Cellular Neurophysiology: Resting Membrane Potential

Cellular Neurophysiology: Part 1 - Resting Membrane Potential

Introduction to Cellular Neurophysiology

  • Divisions of Study: This topic will be divided into three sections:

    • Resting membrane potential.

    • Action potentials.

    • Synapses.

  • Neuron Anatomy (from Vander's human physiology):

    • Cell Body (Soma): Contains the nucleus and most organelles.

    • Dendrites: Receive information and carry it towards the cell body.

    • Axon: Long projection that carries information away from the cell body; can have branches.

    • Initial Segment (Trigger Zone / Axon Hillock):

      • Anatomically, this is where an action potential starts.

      • Distinguished by a very high concentration of voltage-gated sodium channels.

Understanding Membrane Potential

  • Definition of a Potential: A potential starts with a charge separation.

    • In cells, there is a charge separation across the cell membrane.

    • Ions separated by a tiny space are attracted to one another, creating a force.

    • The force (and thus the size of the potential) increases with:

      • Greater charge separation.

      • Smaller distance between separated charges.

    • Cell membranes are nanometers thick, so few separated charges can create a significant force/potential.

  • Resting State: Polarization:

    • In a resting, polarized cell, positive charges (e.g.,Na+e.g., Na^+) are associated with the outside of the membrane.

    • Negative charges (e.g.,phosphateions,ortheabsenceofpositivechargese.g., phosphate ions, or the absence of positive charges) are associated with the inside.

  • Principle of Electroneutrality:

    • It is crucial to understand that the fluid inside the cell is not overall negatively charged, nor is the extracellular fluid overall positively charged.

    • If all positive and negative charges in either fluid compartment were summed, they would add up to zero.

    • Electroneutrality means you cannot have a solution with just one type of ion; anions and cations must be balanced.

    • The membrane potential arises from an uneven distribution of ions relative to the membrane, not from an overall charge imbalance in the bulk solutions.

    • Due to the thinness of the membrane, only a small number of ions need to be separated to create a significant membrane potential.

Measuring Membrane Potential

  • Methodology:

    • A recording electrode is inserted into the cell.

    • A reference electrode is placed in the extracellular fluid.

    • These electrodes are connected to a circuit, and the potential difference (voltage) between them is recorded.

  • Voltmeter Interpretation:

    • A voltmeter needle pointing towards the negative indicates a negative potential inside the cell relative to the outside.

    • If there were no potential difference, the needle would point to zero.

    • If the electrodes were switched (reference inside, recording outside), a positive potential would be recorded (outside is positive relative to the negative inside).

  • Typical Resting Membrane Potentials:

    • Central Nervous System (CNS) Neuron: −70mV-70 mV (meaning the inside is −70mV-70 mV relative to the outside, which is considered $0 mV).</p></li><li><p>SkeletalMuscleCell:).</p></li><li><p>Skeletal Muscle Cell:-90 mV</p></li><li><p>SANode:Between</p></li><li><p>SA Node: Between-40 mVandand-60 mV</p></li><li><p>EpithelialCell:</p></li><li><p>Epithelial Cell:-30 mVoror-40 mV</p></li></ul></li></ul><h4id="10d25bec−8cad−4f7f−b479−86fd933f72fa"data−toc−id="10d25bec−8cad−4f7f−b479−86fd933f72fa"collapsed="false"seolevelmigrated="true">GenerationandMaintenanceoftheRestingMembranePotential(RMP)</h4><ul><li><p><strong>PrimaryFactor:</strong>ThemajorityoftheRMPisgeneratedaspotassium(</p></li></ul></li></ul><h4 id="10d25bec-8cad-4f7f-b479-86fd933f72fa" data-toc-id="10d25bec-8cad-4f7f-b479-86fd933f72fa" collapsed="false" seolevelmigrated="true">Generation and Maintenance of the Resting Membrane Potential (RMP)</h4><ul><li><p><strong>Primary Factor:</strong> The majority of the RMP is generated as potassium (K^+)leaksfromthecell.</p><ul><li><p>Potassiumhasahighconcentrationinsidethecell.</p></li><li><p>Itleavesthecellthrough<strong>leakchannels</strong>,leavingbehindarelativelynegativechargewithinthecell.</p></li></ul></li><li><p><strong>Whyspecificionsandchargesbehaveastheydo:</strong></p><ul><li><p><strong>Potassium() leaks from the cell.</p><ul><li><p>Potassium has a high concentration inside the cell.</p></li><li><p>It leaves the cell through <strong>leak channels</strong>, leaving behind a relatively negative charge within the cell.</p></li></ul></li><li><p><strong>Why specific ions and charges behave as they do:</strong></p><ul><li><p><strong>Potassium (K^+)leaves:</strong>Highintracellular) leaves:</strong> High intracellularK^+concentrationdrivesitout.</p></li><li><p><strong>Sodium(concentration drives it out.</p></li><li><p><strong>Sodium (Na^+)doesnotreadilyenter:</strong>Therearenotmanysodiumleakchannelsopenatrest.</p></li><li><p><strong>Negativechargesdonotfollow) does not readily enter:</strong> There are not many sodium leak channels open at rest.</p></li><li><p><strong>Negative charges do not followK^+$$ out: Most negative charges inside the cell are associated with large proteins and phosphates that cannot cross the membrane.

  • Dynamic Nature of Neurons: While epithelial cells might be