E

Physiological Psychology - Neuron Function and Brain Organization (Lecture 3)

Neuron Structure and Communication

  • Electro-chemical process governing how neurons transmit information within and between cells.
  • Located in intra-cellular and extra-cellular fluids; communications rely on chemical signals across gaps (synapses) and electrical signals along membranes.
  • Key neuron structures:
    • Dendrites: branches receiving messages
    • Soma (cell body): metabolic center
    • Axon: passes messages to other neurons; ends in axon terminal button
    • Synapse: the gap where axon terminal of one neuron meets a dendrite of another
    • Axon collateral: side branch of the axon
    • Myelin sheath: insulating layer around many axons to speed transmission
    • Nodes of Ranvier: gaps in myelin that facilitate rapid conduction
    • Neurilemma: outer membrane of the axon (part of the myelinated fiber in the peripheral nervous system)
    • Axon terminals: terminal buttons that release neurotransmitters
    • Semi-permeable membrane: selective barrier that maintains ion gradients
  • Overall process: intra- and extracellular mixing of ions, diffusion, and electrostatic forces drive signaling across membranes and synapses.

Resting Potential, Threshold, and Action Potential

  • Membrane potential varies with ion distribution across the membrane.
  • Resting potential: V_{rest} \,\approx\, -70\ \text{mV}
  • Threshold for firing: V_{th} \,\approx\, -50\ \text{mV}
  • Action potential (AP): rapid depolarization/repolarization sequence reaching about V_{AP} \,\approx\, +30\ \text{mV}
  • All-or-nothing principle: once threshold is reached, AP occurs completely; magnitude remains constant.
  • Directionality: signaling within a neuron is one-way from dendrites/soma toward axon terminals.

Ions, Channels, and Voltage Changes

  • Action potentials involve voltage-gated ion channels (ion flow drives the change in membrane potential).
  • Major ions: \mathrm{Na}^+\, ,\; \mathrm{K}^+\, ,\; \mathrm{Cl}^-
  • Phases of an AP:
    • Rising (depolarization): Na^+ flows in via voltage-gated Na^+ channels
    • Peak: Na^+ channels inactivated; Na^+ influx ceases
    • Repolarization: K^+ flows out via voltage-gated K^+ channels
    • Hyperpolarization: continued K^+ efflux lowers potential below resting briefly
  • Refractory period: short interval after an AP during which neuron is less excitable; neurons can fire up to approximately 1000\ \text{AP/s} under some conditions.

Ionic Mechanisms and the Diffusion/Electrostatic Forces

  • Outside of cell: high Na^+ and Cl^- concentrations; inside: high K^+ concentration and negative charge.
  • Diffusion: ions move from high to low concentration.
  • Electrostatic forces: charged ions move toward opposite charges.
  • During resting state: inside is relatively negative; outside is relatively positive.
  • The balance of diffusion and electrostatic forces maintains the resting potential and drives APs when threshold is reached.

The Sodium-Potassium Pump

  • Maintaining ionic gradients requires energy.
  • Mechanism: Na^+/K^+ pump uses energy (ATP) to move ions against gradients:
    • 3\ Na^+\ \text{ions are pumped out}
    • 2\ K^+\ \text{ions are pumped in}
  • This helps restore and maintain the resting potential after activity and supports the refractory period.

Synaptic Transmission: Chemical Communication

  • An action potential arriving at the axon terminal triggers chemical communication across the synapse.
  • Steps:
    1) Action potential reaches axon terminal.
    2) Voltage-gated Ca^{2+} channels open in the presynaptic membrane.
    3) Ca^{2+} influx causes synaptic vesicles to move toward the membrane and fuse with it via microtubules.
    4) Vesicles release neurotransmitters into the synaptic gap (synaptic cleft).
    5) Neurotransmitters bind to matching receptors on the postsynaptic dendrite or soma.
    6) Binding alters postsynaptic membrane potential, making the receiving cell more or less likely to fire (excitatory or inhibitory postsynaptic potentials).
    7) Signal ends via reuptake, enzymatic degradation, or diffusion away from the synapse.
  • Vesicle dynamics (illustrated): vesicles within presynaptic terminal release contents; some vesicle material is recycled; vesicle transport can be retrograde to the soma; vesicles may be refilled at the axon terminal.
  • Postsynaptic effects depend on receptor type and ion flow.

Receptors, Postsynaptic Potentials, and Signal Integration

  • Binding site and ion channel dynamics:
    • Open ion channels cause changes in membrane potential.
    • EPSP (excitatory postsynaptic potential): Influx of Na^+ leads to depolarization.
    • IPSP (inhibitory postsynaptic potential): Influx of Cl^- or efflux of K^+ leads to hyperpolarization.
  • The net pattern and timing of EPSPs and IPSPs determine whether the postsynaptic neuron reaches threshold and fires.

Neurotransmitters, Neuromodulators, and Hormones

  • Neurotransmitters: chemical messengers released at synapses.
    • Dopamine, Acetylcholine, Serotonin
    • Neuropeptides or neuromodulators (often modulatory effects, slower timescale)
  • Hormones: part of the endocrine system; influence brain function alongside neurotransmitters.
  • Neurotransmitter actions include reuptake and enzymatic deactivation to terminate signaling.

Endocrine System: Glands and Hormones

  • Pineal gland: helps regulate body rhythms and sleep cycles.
  • Pituitary gland: influences growth and lactation; regulates activity of other glands.
  • Thyroid gland: regulates metabolic rate.
  • Adrenal glands: secrete hormones linked to arousal, stress response, salt balance, and sexual functioning.
  • Pancreas: releases insulin to regulate blood sugar and hunger.
  • Testes: secrete testosterone; influences male sexual function.
  • Ovaries: secrete estrogen; influences female sexual function.

The Nervous System: Organization and Pathways

  • Nerves and branches:
    • Myelination speeds transmission; nodes of Ranvier increase conduction velocity.
    • Central nervous system (CNS): brain and spinal cord.
    • Peripheral nervous system (PNS): nerves carrying information to/from spinal cord/brain.
    • Somatic nervous system: controls voluntary movements and conveys sensory information.
    • Autonomic nervous system: controls involuntary functions; subdivides into sympathetic and parasympathetic branches.
  • Sympathetic: emergency response, arousal; raises heart rate and blood pressure; suppresses digestion; part of the 'fight or flight' response.
  • Parasympathetic: conserves energy; lowers heart rate and blood pressure; supports digestion and normal breathing; works in opposition to sympathetic.

Autonomic Nervous System: Detailed Roles

  • Sympathetic functions (examples):
    • Constricts some pupils; stimulates some tear production; inhibits digestion; increases heart rate and respiration; stimulates sweat glands; releases adrenaline (epinephrine) and glucose mobilization from liver; relaxes bladder; inhibits digestion.
  • Parasympathetic functions (examples):
    • Dilates pupils; stimulates saliva and tear production; slows heart rate; constricts respiration and blood vessels; stimulates digestion; contracts bladder; inhibits ejaculation in males; etc.
  • The two branches work in opposition at all times to regulate the internal environment.

Spinal Cord and Reflexes

  • Spinal cord enables many automatic behaviors (spinal reflexes) to speed responses to injury.
  • Reflex arc: stimulus -> pain receptor -> sensory neuron -> connector neuron in spinal cord -> motor neuron -> effector cells (muscle contraction).
  • Role in balance and automatic responses independent of the brain.

Studying the Brain: Methods

  • Ablation: destruction or removal of brain tissue (invasive).
  • Electrode stimulation: surface stimulation (can be non-invasive or during surgery) or deep lesioning using implanted electrodes (invasive).
  • ESB (Electrical Stimulation of the Brain): used during brain surgery to map functions.
  • Single-cell recording electrodes: microelectrodes for recording activity from individual neurons (invasive).
  • EEG (Electroencephalography): network of electrodes on the scalp; non-invasive; measures global brain activity.

Brain Imaging Techniques

  • CT scan (Computerized Tomography): specialized X-ray imaging for structural detail.
  • MRI (Magnetic Resonance Imaging): uses strong magnets to image brain structure.
  • fMRI (Functional MRI): measures brain activity by detecting changes in blood flow related to neural activity.
  • PET scan (Positron Emission Tomography): injects radiolabeled glucose to image regions with high metabolic activity.

Seeing, Hearing, Speaking, Thinking: Brain Functions

  • Different brain regions contribute to sensory processing, language, action, and cognition.
  • Lateralization often reflects specialization of hemispheres (see below).

Hemispheric Specialization

  • Left hemisphere: language processing and related tasks.
  • Right hemisphere: perception of patterns, facial recognition, emotion detection, and understanding subtleties of speech.
  • This specialization supports how people process different types of information and can influence recovery after brain injury.

The Cerebral Cortex: Cortex and Lobes

  • Cerebral cortex: outer layer of neural tissue; folds increase surface area; about 3 mm thick; contains ~70% of neurons in the CNS.
  • 2 hemispheres connected by the corpus callosum (bundle of axons) that enables inter-hemispheric communication.
  • Hemispheric control: each half largely controls and receives sensory information from the opposite side of the body; damage to one side can cause paralysis on the opposite side.
  • Split-brain procedures illustrate lateralization by severing the corpus callosum.
  • The cortex is organized into lobes with specialized but interconnected regions.

Lobes of the Cortex and Functional Areas

  • Occipital lobe: visual processing.
  • Temporal lobe: auditory processing and language; contains association areas.
  • Parietal lobe: somatosensory cortex; maps body sensations; includes association areas.
  • Frontal lobe: motor cortex and association areas; prefrontal cortex supports higher-level functions and inhibition of brainstem-mediated behaviors.
  • Primary motor cortex (frontal lobe): contains motor homunculus (representation of body parts).
  • Primary somatosensory cortex (parietal lobe): somatic sensation map.
  • Prefrontal cortex: planning, decision-making, inhibition, executive functions.
  • Broca's area (usually left hemisphere): language production; association cortex; damage => Broca's aphasia (telegraphic speech) with preserved meaning.
  • Wernicke's area (in temporal lobe): language comprehension; association cortex; damage => Wernicke's aphasia (fluent speech but with impaired meaning).
  • Orbitofrontal and other prefrontal areas contribute to complex tasks and social behavior.

Localization of Function and Association Cortex

  • Localization of Function: distinct brain regions specialized for specific processes (e.g., motor cortex, sensory cortex, visual areas, auditory areas).
  • Association Cortex: integrates information from multiple senses and supports memory and higher-order processing.

Agnosia and Related Deficits

  • Agnosia: inability to grasp meaning of stimuli (words, objects, pictures).
  • Prosopagnosia (facial agnosia): inability to recognize faces; related to fusiform face area.
  • Deficits illustrate how specific brain regions contribute to perceptual and cognitive functions.

Subcortex and Limbic System

  • Subcortex includes: brainstem (medulla, pons), cerebellum, limbic system (thalamus, hypothalamus, hippocampus, amygdala).
  • Key structures:
    • Thalamus (diencephalon): relay station for sensory and motor signals; contributes to consciousness and alertness.
    • Hypothalamus: regulates hormones, body temperature, hunger, thirst, circadian rhythms; connects to pituitary.
    • Hippocampus: memory formation and spatial navigation.
    • Amygdala: emotion processing, fear, reward; influence on memory encoding.
    • Fornix: major output tract of the hippocampus; part of the limbic system circuitry.
    • Cingulate gyrus: involved in emotion formation and processing, learning, and memory; part of the limbic system.
    • Mammillary bodies: connected to hippocampus via fornix; involved in recollective memory.
  • Brainstem components:
    • Medulla: autonomic and reflex control (breathing, heart rate).
    • Pons: bridge in brainstem; relay center.
    • Reticular formation: arousal and attention; sleep-wake regulation.
  • Cerebellum: balance, posture, coordination, motor learning and memory of skills.

Neuroplasticity and Neurogenesis

  • Neuroplasticity refers to the brain's ability to change in response to experience or injury.
  • Key processes:
    • Neurogenesis: birth of new neurons.
    • Migration: new neurons move to their destined locations.
    • Arborization: growth of dendrites and axons to form connections (two phases: growth and refinement).
    • Cell death: pruning of unused neurons to optimize networks.

Localization Across Species

  • Localization of function can be compared across species (e.g., squirrel monkey, cat, rhesus monkey, dog, chimp, human).
  • Comparative neuroanatomy helps understand evolutionary changes in brain organization and function.

Summary of Key Concepts and Equations

  • Resting potential: V_{rest} \approx -70\ \,\text{mV}
  • Threshold: V_{th} \approx -50\ \text{mV}
  • Action potential: V_{AP} \approx +30\ \text{mV}
  • All-or-nothing: AP occurs fully once threshold is reached.
  • Ion dynamics: Na^+ influx during rising phase; K^+ efflux during repolarization; Cl^- can contribute to IPSP.
  • Neurotransmission steps: AP -> Ca^{2+} influx -> vesicle release -> neurotransmitter binding -> EPSP/IPSP -> potential next-fire -> reuptake/enzymatic degradation.
  • Na^+/K^+ pump: 3\ Na^+ \, out,\ 2\ K^+ \, in per ATP hydrolysis; maintains gradients and resting potential.
  • Sympathetic vs Parasympathetic: opposing branches of the autonomic nervous system regulating involuntary functions.
  • Brain imaging modalities: CT, MRI, fMRI, PET describe structural vs functional imaging capabilities.
  • Broca's vs Wernicke's areas: language production vs language comprehension; lesions produce distinct aphasias.
  • Agnosia and prosopagnosia highlight regional specialization for perception and memory.
  • Spinal reflex arcs: rapid, automated responses to stimuli without brain input; sensory -> interneuron -> motor.
  • Cortex organization: lobes (occipital, temporal, parietal, frontal) with motor and sensory maps; corpus callosum enables interhemispheric communication.
  • Neuroplasticity: neurogenesis, migration, arborization, and programmed cell death shape learning and recovery.

Quick Cross-Referencing Concepts

  • Dendrites/soma/axon alignment governs reception, integration, and transmission of signals.
  • Resting and action potentials depend on ionic gradients and channel dynamics.
  • Synaptic transmission bridges electrical signaling to chemical signaling and back to electrical on the postsynaptic side.
  • Higher cognition relies on cortical networks plus subcortical modulation (limbic system, thalamus, hypothalamus).
  • Studying the brain uses invasive and non-invasive methods to map structure, function, and connectivity, including cross-species comparisons to infer evolutionary adaptations.