Neuron Structure and Function

Neuron Structure

• Neurons have a distinct structure: cell body (soma), dendrites, and an axon.
• Dendrites receive signals from other neurons or sensory receptors.
• The axon carries signals away from the cell body.

Resting Membrane Potential

• Neurons have a resting membrane potential—an electrical charge across the cell membrane when not actively transmitting signals.
• This potential is mainly maintained by selective permeability of the cell membrane to ions like potassium (K+) and sodium (Na+).
• The resting potential is approximately -70 mV.

Action Potential

• The action potential is the basic unit of communication within a neuron.
• It's an electrical impulse traveling along the axon.
• A strong enough stimulus triggers depolarization, opening voltage-gated sodium channels.
• This allows sodium ions to enter the cell, rapidly changing the membrane potential.
• The action potential exhibits an "all-or-none" response.

Propagation of Action Potential

• Saltatory conduction is the process of action potential propagation down the axon.
• This involves jumping between nodes of Ranvier, significantly speeding signal transmission.
• Myelin sheaths, surrounding the axon in some neurons, facilitate this process.

Withdrawal Reflex

• This describes a simple nervous system function.
• A painful stimulus causes the hand to pull away from the source (e.g., a hot iron).
• Sensory neurons in the hand detect the pain, sending signals to the spinal cord.
• Interneurons in the spinal cord relay the signal to motor neurons that trigger a muscular response to withdraw the hand.
• In some cases, the brain can inhibit this reflex to prevent unwanted actions (e.g., dropping a casserole).

Role of Inhibition

• Inhibitory signals from the brain can prevent the withdrawal reflex.
• This is demonstrated by the figure showing brain activity preventing the reflex, like not dropping a casserole when burned.

Squid Axon Significance

• Squid or "Kalamar" is scientifically significant because of its large axon.
• The giant axon of a squid is approximately 0.5 mm in diameter, much larger than mammalian axons.
• The large size of the squid axon makes it easier to study action potentials.

Measuring Electrical Potential

• Microelectrodes are used to record activity of individual neurons.
• The glass microelectrode, filled with potassium chloride, is critical for this measurement.
• The inside of the axon is negatively charged with respect to the outside, differing by approximately 70 mV.
• This difference in charge is called the membrane potential, which is often referred to as resting potential because of its measured value when the neuron is not active.

Studying the Axon

• The figure shows how an axon can be stimulated, while its membrane potential can be recorded, measuring electrical changes.

Terms

• Depolarization: Reduction in membrane potential toward zero
• Hyperpolarization: Increase in membrane potential relative to the resting state.
• Threshold of excitation: The membrane potential value to produce an action potential.
• Action potential: The rapid change in membrane potential during signal transmission.

The Action Potential

• Each stimulus briefly depolarizes the membrane potential.
• At 4 milliseconds (msec), the membrane potential reverses inside becoming positive & outside becoming negative, then swiftly returns to normal.
• This entire process, including hyperpolarization, takes around 2 msec.

Diffusion, Electrolyte, and Ions

• Diffusion: Movement of molecules from high to low concentration.
• Electrolyte: An aqueous solution of ionized material (acid, base, or salt)
• Ion: Charged molecules (cations positive, anions negative)
• Electrostatic pressure: Force between oppositely charged particles.
• Intracellular fluid: Fluid inside the cell
• Extracellular fluid: Fluid outside the cell

Ions in Extra Cellular & Intracellular Fluid

• The fluids inside and outside the cells contain various ions.
• Important ions include organic anions, chloride ions, sodium ions, and potassium ions.
• These ions have different concentrations inside and outside cells, contributing to the membrane potential.

Forces Acting on Ions

• Diffusion and electrostatic pressure are the forces acting on ions, affecting movement across the cell membrane.

Action Potential - Sodium-Potassium Transporter

• A sodium-potassium transporter is situated in the neuronal membrane.
• This transporter pumps 3 sodium ions out and 2 potassium ions in, maintaining optimal ion concentrations.

Action Potential Mechanisms

• When ion channels are open, ions travel across the membrane.
• The membrane is a double layer of lipid molecules with proteins that facilitate ion movement.
• The protein constituents, called ion channels, are crucial for this movement.

Movements of Ions during Action Potential

• Sodium channels open: Sodium rush into the cell and cause depolarization.
• Sodium channels close, Potassium channels open: Potassium flows out of the cell, causing repolarization.
• The action potential rapidly changes the membrane potential, facilitating signal transmission.