Neuroscience: Action Potential and Propagation

Action Potential

• Action potentials are triggered when the membrane potential of a neuron reaches -55 mV (the threshold).
• This “all-or-none” law dictates that depolarization will occur if the threshold is reached, but not if it is not.
• Depolarization only occurs in a localized section of the neuron.
• Voltage-dependent sodium (Na+) channels open in that section.
• Action potentials can decrease in strength as they travel down the axon.
• However, if the signal stays above -55 mV, the process will repeat, leading to a domino effect of action potential generation.

Action Potential Propagation

• Most neurons are covered in a myelin sheath made by oligodendrocytes (CNS) or Schwann cells (PNS).
• Myelin protects the neuron.
• Ions can only be exchanged at the Nodes of Ranvier, gaps in the myelin sheath.
• Conduction under the sheath is decremental, but action potentials are re-triggered at each node.
• Myelinated neurons have faster conduction speeds than unmyelinated neurons, a process known as Saltatory Conduction.
• Saltatory Conduction saves energy by only using Na+-K+ pumps at the nodes.
• Unmyelinated neurons do not jump between nodes, making them less efficient and slower; they are often somatosensory "pain" neurons.

Action Potential Purpose

• Action potentials are used for communication between neurons.
• They trigger the release of neurotransmitters into the synaptic cleft.
• A synapse is the point where neurons communicate.
• Neurotransmitters are released from the pre-synaptic neuron and bind to specific receptors on the post-synaptic neuron.
• The space between neurons is the synaptic cleft.
• Ca2+ channels, which are voltage-dependent (Cav), open when an action potential arrives at the terminal button.
• At rest, there is a higher concentration of Ca2+ outside the neuron, leading to an influx when the channels open.
• This influx of Ca2+ causes fusion of synaptic vesicles (which store neurotransmitters) with the membrane.
• They break open and release their contents into the synaptic cleft.

Cannabis History and Politics

• Cannabis has been used for thousands of years, with evidence of its use in ancient Central and South Asia.
• The plant genus Cannabis is a member of the Cannabaceae family, with three primary species: Cannabis sativa, Cannabis indica, and Cannabis ruderalis.
• Cannabis high in THC and low in CBD is referred to as “marijuana.”
• Hemp is defined as a cannabis plant with less than 0.3% THC.
• Both hemp and marijuana plants can be used to derive CBD.
• Cannabis plants contain at least 489 distinct compounds belonging to 18 different chemical classes.
• Even the Founding Fathers of the United States grew hemp.
• George Washington and Thomas Jefferson grew cannabis on their plantations, and Washington was interested in its medical properties.
• Early 19th-century Irish physician William O’Shaughnessy studied cannabis in India and returned with a large supply to treat various conditions.
• Sir William Osler, known as the father of modern medicine, advocated for cannabis use in migraine in the early 20th century.
• Cannabis fell out of favor in the 20th century due to political factors, including propaganda that it was a drug of abuse and association with criminal activity.
• The Marijuana Tax Act of 1937, despite opposition from the American Medical Association, imposed heavy taxes and fines on any use of cannabis.
• The mischaracterization of cannabis as a harmful drug continued in the 1960s, linking it to the “psychedelic hippie counterculture.”
• In 1970, marijuana was classified as a Schedule 1 substance, despite evidence of medicinal benefits.
• The Schedule 1 classification has hindered research on its benefits, leading to a lack of public understanding of its therapeutic potential.
• Despite this, the Department of Health and Human Services filed a patent for "cannabinoids as antioxidants and neuroprotectants" in 2001.
• The Federal Drug Administration (FDA) approved synthetic versions of Δ9-THC for medicinal use, contradicting the Schedule 1 status.
• Over the past two decades, multiple medical societies have pushed for the re-scheduling of marijuana.
• Politics played a significant role in the legalization of cannabis, largely due to revenue generation through taxes.
• As of May 16, 2024, a federal initiative to re-schedule cannabis to Schedule 3 has begun.

“Drugs” Throughout History

• The use of drugs for medicinal, ritual, and recreational purposes dates back to the earliest civilizations.
• Most of these drugs were plant-based.
• Evidence indicates that our ancestors understood the principles and methods for using chemical substances appropriately.
• Some of the drugs discovered in ancient times are still in use today, often with isolated active ingredients for greater potency, with both positive and negative consequences.
• Acceptance of specific substances depended on the societal norms of a culture or time period.
• There is a long history of using plants for medicinal purposes.

Next Class Readings

• Glia cells
• Neural Communication
• Ion Channels
• Neurotransmitters (Part A and Part B)
• Modern Drug Development.

Metabotropic Receptors

• Also known as G-protein coupled receptors
• Have a binding site but no channel
• Trigger a second messenger system through G-proteins
• Can indirectly open ion channels through intracellular processes
• Slower than ionotropic receptors
• Drugs can act at the binding site, the second messenger system, or on the ion channel

Second messenger system

• Neurotransmitter binds to the receptor and activates G-proteins
• This triggers the production of second messengers which change the conformation of nearby ion channels
• Second messengers can also affect other cell functions.
• Examples of second messengers include cAMP and cGMP

Autoreceptors and Heteroreceptors

• Located on the terminals of presynaptic neurons
• Typically inhibit the production of the same neurotransmitter (autoreceptor) or other neurotransmitters (heteroreceptor)
• This is a form of feedback inhibition
• This is also a site of drug action (to slow or increase production of neurotransmitters)

Neurotransmitter communication

• Brief and stopped by reuptake and deactivation
• Reuptake: transporters bring neurotransmitters back to the neuron
• Deactivation: enzymes break down the neurotransmitters
• Ensures lack of buildup of transmitters
• Allows for reaction to new signaling/information
• Drugs can interrupt or aid these processes

Ion channels on the Post-Synaptic Neurons

• The transmitters released from presynaptic neurons determine excitation or inhibition of the post-synaptic neuron
• Influx/outflux of ions determines the outcome (excitatory or inhibitory)
• Ions involved include Na+, Ca2+, K+, Cl-
• Ion flow follows diffusion and electrostatic principles
• Triggered directly from ionotropic and indirectly from metabotropic receptors

Excitatory Postsynaptic Potential (EPSP)

• When positive ions flow in (Na+ and Ca2+)
• Makes the inside less negative (depolarized)

Inhibitory Postsynaptic Potential (IPSP)

• When negative ions flow in (Cl-), or positive ions flow out (K+)
• Makes the inside more negative (hyperpolarized)

Excitation or Inhibition

• The net effect of EPSPs and IPSPs contribute to the firing of the neuron (excitation or inhibition)
• Overwhelming EPSPs trigger an action potential at the axon hillock
• More IPSPs keep the neuron hyperpolarized, preventing action potentials

Voltage Dependent Channels: Kv

• Allow K+ flow out of the neuron (repolarization)
• Mainly activated by high (positive) voltages
• Kv1 to Kv12 with many subtypes, the most abundant is Kv1.1-1.8

Ligand Gated Channels

• Opening and closing depends on the presence of a ligand

Ligand Gated Channels: P2X

• Activated by ATP and analogs
• When activated allows in-flow of Na+ and Ca2+ (depolarization)
• Also allows flow of K+
• 7 subtypes: P2X1-P2X7

Ligand Gated Channels: iGlutamate

• Also known as Ionotropic Glutamate receptors
• When activated allows in-flow of Na+ (depolarization)
• Also allows flow of K+
• There are also metabotropic glutamate receptors
• Ligands activate different 3 subtypes named after their distinct ligands: NMDA, AMPA, and KA

iGlutamate Subtypes

• NMDA: Allows in-flow of Ca2+, requires glycine with glutamate for activation
• AMPA: Activated by glutamate
• KA: Activated by glutamate

Ligand Gated Channels: 5-HT3

• Activated by serotonin (5-HT)
• 5 receptor subtypes: A-E
• When activated allows in-flow of Na+ and Ca2+ (depolarization)
• Also allows flow of K+
• Most serotonin receptors are metabotropic

Ligand Gated Channels: Transient Receptor Potential (TRP) Cation Channels

• When activated allows in-flow of Na+ and Ca2+ (depolarization)
• Also allows flow of K+
• Activated by numerous ligands (specific channels)
• Diverse functions inside and outside the nervous system

TRP Channel subtypes

• TRPV1: Heat (>43°C), acid, capsaicin, endogenous lipids
• TRPA1: Allyl isothiocyanate (wasabi, mustard oil), eugenol (clove oil), cinnamaldehyde, gingerol, 4-HNE, environmental toxins, mechanical transduction
• TRPM8: Menthol, eucalyptol, cool/cold
• TRPV3: Carvacrol (thyme, oregano), camphor, warm
• TRPV4: Warm, anandamide

Ligand Gated Channels: GABA Receptors (GABAA)

• When activated allows in-flow of Cl- (hyperpolarization)
• Most abundant inhibitory neurotransmitter
• ~20% of all neurons
• Activated by GABA
• GABAC is also ligand gated, GABAB is not (metabotropic)

Ligand Gated Channels: Nicotinic Acetylcholine Receptors (nAChR)

• When activated allows in-flow of Na+ and Ca2+ (depolarization)
• Also allows flow of K+
• Activated by acetylcholine (ACh) and nicotine
• Many subtypes, most notable α7, α4β2
• Can be both pre-synaptic and post-synaptic
• Needs 2 ACh molecules for its activation

Glial cells

• Have a 1:1 ratio in the brain with neurons
• Supply nutrients, physical support and barrier, debris removal, and functional interaction (release transmitters)
• Part of the blood-brain barrier
• Reuptake of neurotransmitters

Neurotransmitters

• Chemical substances that communicate across a synapse
• About 100 known neurotransmitters
• 10 neurotransmitters are the most prevalent

Monoamine Neurotransmitters

• Dopamine
• Norepinephrine/Noradrenaline
• Epinephrine/Adrenaline
• Serotonin
• Histamine

Dopamine

• G-coupled (metabotropic) receptors
• D1-D5 receptors
• D1 and D5 excitatory (“D1-like”)
• D2 (also autoreceptor), D3, and D4 inhibitory (“D2-like”)

Dopamine Functions

• Motor function
• Reward
• Cognition
• Learning
• Reward/aversion
• Motivation
• Pleasure

Dopamine pathways

• There are three main pathways:
  • Mesolimbic Pathway: Connects the ventral tegmental area (VTA) in the midbrain to the nucleus accumbens (NAc) and amygdala in the forebrain. This pathway is important for reward, motivation, and pleasure.
  • Mesocortical Pathway: Connects the VTA to the prefrontal cortex (PFC). This pathway is involved in executive functions, planning, decision-making, and working memory.
  • Nigrostriatal Pathway: Connects the substantia nigra in the midbrain to the dorsal striatum. This pathway is primarily involved in motor control and movement.

Metabotropic Receptors

• Also known as G-protein coupled receptors
• Only have a binding site, no channel
• Trigger a second messenger system through G-proteins
• Can open ion channels indirectly, using intracellular processes
• Action on ion channels is slower than ionotropic receptors
• Can be targeted by drugs that act on the binding site, second messenger system, or ion channel

Second Messenger System

• Neurotransmitter binds to receptor and activates G-proteins
• G-proteins trigger the production of second messengers
• Second messengers can change the conformation of nearby ion channels
• Second messengers can also affect other cell functions
• Different types of second messenger systems exist
• Examples of second messengers include cAMP and cGMP

Autoreceptors and Heteroreceptors

• Receptors on terminals of presynaptic neurons
• Inhibit the production of neurotransmitters (feedback inhibition), either their own (autoreceptors) or of other neurotransmitters (heteroreceptors)
• Are sites of drug action, used to slow or increase neurotransmitter production

Regulating Neurotransmitter Communication

• Neurotransmitter communication is brief and stopped by:
  • Reuptake: transporters bring neurotransmitters back to the neuron, then recycled or degraded
  • Deactivation: enzymes break down neurotransmitters in the synaptic cleft, astrocytes, or the neuron itself
• Ensures a lack of neurotransmitter buildup, reinstating communication dynamics
• Allows the neuron to react to new signaling information, as buildup would “clog” the synaptic cleft
• Drugs are created to interrupt or aid these processes

Ion Channels on Post-Synaptic Neurons

• Neurotransmitters released from presynaptic neurons determine the excitation or inhibition of the postsynaptic neuron
• The net influx/outflux of ions through ion channels determines the outcome, either excitation or inhibition
• Ions involved include Na+, Ca2+, Cl-, and K+
• Ion flow follows the same principles as in action potentials (diffusion, electrostatic)
• Ion flow is triggered directly by ionotropic receptors and indirectly by metabotropic receptors
• If positive ions flow in, an excitatory postsynaptic potential (EPSP) is generated, depolarizing the neuron
• If negative ions flow in, or positive ions flow out, an inhibitory postsynaptic potential (IPSP) is generated, hyperpolarizing the neuron
• The net EPSP and IPSP effect contributes to the firing of the neuron:
  • If EPSPs are overwhelming, the signal hits the axon hillock, and an action potential triggered
  • If more IPSPs are present, the neuron remains hyperpolarized, preventing an action potential

Kv Channels

• Allow the flow of K+ out of the neuron, helping with repolarization
• Mainly activated by high (positive) voltages, with some exceptions
• Many subtypes, from Kv1 to Kv12, with various sub-subtypes
• Most abundant and diverse

P2X Receptors

• Activated by ATP and analogs
• When activated, allows influx of Na+ and Ca2+, resulting in depolarization
• Also allows flow of K+
• Seven subtypes, P2X1-P2X7

Ionotropic glutamate Receptors (iGluR)

• When activated, allows influx of Na+ leading to depolarization
• Also allows flow of K+
• Three subtypes are named after distinct ligands that activate them:
  • NMDA (N-methyl-D-aspartate)
  • AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid)
  • Kainate (KA)
• NMDA related channels also allow influx of Ca2+
• iGluRs are important for glutamate’s essential role in neuron excitability
• They are found throughout the body and are involved in many functions inside and outside of the nervous system

Serotonin Receptor 3 (5-HT3)

• Activated by serotonin (5-HT)
• Five receptor subtypes, A-E
• When activated, allows influx of Na+ and Ca2+, resulting in depolarization
• Also allows flow of K+
• Most serotonin receptors are metabotropic

Transient Receptor Potential (TRP) Cation Channels

• When activated, allows the influx of Na+ and Ca2+, resulting in depolarization
• Also allows flow of K+
• Activated by numerous ligands, varying by specific channel type:
  • TRPV1: heat, acid, capsaicin, endogenous lipids
  • TRPA1: allyl isothiocyanate, eugenol, cinnamaldehyde, gingerol, 4-HNE, environmental toxins, mechanical transduction
  • TRPM8: menthol, eucalyptol, cool/cold
  • TRPV3: carvacrol, camphor, warm
  • TRPV4: warm, anandamide
• Diverse functions inside and outside the nervous system

-Aminobutyric acid Receptor A (GABAA)

• When activated, allows influx of Cl-, causing hyperpolarization
• The most abundant inhibitory neurotransmitter
• ~20% of all neurons have GABAA receptors
• Activated by GABA
• Receptors are found throughout the Central Nervous System

Nicotinic Acetylcholine Receptors (nAChR)

• When activated, allows influx of Na+ and Ca2+, resulting in depolarization
• Also allows flow of K+
• Activated by acetylcholine (ACh) and nicotine
• Many subtypes, most notable are 7 and 4β2
• Can be both presynaptic and postsynaptic
• Requires two ACh molecules for activation

Take Home Message

• Neurotransmitter release facilitates communication between neurons
• Ion channels play key roles in action potential generation and neuronal signaling, both excitatory and inhibitory
• Opening and closing of ion channels can be voltage or ligand dependent
• Diverse ion channels exist with various roles inside and outside the nervous system
• Receptors and channels are primary mechanisms for drugs to alter normal or pathological communication between neurons

Neurotransmitters

• Chemical substances packaged in synaptic vesicles and released by neurons to communicate across a synapse with another neuron, muscle cell, organ, or a hormone-producing cell in an endocrine gland
• About 100 neurotransmitters are known
• 10 are most prevalent

Monoamines

• Dopamine (DA)
• Norepinephrine/Noradrenaline (NE/NA)
• Epinephrine/Adrenaline
• Serotonin (5-HT)
• Histamine

Dopamine (DA)

• G-coupled (metabotropic) receptors
• D1-D5 receptors
  • D1 and D5 are excitatory, “D1-like”
  • D2 (also autoreceptor), D3, and D4 are inhibitory, “D2-like”
• Involved in:
  • Motor function
  • Reward
  • Cognition
  • Learning
  • Reward/aversion
  • Motivation
  • Pleasure

This quiz covers the essential concepts of action potential and its propagation in neurons. Explore the all-or-none principle, the role of sodium channels, and the importance of the myelin sheath in neuronal conduction. Test your understanding of how action potentials are generated and transmitted!