gyri).

Aphasia: Language Impairments

  • Aphasia: Disruption of the ability to understand or produce language.

  • Wernicke’s aphasia: Impaired comprehension of speech and difficulty selecting appropriate words.

  • Broca’s aphasia: Understands language plausibly but speaks slowly and laboriously.

The Endocrine System: Glands and Hormones

  • Pituitary gland and hypothalamus:

    • Pituitary: Known as the master gland; secretes growth hormone.

    • Hypothalamus: Regulates pituitary activity; involved in various homeostatic processes.

    • Oxytocin: Stimulates labor in pregnant women and is associated with maternal behavior (cuddling and caring for offspring).

  • Pineal gland: Secretes melatonin, which helps regulate the sleep-wake cycle.

  • Thyroid gland: Produces thyroxin (thyroxine), which affects the body's metabolism.

  • Adrenal glands: Secrete corticosteroids, which increase resistance to stress, promote muscle development, and mobilize energy by releasing stored sugar during emergencies.

  • Gonads (Testes and Ovaries): Produce steroids including testosterone and estrogen.

  • Puberty notes:

    • During puberty, testosterone stimulates muscle and bone growth and the development of primary and secondary sex characteristics.

    • Ovaries produce estrogen and progesterone, as well as some testosterone.

Connections and Implications

  • Functional integration: The nervous and endocrine systems coordinate to regulate behavior, cognition, emotion, and physiological homeostasis.

  • Clinical relevance: Dopamine imbalances relate to schizophrenia; acetylcholine deficits relate to memory disorders like Alzheimer's disease; GABA modulates anxiety; serotonin and norepinephrine influence mood and arousal; autonomic balance (sympathetic vs parasympathetic) affects stress responses and everyday functioning.

  • Real-world relevance: Understanding synaptic transmission informs pharmacology (e.g., antipsychotics, antidepressants), neurology, and psychology.

  • Ethical/philosophical implications: Case studies like Phineas Gage illustrate how brain structure correlates with personality and behavior, raising questions about mind–brain identity and responsibility.

Quick Reference: Key Numbers and Constants

  • Neurons in the human brain: > 101110^{11} neurons.

  • Neuron signal speeds:

    • Nonmyelinated: about 2 mph2\ \text{mph}

    • Myelinated: up to 225 mph225\ \text{mph}

  • Typical travel time from toe to brain: t≈0.02 st \approx 0.02\ \text{s}

  • Resting membrane potential: Vrest≈−70 mVV_{rest} \approx -70\ \text{mV}

  • Threshold potential: Vth≈−55 mVV_{th} \approx -55\ \text{mV}

  • Action potential amplitude: can reach up to about +40 mV+40\ \text{mV} during peak depolarization

  • Synapse formation and pruning timeline: starts prenatally (36 weeks gestation) and continues through early childhood up to at least 6 years and beyond.

  • Hormonal players: oxytocin, melatonin, thyroxine (thyroxin), corticosteroids, testosterone, estrogen, progesterone.

Connections to Foundational Principles

  • Structure–function relationship: Neuron morphology (dendrites, axon, myelin) dictates signaling speed and integration.

  • Electrical and chemical signaling: Action potentials (electrical) and neurotransmitter release at synapses (chemical) together enable rapid, precise communication.

  • Plasticity: Synapse formation and pruning illustrate neural plasticity essential for learning and development.

  • System integration: CNS and PNS coordination with endocrine signals underpins behavior, cognition, sleep, stress response, and homeostasis.

  • Clinical translation: Descriptions of aphasia, Gage’s case, and neurotransmitter roles anchor theoretical knowledge to real-world human outcomes.

Summary Takeaways

  • The nervous system is composed of neurons and glial cells, with myelination accelerating transmission.

  • Neurons communicate via electrical impulses and chemical synapses, obeying the all-or-none principle and featuring refractory periods.

  • The brain is organized into CNS and PNS, with distinct regions and systems (brainstem, limbic system, cortex, autonomic and somatic divisions).

  • Neurotransmitters link neurons across synapses, with specific roles for acetylcholine, dopamine, norepinephrine, serotonin, GABA, and endorphins.

  • The endocrine system works in concert with the nervous system to regulate physiology and behavior through hormones.