Week 3 Neurobiology and Neural Communication Vocabulary

Subcortical Structures and Central Brain Systems

• Functional organization of subcortical structures and thalamic nuclei:

  • Lateral Geniculate Nucleus (LGN): Primary thalamic relay nucleus for visual sensory information.

  • Medial Geniculate Nucleus (MGN): Primary thalamic relay nucleus for auditory sensory information.

  • Ventral Posterolateral Nucleus (VPL): Primary thalamic relay nucleus for somatosensory information originating from the body.

  • Ventral Posteromedial Nucleus (VPM): Primary thalamic relay nucleus for somatosensory information originating from the head and face.

• Limbic System:

  • A neural network implicated in processing emotion, motivation, learning, memory, and autonomic regulatory responses.

  • Autonomic responses encompass functions that the body handles automatically without conscious oversight, such as respiration and digestion.

  • Hippocampus: A bilateral structure named from the Latin word for "seahorse" due to its visual morphology; plays an essential role in learning and memory processing.

  • Hypothalamus: A small subcortical region critical for regulating emotional behavior and maintaining homeostasis (physiological internal balance). Functions in coordination with the pituitary gland and adrenal glands; drives fight-or-flight responses outside of conscious awareness in response to environmental threats.

• Basal Ganglia:

  • Subcortical nuclei located beneath the cerebral cortex.

  • Functionally modulates motor action through reciprocal loop interactions with the frontal lobes.

• Autonomic Nervous System (ANS):

  • Sympathetic Branch: Activated under conditions of threat or stress, driving the fight-or-flight response.

  • Parasympathetic Branch: Active when not under threat, maintaining baseline physiological balance and rest-and-digest operations.

• Methodological Axiom:

  • "The rat is always right": A foundational principle stating that an organism's physiological or behavioral response is an accurate reflection of neural processes; the task of investigation is to determine the exact neural mechanism driving the observed response.

Neuroglia: Classification and Functional Roles

• Glial Cells of the Central Nervous System (CNS):

  • Astrocytes: Star-shaped glial cells that form crucial structural support networks within the brain. Astrocytes create the blood-brain barrier via specialized fatty end-feet wrapped around cerebral blood vessels, regulating chemical access to neuronal tissue.

  • Oligodendrocytes: Myelin-forming glial cells within the CNS. A single oligodendrocyte extends multiple cellular processes to wrap myelin sheaths around multiple distinct axons (and multiple axon segments) simultaneously.

  • Microglial Cells: Small, phagocytotic immune cells in the CNS responsible for engulfing, clearing, and getting rid of damaged cells and cellular debris.

• Glial Cells of the Peripheral Nervous System (PNS):

  • Schwann Cells: Myelin-forming glial cells within the PNS. Unlike CNS oligodendrocytes, a single Schwann cell forms a myelin sheath around only one single axon segment along a peripheral nerve fiber.

• Nodes of Ranvier:

  • Uninsulated gaps along a myelinated axon between adjacent glial sheaths where the axon membrane is exposed, facilitating rapid electrical transmission.

Anatomic Architecture of the Neuron

• Cytological Features Shared with Body Cells:

  • Neurons possess a cell body (soma) containing cytoplasm, a nucleus, a cell membrane, and standard cellular organelles.

  • The neuronal membrane consists of a lipid bilayer separating the intracellular fluid inside the cell from the extracellular space outside.

• Specialized Neurites (Cellular Extensions):

  • Neurons extend protruding processes from the cell body known as neurites, which are categorized into dendrites and axons.

• Distinctions Between Axons and Dendrites:

  • Direction of Signal Conduction: Dendrites convey electrical signals toward the cell body; axons convey electrical signals away from the cell body toward target cells (other neurons, muscle tissues, or glands).

  • Surface Topography: Dendrites exhibit a rough surface due to the presence of dendritic spines; axons exhibit a smooth surface structure.

  • Numerical Distribution: Neurons typically feature many dendrites radiating into a complex dendritic tree; neurons typically feature only one single axon (or occasionally none).

  • Protein Synthesis Capabilities: Dendrites contain ribosomes that allow local protein synthesis; axons contain no ribosomes.

  • Myelination: Axons can be insulated by a fatty myelin sheath; dendrites are never myelinated.

  • Branching Pattern: Dendrites branch extensively near the cell body; axons branch further away from the cell body toward terminal branches.

• Functional Sub-regions of Neurons:

  • Cell Body (Soma): The life-support center of the neuron containing the cell nucleus and primary organelles.

  • Axon Hillock: The specialized junction connecting the cell body to the initial segment of the axon. Serves as the site where sub-threshold membrane potentials are integrated to determine if an action potential will fire.

  • Axon Trunk & Initial Segment: Long process that functions as a conduit for conducting electrical signals over distance.

  • Terminal Branches & Presynaptic Terminals: Distal ends of an axon that form synaptic junctions (tiny physical gaps) with postsynaptic cells.

Morphological Categories and Signal Flow

• Morphological Classification of Neurons:

  • Unipolar Neurons: Feature a single protruding process extending from the cell body; highly common in invertebrate nervous systems.

  • Bipolar Neurons: Feature two distinct processes (one axon and one dendrite) extending from opposite poles of the cell body; found in specialized sensory pathways.

  • Pseudounipolar Neurons: Feature a single neurite extending from the cell body that branches early into two functional axon arms (one central and one peripheral); found extensively in the peripheral nervous system.

  • Multipolar Neurons: Feature multiple dendrites radiating from the cell body alongside a single axon; the most prevalent morphological class in the central nervous system.

• Directional Pathway of Information Flow:

  • Standard sequential signal direction in most neurons follows: Synapse $\rightarrow$ Dendrite $\rightarrow$ Soma (Cell Body) $\rightarrow$ Axon $\rightarrow$ Synapse.

Conduction Biophysics: Electrotonic vs. Active Propagation

• Passive / Electrotonic Conduction (Decremental Spread):

  • Refers to the passive, sub-threshold spread of electrical current along the neuronal membrane.

  • Decremental nature: Signal amplitude decays exponentially as a function of the distance it travels away from the point of current injection.

  • Voltage drop threshold: At a characteristic length $l_c$, membrane voltage drops to approximately $\frac{1}{3}$ ($37\%$) of its maximum initial value.

  • Cable / Live-Wire Analogy: Plugging an electrical wire into an outlet. Grabbing a short $2\,\text{ft}$ live cable delivers a severe electric shock because current is unattenuated; however, if the wire is extremely long, electrical resistance causes current to dissipate over distance so that grabbing it causes no harm. Thus, signal strength in electrotonic conduction is strictly a function of travel distance.

  • Graded Potentials: Sub-threshold fluctuations in membrane potential whose magnitude is directly proportional to the strength of the input stimulus.

  • Depolarization: A change in membrane potential in the positive direction relative to resting state (a decrease in internal negative charge).

  • Hyperpolarization: A change in membrane potential in the negative direction relative to resting state (an increase in internal negative charge).

• Active Conduction / Action Potential Propagation (Non-decremental):

  • Active conduction relies on the continuous regeneration of electrical current along the axon membrane via voltage-gated ion channels.

  • Non-decremental nature: The action potential signal never depletes, decays, or loses amplitude regardless of axon length.

  • Invariable signal parameters: The shape, size, amplitude, and speed of an action potential cannot be modified once triggered.

  • Historical Research Preparation: Electrophysiological principles of active axonal conduction were demonstrated using the Giant Axon of the Squid (Hodgkin & Huxley).

Action Potential Electrophysiology and Ion Channel Dynamics

• Action Potential Fundamentals:

  • Definition: A brief electrical impulse that serves as the basis for active information conduction along an axon.

  • Resting Potential: The baseline electrical charge of an unstimulated neuron, measured at $-70\,\text{mV}$.

  • Threshold of Excitation / Activation: The critical level of depolarization at the axon hillock (typically $-55\,\text{mV}$ to $-60\,\text{mV}$) that must be exceeded to initiate an action potential.

  • All-or-None Law: A neuron either fires a complete, full-blown action potential or no action potential occurs at all. Each firing produces an impulse of identical amplitude, shape, and speed, completely independent of the intensity of the stimulus that initiated it (provided threshold is reached).

• Phase-by-Phase Ionic Mechanisms of the Action Potential:

  • Resting State: Membrane potential rests at $-70\,\text{mV}$.

  • Phase A (Threshold Activation & $\text{Na}^+$ Opening): Depolarizing stimuli reach the threshold of activation at the axon hillock, causing voltage-gated Sodium ($\text{Na}^+$) channels to open. $\text{Na}^+$ ions rapidly enter the intracellular fluid down concentration and electrical gradients.

  • Phase B (Depolarization Rise & $\text{K}^+$ Opening): Rapid influx of $\text{Na}^+$ causes steep depolarization; voltage-gated Potassium ($\text{K}^+$) channels open, initiating an outward flow of $\text{K}^+$ ions.

  • Phase C (Peak Depolarization & $\text{Na}^+$ Inactivation): Membrane potential reaches its peak at approximately $+40\,\text{mV}$. Voltage-gated $\text{Na}^+$ channels become refractory / inactivated, preventing any further $\text{Na}^+$ influx.

  • Phase D (Repolarization via $\text{K}^+$ Efflux): Inactivated $\text{Na}^+$ channels block entry while $\text{K}^+$ ions continue exiting the cell, rapidly driving membrane potential back down toward negative values.

  • Phase E ($\text{K}^+$ Closing & $\text{Na}^+$ Channel Reset): Membrane voltage drops past threshold; $\text{K}^+$ channels begin closing, and $\text{Na}^+$ channels reset from refractory/inactivated back to closed resting states.

  • Phase F (Hyperpolarization Undershoot & Recovery): Transient hyperpolarization occurs because $\text{K}^+$ channels close slowly, allowing excess $\text{K}^+$ to accumulate outside. As extra external $\text{K}^+$ diffuses away, membrane potential returns to baseline resting state ($-70\,\text{mV}$).

  • Refractory Periods: Absolute refractory period occurs during peak firing when $\text{Na}^+$ channels are inactivated; during this interval, no second action potential can fire regardless of stimulus strength.

• Stimulus Intensity Coding (Rate Law):

  • Because action potential amplitude and velocity are fixed by the All-or-None law, neurons encode the strength of an input stimulus by altering their rate (frequency) of firing over time.

  • Weak Stimulus: Produces a low firing rate / frequency of action potentials over time.

  • Strong Stimulus: Produces a high firing rate / frequency of action potentials over time.

Synaptic Transmission and Neurotransmitter Dynamics

• Structural Elements of the Chemical Synapse:

  • Presynaptic Side: Terminal ending of the sending axon containing energy-providing mitochondria and membrane-bound synaptic vesicles loaded with packets of neurotransmitter molecules.

  • Synaptic Cleft: A tiny fluid-filled physical gap separating presynaptic and postsynaptic neuronal membranes.

  • Postsynaptic Side: Specialized membrane region on the receiving dendrite or soma containing specific neurotransmitter receptor sites.

• Step-by-Step Mechanism of Chemical Transmission:

  1. An action potential propagates down the axon and reaches the presynaptic axon terminal.

  2. Depolarization triggers synaptic vesicles to fuse with the presynaptic cell membrane.

  3. Neurotransmitter molecules are released out of vesicles into the synaptic cleft via exocytosis.

  4. Neurotransmitters diffuse across the synaptic cleft gap and bind to receptor sites on the postsynaptic membrane.

  5. Receptor binding induces active postsynaptic currents (graded depolarizations or hyperpolarizations) in the receiving neuron.

• Five Mechanisms for Modulating Neurotransmitter Concentration in the Synaptic Cleft:

  • Mechanism A (Reuptake): Specialized presynaptic transporter proteins actively reabsorb intact neurotransmitter molecules from the synaptic cleft back into the presynaptic terminal for repackaging or reuse.

  • Mechanism B (Enzymatic Deactivation / Degradation): Specific enzymes located within the synaptic cleft metabolize and break down neurotransmitter molecules into inactive chemical fragments.

  • Mechanism C (Glial Cell Degradation): Adjacent glial cells (astrocytes) actively take up neurotransmitter molecules from the cleft and metabolize/degrade them.

  • Mechanism D (Autoreceptors): Specialized receptors situated on the presynaptic terminal membrane bind the neuron's own released neurotransmitters, initiating internal feedback loops that adjust or inhibit further neurotransmitter release.

  • Mechanism E (Diffusion): Neurotransmitter molecules passively diffuse out of the synaptic cleft away from receptor sites into the surrounding extracellular fluid.