Introduction to Excitable Cells

What are the three basic functions of neurones?

  • Receive signals

  • Communicate signals to target cells

  • Integrate incoming signals

What factors affect the rate of impulse propagation in neurones?

  • Degree of myelination

  • Axonal diameter

  • Temperature

How does myelination affect impulse transmission in neurones?

  • Myelinated axon: Faster transmission via saltatory conduction

  • Unmyelinated axon: Slower transmission via contiguous conduction

What is contiguous conduction?

  • Propagation of the nerve impulse along an unmyelinated axon.

How does axonal diameter affect the speed of impulse conduction?

  • Larger diameter: Faster conduction (due to larger membrane area, more channels, and lower resistance).

  • Smaller diameter: Slower conduction.

How does temperature influence the speed of nerve impulse conduction?

  • Cooler temperature: Slower conduction (slower ion channel opening).

  • Warmer temperature: Faster conduction.

What is membrane potential?

  • Distribution of charge across the cell membrane, measured in millivolts (mV).

What does it mean when a membrane is said to be polarized?

  • There is a potential difference across the membrane, with the inside being more negative than the outside.

What factors determine membrane potentials?

  • Ion concentration on the inside and outside of the cell

  • Permeability of the membrane to specific ions

  • Activity of electrogenic pumps maintaining ion gradients

Give examples of electrogenic pumps.

  • Ca²⁺ transport pumps

  • Na⁺/K⁺ ATPase

What are the two main methods of ion transport across membranes?

  • Ion channels

  • Ion pumps

How do ion channels work?

  • They allow ions to flow passively down their electrical and concentration gradients, with their flow regulated by conformational changes (gates opening or closing).

What are the two types of ion channels?

  • Leak channels

  • Gated channels

What are the types of gated ion channels?

  • Ligand-gated: Respond to binding of a ligand (chemical messenger)

  • Mechanically-gated: Respond to stretch and pressure

  • Voltage-gated: Open or close due to changes in membrane potential

What is the function of ion channels?

  • To control the flow of ions across the neuronal membrane, which is needed for rapid changes in membrane potential, such as those involved in action potentials.

How do ion channels differ from ion pumps?

  • Ion channels: Passive flow of ions down the concentration gradient.

  • Ion pumps: Active transport of ions against the concentration gradient.

What is the function of ion pumps?

  • To maintain ionic gradients across the membrane, which are necessary for action potentials.

Why do ion pumps require energy?

  • Because they move ions against their concentration gradient.

What is the role of membrane proteins in ion transport?

  • Membrane proteins act as ion pumps to transport ions against their concentration gradients and maintain ionic balance.

What are the two types of active transport for ions?

  • Primary active transport

  • Secondary active transport