Neurotransmission

Communication Within a Neuron

• Axon: The structure that transmits signals away from the cell body.
• Axon Hillock: The area where the axon originates from the cell body, and where the action potential is initiated.
• Action Potential Movement: The action potential travels down the axon towards the terminal, leading to neurotransmitter release.

Blood-Brain Barrier (BBB)

• Selective Permeability: The BBB is highly selective, limiting the passage of substances into the brain.
• Permeable Substances: Only tiny, water-soluble substances can diffuse through the gaps in the BBB. Some fat-soluble substances can pass through cell membranes.
• Active Transport: Other substances require active transport mechanisms to cross the BBB.
• Drug Engineering: Medicines must be engineered to bypass or pass through the BBB.

Brain Damage at the Cellular Level

• Alzheimer’s Disease: Characterized by the buildup of protein aggregates:
  • Beta-amyloid plaques: outside of cells
  • Tau tangles: inside cells
• Cerebrospinal Fluid (CSF): The amount of CSF volume does not change in the Alzheimer's Disease brains, but clearance and composition do.

Causes and Contributing Factors of Cell Dysfunction

• Incomplete Understanding: The exact reasons for cell death/dysfunction are not fully known.
• Blood-Brain Barrier Disruption: May contribute to neurological disorders such as:
  • Alzheimer’s disease
  • Parkinson’s disease
  • Stroke
  • Epilepsy
• Repair Strategy: Enhancing the repair of a damaged blood-brain barrier is a potential therapeutic strategy for treating neurological diseases.

Action Potentials

• Definition: Electrical signals sent from the cell body to the terminal button, causing the release of neurotransmitters.

Action Potential Generation and Propagation

• Basic electrical properties and definitions.
• How the resting membrane potential is established.
• Membrane machinery involved.
• Ion flow through the machinery when excited (during an action potential).
• How resting membrane potential is re-established.
• How action potentials are propagated down myelinated (saltatory) and unmyelinated (continuous) axons.
• Rate Law

Cell Membrane

• Structure: Spherical phospholipid bilayer with protein channels.
• Selective Permeability: Ions can flow through gated and un-gated channels.

Ionic Distribution

• Resting State: The distribution of ions across the cell membrane at rest.
• Charge: Outside of the cell is positively charged, while the inside is negatively charged.
• Ions Present:
  • Outside: Na^+, Cl^-
  • Inside: K^+, negatively charged proteins

Ions and Charge

• Ions: Charged particles.
• Examples:
  • Chloride (Cl^-
  • Sodium (Na^+
  • Potassium (K^+
• Anion: Negatively charged ion (e.g., chloride ion).
• Cation: Positively charged ion (e.g., sodium ion).

Key Definitions: Electricity

• Electricity: The flow of electric charge.
• Charge: A basic property of matter; can be positive or negative.
• Electrostatic Force: The force between charges (opposites attract, likes repel).
• Conventional Current: The flow of positive charges.

Key Definitions: Neuronal Electricity

• Electricity: Flow of ions.
• Ions: Charged particles.
• Electrostatic Force: The force between charges, i.e., opposites attract & likes repel.
• Current: Flow of positive charges (rate of flow; Amps).
• Voltage:
  • Difference in charge between two points
  • The “push” that causes the ions to flow
  • Electropotential difference (Volts)

Voltage

• Definition: Difference in charge between two points.
• Charge Separation: Buildup of charge separation creates a force on ions (ionic distribution).
• Ion Flow: When a channel opens, the force acts on ions, causing them to flow.
• Current and Electricity: Flow of positive ions = current or electricity.
• Measurement: Voltage can be measured.
• Relative Measure: Always recorded against a reference or ground electrode.

Ions Contributing to Resting Potential

• Sodium (Na^+
• Chloride (Cl^-
• Potassium (K^+
• Negatively charged proteins (A^-
  • Synthesized within the neuron
  • Found primarily within the neuron
• Mnemonic: Salty banana.

Neuron’s Resting Membrane Potential

• Membrane Potential: Difference in electrical charge between inside and outside of the cell.
• Polarization: Inside of the neuron is negative with respect to the outside.
• Resting Membrane Potential: Approximately -70mV.

Forces Moving Ions

• Concentration Gradient: Particles tend to move down their concentration gradient.
• Electrostatic Gradient: Like repels like, opposites attract.

Ionic Distribution and Gradients

• Concentration and electrostatic gradients affect each ion differently.
  high concentration of potassium inside the cell.
  high concentration of Cl- and Na + outside the cell.

• **Sodium (Na^+

  • Driven IN by both electrostatic and concentration gradients.

• **Potassium (K^+

  • Driven IN by the electrostatic gradient and OUT by its concentration gradient.

Ion Channels

• Allow ions to pass through the cell membrane.
• Ions can enter or leave the cell through them.
• Some are un-gated, while others are gated.

Sodium-Potassium Pump

• Function:
  • Exchanges 3 sodium ions (Na^+$) pumped out
  • Exchanges 2 potassium ions (K^+$) pumped in
• Location: Found in the cell membrane.
• Energy Source: Powered by ATP.

Resting Membrane Potential Maintenance

• Potassium leak channels allow passive leaking of K^+ out of the cell.
• The sodium-potassium pump exchanges 3 Na^+ for 2 K^+.

Action Potential (AP)

• Definition: Electrical signals sent from the cell body to the terminal button that lead to neurotransmitter release.
• Initiation: Cell responds to a depolarizing (more positive) stimulus at the axon hillock by rapidly changing its membrane potential through voltage-gated Na^+ and K^+ channels.

Neural Membranes and Ion Channels

• Contain thousands of ion channels.

• Voltage-gated ion channels

  • Sensitive to the charge distribution (voltage) across the cell membrane
  • Voltage-gated sodium channels & voltage-gated potassium channels

• Action Potential Results: A positive change in membrane potential causes an abrupt influx of Na^+ ions into the cell, followed by an efflux of K^+ ions out of the cell.

Initiation of Action Potential

• Triggered by positive changes in membrane potential at the axon hillock.
• When the threshold of excitation is reached, voltage-activated ion channels open, and the neuron “fires” a full-size AP.
• All-or-None Principle: The action potential either occurs or it does not.

Ion Movements During an Action Potential

• K^+ leak channels
• Voltage-gated sodium channels open
• Na-K pump
• Voltage-gated potassium channels open.
• Voltage-gated sodium & potassium channels closed.

Voltage Changes During Action Potential

• The opening and closing of voltage-activated sodium and potassium channels during the three phases of the action potential:
  • Depolarization
  • Repolarization
  • Inactivation

Sodium Channels in Three States

• Closed (Resting, Reset): Non-conducting.
• Open (Activated): Conducting.
• Inactivated (Refractory): Non-conducting.

Ion Movements During Action Potential

1. Na^+ channels open, Na^+ begins to enter the cell.
2. K^+ channels open, K^+ begins to leave the cell.
3. Na^+ channels become refractory, no more Na^+ enters the cell.
4. K^+ continues to leave cell, causes membrane potential to return to resting level.
5. K^+ channels close. Na^+ channels reset.
6. Extra K^+ outside diffuses away.

Action Potential Mnemonic

• LEAP into Action!
• L: less negative
• E: excited
• AP: action potential

Re-establishing Resting Membrane Potential

• Na^+- K^+ pump and leak channels help re-establish resting membrane potential.
• The pumps re-charge the membrane, resetting it for the next action potential.

Refractory Periods

• Absolute Refractory Period: (1-2 msec) – impossible to initiate another action potential.
• Relative Refractory Period: Harder to initiate another action potential.
• Prevent the backward movement of APs and limit the rate of firing.

Saltatory Conduction

• Conduction of the action potential along a myelinated axon.
• Conduction OR propagation of AP = movement of message down the axon.

Saltatory Conduction Details

• Passive conduction (instant and decremental) along each myelin segment.
• “Jumps” from node to node.
• Faster conduction than in unmyelinated axons.

Continuous Conduction

• Conduction of the action potential down an unmyelinated axon.
• Requires more channels, pumps, and energy.

Questions to Consider

• At which feature of the neuron is the first AP produced?
• At rest, which force(s) are driving the K^+ out of the cell?
• Why does the cell hyperpolarize following the falling phase/repolarization phase of the AP?

Glial Cells

• Functions:
  • Provide physical support & nourishment to neurons.
  • Help control chemical composition via BBB.
  • Clean-up debris & form scar tissue.
  • Primary immune cells of the nervous system.
  • Respond to injury & disease.
  • Clean up debris & form scar tissue.
  • Produce myelin in the CNS (one cell produces many myelin sheaths).
  • Produce myelin in the PNS (one cell produces one myelin sheath).

Mapping the Mouse Brain

• Scientists created the first full cellular map of a mammalian brain (mouse).
• Identified over 5,300 cell types, far more than known before, and pinpointed their locations within the brain’s intricate geography.

Neuron Count in Human Brain

• 86 billion neurons.

• Do glial cells outnumber neurons? Yes, no, maybe in some structures?
  Neuroanatomical Techniques

• Golgi stain

• Nissl stain

• Immunohistochemistry

• Neuroanatomical tracing

  • Retrograde
  • Anterograde

Staining Process

• Cut tissue into slices along the anatomical plane of interest.
• Soak or wash in the staining solution(s).
• Mount them on microscope slides.

Golgi Stain

• Study of individual neurons with great detail on cell silhouettes, but without internal details.

Nissl Stain

• Highlights cell bodies of all neurons; allowed estimation of cell density in tissue.
• Helps distinguish gray matter from white matter.

Immunohistochemistry (IHC)

• Helps visualize cells based on the proteins they express.
• Uses modified antibodies to add a visible compound to neurons of interest (i.e., neurons that release a certain neurotransmitter).
• Slices soaked in Primary antibody then slices are soaked in Secondary antibody with a visible element to make it easier to visualize.

Neuroanatomical Tracing

• Helps visualize cells based on their relative location (cell bodies & projections).
• Conducted in intact brains with injection of fluorescent dye into specific sites.
  • Anterograde: from the soma to the terminal button.
  • Retrograde: from the terminal buttons to the soma.

Neuroanatomical Tracing Example

• Inject a combo of an Anterograde Tracer and a Retrograde Tracer into Area A
• Visualize the location of cells projecting from A to B as well as cells projecting from B to A

Retrograde Tracer Injection

• If you are studying a neuron that projects from a rostral region of the brain to a caudal region of the brain, where would you inject a retrograde tracer to be taken up by that neuron?
  • Inject in the Caudal region (BACK).

Neural Development

• How do cells develop and find their targets?

Phases of Development

1. Creation of neural tube
2. Neural proliferation
3. Neural migration
4. Axon growth and synapse formation
5. Neuron death and synapse refinement

Creation of the Neural Tube

• Note: you do not need to memorize the cell types or steps. Just appreciate that the neural tube is created.

Neural Proliferation

• Cells multiply and form three swellings:
  • Forebrain
  • Midbrain
  • Hindbrain

Neural Migration

• Once cells have proliferated, they migrate (move) to their final destinations to form structures and brain areas.
• Migrating cells are immature, lacking axons and dendrites.
• Migrating cells move in two directions by two different means.
• Cortical layers form in an “inside out pattern”.

Directions of Neural Migration

• Tangential migration
• Radial migration

Methods of Migration

• Somal Translocation (Radial or Tangential)
• Glia-Mediated Migration (Radial Only).

Inside Out Development

• Process is supported by an early progenitor cell called a radial glial cell
• Note: Numbers indicate the order of layer development NOT anatomical reference

Cortical Layers

• Meninges, Marginal zone, Cortical plate, Subplate, Intermediate zone, Subventricular zone, Ventricular zone

Development Processes

• Reliably get specific cell types to the same destination
• Interneurons, Excitatory Spiny Neurons, Inhibitory Aspiny Neurons

Axon Growth and Synapse Formation

• At the growing tip of each axon, the growth cone extends and retracts as if exploring to find its way.

Neuron Death and Synapse Refinement

• Overproduction: During development, many neurons die (programmed cell death or Apoptosis).
• (Differs from necrosis, which is unprogrammed form of cell death that occurs in response to overwhelming chemical or physical insult).
• Neurons die due to the failure to find synaptic partners and compete for neurotrophic factors provided by targets (e.g., NGF, nerve growth factor).

Synapse Rearrangement and Refinement

• Axons are fine-tuning connections, forming and reforming connections.
• Synapse refinement is likely influenced by chemical signals and by spontaneous and experience-evoked neural activity.

Refining Neural Connections

• Overproduction and Pruning: Experience shapes brain architecture by the early overproduction of neurons, followed by later apoptosis and refinement of synaptic connections based on learning and exposure to stimuli.

Brain Development Timeline

• Begins during the prenatal period and extends through adulthood
• Note: new synapses can form in adulthood. This development phase (synaptogenesis) is referring to the first synaptic connections

Postnatal Brain Development

• Continues after an animal is born.
• The human brain continues to develop for at least two decades
• Then even after that, the adult brain continues to undergo subtle changes throughout life
• Experience affects brain development and maturation

Experience Effects

• Development:
  • If crossed eyes are not corrected during development (1-3 years), depth perception may be irrevocably impaired (use it or loose it).
• Adulthood:
  • Individuals who learn braille will have a larger portion of the sensory area of their brain that respond to finger sensation.

Neurogenesis

• The process by which new neurons are formed in the brain.
• Once thought to stop in adulthood.
• Now, we know new neurons are formed in (at least) two brain areas.

Vulnerability of the Developing Brain

• More than 200 genetic mutations associated with developmental disability.
• More vulnerable than the mature brain to malnutrition, toxic chemicals, and infections.
• Examples:
  • Fetal alcohol syndrome: Dendrites tend to be short, with few branches (toxin).
  • Rett Syndrome: Anomaly of brain development with developmental disability affecting mainly girls older than 1-2 years. Associated with lack of dendritic development (genetic mutation).
  • Phenylketonuria (PKU): Lack enzyme to convert protein phenylalanine to tyrosine. If consume food with phenylalanine, it accumulates in the blood and interferes with myelination (metabolic disorder).

Damage in Developed Brains

• Parkinson’s disease: degeneration of pigmented cells in the midbrain
• Alzheimer’s disease: buildup of protein aggregates outside of cells (beta-amyloid plaques) and inside cells (tau tangles).

APs vs. PSPs

Functional Circuits

• Neurons connected to many other neurons through electrical and chemical activity.
• Connected neurons form anatomical structures & functional circuits that depend on excitatory and inhibitory signals.

Withdrawal Reflex

• Sensory neuron excites motor neuron.
• Interneuron excites a motor neuron, causing muscular contraction.
• This muscle causes withdrawal from the source of pain.
• Excitatory synapse
• Inhibitory synapse
• EPSP + EPSP temporal summation

Cortical Input and Withdrawal

• Inhibitory signals arising from the brain can prevent the withdrawal reflex from causing the person to drop the cup.
• Interneuron inhibits the motor neuron, preventing muscular contraction.
• A simultaneous IPSP and EPSP cancel each other out
• EPSP + IPSP Spatial summation

Communication Within and Between Neurons

• Within a Neuron: Axon hillock and action potential causing neurotransmitter release.
• Between Neurons: Presynaptic neuron releases neurotransmitters that bind to postsynaptic receptors, triggering postsynaptic potentials.

Steps from Signal Receipt to Chemical Transmission

1. PSPs are elicited in the dendrites and cell body.
2. Postsynaptic potentials (PSPs) are conducted decrementally in the cell body on their way to the axon.
3. When the summated PSPs exceed the threshold of excitation at the axon, an action potential, AP, is triggered.
4. The AP is conducted nondecrementally down the axon to the terminal button.
5. Arrival of the action potential (AP) at the terminal button triggers calcium entry and neurotransmitter release via exocytosis.
6. Neurotransmitters bind to postsynaptic receptors

Multipolar Neuron Signals

1. Postsynaptic potentials (PSPs) are elicited on the cell body and dendrites.
2. PSPs are conducted decrementally to the axon.
3. When the summated PSPs exceed the threshold of excitation at the axon initial segment, an action potential (AP) is triggered.
4. The AP is conducted nondecrementally down the axon to the terminal button.
5. Arrival of the AP at the terminal button triggers exocytosis.

Key Features of Chemical Signals

1. The receptors
2. The neurotransmitters

Chemical Diffusion

• At some synapses, neurochemicals remain local.
• While at others, they diffuse long distances.
• Synaptic Transmission
• Volume Transmission

Differentiating Characteristics of Receptors

• Type: ionotropic vs metabotropic
• Effect on cell:
  • Depolarize or hyperpolarize (depends on ion channels that are opened)
  • Increase excitability or decrease excitability (depends on longer-lasting effects of enzymatic cascades)
• Location: can be found postsynaptically OR presynaptically OR outside of the synaptic cleft
• Neurotransmitter (NT) that activates it:
  • Many NT found in CNS
  • Not 1:1; Multiple, different receptors can be activated by the same type of NT

Cell Signaling Context

• Signal transduction: The process in which binding of an extracellular messenger (chemical or physical signal) to the cell surface receptor is translated into changes in the cell.
• Those changes, also called signaling cascades, involve a series of molecular events between MANY molecules that lead to:
  • Secretion of substances from the cell
  • Opening of ion channels
  • Transcription (gene expression)
• This occurs in MANY cell types; not just neurons.
• It allows cells to respond to the information they receive.

Ionotropic vs. Metabotropic Receptors

• Ionotropic receptors: Ligand-activated ion channels.
  • Fast opening of the channel
  • EPSP or IPSP
• Metabotropic receptors: ligand-activated protein, often associated with G proteins.
  • Either coupled to ion channel or 2nd messenger.
• effects are slower, longer-lasting, more diffuse, and more varied than ionotropic receptors.

Metabotropic Receptors and 2nd Messengers

• NT is the 1st messenger.
• The 2nd messenger is activated by G protein and can have a variety of effects (e.g., enter the nucleus, bind to DNA, alter gene expression).
• Structural changes = plasticity

Effect on Cell

• The nature of the PSP at a particular synapse is determined by the postsynaptic receptor.
• More specifically, by the particular type of ion channel they open.

Receptor Location

• Can be found postsynaptically OR presynaptically OR outside of the synaptic cleft

Receptors on the Axon Terminal

• Presynaptic receptors: Situated on the axon side of the synaptic cleft.
• Autoreceptors:
  • Receptors that respond to the NT that they themselves release.
  • Usually metabotropic.
• Neuromodulatory receptors:
  • Usually away from the synapse.
  • Modulate the effect of neurotransmitters.
  • Usually metabotropic.

NT Release

• NT are released at directed chemical synapses
• NT are also released at nondirected chemical synapse

Neurotransmitter Substance Definition

• Produced by the cell
• Present at the axon terminal (or in varicosities)
• External application mimics the natural effect
• There exists a mechanism to STOP the effect of the NT:
  • Diffusion
  • Reuptake
  • Enzymatic degradation

Neurotransmitter Action

1. Neurotransmitter molecules are synthesized from precursors under the influence of enzymes.
2. Neurotransmitter molecules are stored in vesicles.
3. Neurotransmitter molecules that leak from their vesicles are destroyed by enzymes.
4. Action potentials cause vesicles to fuse with the presynaptic membrane and release their neurotransmitter molecules into the synapse.
5. Released neurotransmitter molecules bind with autoreceptors and inhibit subsequent neurotransmitter release.
6. Released neurotransmitter molecules bind to postsynaptic receptors.
7. Released neurotransmitter molecules are deactivated by either reuptake or enzymatic degradation.

Neurotransmitter Molecules

• Small: Synthesized in the terminal button and packaged in synaptic vesicles.
• Large: Assembled in the cell body, packaged in vesicles, and then transported to the axon terminal (via anterograde axoplasmic transport).

Small-Molecule Neurotransmitters

• Amino acids – the building blocks of proteins
  glutamate, GABA
• Monoamines – all synthesized from a single amino acid
  dopamine, Norepinephrine , Serotonin and Histamine
• Acetylcholine
• Unconventional NT *Endocannabinoids