Comprehensive Study Guide on Neurophysiology and Action Potentials

Structural and Functional Segments of the Neuron

• The neuron is organized into distinct functional segments that govern electrical signal reception, integration, conduction, and transmission:
  • Receptive Segment: Composed of the dendrites and the neurosoma (cell body). This segment receives chemical signals (neurotransmitters) from prior neurons.
  • Initial Segment: Composed of the axon hillock (often termed the "trigger zone"). This segment integrates incoming electrical signals through summation and determines whether an action potential will be generated.
  • Conductive Segment: Composed of the axon. This segment conducts the action potential along its entire length toward the axon terminals without loss of signal strength.
  • Transmissive Segment: Composed of the synaptic bulbs (axon terminals). This segment converts electrical signals back into chemical signals by releasing neurotransmitters into the synaptic cleft.
• Ion Channel Distribution Across Segments:
  • The receptive segment contains chemically gated ion channels (ligand-gated channels) that respond to neurotransmitter binding.
  • The initial segment and conductive segment contain voltage-gated ion channels (specifically voltage-gated $\text{Na}^+$ and voltage-gated $\text{K}^+$ channels) that respond to changes in membrane potential.

Receptive Segment Physiology: Local and Graded Potentials

• Chemical neurotransmitters released from presynaptic neurons (such as serotonin, ethylamine, or norepinephrine) bind to chemically gated ion channels on the receptive segment.
• Binding of neurotransmitters produces graded potentials (also termed local potentials or postsynaptic potentials):
  • Graded potentials are small-scale, short-lived electrical fluctuations across the plasma membrane.
  • They are variable in magnitude ("graded"), ranging from very weak to very strong depending on the stimulus strength and amount of neurotransmitter bound.
  • They travel short distances across the dendrites and neurosoma toward the initial segment.
• Scale and Spatial Proportions of Neuronal Processing:
  • If the neurosoma (cell body) were scaled to the size of a tennis ball held in hand, its surrounding dendritic tree would be vast enough to fill a 30-to-40-person classroom from floor to ceiling.
  • The associated axon at this scale would stretch approximately one mile in length.
  • A single neuron receives millions of simultaneous incoming presynaptic signals across its dendritic field, requiring real-time integration to determine output.

Types of Postsynaptic Potentials

Excitatory Postsynaptic Potentials (EPSP)

• Definition: An excitatory postsynaptic potential (EPSP) is a temporary, local depolarization that brings the membrane potential closer to the threshold required to trigger an action potential.
• Mechanism of Generation:
  1. A presynaptic neuron releases a neurotransmitter across the synaptic cleft.
  2. The neurotransmitter binds to chemically gated cation channels (specifically chemically gated $\text{Na}^+$ channels) on the postsynaptic membrane.
  3. Because sodium ($\text{Na}^+$) concentration is significantly higher outside the neuron than inside, opening these channels causes $\text{Na}^+$ to rapidly flow into the cell down its chemical concentration gradient.
• Electrical Effect:
  • The influx of positively charged sodium ions ($\text{Na}^+$) adds positive current to the inside of the cell.
  • This drives the membrane potential in a positive direction (making it less negative than the resting value of $-70\,\text{mV}$).
  • This shift toward a less negative/more positive voltage is termed depolarization (movement away from the resting state in a positive direction).

Inhibitory Postsynaptic Potentials (IPSP)

• Definition: An inhibitory postsynaptic potential (IPSP) is a temporary, local hyperpolarization that moves the membrane potential further away from the threshold, making action potential generation less likely.
• Physiological Importance: Inhibitory signals are essential for maintaining nervous system balance. They exert a calming effect on the nervous and muscular systems, enabling processes such as falling asleep, decompressing after cognitive exertion, and allowing skeletal muscles to relax.
• Mechanism of Generation:
  1. Inhibitory neurotransmitters bind to chemically gated potassium ($\text{K}^+$) channels or chemically gated chloride ($\text{Cl}^-$) channels.
  2. Potassium Flow: Potassium is the primary intracellular cation (highest inside the neuron). Opening $\text{K}^+$ channels causes positively charged $\text{K}^+$ ions to flow out of the cell down their concentration gradient.
  3. Chloride Flow: Chloride is an extracellular anion (highest outside the neuron). Opening $\text{Cl}^-$ channels causes negatively charged $\text{Cl}^-$ ions to flow into the cell down their concentration gradient.
• Electrical Effect:
  • Loss of positive charge ($\text{K}^+$ leaving) or gain of negative charge ($\text{Cl}^-$ entering) increases the net negative charge inside the cell.
  • This drives the membrane potential below $-70\,\text{mV}$.
  • This change is termed hyperpolarization (movement beyond the resting membrane potential in an excessively negative direction).

Signal Integration: Summation at the Initial Segment

• Location: Summation occurs in the initial segment, specifically at the axon hillock (the "trigger zone"), where the axon originates from the neurosoma.
• Mechanism of Summation:
  • The initial segment functions as a computational decision center by mathematically summing all incoming signals (both EPSPs and IPSPs) arriving simultaneously from the receptive segment.
  • EPSPs act as positive inputs ("votes in favor" of generating an action potential).
  • IPSPs act as negative inputs ("votes against" generating an action potential).
• Threshold Voltage:
  • Threshold is defined as the minimum membrane potential change required to open voltage-gated ion channels and initiate an action potential.
  • The standard threshold value in a typical neuron is $-55\,\text{mV}$.
  • If the net algebraic sum of EPSPs and IPSPs raises the membrane potential from the resting level of $-70\,\text{mV}$ up to the threshold of $-55\,\text{mV}$, an action potential is triggered.

The All-or-None Principle and Signal Propagation

• Action potentials adhere strictly to the All-or-None Principle:
  • All: If the membrane potential at the axon hillock reaches threshold ($-55\,\text{mV}$), an action potential will fire completely and uniformly every single time. It travels down the conductive segment with zero loss of signal amplitude or intensity.
  • None: If incoming signals fail to reach threshold (e.g., stopping at $-56\,\text{mV}$), no action potential will fire ($0\%$ output).
• Analogy to Mechanical Triggers: Pulling the trigger on a firearm produces an all-or-none result; once the threshold pressure is applied to the trigger, the bullet exits the barrel at maximum velocity. It cannot fire partially, nor can it reverse direction mid-flight.
• Directionality:
  • Action potentials travel exclusively in the anterograde direction (forward from the axon hillock down the length of the axon to the synaptic bulbs).
  • They never propagate in the retrograde direction (reverse).

Electrophysiology and Phases of the Action Potential

Key Baseline Voltages

• Resting Membrane Potential (RMP): $-70\,\text{mV}$
• Threshold Voltage: $-55\,\text{mV}$
• Isoelectric / Neutral Voltage: $0\,\text{mV}$ (no net electrical potential difference between intracellular and extracellular environments)
• Peak Depolarization Potential: $+30\,\text{mV}$

Sequential Phases of the Action Potential Curve

1. Resting State and Sub-threshold Potentials:
   • The neuron rests at $-70\,\text{mV}$.
   • Chemically gated channels in the receptive segment produce local EPSPs that elevate the voltage incrementally toward threshold.
2. Reaching Threshold and Opening of Fast $\text{Na}^+$ Channels:
   • When local voltage accumulation reaches $-55\,\text{mV}$ at the axon hillock, the gating mechanism of voltage-gated sodium channels is triggered.
   • Voltage-gated $\text{Na}^+$ channels are extremely fast to open.
   • High concentrations of dense $\text{Na}^+$ channels at the axon hillock open almost instantaneously.
3. Depolarization Phase ($-55\,\text{mV}$ to $+30\,\text{mV}$):
   • Extracellular $\text{Na}^+$ floods into the cytoplasm down both its concentration and electrical gradients.
   • This massive influx of positive charge causes a rapid spike in membrane potential, depolarizing the cell through $0\,\text{mV}$ up to a peak of $+30\,\text{mV}$.
4. Inactivation of Sodium Channels and Opening of Slow $\text{K}^+$ Channels:
   • At $0\,\text{mV}$, voltage-gated $\text{Na}^+$ channels begin inactivation.
   • Inactivation involves a conformational change in the channel protein that obstructs ion entry, progressively slowing and eventually halting $\text{Na}^+$ influx by the time the potential reaches $+30\,\text{mV}$.
   • Simultaneously, voltage-gated potassium channels—which were also stimulated when threshold ($-55\,\text{mV}$) was originally passed—finally open fully.
   • Potassium channels are inherently slow or "pokey" to open, taking the entire duration of the depolarization phase to achieve full open conformation.
5. Repolarization Phase ($+30\,\text{mV}$ to $-70\,\text{mV}$):
   • At $+30\,\text{mV}$, all $\text{Na}^+$ channels are inactivated (no further positive influx).
   • All voltage-gated $\text{K}^+$ channels are fully open.
   • Positively charged $\text{K}^+$ ions rapidly exit the cell down their chemical gradient.
   • The loss of positive intracellular charge causes repolarization, returning the membrane potential back down toward its resting value ($-70\,\text{mV}$).
6. Hyperpolarization Phase (Below $-70\,\text{mV}$):
   • Voltage-gated $\text{K}^+$ channels are slow to close ("pokey") just as they were slow to open.
   • As the membrane potential drops back down to $-70\,\text{mV}$, these channels fail to close immediately, allowing continuous outward efflux of $\text{K}^+$.
   • The membrane potential undershoots RMP, entering a state of hyperpolarization where voltage becomes excessively negative (more negative than $-70\,\text{mV}$).
7. Restoration of Resting State:
   • Voltage-gated $\text{K}^+$ channels finish closing.
   • The continuously active sodium-potassium pump ($\text{Na}^+/\text{K}^+$ ATPase) pumps sodium back out of the cell ($3\,\text{Na}^+$ out) and potassium back into the cell ($2\,\text{K}^+$ in).
   • This restores both the original chemical ion distributions and the baseline resting membrane potential of $-70\,\text{mV}$.

Time Course and Real-Time Kinematics

• Depolarization and Repolarization Spike: The upward spike ($-70\,\text{mV}$ to $+30\,\text{mV}$) and rapid repolarization occur almost instantaneously, taking only a couple of milliseconds ($\text{ms}$).
• Hyperpolarization Duration: The hyperpolarization phase is comparatively prolonged, extending up to approximately $50\,\text{ms}$. This extended timeframe reflects the slow closing kinetics of voltage-gated potassium channels and the duration required for $\text{Na}^+/\text{K}^+$ pumps to fully restore ionic equilibrium.