Excitable Tissues

Overview – Excitable Tissues

Excitable tissues refer primarily to nervous and muscle tissues, which are capable of generating and propagating electrical impulses in response to specific stimuli. This electrical activity underpins critical physiological processes such as neural signalling and muscle contraction. The basis of this excitability lies in the membrane potential—a voltage difference maintained across the cell membrane by the selective movement of ions, primarily sodium (Na⁺) and potassium (K⁺). A sound understanding of excitable tissues is crucial for mastering physiology, neurobiology, and many clinical conditions involving nerve or muscle dysfunction.

Membrane Potential

• Membrane potential = voltage across the plasma membrane
• Caused by unequal distribution of Na⁺ and K⁺ ions across the membrane
• Maintained by selective permeability of ion channels
• Present in all living cells (approx. –20 to –200 mV)
• Excitable tissues experience changes in membrane potential when stimulated → leads to cellular activation

Factors Influencing Membrane Potential

• Relative permeability of the membrane to specific ions
• Concentration gradients of ions (particularly K⁺ and Na⁺)
• Electrochemical gradients

Resting Membrane Potential (RMP)

• Baseline potential of unstimulated cells
• In neurons: ~ –70 mV
• Established primarily due to:
  • Greater passive diffusion of K⁺ out of the cell
  • Limited Na⁺ entry due to low membrane permeability
• Net effect:
  • Inside becomes negative
  • Outside becomes positive

Role of Na⁺/K⁺ ATPase

• Maintains the concentration gradients of Na⁺ and K⁺
• Pumps 3 Na⁺ out and 2 K⁺ in using ATP
• Prevents dissipation of gradients due to passive leakage
• Critical for sustaining RMP and cellular excitability

Excitable Cells: Nerves & Muscle

• Stimuli open gated ion channels (e.g. voltage-gated, ligand-gated, mechanically gated)
• Alters membrane permeability → modifies membrane potential
• If threshold is reached, an action potential is generated and propagated

Neuronal Action Potential

Phase 1 – Resting State

• High K⁺ permeability; low Na⁺ permeability
• Inside is negative, outside is positive
• All gated Na⁺ and K⁺ channels are closed

Phase 2 – Depolarisation

• Triggering stimulus opens some Na⁺ channels → Na⁺ enters
• If threshold (~ –55 mV) is reached:
  • Voltage-gated Na⁺ channels open
  • Rapid Na⁺ influx → membrane potential rises to ~ +30 mV
  • Na⁺ channels then inactivate

Phase 3 – Repolarisation

• At +30 mV: voltage-gated K⁺ channels open
• K⁺ efflux → membrane becomes more negative again
• Returns to near RMP

Phase 4 – Hyperpolarisation

• K⁺ channels remain open briefly after repolarisation
• MP drops below RMP (undershoot)
• Na⁺/K⁺ ATPase restores normal ion balance and RMP

Refractory Periods

Absolute Refractory Period

• No new action potential possible
• Includes depolarisation and part of repolarisation
• Prevents overlap of impulses and ensures unidirectional flow

Relative Refractory Period

• New action potential possible only with a stronger-than-usual stimulus
• Occurs during late repolarisation and hyperpolarisation phases
• Ensures proper interval between successive action potentials

Summary – Excitable Tissues

Excitable tissues, including nerve and muscle cells, exhibit dynamic changes in membrane potential in response to stimuli. These changes enable the generation and propagation of action potentials, forming the basis of neuromuscular signalling and physiological responsiveness. Ion gradients, selective membrane permeability, and Na⁺/K⁺ ATPase are all essential in this process. For a deeper understanding, see our Cell Biology & Biochemistry Overview page.