Human Physiology – Chapter 7 (Part 1): Resting Membrane Potential & Graded Potentials

Nervous System Architecture and Functional Map

  • Major Structural Divisions

    • Central Nervous System (CNS): brain + spinal cord.

    • Peripheral Nervous System (PNS): every neural structure whose cell body lies outside the brain or spinal cord.

  • Directional Terminology for Information Flow

    • Afferent / Sensory / Ascending ("A for Arriving") → signals to the CNS.

    • Efferent / Motor / Descending ("E for Exiting") → signals from the CNS to effectors.

  • Sensory (Afferent) Sub-classes

    • Somatic General Senses: touch, temperature, pressure, pain (4).

    • Special Senses: sight, smell, taste, hearing, balance (5).

    • Visceral Senses: internal organ status; imprecise localization (explains referred pain such as left-arm numbness during a myocardial infarction).

  • Motor (Efferent) Sub-classes

    • Somatic: voluntary skeletal-muscle control.

    • Autonomic (involuntary):

    • Sympathetic → “fight or flight,” gas pedal, targets cardiac muscle, smooth muscle, glands, adipose tissue.

    • Parasympathetic → “rest & digest,” brakes, targets primarily digestive tract and other visceral organs.

    • Enteric Nervous System → intrinsic GI tract network (sometimes listed separately; for BIO 32, grouped under autonomic).

    • NOTE: Sympathetic and parasympathetic divisions are reciprocally active except during sexual climax, when both fire concurrently.

Neuronal Cell Types

  • Two broad cell categories in nervous tissue: neurons (excitable) and neuroglia (support; not covered in detail here).

  • Structural Classification (morphology)

    • Anaxonic – no discernible axon.

    • Bipolar – one dendritic process + one axon separated by soma (typical of retina, olfactory epithelium).

    • Pseudounipolar – single elongated process with soma off to the side; peripheral + central axon segments (prototype 1st-order sensory neuron).

    • Multipolar – many dendrites, one axon (prototype motor neuron/interneuron).

  • Functional Classification

    • Sensory (afferent) → usually bipolar or pseudounipolar.

    • Motor (efferent) → typically multipolar.

    • Interneurons → processing elements inside CNS (mostly multipolar).

  • Central vs. Peripheral Neuron Rule

    • Location of cell body (soma) determines classification.

    • Soma inside brain/spinal cord → central neuron.

    • Soma outside CNS (e.g., dorsal-root ganglion) → peripheral neuron.

Resting Membrane Potential (RMP): The Cellular “Battery”

  • Plasma membrane (phospholipid bilayer) can separate charges → stores potential energy (measured in volts).

    • Only the two leaflets carry the separated charges; bulk cytosol & extracellular fluid remain electrically neutral.

  • Key Molecular Equipment

    • Leak Channels (Always Open)

    • K⁺ leak channels (many more).

    • Na⁺ leak channels (≈ 25× fewer → membrane is 25× more permeable to K⁺).

    • Na⁺/K⁺ ATPase (Pump)

    • Primary active transport; hydrolyzes ATP.

    • Moves 3  Na+<em>in→out3\;\text{Na}^+<em>{\text{in}→\text{out}} and 2  K+</em>out→in2\;\text{K}^+</em>{\text{out}→\text{in}} each cycle.

    • Establishes and maintains RMP.

  • Sequence for Establishing RMP

    1. Start at 0  mV0\;\text{mV} (no separation).

    2. Chemical (concentration) gradients drive K⁺ efflux & Na⁺ influx via leak channels.

    3. Because of 25× higher K⁺ permeability → more positive charge leaves → inside becomes negative relative to outside.

    4. Developing negativity creates an electrical force:

    • Slows K⁺ efflux (like charges repel).

    • Enhances Na⁺ influx (opposites attract).

    1. System approaches electro-chemical equilibrium: RMP in neurons ≈ −70  mV-70\;\text{mV}.

    2. Na⁺/K⁺ ATPase continuously offsets leak to keep RMP steady; otherwise gradients would dissipate to 0  mV0\;\text{mV}.

  • Equilibrium Potentials (Nernst)

    • Potassium: EK≈−94  mVE_K \approx -94\;\text{mV}

    • Sodium: ENa≈+60  mVE_{Na} \approx +60\;\text{mV}

    • Neuronal RMP closer to EKE_K because of higher K⁺ permeability.

  • Variability Across Cell Types

    • General range: −5 to −100  mV-5 \text{ to } -100\;\text{mV}.

    • Cardiomyocytes: ≈−90  mV\approx -90\;\text{mV}.

  • Clarifying the “Negative” Sign

    • “−70  mV-70\;\text{mV}” does not mean interior is full of anions. It means the inner leaflet is less positive than the outer leaflet.

Electrical Signalling in Excitable Tissues

Two Major Signal Classes

  1. Graded Potentials (GPs) – variable amplitude, short-distance.

  2. Action Potentials (APs) – all-or-none, long-distance, constant amplitude (preview; full treatment in next lecture).

Graded Potentials: Types & Properties

  • Sub-types

    1. Receptor (sensory transduction) potential.

    2. Generator potential.

    3. Postsynaptic potential (PSP) – focus of this lecture.

  • Key Characteristics

    • Amplitude proportional to stimulus strength.

    • Decremental propagation: voltage decays with distance (hence “short-distance communication”).

    • Can be summed spatially & temporally: multiple GPs arriving close in space/time add together to overcome decay.

    • Objective (for PSPs): reach the axon hillock (delta-shaped junction between soma & axon) with sufficient amplitude to trigger an AP.

  • Terminology Note: Replace textbook term “conduction” with propagation; ionic movement, not electron conduction, drives biological currents.

Action Potentials (Preview)

  • Suited for both short & long distances because amplitude is non-decremental: remains at 100 % from axon hillock to synaptic terminals.

  • Governed by all-or-none principle; cannot exist at partial strength.

  • Sustained by voltage-gated channels → positive feedback loops.

  • Drawback: no summation possible; each AP must finish before the next starts (refractory periods).

Real-World & Conceptual Connections

  • All excitable cells (neurons, skeletal muscle fibers, cardiomyocytes, smooth‐muscle cells, pacemaker cells) exploit the same RMP & electrical signalling logic.

  • Analogy: RMP is a mini-battery (≈ −0.07  V-0.07\;\text{V}) compared with household AA batteries (≈ 1.5  V1.5\;\text{V}) or nine-volt batteries (≈ 9  V9\;\text{V}). The membrane constantly recharges itself via ATP expenditure.

  • Clinical link: understanding referred pain maps (visceral vs somatic projection) aids diagnosis of heart attack, gall-bladder disease, etc.

  • Ethical / physiological implication: pharmacologic agents or toxins that block Na⁺/K⁺ pumps (e.g., ouabain) or leak channels can depolarize cells lethally.

Summary Checklist

  • [x] CNS vs PNS, afferent vs efferent.

  • [x] Somatic vs special vs visceral senses.

  • [x] Somatic vs autonomic motor; sympathetic, parasympathetic, enteric roles.

  • [x] Neuron structural & functional classifications; central vs peripheral neuron rule.

  • [x] Biophysical basis of RMP: leak channels, concentration gradients, electrical forces, Na⁺/K⁺ ATPase.

  • [x] Equilibrium potentials E<em>K,E</em>NaE<em>K, E</em>{Na}; why neuronal RMP ≈ −70  mV-70\;\text{mV}.

  • [x] Graded vs action potentials; PSP significance; distance decrement; summation.

  • [x] Preview of action potential mechanics and importance.