Previouse Notes:

https://mednotesbyrazaullah.blogspot.com/2026/08/roles-of-other-ions-during-action.html

Rhythmicity of Excitable Tissues (Repetitive Discharge)

Rhythmicity refers to the ability of certain excitable tissues to generate repetitive, self-induced electrical discharges without external stimulation. This automaticity is essential for rhythmically controlling cardiac contractions, intestinal peristalsis, and the neuronal pacing of breathing. For a tissue to display continuous rhythmicity, its resting membrane potential must be naturally permeable enough to sodium or calcium ions to allow spontaneous depolarization toward threshold.

In cardiac pacemaker cells (like those in the SA node), the resting membrane potential is relatively high around -60{ mV} which keeps voltage-gated fast sodium channels permanently inactivated. Instead, slow sodium-calcium channels allow a steady, slow leakage of positive ions into the cell. This inward leak gradually elevates the membrane potential until it reaches the threshold voltage, triggering a slow-channel action potential.

Normal Physiological Occurrence:

• Causes rhythmical heart beating.
• Drives intestinal peristalsis.
• Controls neuronal events like breathing.

Induced Repetitive Discharge:

• Low threshold triggers repetitive discharges elsewhere.
• Veratrine increases sodium permeability.
• Low calcium concentration increases sodium permeability.
• High sodium permeability excites stable nerve/skeletal fibers.

Mechanism of Spontaneous Rhythmicity

• Requires natural sodium or calcium permeability.
• Heart resting potential is -60\{ to }-70\{ mV}.
• Resting potential fails to close sodium/calcium channels.
• Inward sodium/calcium flow increases voltage.
• Voltage rise opens more channels in a positive-feedback style.
• Triggers self-induced action potential cycle.

Prevention of Immediate Re-excitation:

• High potassium conductance follows action potential.
• Outward potassium flow increases internal negativity.
• Excess internal negativity causes hyperpolarization.
• Hyperpolarization draws potential near potassium Nernst potential.
• Hyperpolarization delays the next action potential.
• Decay of potassium conductance restores threshold voltage.

Fiber Types in Nerve Trunks

• Large nerve fibers are myelinated.
• Small nerve fibers are unmyelinated.
• Unmyelinated fibers outnumber myelinated fibers two to one.

Structure of Myelinated Fibers:

• Axon core conducts action potential.
• Axoplasm fills the central axon core.
• Thick myelin sheath surrounds the axon.
• Nodes of Ranvier interrupt myelin every 1\{ to }3\{ mm}.

Role of Schwann Cells:

• Schwann cells envelop the axon.
• Schwann cells rotate, depositing sphingomyelin lipid layers.
• Sphingomyelin acts as an electrical insulator.
• Decreases membrane ion flow 5000-fold.

Saltatory Conduction in Myelinated Fibers

Mechanism of Saltatory Conduction:

• Ions cannot flow through thick myelin sheaths.
• Ions flow easily through nodes of Ranvier.
• Action potentials occur only at the nodes.
• Current flows through extracellular fluid and axoplasm.
• Impulse jumps sequentially from node to node.

Advantages of Saltatory Conduction:

• Increases nerve conduction velocity 5- to 50-fold.
• Conserves energy by depolarizing only the nodes.
• Requires minimal energy to restore concentration gradients.
• Lower membrane capacitance allows rapid repolarization.

Velocity of Conduction:

• Small unmyelinated fibers conduct as slow as 0.25\{ m/sec}.
• Large myelinated fibers conduct as fast as 100\{ m/sec}.

Excitation The Process of Eliciting Action Potentials

Causes of Membrane Excitation:

• Inward sodium diffusion triggers regenerative action potentials.
• Mechanical disturbance excites skin sensory nerve endings.
• Chemical neurotransmitters transmit signals between brain neurons.

Electrical current transmits signals in cardiac/smooth muscle:

Electrical Stimulation Mechanics

• A negative electrode (cathode) stimulates the excitable membrane.
• Negative current decreases voltage outside the membrane.
• Reduced membrane potential opens voltage-gated sodium channels.
• A positive electrode (anode) increases voltage outside the membrane.
• A positive electrode causes hyperpolarization, decreasing excitability.

Threshold and Acute Local Potentials: 

• Weak stimuli fail to trigger automatic action potentials.
• Subthreshold stimuli cause temporary local potential changes.
• These localized potential changes are acute local potentials.
• Non-propagated local changes are acute subthreshold potentials.