Bioelectricity - MBBS 1st Year Physiology

1. What is Bioelectricity?

2. Resting Membrane Potential (RMP)

• Pumps 3 Na⁺ out for every 2 K⁺ in - creating a net deficit of positive ions inside.
• This pump is the primary driver of the RMP.

• The resting membrane is ~100x more permeable to K⁺ than Na⁺.
• K⁺ continuously leaks out down its concentration gradient, carrying positive charges out and making the inside more negative.

• Large negatively charged proteins inside the cell cannot cross the membrane, contributing to the negative interior.

Nernst Equation is used to calculate the equilibrium potential for individual ions. The Goldman-Hodgkin-Katz equation accounts for multiple ions and gives a more accurate RMP.

3. Action Potential (AP)

Phases of Action Potential:

• Membrane is polarized. No signal being transmitted.

• Stimulus depolarizes membrane to threshold (~-55 mV).
• Voltage-gated Na⁺ channels rapidly open → massive Na⁺ influx.
• Membrane potential shoots from -70 mV to +35 mV (overshoot).

• Na⁺ channels inactivate (close) within <1 ms.
• Voltage-gated K⁺ channels open (slower) → K⁺ rushes out.
• Membrane potential returns to -70 mV.

• K⁺ channels stay open slightly longer than needed.
• Membrane goes below -70 mV transiently (e.g., -75 to -80 mV).
• Returns to resting potential when K⁺ channels close.

Ion Channel Events Summary:

Na⁺ conductance increases ~5000-fold at onset; K⁺ conductance increases only ~30-fold and more slowly.

4. Threshold and All-or-None Law

• A stimulus must depolarize the membrane to threshold (~-55 mV) to trigger an AP.
• Once threshold is reached, the AP is all-or-none - it fires at full amplitude regardless of stimulus strength.
• Subthreshold stimuli produce only local, graded potentials that fade away.

5. Refractory Periods

6. Propagation of Action Potential

• At the site of depolarization, local currents flow from the active (+ve inside) region to the adjacent resting (-ve inside) region.
• This depolarizes the adjacent membrane to threshold, triggering a new AP there.
• The AP thus travels along the fiber in one direction (the refractory period behind prevents backward conduction).

• In myelinated nerves, the AP "jumps" from one node of Ranvier to the next.
• This is much faster and energy-efficient than continuous conduction in unmyelinated fibers.
• Conduction velocity: up to 120 m/s in large myelinated fibers vs. ~0.5 m/s in small unmyelinated C fibers.

7. Importance / Clinical Relevance

• Local anesthetics (e.g., lidocaine) block voltage-gated Na⁺ channels → prevent AP generation → pain relief.
• Cardiac arrhythmias involve abnormal membrane potentials in cardiac muscle.
• Nerve conduction studies (NCS) measure AP velocity to diagnose neuropathies.
• EEG / ECG / EMG all record bioelectric potentials from brain, heart, and muscle respectively.

All-or-None Law

Definition

"No action potential occurs if the stimulus is subthreshold in magnitude, and the action potential has a constant amplitude and form at any stimulus strength above the threshold intensity."

• Ganong's Review of Medical Physiology, 26th Ed.

The Core Concept: Threshold

• Every excitable membrane has a threshold potential (approximately -55 mV in nerve fibers).
• If a stimulus depolarizes the membrane to or beyond threshold → action potential fires fully.
• If the stimulus is subthreshold → no action potential; only a local, decaying electrotonic potential occurs.

Why is it "All-or-None"? - The Ionic Basis

1. Membrane depolarizes to threshold (-55 mV)
2. Voltage-gated Na⁺ channels open → Na⁺ rushes in
3. This further depolarizes the membrane → more Na⁺ channels open
4. The process becomes self-sustaining and explosive - it cannot be "half-done"
5. The AP always peaks at the same level (~+35 mV) because it is limited by the Na⁺ equilibrium potential

"The firing of an action potential is a binary, all-or-none event; that is, the spike has a constant, nongraded voltage peak that occurs only if the depolarizing stimulus exceeds the threshold."

• Medical Physiology (Boron & Boulpaep)

All-or-None vs. Graded Potentials

Applies to Individual Fibers - Not Whole Nerves

• The all-or-none law applies to a single nerve fiber or muscle fiber.
• A whole nerve trunk contains thousands of fibers with different thresholds, so a stronger stimulus recruits more fibers, giving a graded compound response - this is called recruitment or the size principle.
• This is why squeezing harder gives a stronger muscle contraction - more motor units are recruited, not bigger APs per fiber.

Exceptions / Where It Does NOT Apply

• Rods and cones of the retina: Their output signal is graded (hyperpolarizing), not all-or-none. The strength of the signal is proportional to light intensity. (Guyton & Hall)
• Receptor potentials (e.g., in mechanoreceptors): These are graded.
• Synaptic potentials (EPSPs/IPSPs): Graded responses.
• These graded signals eventually summate at the trigger zone (axon hillock) to decide whether to fire an all-or-none AP.

Clinical Significance

• Local anesthetics (lidocaine, bupivacaine): Block Na⁺ channels → prevent threshold from being reached → no AP → no pain signal.
• Cardiac muscle: Follows the all-or-none law per cell - the heart contracts fully or not at all (important in understanding arrhythmias).
• Nerve conduction studies: Measure the all-or-none AP response to assess nerve integrity.

• Stronger stimulus = more fibers recruited, NOT a bigger AP per fiber.
• The all-or-none law ensures reliable, consistent signal transmission over long distances without decay.

HLA (Human Leukocyte Antigen) Genes

1. What are HLA Genes?

2. Chromosomal Location

• Located on the short arm of chromosome 6 (6p21.3)
• Spans approximately 4000 kilobases (kb) of DNA
• Contains over 224 identified gene loci
• Map order (distal → centromere): HLA-A → HLA-C → HLA-B → HLA-DR → HLA-DQ → HLA-DP

Map of the human MHC locus - Cellular and Molecular Immunology

3. Classes of HLA Genes

Class I (HLA-A, HLA-B, HLA-C)

• Encode single α-chain proteins that pair with β2-microglobulin (encoded on chromosome 15)
• Present endogenous (intracellular) antigens - e.g., viral peptides, tumor antigens
• Expressed on virtually all nucleated cells
• Recognized by CD8+ cytotoxic T lymphocytes (CTLs)

Class II (HLA-DP, HLA-DQ, HLA-DR)

• Encode heterodimers (α + β chains) - each locus has separate A gene (α) and B gene (β)
• Present exogenous (extracellular) antigens - e.g., bacteria, phagocytosed microbes
• Expressed only on antigen-presenting cells (APCs): Dendritic cells, B lymphocytes, Macrophages, Thymic epithelial cells
• Recognized by CD4+ helper T lymphocytes

Class III

• Encoded between Class I and II regions
• Encode complement proteins: C2, C4A, C4B, Factor B
• Encode cytokines: TNF-α, Lymphotoxin-α (LTα), Lymphotoxin-β (LTβ)
• Also encode proteins involved in antigen processing: TAP1, TAP2, Tapasin, Proteasome subunits

4. Non-Classical HLA Genes

• HLA-E, HLA-F, HLA-G: Class I-like molecules; important for NK cell regulation
• HLA-G: Expressed at the maternal-fetal interface; protects the fetus from maternal immune attack
• MIC-A, MIC-B: Stress-induced molecules; recognized by NK cells

5. Inheritance

• HLA genes are tightly linked - they segregate en bloc as a haplotype
• Each individual inherits one haplotype from each parent (two total)
• Alleles show codominant expression - both alleles are expressed
• Follow Mendelian inheritance - 4 possible combinations among siblings
• Probability of two siblings being HLA-identical = 25%
• Crossing over within the MHC occurs in ~4% of meioses

6. Polymorphism

• HLA genes are the most polymorphic in the human genome
• Hundreds to thousands of alleles exist at each locus
• Alleles are named systematically:
  • e.g., HLA-A*0201 = HLA-A gene, allele group 02, subtype 01
  • e.g., HLA-DRB1*0401 = DR beta-1 gene, allele 04, subtype 01
• This polymorphism ensures that populations can respond to a wide variety of pathogens

7. Linkage Disequilibrium

• Certain HLA alleles occur together on the same haplotype more frequently than expected by chance - this is called linkage disequilibrium (LD)
• Classic example: A1-Cw7-B8-DR17(3)-DQ2 haplotype in Caucasians - seen ~4x more often than expected
• Extends from HLA-A through HLA-DP

8. HLA and Disease Associations (High-Yield for Exams)

Mechanism: HLA-B27 in ankylosing spondylitis causes heavy chain misfolding → inefficient antigen presentation → triggers autoimmune inflammation. (Thompson & Thompson Genetics, 9th Ed.)

9. HLA in Transplantation

• HLA matching between donor and recipient is critical for preventing graft rejection
• Standard matching involves at least 3 loci: HLA-A, HLA-B, HLA-DR
• Better HLA match → lower risk of rejection → longer graft survival
• Panel Reactive Antibody (PRA) test: measures pre-formed antibodies against ~80% of HLA antigens in the donor panel
• A crossmatch test is done before transplant to detect pre-formed donor-specific antibodies

10. HLA and Immune Function - Key Points

• MHC/HLA expression is upregulated by cytokines:
  • Class I: increased by IFN-α, IFN-β, IFN-γ (especially during viral infection)
  • Class II: increased by IFN-γ (e.g., during inflammatory states)
• This ensures that during an infection, more antigen is presented to T cells, amplifying the immune response

• Class I = One chain = One cell (all nucleated) = CD8 (cytotoxic)
• Class II = Two chains = Two types of APCs = CD4 (helper)
• B27 = Back pain (Ankylosing Spondylitis)
• HLA-DR4 = RA (4 letters)