Nerve Impulse & Neuron Structure

Nerve Impulse Transmission

• Nerve impulse transmission is related to sodium (Na+) and potassium (K+) concentrations, local anesthetic drugs, viral transmission through nerves, and demyelinating diseases.

Intended Learning Outcomes

• Intended learning outcomes include describing the electrical properties of a neuron and comparing the ionic properties of resting membrane potential and action potential.
• Also, describe the generation and conduction of action potential.

Axonal Transport

• Axonal transport is how materials move along a neuron's axon
• It is necessary for neuronal function, and allows movement of organelles, proteins, and vesicles between the cell body (soma) and the axon terminal

Neuron Structure

• The cell body (soma) contains the nucleus and organelles and facilitates protein synthesis
• The axon extends from the soma, transporting materials
• The axon terminal releases neurotransmitters for communication with other neurons

Microtubules

• These are the green structures in the axon, which form tracks for intracellular transport
• They facilitate the movement of cargo via molecular motors

Axonal Transport Types

• Key difference in axonal transport types are direction of movement

Anterograde Transport

• Anterograde transport moves away from the cell body toward the axon terminal.
• This type uses kinesin motor proteins to deliver vesicles, mitochondria, and proteins.
• It supports synaptic function by delivering necessary materials.

Retrograde Transport

• Retrograde transport moves toward the cell body from the axon terminal.
• This type uses dynein motor proteins and recycles materials, signaling molecules, and damaged organelles back to the soma for degradation or reuse.

Synapse and Cargo Release

• The axon terminal releases transported vesicles
• It is crucial for neurotransmitter release and neuronal communication.

Resting Membrane Potential

• The inside of the cell is negative relative to the outside when at rest in a polarized state
• Neurons have a resting membrane potential of around -70 mV.
• It exists in other cells like muscle and heart cells
• It is mainly maintained due to movement of ions across the membrane

Resetting Membrane Potential

• Resetting membrane potential refers to the electrical charge difference across the cell membrane when a neuron or muscle cell is not actively sending signals
• Typically around -70 mV in neurons, making the inside more negative
• It is a property needed for excitability in neurons, muscles, and the heart

Factors Maintaining Resting Membrane Potential

• Selective Permeability of the membrane allows K+ to permeate better than Na+

• More K+ leaking makes the inside more negative

• The sodium-potassium pump (Na+/K+ ATPase) actively pumps 3 Na+ out and 2 K+ in to maintain a higher sodium concentration outside and higher potassium concentration inside

• The sodium-potassium pump requires ATP (active transport)

• Negatively charged proteins inside the neuron contribute to the negative charge inside

Ion Distribution Across Membrane

• The voltmeter measures the resting membrane potential (-70mV)
• The voltmeter measures around -70 mV typically
• Ion channels and the Na+/K+ pump help maintain the difference in charge

Interstitial Fluid Composition

• There is a high Na+ concentration in the extracellular environment

Cytosol Composition

• High K+ and negative proteins
• Higher concentration of potassium ions (K+) and negatively charged proteins contribute to the negative resting potential found in the cytosol

Na+/K+ Pump

• The Na+/K+ active transport pump pumps 3 Na+ out of the cell and 2 K+ in to maintain the high Na+ concentration outside
• ATP is used to help sustain resting potential.

K+ Leak Channels

• The K+ passive transport leak channels move K+ out of the cell using more channels than Na+
• It is the reason the resting membrane potential is negative.

Na+ Leak Channels

• A small amount of Na+ enters the cell through leak channels
• Inward Na+ movement is minimal because these channels are less permeable

Negatively Charged Proteins Inside the Cell

• They cannot cross the membrane and are in the intracellular space
• They contribute to the negativity

Resting Membrane Potential

• K+ moves out, and Na+ moves in, passively
• It is an equal amount of opposite forces in ions, where gradients are being challenged
• For every neuron, Na+ is actively transported out and K+ is actively transported in an ATP dependent process

Governing Principles of Resting Membrane Potential

• K+ diffuses out and Na+ moves into the cell because of concentration needing to be stabilized

• Leak channels allow the cell to passively transport

Opposing Forces at Equilibrium

• The concentration gradient drives K+ out and Na+ in because ions move from high to low concentration
• The electrical gradient attracts positive ions such as K+ inside and repels positive ions like Na+ outside
• At rest, these forces reach an equilibrium, resulting in a stable RMP.

Active Transport

• Na/K ATPase pump helps to keep cells at equilibrium
• Na+ is pumped out of the cell, and 2 K+ is pumped into the cell which requires ATP and ensures that the resting membrane potential remains

Membrane Excitability

• Membranes of nerve and muscle cells are easily excitable and can be depolarized

Membrane Depolarization

• Neurons and muscle cells have excitable membranes, changing rapidly with stimuli

Key Points:

• Key points for excitable membranes are for the negative interior state to be polarized
• Stimulus depolarizes and opens ion channels in the membranes to propagate an action potential
• Na+ rushes into the cell, making it less negative and leading to potassium leaving for resting potential

Electrical Property

• Nerve cells have a low excitation threshold, where stimuli open ion pathways and move via gates to trigger an action potential

• The stimulus can be electrical, chemical, or mechanical (e.g., neurotransmitters, pressure, or direct electrical stimulation).

Electrical Property

• Depolarization starts, increasing at 15mV, reaching a firing level or threshold to be reversed for a rapid decline to resting level

Electrical Properties

• Neurons generate and transmit signals using ion movement

Membrane Depolarization

• At rest, the inside is negative (-70mV)
• A stimulus opens Na+ channels and triggers the rush of it to depolarize
• This occurs by way of neurotransmitters or mechanical pressure

Initial Depolarization/Threshold

• RMP is -70 m
• -55mV = threshold, activating Na+ channels for rapid entry
• Entry will lead to more entry and positive feedback

Rapid Depolarization

• Crossing of threshold leads to +30 mV due to a great positive charge
• When nearing peak, Na channels are closed, and K+ channels begin opening

Returning to Baseline

• Voltage-gated potassium channels facilitate K+ for repolarization for a drop to a resting state (-70mV)

Hyperpolarization/Resetting

• Too much K+ leaving will drop below equilibrium
• Sodium potassium pumps are used to restore balance with hyperpolarization

Action Potential Summary

• Stimulus → Na+
• Threshold -55mV leads to rapid cycle activation
• Channels are closed and open, making it negative

Polarization

• Polarization starts at membrane potential, and then will trigger depolarization, repolarization, hyperpolarization

Phases of The Action Potential

• The graph illustrates a curve plotting membrane potential (mV) versus time (milliseconds)

Resting Membrane Potential

• At rest, the neuron's inside is negative related to the outside (~ -70 mV).
• The potential is maintained by the Na+/K+ pump that pumps 3 Na+ out and 2 K+ in.

Depolarization

• At -5mV Threshold, Na rushes in, allowing voltage-gated channels to rush in, until +30 mV
• Once threshold is met, the inside becomes less negative and, ultimately, positive.

Action Potential Maximum

• Around +30 mV, sodium channels inactivate
• Potassium channels begin to activate and prepare for polarity to switch

Repolarization Phase

• K+ escapes through voltage-gated potassium channels for negativity
• Repolarization occurs due to K+ release
• Na+ channels are inactivated during the hyperpolarization of the cell to prevent absolute refractory period

Hyperpolarization

• Happens if too many channels are released, and is in effect for the relative refractory period

Return To Resting

• Voltage is gated and channel close
• The pump restores ionic balance to -70mV

Understanding Excitability

• Action potential generation, voltage-gated Na channels, a stimulus

Key Phases of Excitability

• State = -70mV, Na/K is closed and high and ready to respond for depolarization

• State = Stimulus and causes -55mV

• Na begins to respond greatly to stimulation for action potential

Excitability Drops

• Na+ inactivates
• Occurs during the absolute refractory period
• It is low because the membrane recovers from potential charges
• Threshold for firing

What Occurs During States

• Na is closed with quick stimulation for high response channels

• State involves the neuron responding rapidly to stimuli to cause rapid depolarization

• Nearing peak action potential makes it start decreasing because voltage gates close

Action Potential

• Depolarization occurs

• Artifact after firing, returning in equilibrium

• Includes an overshoot

Membrane Resting Potential

• It is negative, with K leakage, and ATP transport for Na/K to maintain potential charge

Latent Period

• Action potential is only triggered if electrical messages can bring it to -55mV