Excitable Cells and Action Potentials

TRPV-1 Channel

• This refers to a specific type of ion channel, the transient receptor potential vanilloid 1 channel, involved in sensory transduction.
• The diagram shows the channel spanning the cell membrane, with extracellular and intracellular regions indicated.

Excitable Cells

• Lecture by Dr. Paolo Actis from the University of Leeds, p.actis@leeds.ac.uk.

Key Discoveries and Techniques

• Action Potential: Groundbreaking research recognized with the 1963 Nobel Prize.
• Patch Clamp Technique: Revolutionized the study of ion channels; Nobel Prize in 1991.
• Sodium-Potassium Pump: Vital for maintaining cell resting potential; Nobel Prize in 1997.
• Potassium Leak Channel: Important for setting resting membrane potential; Nobel Prize in 2003.
• Ion Channel Receptors for Temperature and Touch: Recognized with the 2021 Nobel Prize, highlighting the importance of these channels in sensory perception.
• Importance: Understanding excitable cells is crucial for neuroscience, physiology, and medicine.

Ion Concentrations in Human Cells

• Illustrates the differing concentrations of key ions inside and outside a typical human cell.
• Potassium (K+): High intracellular concentration (150 mM) and low extracellular concentration (5 mM).
• Sodium (Na+): Low intracellular concentration (5-15 mM) and high extracellular concentration (145 mM).
• Chloride (Cl-): Low intracellular concentration (4 mM) and high extracellular concentration (110 mM).
• Calcium (Ca2+): Very low intracellular concentration (0.0001 mM) and higher extracellular concentration (2.5-5 mM).

Ion Distribution and Membrane

• Illustrates the distribution of sodium (Na+) and potassium (K+) ions across a cell membrane (lipid bilayer).
• Highlights the presence of the sodium-potassium pump, which actively transports Na+ out of the cell and K+ into the cell, maintaining the concentration gradients.

Nernst Equation

• The Nernst Equation is presented, which allows the calculation of the equilibrium potential for an ion based on its concentration gradient across a membrane.
• $E = \frac{RT}{nF} ln \frac{[\text{ion}]<em>{out}}{[\text{ion}]</em>{in}}$
• Where:
  • R is the universal gas constant (8.314 J.K-1.mol-1).
  • T is the temperature in Kelvin.
  • n is the valence of the ionic species.
  • F is the Faraday's constant (96,485 C mol-1).

Nernst Equation for Potassium

• Simplified Nernst equation for potassium (K+).
• $E = 62 \text{mV} \cdot log<em>{10} \frac{[K^+]</em>{out}}{[K^+]_{in}}$
• Example: $E = 62 \text{mV} \cdot log_{10} (\frac{5}{150}) = -62 \text{mV} \cdot (7)$

Nernst Equation for Chloride

• Nernst equation for chloride (Cl-), noting the negative charge.
• $E = 62 \text{mV} \cdot log<em>{10} \frac{[Cl^-]</em>{in}}{[Cl^-]_{out}}$

Nernst Equation for Calcium

• Nernst equation for calcium (Ca2+), accounting for its +2 charge.
• $E = \frac{62}{2} \text{mV} \cdot log<em>{10} \frac{[Ca^{2+}]</em>{out}}{[Ca^{2+}]_{in}}$

GHK Equation

• GHK (Goldman-Hodgkin-Katz) Equation: Used to calculate the membrane potential taking into account the permeability and concentrations of multiple ions.
• $V<em>m = \frac{RT}{F} \ln \frac{P</em>K[K^+]<em>{out} + P</em>{Na}[Na^+]<em>{out} + P</em>{Cl}[Cl^-]<em>{in}}{P</em>K[K^+]<em>{in} + P</em>{Na}[Na^+]<em>{in} + P</em>{Cl}[Cl^-]_{out}}$
  • Where P represents the permeability of the respective ion.

Typical Ion Concentrations - Squid Axon vs. Mammalian Cell

• Comparison of ion concentrations in squid axon and mammalian cells.
• Squid Axon:
  • K+: Intracellular = 400 mM, Extracellular = 10 mM
  • Na+: Intracellular = 50 mM, Extracellular = 440 mM
  • Cl-: Intracellular = 40-150 mM, Extracellular = 560 mM
• Mammalian Cell:
  • K+: Intracellular = 150 mM, Extracellular = 5 mM
  • Na+: Intracellular = 5-15 mM, Extracellular = 145 mM
  • Cl-: Intracellular = 4 mM, Extracellular = 110 mM
• Relative Permeabilities: pK:pNa:pCl = 1:0.05:0.45

Action Potential

• Illustrates the phases of an action potential.
• Resting state: Membrane potential at -70 mV.
• Stimulus: Causes depolarization.
• Threshold: If the membrane potential reaches the threshold (-55 mV), an action potential is triggered.
• Depolarization: Na+ ions rush into the cell, causing a rapid increase in membrane potential.
• Repolarization: K+ ions flow out of the cell, restoring the membrane potential.
• Hyperpolarization: Membrane potential becomes more negative than the resting state before returning to normal.

Neuron Structure

• Diagram of a neuron, highlighting:
  • Cell body (soma).
  • Dendrites.
  • Axon: Ranging from less than 1 mm to over 1 m in length.
  • Nerve terminal.
  • Terminal branches of axon.

Squid Giant Axon

• Highlights the historical significance of the squid giant axon in neuroscience research due to its large size.

Action Potential Comparison

• Comparison of action potentials in a nerve cell and a cardiac myocyte, showing differences in duration and shape.
• Nerve Cell: Fast action potential.
• Cardiac Myocyte: Longer action potential (plateau phase).

Patch Clamp Technique

• Illustrates the patch clamp technique, used to study ion channel activity.
• Components:
  • Electrode.
  • Patch pipette.
  • Operating amplifier.
  • Feedback resistor.
  • Pipette solution.

Electromyography (EMG)

• EMG: A technique to evaluate and record the electrical activity of skeletal muscles.
• The EMG signal represents the sum of action potentials from muscle fibers.

Silicon Probes for Neural Recording

• References a Nature article from 2017 on fully integrated silicon probes for high-density recording of neural activity.
• Jun et al., Nature, 2017.

Neural Probe Design

• Diagrams illustrating the design and specifications of silicon probes for neural recording.
• Key features include:
  • Connector for data cable.
  • Headstage.
  • Sites.
  • Detachable connector.
  • Shank.
  • Flex cable.

Neural Recording Data

• Data from neural recordings, showing activity in different brain regions (V1B, MEnt, V2L).
• Measurements include r.m.s. noise and site impedance.

Ultrasonic Neural Dust

• References a Neuron article from 2016 on wireless recording in the peripheral nervous system using ultrasonic neural dust.
• Seo et al., Neuron, 2016.

Neural Dust Components

• Illustrates the components of neural dust particles.
• Key components include:
  • ASIC (Application-Specific Integrated Circuit).
  • Piezoelectric material.
  • Recording pads.

Further Reading

• Links to resources for further learning:
  • Membrane Potential: Khan Academy (https://goo.gl/ttKeSE)
  • Action Potential: Khan Academy (https://goo.gl/6zrsro)
  • Nernst Equation Practice: (https://goo.gl/b8e9fw)
  • Labster Labs: Sensory Transduction and Action Potential Lab
  • Patch Clamp Technique: Leica Microsystems (https://www.leica-microsystems.com/science-lab/the-patch-clamp-technique)
  • Book: Essential Cell Biology (Alberts, Bray, Johnson et al.)