Psyc 280: Unit 3- Neurophysiology

What is the resting potential

• Resting potential = electrical charge difference across a neuron’s membrane at rest

• Inside of the neuron is more negative than the outside

• Typical value is about −70 mV

Vrest≈−70 mVVrest​≈−70 mV

• Caused by unequal distribution of ions (Na⁺ and K⁺)

• Potassium (K⁺) leaks out more easily than sodium (Na⁺) enters

• Sodium–potassium pump helps maintain the difference

• Keeps the neuron ready to send nerve impulses (action potentials)

What are the changes in membrane potential

• Resting potential: neuron is at rest, about −70 mV

• Depolarization: membrane becomes less negative (more positive)

• Repolarization: membrane returns back toward negative

• Hyperpolarization: membrane becomes more negative than resting potential

• Action potential: rapid change in membrane potential that sends a nerve signal

What is summation

• adding together of nerve signals in a neuron

• Helps decide whether the neuron will fire an action potential

• Occurs at the cell body and axon hillock

• 2 types

Graded potentials

• Arising in the dendrites and somata spread passively

• Side of potential rapid decreases with distance from origin

• undergo summation

Temporal summation

is a neurophysiological process where a single presynaptic neuron fires in rapid succession

Spatial summation

a neurological process where multiple presynaptic neurons release neurotransmitters onto a single postsynaptic neuron simultaneously

What happens in the action potential

• What happens when threshold is exceeded rapidly

• Rapid Electrical signal sent along neuron

• Membrane

• All or nothing: stimulus is independent of AP size and shape

• potential quickly changes from negative to positive and back

• −70 mV→+30 mV→−70 mV

Graded vs action potential

• Analog vs digital

• Graded potential

  • Small, local change in membrane potential

  • Strength varies with stimulus

  • Can decrease over distance

  • Occurs in dendrites and cell body

  • Can summate

  • May be depolarizing or hyperpolarizing

• Action potential

  • Large, rapid change in membrane potential

  • Same strength every time (all-or-none)

  • Does not decrease over distance

  • Occurs in axon

Where do the graded potentials come from?

• generated in the dendrites and cell body (soma) of neurons.

• They happen when ligand-gated or mechanically gated ion channels open or close.

• A stimulus causes ions (like Na⁺, K⁺, or Cl⁻) to move across the membrane.

• can be:

  • Depolarizing (membrane becomes more positive)

  • Hyperpolarizing (membrane becomes more negative)

• Their size varies with stimulus strength (“graded” = variable size).

• They spread only a short distance and weaken as they travel.

• If enough depolarization reaches the axon hillock, it can trigger an action potential.

Excitatory synapse (Excitatory post synaptic potential, EPSP)

Produces depolarization

Inhibitory synapse (Inhibitory post-synaptic potential, IPSP)

Produces a hyper polarization

What are the variations in this signalling strategy

• threshold differences between cells

• AP’s differ in shape between cells

• Differences between cells in rate of AP discharge for same amount of depolarization

• Hebbian synapses

• Temperature, hormone, etc. sensitivity

Ionic mechanisms

• Ionic mechanisms = movement of ions across a neuron membrane

• Ions move through ion channels

• Main ions:

  • Na⁺ (sodium)

  • K⁺ (potassium)

  • Ca²⁺ (calcium)

  • Cl⁻ (chloride)

• Ion movement changes membrane voltage

• Na⁺ in → membrane becomes more positive

• K⁺ out → membrane becomes more negative

• Creates nerve signals:

  • Resting potential

  • Graded potentials

  • Action potentials

• Controlled by:

  • Voltage-gated channels

• anion= negative charge

• Cation = positive charge

Concentration gradient

the gradual difference in the concentration of a dissolved substance (solute) between two regions, such as across a semi-permeable cell membrane

electrostatic gradient

The difference in electrical charge across a membrane (membrane potential), driving ions toward regions of opposite charge

What is the neuronal membrane

• Outer covering of a neuron

• Thin membrane (phospholipid bilayer)

• Separates inside of cell from outside

• Controls what enters/leaves cell

• Has ion channels (Na⁺, K⁺, etc.)

• Has pumps (like Na⁺/K⁺ pump)

• Helps create electrical signals

• Receives neurotransmitters (receptors)

What is nernest equation

• describes distribution of charge acreee a permeable membrane

• Almost perfect prediction of resting potential:

  • For squid axon, products -75 my rather than the -70 my observed

  • There is some leakage of ions, 3 NA in and 2 K out (sodium potassium pump)

  • When balances, leakage would bring resting potential to 0

  • Active transport

Ionic basis of graded potentials

• Movement of ions across the neuronal membrane

• Happens through ligand-gated or mechanically gated ion channels

• Mainly involves:

  • Na⁺ entering → depolarization

  • K⁺ leaving → hyperpolarization

  • Cl⁻ entering → hyperpolarization

• Ion flow changes membrane potential locally

• Size depends on stimulus strength

• Decays as it spreads (no regeneration)

• synaptic events can cause gated ion channels to open

Excitatory synapse

• NA opens an/ or K closed

• Na rushes in (concentration gradient) and decreases the local potential= EPSP

Inhibitory synapse

• K opened and/ or Na closed and/ or Cl opened

• Cations move out or anions move in, increasing the potential= IPSP

Ionic basis of the action potential

• There are 3 ways that gated channels can be opened

• Axon hillock

• Propagation of the action potential

• Saltatory conduction

What are the stages in the axon hillock (fire control centre)

Start with many voltage- gated Na+ channels

1. supra-threshold EPSP opens some, depolarizatiom increases

2. Increasing depolarizatiom opens more and more Na channels

   1. Na rushes in

   2. Membrane potential collapsed

   3. Potential overshoots 0 my, up to +40 my

Then, Na channels close so that K channels open, membrane potential begins to return to equilibrium, -70 mv (Decay phase, which membrane is refractory, can’t fire again, one direction only)

What is the propagation of the action potential

• Neurons have a “distance problem” — they must send a potential a very long way down the axon. (If the cell body was 6 inches, the axon would be up to a mile long.)

• In somatic potentials, potential spreads passively, or “decrementally,” getting smaller and smaller.

• Axons send a “regenerative” potential instead.

  • A spike of identical amplitude is recreated all down the axon, due to the presence of voltage-gated Na⁺ channels.

What is saltatory conduction

• Saltatory conduction occurs in myelinated axons

• The action potential “jumps” from one Node of Ranvier to the next

• Myelin acts as an insulator

• Voltage-gated Na⁺ channels are concentrated at the nodes

• The signal is regenerated at each node

• Makes nerve impulse transmission:

  • Faster

Synaptic events: what happens at the end of the axon?

• There are many voltage gates Ca channels, arrrival of AP causes them to open

• Ca rushes in

• Calcium current causes vesicles near presynaptic membrane to fuse and rupture, releasing transmitter into synaptic cleft

• Amount released is proportional to size of calcium current

How much neurotransmitter enters the cleft?Modulation of release

• decrease in potential of the bouton decreases Ca current

  • Less Ca = less release

  • Bouton may be synapses by another, inhibitory bouton

• Auto receptors

How much neurotransmitter enters the cleft? Rapid Quenching of transmitter

• Enzymes breakdown transmitter molecules

  • MAO, AChE etc

  • Breakdown products have little activity

• Reuptake sites

  • Remove transmitter, back into bouton

How much neurotransmitter enters the cleft?Frequency Coding

• the message is encoded in the frequency of AP’s

• The more that arrive, the greater the amount released

Synaptic events: Receptors, what happens on the postsynaptic side?

• transmitter substance (aka ligand) binds to receptors

  • Specialized proteins that bind only one kind of transmitter

  • Lock and key

• 3 families

Chemical receptor type 1: ionotropic

• directed toward ions

• Ligand binds to receptors Specialized proteins site that is part of a channel protein (recognition function)

• Binding causes a change in the electrochemical conformation of the receptor ligand complex

• The change causes the pore to open, and ionic flow to start (this is called the effector function)

• A change in local membrane potential ensues “ligand gated ion channel”

FASTER

Chemical receptor type 2: metabotropic

• directed toward metabolism

• Receptor is not directly linked to a channel, it produces metabolic changes within the neuron

• Couple via a g-protein to a second messenger. May be an enzyme that is liberated and has effects elsewhere (effector function)

  • May lead directly to a membrane conductance change

  • May alter energy use

  • May alter gene transcription

SLOWER

The gap junction

• aka electrical synapse

• Characteristics:

  • Very fast, no synaptic things happening

  • Metabolically cheap

  • No synthesis cheap

• Many in brain, might be for local processing

Variation in synaptic strategies: directed synapses

type of chemical synapse where the site of neurotransmitter release (presynaptic membrane) is in extremely close proximity—approximately 20–50 nm—to the site of neurotransmitter reception (postsynaptic membrane)

Synaptic events: what are these transmitters? RULES

Qualifications for a transmitter:

• presence in bottoms

• Synthesis enzymes present in neuron

• Substance must be released when AP arrives

• Application of transmitter to postsynaptic membrane must provoke an effect

• Blocking the transmitter (with drugs) must shut down the synapse

Vagusstoff

The neurotransmitters

the body’s chemical messengers. They are chemicals used by the nervous system to carry signals (messages) between neurons (nerve cells) or from neurons to target cells like muscles or glands

Subtypes of receptors

• can be many different types of receptors for a given transmitter (master Key)

• Example: serotonin

  • More than 14 subtypes of receptors

  • Some are slow, others are fast

• can have different effects depending on the subtype activated

• Subtypes vary in brain distribution can have functional implications