Animal Body Systems - Neurons

Animal Body Systems - Electrochemical Potentials in Neurons

•  Neural function relies on electrochemical potentials.
• Lectures cover electrochemical potentials in neurons.
•  Supplementary reading recommended: Textbook (15th Edition, Chapter 42, pages 1123-1137).

Review General Concepts

• Organ systems must coordinate within an animal and with the environment.
• Two major systems involved in this coordination are nervous and endocrine.
• These systems work together to create complex homeostasis.

Nervous Systems

• Nervous systems are rapid coordination/regulation systems in all animals except sponges.
• Major roles of nervous systems include:
  • Collecting information from internal or external environments using modified neurons (sensory receptors).
  • Processing and integrating information (evaluating information based on past experiences and genetics.)
  • Transmitting information to coordinate/regulate effector organs/cells.

Terminology

• Neuron: An individual nerve cell.
• Nerve: A bundle of axons (not the entire neuron).
• Axon: Also called a nerve fiber.
• Synapse: The connection between the axon terminal and the effector cell (synaptic cleft).
• Effector: Can be a neuron, muscle cell, or another type of cell (e.g., gland).

Bioelectricity

• Bioelectricity occurs at the membrane; all processes occur here.
• Potential: Difference in electrical charge between regions, measured in volts (V) or millivolts (mV).
• Current: Flow of electrical charge between regions. Opposite charges attract, like charges repel. Differences in charge from inside to outside influence the current
• Membrane potential: Unequal charge distribution across a cell membrane.

Cells are Polarized

• All living cells have a membrane potential (MP).
• The inside of a cell membrane is negative relative to the external side.
• The range of MP is from -10 mV to -90 mV.

Excitable Cells

• Neurons and muscle cells are excitable and have large membrane potentials.
• Mechanisms regulate membrane potentials and currents.
• Three types of membrane potentials include resting, electrotonic, and action potentials.
• Membrane potentials and currents depend on inorganic ions.

Resting Membrane Potential of a Cell

• All cells have a resting MP.
• Measured when the cell is inactive.
• MP results from an unequal distribution of positive and negative charges across the cell membrane.
• The resting potential creates a potential difference across the membrane, which is primarily controlled by Na⁺ and K⁺ ions, and other ions.

Ion Concentrations in Cells

• Extracellular fluid has a high concentration of Na⁺ and a low concentration of K⁺.
• Intracellular fluid has a high concentration of K⁺ and a low concentration of Na⁺.
• Embedded proteins (e.g., sodium-potassium pump) control ion concentrations in cells. Active transport is involved.

Ion Gradients in all Cells

• Ion gradients are maintained by active transport (energy required, requiring ATPases).
• Na⁺/K⁺ ATPase moves three Na⁺ ions out and two K⁺ ions into the cell.
• It's an electrogenic pump.
• It generates a -10mV potential.
• Anionic proteins generate a -5 mV potential.
• All cells have ions gradients

The Additional Potential in Neurons

• Passive diffusion of K⁺ through open K⁺ channels contributes to membrane potential.
• Chemical gradient for K⁺; no electrical gradient.
• ATPase and leak channels together create an electrochemical gradient.

Resting Potential of Neurons

• Na⁺/K⁺ active transport pump sets up concentration gradients for Na⁺ and K⁺ ions.
• Open channels (leak channels) allow K⁺ to flow out freely.
• Negatively charged molecules (anions) in the cell cannot pass through the membrane.
• Standard resting membrane potential value (-70 mV). The value can vary, but is a good baseline.

Membrane Ion Channels

• Ion channels are specific for each ion.
• Leak channels are always open (e.g., K+ channel at rest).
• Other channels are regulated (gated) and can change shape to open/close (e.g., in neurons).
• Gated channels are often voltage-gated in neurons.
• Ion movement depends on the concentration gradient.

Polarization in Cells

• Cells are polarized with a negative inside.
• Cells can depolarize (more +ve inside) or hyperpolarize (more -ve inside).
• Electrotonic potentials (EP) and action potentials (AP) involve changes in potential.
• APs are rapid changes in membrane potential.
• EPs are small changes in membrane potential.

Graded Potentials

• Changes in membrane potential due to ion permeability changes are graded potentials.
• In neurons, these are parts of integration in dendrites and cell bodies.
• Electrotonic potentials are one type of graded potential. Not enough to cause action potential

Electrotonic Potentials

• Electrical currents traveling along the membrane.
• Small changes in potential (a few mV).
• Can depolarize or hyperpolarize.
• Travel a short distance.
• Used to initiate action potentials in axon hillock.
• Also used in conducting APs along axons.

Action Potentials

• Action potentials begin at the axon hillock and propagate down the axon.
• Action potentials are found only in nerve axons.
• Action potentials carry signals from axon hillocks to terminals.
• Depolarization to the level of +35 mV.
• Quick and brief change in potential. Not always a long event.
• Entire axon is involved.
• Relies on ion currents via voltage-gated channels for propagation.

Generation of an Action Potential

• Occurs when a stimulus raises resting potential to threshold value.
• Voltage-gated Na⁺ and K⁺ channels open in the plasma membrane.
• Inward flow of Na⁺ changes potential from negative to positive.
• Potential falls to resting value due to gated K⁺ channels.

Depolarization (Rising Phase of AP)

• Action potential depends on ion currents and voltage-gated channels.
• Na⁺ - voltage gated sodium channel
• K⁺ - voltage gated potassium channel
• NOTE: K⁺ leak channels are always open. This is a crucial ion channel in the maintenance of resting potential.

Falling Phase of AP

• Action potential depends on ion currents and voltage-gated channels.
• K⁺ channel opens. K⁺ flows out.
• Repolarization occurs.

The Hodgkin-Huxley Cycle

• The rise phase of AP is positive feedback.
• Initial depolarization opens Na⁺ channels, increasing permeability to Na⁺.
• Increased Na⁺ flow leads to further depolarization, completing the cycle.

AP Propagation Along Axon

• Action potential is initiated in the axon hillock and is carried down the axon.
• The signal moves without change in strength.
• Dendrites and cell bodies have K⁺ channels reducing signal strength moving back to the soma.

Propagation of an Action Potential

• Generated in one segment from ion flow, depolarizing the next segment.
• This process repeats.
• Applies to both myelinated and unmyelinated axons.

AP Conducton in Unmyelinated Axon

• Reduced threshold at the axon hillock.
• Concentration of Na⁺ channels.
• Current spreads along the membrane toward terminals.

AP Conduction in Myelinated Axon

• Myelin (protein and lipid) insulates the axon, preventing ion flow across the membrane.
• Reduces current loss.
• Concentration of Na⁺ and K⁺ at nodes allows ions to cross the membrane.
• Saltatory conduction occurs (jumping between nodes).

### Something to Think About

• Vertebrates requiring higher action potential conduction velocities.

This quiz delves into the electrochemical potentials in neurons and the coordination between the nervous and endocrine systems within animal body systems. It covers essential concepts as discussed in the 15th Edition textbook, Chapter 42. Test your understanding of neural functions and their role in maintaining homeostasis.