Neurophysiology: Action Potentials lecture 18

• This pre-lecture material assists with lecture preparation and note-taking.
• The pre-lecture material may have differences or additions compared to the live lecture.
• Lecture 18 focuses on Neurophysiology 1: Action Potentials.

Objectives

• Describe three types of gated ion channels associated with neurons.
• Define a local potential.
• Describe how summation occurs at the axon hillock (without calculations).
• Describe the five steps that comprise an action potential.

Neuronal Communication

• Neurons communicate using chemical signals.
• Every neuron-to-neuron synapse includes a pre-synaptic neuron, which releases neurotransmitters.
• It also includes a post-synaptic neuron that detects neurotransmitters and responds by opening ion channels.
• Neurons can function as both pre-synaptic and post-synaptic at the same time.

Chemical to Electrical Signal Conversion

• A chemical signal (neurotransmitter) binds to and opens chemically-gated ion channels.
• Ions flow in or out, which changes the voltage in a localized area of the membrane.
• An electrical signal (action potential) begins if the membrane voltage reaches -60mV at the axon hillock.

Gated Ion Channels

• Gated ion channels open in response to a stimulus.
• Types of gated ion channels include chemically-gated, voltage-gated, and mechanically-gated channels.
• The stimulus is like a 'key' that opens the channel.
• The channel is like a 'door'.
• Ions are like the 'people' that pass through the door when it opens.

Chemically-Gated Ion Channels

• A chemical neurotransmitter binds to the ion channel stimulating it.
• The channel changes shape and opens.
• Ions travel across the membrane driven by their electrochemical gradient.
• The channel closes when the neurotransmitter detaches.

Voltage-Gated Ion Channels

• The membrane depolarizes to a threshold voltage such as -60mV, stimulating the channel.
• The channel changes shape and opens up.
• Ions travel across the membrane by their electrochemical gradient.
• Changes in membrane potential cause the channel to close or inactivate.

Mechanically-Gated Ion Channels

• Deformation or stretching of the membrane stimulates the channel.
• The channel changes shape, and opens.
• Ions travel across the membrane using their electrochemical gradient.
• The channel closes when the membrane returns to its original shape.

Location of Gated Ion Channels

• Chemically-gated Na+ and Cl- channels are found on dendrites and the cell body.
• Mechanically-gated ion channels are on dendrites of some sensory neurons.
• Voltage-gated Na+ and K+ channels are located at the axon hillock and axon.
• Voltage-gated Ca2+ channels are at the axon terminals.

Resting Membrane Potential

• The intracellular space has a more negative charge than the extracellular space, establishing an electrical gradient.
• High concentration of Na+ and low concentration of K+ is outside the cell.
• High concentration of K+ and low concentration of Na+ is inside the cell.

Local Potentials

• Local potentials involve a change in voltage or charge within a specific area of the cell.
• It is an excitatory or inhibitory change in voltage in a localised region of the cell body or dendrites.
• Its located on a post-synaptic cell.
• Local potentials can also be referred to as Post-Synaptic Potentials (PSP).
• They occur on the membrane of the post-synaptic cell.
• They can be called Graded potentials, varying in size based on how many ions enter or exit.

Excitatory Post-Synaptic Potential (EPSP)

• EPSPs are excitatory local potentials.
• A presynaptic neuron releases an excitatory neurotransmitter (e.g., acetylcholine - ACh).
• Chemically-gated Na+ channels open when the neurotransmitter binds.
• Na+ enters the post-synaptic cell, causing the membrane to depolarize (become more positive).

Inhibitory Post-Synaptic Potential (IPSP)

• IPSPs are inhibitory local potentials.
• A presynaptic neuron releases an inhibitory neurotransmitter (e.g., GABA).
• Chemically-gated K+ or Cl- channels open when the neurotransmitter binds.
• K+ exits or Cl- enters the post-synaptic cell, resulting in hyperpolarization (membrane becomes more negative).

Summation of Local Potentials

• Summation is the combination of excitatory and inhibitory potentials.
• Spatial summation involves summed input from multiple pre-synaptic neurons.
• Temporal summation involves summed input from repeated firing of one pre-synaptic neuron.
• Spatial and temporal summation usually take place simultaneously in a complex neural network.

Axon Hillock Summation

• The axon hillock is high in voltage-gated channels.
• Threshold potential (-60mV) is the key that opens voltage-gated channels
• Voltage-gated Na+ channels open at the axon hillock if summation reaches or exceeds -60mV.

Action Potential Steps

• Step 1: Voltage-gated Na+ channels open when the membrane depolarizes to -60mV.
• Step 2: A large influx of Na+ causes a 'rapid depolarization phase'.
• Step 3: Around +30mV, VG Na+ channels become inactivated, stopping Na+ entry.
• Step 3: VG K+ channels open, and K+ exits causing the 'repolarization phase'.
• VG K+ channels open more slowly, triggering at -60mV.
• Step 4: VG K+ channels start to close slowly, and excess K+ exits causing the 'hyperpolarization phase'.
• Step 5: The membrane returns to -70mV as all VG K+ channels close.
• A chemical signal (neurotransmitter) binds to and opens chemically-gated ion channels.
• Ions flow in or out, potentially being either excitatory or inhibitory, and it changes the localized voltage of the membrane.
• Summation occurs when these ion movements are added together.
• Voltage-gated Na+ channels are abundant in the axon hillock.
• An electrical signal (action potential) begins.
• Depolarization, repolarization, and hyperpolarization all result from coordinated ion movement.