Nervous System Overview Quiz

Animal Body Systems - Electrochemical Potentials in Neurons

• Neurons and muscle cells are excitable cells
• They have large membrane potentials
• They have special mechanisms to regulate membrane potentials and currents
• Three types of membrane potentials exist: resting membrane potentials (RMP), electrotonic potentials (EP), and action potentials (AP)

Review General Concepts

• Organ systems must coordinate both within the animal and with the environment
• Two major systems involved in homeostasis are the nervous and endocrine systems

Nervous Systems

• A very rapid coordination/regulation system in all animals except sponges
• Has three roles:
  • Collects information from the internal or external environment using modified neurons (sensory receptors)
  • Processes and integrates information, evaluating based on past experience and/or genetics
  • Transmits information, coordinating and regulating effector organ/cells

Terminology

• Neuron: Individual nerve cell
• Nerve: Bundle of axons (ranging from a few to millions)
• Axon: Nerve fiber
• Synapse: Connection between axon terminal and effector cell
• Effector: Can be a neuron, muscle cell, or any other cell

Bioelectricity

• Bioelectricity occurs at the membrane
• Potential: Difference in electrical charge between regions (measured in volts [V] or millivolts [mV])
• Current: Flow of electrical charge (opposites attract, likes repel)
• Membrane Potential: Unequal charge distribution across a cell membrane

Cells are Polarized

• All living cells are electrically polarized
• Have a membrane potential (MP)
• Inside of the membrane is negative relative to the exterior side
• MP ranges from -10 to -90 mV

Excitable Cells

• Neurons and muscle cells are specially adapted
• Have large membrane potentials
• Have special mechanisms to regulate membrane potentials and currents
• Three types of membrane potentials:
  • Resting membrane potentials (RMP)
  • Electrotonic potentials (EP)
  • Action potentials (AP)

Resting Membrane Potential of a Cell

• All cells have a resting membrane potential
• Measured when the cell is inactive
• The membrane potential results from an unequal distribution of positive and negative charges on either side of the membrane
• This creates a potential difference, or resting potential, across the membrane
• Principle ions involved are Na⁺ and K⁺

Ion Concentrations in Cells

• Extracellular fluid always has high Na⁺ and low K⁺ concentrations.
• Intracellular fluid always has high K⁺ and low Na⁺ concentrations.
• Specific ion concentration amounts are displayed in a table

Ion Gradients in all Cells

• Ion gradients are maintained by active transport
• Na+/K+ ATPase moves 3 Na⁺ out and 2 K⁺ in
• Is an electrogenic pump
• Generates a -10 mV potential
• Anionic proteins generate a -5 mV potential (all cells)

The Additional Potential in Neurons

• By passive diffusion of K⁺ through open K⁺ channels
• Chemical gradient for K+ – no electrical gradient
• ATPase and leak channels create an electrochemical gradient (-55 mV in neurons/muscle)

Resting Potential of Neurons

• Na+/K+ active transport pump sets up concentration gradients of Na⁺ (higher outside) and K⁺ (higher inside)
• Open channel (leak channel) allows K⁺ to flow out freely
• Negatively charged molecules (proteins) inside the cell cannot pass through the membrane
• -10 mV + -5 mV + -55 mV = -70 mV

Membrane Ion Channels

• Are very specific for each ion
• Leak channels are always open (e.g., the K⁺ channel at rest)
• Other channels are regulated (gated)
• In neurons, channels are often voltage-gated
• Ion movement depends on the concentration gradient

Polarization in Cells

• Cells are polarized (negative inside)
• Can also become depolarized (more +ve inside) or hyperpolarized (more –ve inside)
• Happens during electrotonic potentials (EP) or action potentials (AP)
• EP: small changes in membrane potentials
• AP: large and rapid changes in membrane potentials

Graded Potentials

• Changes in membrane potential due to changes in membrane permeability to ions are called graded potentials
• In neurons, graded potentials are part of the integration that takes place in dendrites and cell bodies
• Electrotonic potentials are one type of graded potential

Electrotonic Potentials

• Current (ions) travels along the surface of the membrane
• Small (only a few mV)
• Can depolarize or hyperpolarize
• Only travel a short distance along the membrane
• Used to initiate an action potential (AP) in the axon hillock
• Also used to conduct AP along the axon

Action Potentials

• Initiated at the axon hillock region, found only in axons
• Carries the signal from the axon hillock to terminals
• Special features:
  • Depolarizes the membrane (from -70 to +35 mV)
  • All or nothing, but transient
  • Conducted along the entire axon
  • Relies on ion currents through membrane via voltage-gated ion channels

Generation of an Action Potential

• Generated when stimulus pushes resting potential to threshold value
• Voltage-gated Na⁺ and K⁺ channels open in the plasma membrane
• Inward flow of Na⁺ changes membrane potential from negative to positive peak
• Potential falls to resting value as gated K⁺ channels allow ion to flow out

Depolarization

• AP depends on ion currents and voltage-gated channels
• Na⁺: voltage gated sodium channels
• K⁺: voltage gated potassium channels
• NOTE: K⁺ leak channels are always open -Channels open and close in specific sequences – causing different membrane potentials in different segments of the neuron -Different types of ion channels control the membrane potential

Falling Phase of AP

• AP depends on ion currents and voltage-gated channels
• K⁺ channel opens – K⁺ flows out – repolarization
• K⁺ channel still open at RMP – K⁺ still flows out briefly -hyperpolarization

The Hodgkin-Huxley Cycle

• AP rise phase is positive feedback
• Initial depolarization
• Further membrane depolarization
• Opening of Na⁺ channels increase permeability to Na⁺
• Increased Na⁺ flow

AP Propagation Along Axon

• AP initiated in axon hillock (concentration of Na⁺ channels)
• Conducted unchanged along axon membrane to terminals
• Dendrites & cell body – concentration of K⁺ channels reduces backpropagation into soma

Propagation of Action Potential

• Action potentials move along an axon as the ion flows generated in one segment depolarize the potential in the next segment
• This process happens in both myelinated and unmyelinated axons

AP Conduction in Unmyelinated Axons

• Reduced threshold at axon hillock (spike-initiating zone)
• Concentration of Na⁺ channels
• Current spreads along the membrane toward terminals (new AP)

AP Conduction in Unmyelinated Axon-additional details

• Adjacent (downstream) Na⁺ channels reach threshold directly from the large depolarization of the original segment – a new AP happens further down the axon
• Refractory period prevents backpropagation

AP Conduction in Myelinated Axons-more details

• Myelin (protein and lipid) insulation prevents ions from crossing the membrane – reduces current loss
• Concentration of Na⁺ and K⁺ at nodes – allows ions to cross the membrane

Saltatory Conduction

• In myelinated axons, ions can flow across the plasma membrane only at nodes where the myelin sheath is interrupted
• Action potentials skip rapidly from node to node
• Saltatory conduction allows thousands to millions of fast-transmitting axons to be packed into a relatively small diameter

Something to Think About

• Why might vertebrates require higher action potential conduction velocities?