Nerve Impulse and Action Potential

Action Potential

• Action potential is a momentary change in the voltage difference across the cell membrane of a neuron, characterized by an influx of positively charged sodium ions (Na+)
• This results in the propagation of the impulse along the neuron
• Resting potential of the neuron is approximately -70 millivolts (mV)

Resting Membrane Potential

• The principal ions involved are:
  • Na+ (main extracellular cation)
  • K+ (main intracellular cation)
• Nerves and muscles are excitable tissue, both use this potential by undergoing transient fluctuations in membrane potential
• Two kinds of potential:
  • Action potential
  • Graded potential

Generation of Action Potential

• High concentration of Na+ outside the cell
• A stimulus occurs (e.g. pressure at the finger)
• Na+ rushes through the cell membrane, changing the inner surface to +40 mV
• This causes the inner cell membrane to be positively charged
• This is known as the Depolarisation Stage

Depolarisation Stage

• When the threshold is reached, the sodium channels open
• At this stage, the sodium channels are active, allowing sodium ions to rush into the cell
• The polarity of the inside of the cell becomes positive

Repolarisation Stage

• Membrane becomes more permeable to potassium ions (K+)
• K+ gated ion channels open, allowing K+ to rush out of the cell
• Higher concentration of K+ outside of the cell compared to the intracellular
• This causes the cell membrane to become negatively charged
• K+ release causes the membrane potential to fall below -70 mV (approx. -90 mV)
• This is known as the Repolarisation Stage

Hyperpolarisation Phase

• Too much K+ is released from the cell
• This causes a -90 mV impulse
• This is known as the Hyperpolarisation Phase

Refractory Stage

• The refractory stage of the action potential occurs with the assistance of the Na+/K+ pump
• 3 Na+ ions are received through the membrane and 2 K+ ions are released
• This returns the resting membrane to -70 mV

Propagation of Action Potential

• Action potential occurs at a very small area of the membrane
• Positive charge exists on the outside of the membrane
• Negative charge exists inside of the membrane
• Stimulus occurs, leading to the phases of the action potential
• This causes a "domino" type effect through the cells/neuron until it reaches the end terminal

Myelinated vs. Non-Myelinated Axons

• Myelinated sheath around a nerve
  • Impulses move more quickly
  • Increases information processing speed
  • Found in the central and peripheral nervous system
  • Utilizes a significant amount of metabolic energy to produce the many layers of the sheath
• Non-Myelinated axons
  • Comprise the smaller axons of the CNS
  • Slower conduction time
  • Found in the peripheral nervous system
  • Found in the visceral nervous system (internal organs)

Neurotransmitters

• Acetylcholine
  • Can be found in the neuromuscular junction
  • Signals voluntary muscle movement
• Catecholamines:
  • Dopamine, Norepinephrine, Epinephrine
  • Family of neurotransmitters that 'speed up' heart rate, dilate airways, dilate pupils
  • Signal a sympathetic response
  • Receptor type is alpha and beta
• Serotonin
  • If we block the reuptake pumps, we end up with more Serotonin
• Histamine
• GABA (Gamma-Aminobutyric Acid)
• Glutamate

Synapse

• The neurotransmitter is released into a space called the synapse
• Chemicals released from neurons communicate with other neurons
• Neurotransmitter = The 'Key'
• Receptor = The 'Lock'
• Ways to remove excess neurotransmitters:
  • Diffusion
  • Enzymes
  • Reuptake pumps
  • Astrocyte endfeet