Neurophysiology and Signaling Notes

Lipophilic vs Lipophobic Ligands and Receptors

• Lipophilicity concept: a molecule that likes fat can cross the plasma membrane; the membrane is a phospholipid bilayer with fatty acid tails in its core. Lipophilic ligands cross the membrane and can act inside the cell.
• Lipophilic ligands (lipid-loving): cross the membrane and may bind to intracellular receptors in the cytoplasm or nucleus. Examples include cholesterol-based hormones like estrogen, cortisol, aldosterone, testosterone. These ligands can interact with cytoplasmic receptors or nuclear receptors; some can also have surface receptor interactions (e.g., estrogen can have multiple receptor localizations).
• Lipophobic ligands (charged or polar): cannot cross the phospholipid bilayer and must bind to surface receptors on the cell membrane. Examples include amino-like molecules and catecholamines such as epinephrine.
• Receptor types for lipophobic ligands (surface receptors):
  • Inotropic (ionotropic) receptors
  • Metabotropic receptors
  • Enzyme-linked receptors
• Insulin as a key lipophobic ligand example:
  • Insulin cannot cross the membrane.
  • It binds to a membrane-embedded surface receptor (enzyme-linked receptor; receptor tyrosine kinase type).
  • This activates intracellular signaling cascades, typically via phosphorylation of intracellular enzymes (often a kinase cascade).
  • An intracellular enzyme (denoted as a) becomes phosphorylated, activating downstream signaling.
  • Insulin resistance can occur when insulin receptors are reduced in number or function, reducing efficacy (diabetes context).
• Significance of surface receptor signaling for lipophobic ligands:
  • Requires receptor presence and proper signaling machinery to elicit cellular responses.
  • Activation often involves phosphorylation cascades and second messengers.
• Lipophobic signaling in practice: three main surface receptor families are engaged when the ligand cannot cross the membrane (inotropic, metabotropic, enzyme-linked).
• Lipophilic ligands and their intracellular targets: lipophilic ligands do not require surface receptors; they directly affect intracellular targets once inside the cell.
• Summary concept: location of receptor (surface vs intracellular) dictates mechanism, speed, and scope of response.

Steroid Hormones, Lipophilic Ligands, and Receptor Localization

• Lipophilic ligands can cross the plasma membrane and bind to receptors inside the cell.
• Within the cell, two major receptor localizations are relevant:
  • Cytoplasmic (cytoplasmic) receptor: the ligand-receptor complex may interact with other cytoplasmic proteins and often translocates to the nucleus or directly modulates cytoplasmic targets.
  • Nuclear receptor: the ligand-receptor complex resides in the nucleus and directly binds DNA to regulate gene transcription.
• Estrogen as an example of lipophilic signaling:
  • Can traverse the membrane easily and interact with cytoplasmic receptors or nuclear receptors.
  • In some cells, estrogen binds cytoplasmic receptors that can translocate to the nucleus and regulate gene expression.
  • Estrogen can also have surface receptor interactions in some contexts.
• Classic steroid mechanism (general):
  • Lipophilic steroid hormones (e.g., estrogen, testosterone, aldosterone) cross the membrane and interact with intracellular receptors.
  • Cytoplasmic receptor pathway: ligand binds receptor in cytoplasm; the complex may translocate to the nucleus to affect gene transcription.
  • Nuclear receptor pathway: receptor already in the nucleus; ligand binding directly modulates transcription.
• Thyroid hormone as a notable exception among amines:
  • Although thyroid hormone is an amine, it behaves like a steroid in many respects.
  • Entry into the cell is via a transporter, not a simple diffusion across the membrane.
  • Once inside, T4 is converted to T3 by a cytoplasmic receptor/transformation step; T3 then binds to intranuclear receptors to regulate gene expression.
  • There is a special case where thyroid hormone action involves rapid, non-genomic effects via cytoplasmic mechanisms and longer genomic effects via transcriptional regulation.
• Thyroid hormone signaling pathways:
  • Cytoplasmic receptor route: T4 enters via transporter, is converted to T3, binds cytoplasmic receptor, and may produce rapid effects via activation of existing enzymes (primary/delayed concepts applicable).
  • Nuclear receptor route: T3 (or T4-derived T3) binds nuclear receptors to regulate transcription and protein synthesis (delayed response).
• Primary (immediate) vs delayed (late) responses for steroid hormones:
  • Primary response: immediate, often via activation of enzymes already present in the cell (e.g., phosphorylation cascades).
  • Delayed response: requires transcription and translation to produce new proteins; typically days to weeks.
• Primary vs delayed responses in thyroid hormone signaling:
  • Thyroid hormone can elicit rapid enzyme activation (primary) via cytoplasmic mechanisms in some cells (e.g., heart).
  • Nuclear receptor action leads to transcriptional upregulation and new protein synthesis (delayed).
• Summary cross-link: Steroid hormones use intracellular receptors and can produce both fast (cytoplasmic) and slow (genomic) effects; thyroid hormone is a notable exception among amines that requires intracellular handling to exert its effects, with a mix of transcriptional and non-genomic actions.

Nuclear Receptors, Cytoplasmic Receptors, and Gene Regulation Details

• Cytoplasmic receptors:
  • Receptors located in the cytoplasm that bind lipophilic ligands (e.g., some steroid hormones, thyroid hormone in certain contexts).
  • Binding can lead to translocation into the nucleus and gene regulation, or can activate cytoplasmic enzymes directly.
• Nuclear receptors:
  • Receptors located in the nucleus that bind lipophilic ligands and directly regulate transcription by binding to DNA at promoter/enhancer regions.
  • This typically modulates production of specific proteins over longer time frames.
• Thyroid hormone specifics:
  • T4 (thyroxine) enters cell via transporter; inside the cytoplasm, it can be processed to T3; the T3-receptor complex influences gene transcription in the nucleus.
  • There are also non-genomic, rapid actions that may occur through different cytoplasmic routes.
• Receptor localization determines response type and kinetics:
  • Cytoplasmic receptor actions tend to be faster, often involving phosphorylation cascades.
  • Nuclear receptor actions involve transcriptional changes and synthesis of new proteins, with a longer lag.
• Visualizing the steroid signaling with a generic cartoon concept:
  • Top: steroid hormone (lipophilic) diffuses across the membrane (blue plasma membrane) and binds to a receptor.
  • Pathway A (cytoplasmic receptor): Receptor-ligand complex acts in the cytoplasm and may affect immediate signaling or translocate to the nucleus.
  • Pathway B (nuclear receptor): Receptor-ligand complex binds DNA to regulate transcription, producing proteins over time.

Special Case: Estrogen and Steroid Hormone Signaling Variability

• Estrogen as a complex lipophilic ligand:
  • Can bind cytoplasmic receptors and translocate to the nucleus to regulate genes.
  • In some cells, estrogen also interacts with nuclear receptors and sometimes surface receptors.
  • This multitier signaling contributes to tissue-specific responses.
• Other steroid hormones (testosterone, aldosterone, cortisol):
  • Generally cross the membrane and act via intracellular receptors, leading to gene regulation and classical genomic effects.
  • Early responses may appear via cytoplasmic signaling cascades; delayed responses occur via gene transcription.

Action Potentials, Excitable Cells, and Neurophysiology Overview

• Central nervous system information flow:
  • Sensory information (e.g., smell, sight, pain) travels via sensory fibers to the CNS where it is processed.
  • The CNS sends information outward via efferent signals to effectors such as skeletal muscle for contraction.
  • Local anesthetics block action potentials in sensory nerves, reducing pain without completely paralyzing movement (often selectively affecting small-diameter C fibers).
• Neurons and fibers:
  • Neuron structure: soma (cell body, nucleus), dendrites (receive information), axon (sends information away), axon hillock (site of action potential initiation).
  • Axon terminals form synapses with target cells (neurons, muscle, glands).
  • Neurons can be broadly categorized by function/location: upper motor neuron (brain; synapses onto lower motor neuron) and lower motor neuron (spinal cord; synapses on muscle).
  • Upper motor neuron lesions vs lower motor neuron lesions have different clinical presentations due to circuitry differences.
• Sensory and motor fibers:
  • C fibers: small-diameter, unmyelinated, transmit pain signals to the brain.
  • Large-diameter motor fibers: typically myelinated, fast conduction to skeletal muscles for contraction.
  • Myelination and fiber diameter influence conduction speed: larger diameter and myelination yield faster transmission.
• Fiber conduction speed factors:
  • Axon diameter: larger diameter = faster conduction; smaller diameter = slower conduction.
  • Myelination: myelinated fibers conduct faster via saltatory conduction, with nodes of Ranvier interspersed along the axon.
• Nodes of Ranvier:
  • Gaps in the myelin sheath where voltage-gated channels are concentrated; essential for rapid depolarization and action potential regeneration.
• Local anesthetics and selective effects:
  • Local anesthetics primarily block action potentials in sensory (pain) fibers (C fibers) and do not completely block motor neurons, enabling some movement.

Action Potentials: Generation, Measurement, and Encoding of Stimuli

• Exitable cells:
  • Cells capable of generating rapid changes in membrane potential, i.e., action potentials; includes neurons, skeletal muscle, and certain endocrine cells like pancreatic beta cells with specialized signaling (not the main focus here).
• Measuring action potentials (conceptual):
  • Time axis in milliseconds; current axis in picoamperes (pA).
  • Resting state: neuron at rest with no action potential; a stimulus induces an action potential.
  • Typical experimental recording shows a current spike during stimulation that quickly returns to baseline when stimulus is removed.
  • Example numbers from demonstrations: stimulus of $25 ext{ mV}$ can evoke an inward current of about $3 ext{ pA}$, though actual values vary by preparation.
• Sodium channels and inward current:
  • Action potentials begin with opening of voltage-gated sodium channels.
  • When opened, Na+ ions enter the cell, making the intracellular side more positive (inward current).
  • This inward current is referred to as I_Na and is an inward current because Na+ moves into the cell.
  • The resting membrane potential is a baseline negative value inside the cell; opening Na+ channels depolarizes the membrane (reducing the negative potential).
• Potassium channels and outward current:
  • As part of the repolarization phase, voltage-gated K+ channels open, allowing K+ to exit the cell.
  • Potassium efflux makes the interior more negative (outward current, I_K), leading to hyperpolarization (membrane potential becomes more negative than resting, depending on the cell).
• Spatial and temporal aspects of action potentials:
  • Initiation typically occurs at the axon hillock, a region with high density of voltage-gated channels, enabling reliable threshold crossing.
  • Propagation speed is influenced by axon diameter and myelination status (see above).
• Encoding of stimulus intensity by firing rate:
  • The brain encodes stimulus intensity largely by the frequency of action potentials (firing rate), measured in Hz (cycles per second): $ext{Hz} = ext{cycles/s}$.
• Resting potential and hyperpolarization concepts:
  • Resting potential is a specific membrane potential when the neuron is not firing; following an action potential, potassium efflux can lead to a transient hyperpolarization, where the membrane potential is below the resting value.
• Quick recap of core ions and directions:
  • Sodium (Na+) outside the cell is high; inside is low. Opening Na+ channels allows Na+ to enter (inward current) and depolarize the cell.
  • Potassium (K+) inside is high; outside is low. Opening K+ channels allows K+ to leave (outward current) and hyperpolarize the cell.

Neuron Structure, Synapses, and the Synaptic Cleft

• Neuron morphology overview:
  • Dendrites: receive information; multiple dendrites possible.
  • Soma (cell body): contains nucleus.
  • Axon: conducts action potentials away from the soma; usually a single axon per neuron; may have collaterals.
  • Axon hillock: region where the action potential is initiated due to high density of voltage-gated channels.
  • Terminal arborizations and synaptic boutons: end regions where neurotransmitter is released into the synaptic cleft.
• Synapse components:
  • Presynaptic cell: the neuron that releases neurotransmitter from its synaptic vesicles in the presynaptic terminal.
  • Synaptic bouton: swelling at the end of the presynaptic terminal where neurotransmitter-containing vesicles are released.
  • Synaptic cleft: small gap between presynaptic and postsynaptic cells where neurotransmitter diffuses.
  • Postsynaptic cell: the cell that has receptors for the neurotransmitter.
  • Receptors on the postsynaptic cell bind the neurotransmitter and translate the signal into a cellular response (e.g., opening ion channels, activating enzymes).
• The two main synapse types mentioned:
  • Chemical synapses: neurotransmitter release and receptor binding mediates signaling; can be neuron-to-neuron, neuron-to-muscle, or neuron-to-gland.
  • Electrical synapses: less emphasized here; will be covered later in the context of cardiac tissue.
• Ranvier nodes and myelination in conduction:
  • Myelin sheath increases conduction speed; gaps are the nodes of Ranvier where voltage-gated channels are concentrated; saltatory conduction allows rapid signal propagation.

Axonal Transport: Anterograde and Retrograde Movement

• Axonal transport basics:
  • Materials produced in the soma (proteins, mitochondria, membrane components) are transported along the axon in vesicles to the axon terminal (anterograde transport).
  • Transport is mediated by motor proteins such as kinesin (anterograde).
  • Backward transport (retrograde) returns materials to the soma for recycling or signaling; dynein is a motor protein responsible for retrograde transport.
• Why axonal transport matters:
  • Enables maintenance and growth of the distant axon terminal; supplies components needed for neurotransmitter synthesis and membrane expansion.
  • Retrograde transport provides a pathway for signaling from the nerve terminal back to the soma.
• Viruses exploiting axonal transport:
  • Rabies and herpes viruses can hitch a ride on axonal transport machinery via motor proteins to reach the central nervous system, illustrating how viruses exploit this system to spread within the nervous system.

The Synapse in Action: Neurotransmitter Release and Receptors

• Presynaptic neurotransmitter release:
  • Neurotransmitter is stored in vesicles within the presynaptic terminal.
  • Upon stimulation, vesicles fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft.
  • Neurotransmitter diffuses across the cleft and binds to receptors on the postsynaptic cell.
• Postsynaptic receptors and responses:
  • Binding to receptors elicits a response in the postsynaptic cell, which can be excitatory or inhibitory:
  • Ligand-gated ion channels (ionotropic) open to allow ion flow directly.
  • G-protein coupled receptors (metabotropic) trigger intracellular signaling cascades.
  • Enzyme-linked receptors activate intracellular enzymes (e.g., kinases) upon ligand binding.
• Conceptual note on signal specificity:
  • The exact effect depends on the receptor type, the ligand, and the cell context.

Quick Clinical and Conceptual Links

• Local anesthetics and sensory nerves:
  • Local anesthetics block action potentials in sensory nerves (e.g., pain pathways) to prevent pain sensation during procedures.
  • They often do not completely block motor neurons, allowing movement to remain.
• Clinical relevance of receptor localization and signaling:
  • Receptor density and localization influence responsiveness to hormones and neurotransmitters (e.g., insulin receptor signaling in diabetes).
  • Understanding the location and type of receptor helps predict the onset and duration of drug effects (fast cytoplasmic signaling vs slower genomic effects).

Key Connections Across Topics

• Membrane permeability determines the signaling route:
  • Lipophilic ligands cross membranes and act via intracellular receptors (cytoplasmic or nuclear); speed depends on receptor type and downstream signaling.
  • Lipophobic ligands bind surface receptors and trigger signaling cascades without entering the cell.
• Hormone signaling and rapid vs delayed effects:
  • Steroids and thyroid hormone demonstrate rapid (primary) and delayed (genomic) responses depending on whether signaling occurs through cytoplasmic or nuclear receptors.
• Neuron structure underpins rapid signal transmission:
  • The axon hillock initiates action potentials; myelination and axon diameter determine conduction speed; nodes of Ranvier enable fast propagation.
• Synapses convert electrical signals to chemical signals and back:
  • Action potentials trigger neurotransmitter release at the presynaptic terminal; postsynaptic receptors interpret this chemical signal to generate a new electrical signal.
• Axonal transport ties cell body to distant terminals:
  • Anterograde and retrograde transport ensure ongoing maintenance, signaling, and pathogen transport within neurons.

Quick Glossary of Terms Mentioned

• Lipophilic: lipid-loving; crosses the lipid bilayer readily.
• Lipophobic: lipid-phobic; cannot cross the lipid bilayer; requires surface receptor.
• Surface receptor: receptors embedded in the plasma membrane that bind ligands outside the cell.
• Cytoplasmic receptor: receptor located in the cytoplasm that binds ligand and mediates cytoplasmic signaling or translocates to the nucleus.
• Nuclear receptor: receptor located in the nucleus that binds ligand and directly regulates gene transcription.
• Steroid hormone: lipid-soluble hormones that typically signal via intracellular receptors.
• Thyroid hormone: amine hormone that behaves like steroids in signaling, with a transporter-mediated entry and both cytoplasmic and nuclear actions.
• Ionotropic receptor: ligand-gated ion channel; immediate ion flow upon ligand binding.
• Metabotropic receptor: G-protein-coupled receptor; initiates intracellular signaling cascades.
• Enzyme-linked receptor: receptor enzymatic activity activated by ligand binding (e.g., receptor tyrosine kinase).
• Axon hillock: region where the action potential is initiated.
• Nodes of Ranvier: gaps in myelin with high density of voltage-gated channels.
• Anterograde transport: movement of materials from soma to axon terminals; motor protein is kinesin.
• Retrograde transport: movement from axon terminals back to soma; motor protein is dynein.
• Synaptic bouton: presynaptic terminal area where neurotransmitter-containing vesicles release neurotransmitter.
• Synaptic cleft: the gap between presynaptic and postsynaptic membranes where neurotransmitter diffuses.
• Action potential: rapid, transient electrical impulse that travels along the neuron.
• Hz: hertz, cycles per second, a unit of frequency used to describe firing rate of neurons.

$25 ext{ mV}$
$3 ext{ pA}$
$ext{Hz} = ext{cycles per second}$

• Common rest membrane potential: typically around V_m
  oughly -70 ext{ mV} (negative inside).

End of Notes