Excitable Membranes

Learning Outcomes

  • Define Excitable Cells/Tissues

    • Importance of excitable cells in signaling and coordination within the body.

  • Excitation, Irritation, and Stimulus

    • Differentiate these key terms.

  • Basic Properties of Excitable Tissues

    • Understanding how excitability works.

  • Resting Membrane Potential and Action Potential

    • Explanation of these fundamental concepts.

  • Stimulation of Excitable Cells

    • Awareness of how these cells are activated.

  • Cystic Fibrosis Transmembrane Conductance Regulator (CFTCR)

    • Impact on membrane polarization and salt-water balance.

Membrane Potential

Overview

  • Cell Membrane Function

    • Acts as a barrier between extracellular and intracellular fluid.

  • Ion Permeability

    • Different permeabilities for various ions essential for membrane potential generation.

  • Charge Distribution

    • The differential concentration of cations and anions results in a membrane potential.

Ion Concentrations

  • Extracellular vs. Intracellular Concentrations (mM)

    • Na+: 150 (Extracellular) | 15 (Intracellular) | RP=1

    • K+: 5 (Extracellular) | 150 (Intracellular) | RP=25-30

    • A-: 0 (Extracellular) | 65 (Intracellular) | RP=0

Characteristics of Membrane Potential

  • Definition

    • Difference in charge across the membrane, measured in millivolts (mV).

  • Typical Resting Potential

    • Generally around -70mV; the inside of the cell is negatively charged.

  • Influence of Ion Flow

    • Changes in ion movement across the membrane can alter membrane potential.

Measurement of Membrane Potential

  • Voltmeter Setup

    • Reference electrode placed outside, recording electrode inside via microelectrode.

Changes in Membrane Potential

  • States:

    • Polarized/Resting at -70mV

    • Hyperpolarized -80

    • Depolarized 0

  • Importance of States

    • Dictate the excitability of the cell and response to stimuli.

Excitable Tissues

  • Definition

    • Tissues capable of rapid, transient changes in membrane potential, allowing responses to stimuli.

  • Types of Excitable Tissues

    • Muscle, nerve, and some gland cells are all excitable tissues.

Membrane Potential and Transporters

  • Role of Ion Transporters

    • Maintain the membrane potential and manage permeability for various ions.

  • Excitation Results

    • Activation of ion channels facilitating Na+ or Ca2+ entry leads to depolarization.

Ion Channels and Pumps

Types of Ion Channels

  • Gated Ion Channels

    • Open in response to specific stimuli (voltage(change in membrane potential), ligand(binding nureotransmitter), stretch).

Ion Pumps

  • Function

    • Actively transport ions against gradients, critical for maintaining potential.

    • Examples include Na+/K+ ATPase and the movement of ions like Na+, K+, and Cl-.

Excitation

  • Definition of Excitation

    • Transition from resting potential to action potential in response to stimuli.

    • electrical response

  • Responses

    • Resulting in muscle contractions, chemical releases in neurons or glandular tissues.

Other Properties of Excitable Tissues

  • Conductivity

    • Ability to transmit excitation impulses through tissues.

  • Contractility

    • Muscle's ability to change in response to stimuli.

  • Lability and Frequency Variation

    • Adjusting action potential frequencies for varying responses.

Stimulus

  • Definition

    • Detectable environmental changes causing responses in cells.

  • Types of Stimuli

    • External: temperature, touch, odors.

    • Internal: biological factors like hormones, physical factors like blood pressure, or chemicals like blood glucose.

Irritation in Non-Excitable Cells

  • Definition

    • Changes in environment affecting non-excitable cells like red blood cells and fibroblasts.

  • Responses

    • May result in electrical, metabolic, or growth changes.

    • hyperplasia /hypertrophy

Resting Membrane Potential Generation

  • Factors

    • Ion distribution and membrane permeability are key.

Na+/K+ ATPase Activity

  • Transport Mechanism

    • Pumps Na+ out and K+ in, crucial for maintaining negative resting potential.

Mechanisms of Excitation

  • Rapid Depolarization

    • Requires increased permeability for Na+ or Ca2+. Ion channels must be activated.

Action Potential in Nerve Cells

Steps in Action Potential Generation

  1. Resting Potential

  2. Stimulus Activation

  3. Threshold Reached

  4. Na+ Channels Open

  5. K+ Efflux and Repolarization

  6. Hyperpolarization and Recovery

Cystic Fibrosis Transmembrane Conductance Regulator (CFTCR)

  • Function

    • Cl- channel involved in ion and water transport; critical in the lungs.

  • Cystic Fibrosis Effects

    • Mutations reduce function, leading to mucus accumulation and respiratory issues.

CFTCR in Excitable Cells

  • Nervous System Role

    • Affects nerve cell excitability; Cl- currents vary between PNS and CNS.

CFTCR and Membrane Depolarization Process

  1. Initial Stimulus

  2. Activation Sequence

  3. Depolarization

  4. Action Potential Induction