Human Physiology of the Nervous System: Cellular Structure to Systemic Control

Neuronal Signalling: Membrane Potentials

The ability of a neuron to "fire" depends on the electrical charge across its membrane. At rest, a neuron maintains a resting membrane potential ($V_m$), typically around $-70\text{ mV}$. This negative internal charge is established by two main factors: the concentration gradients of ions (primarily $\text{Na}^+$ and $\text{K}^+$) and the membrane's unequal permeability to these ions. Specifically, the membrane has more "leak channels" for $\text{K}^+$, allowing it to exit the cell more easily than $\text{Na}^+$ can enter.

The $\text{Na}^+/\text{K}^+$-ATPase pump is vital for this process, as it actively uses energy to maintain these concentration gradients by pumping $\text{Na}^+$ out and $\text{K}^+$ in.

Equilibrium and the Nernst Equation

An ion reaches its equilibrium potential ($E_{ion}$) when the chemical force (moving from high to low concentration) is exactly balanced by the electrical force (moving opposite to the charge). We calculate this specific voltage using the Nernst equation:

$E_{ion} = 61 \cdot Z \cdot \log\left(\frac{C_{out}}{C_{in}}\right)$

Where $Z$ is the ion's valence and $C_{out}/C_{in}$ is the ratio of extracellular to intracellular concentration.

Student Note: Concentration Sensitivity Not all concentration changes are equal. Because the intracellular concentration of $\text{K}^+$ is much higher than the extracellular concentration, a small change in extracellular $\text{K}^+$ (e.g., $1\text{ mM}$) has a much larger percentage impact on the resting membrane potential than an equivalent change inside the cell.

The Action Potential and Synaptic Transmission

Communication occurs through two types of electrical changes: graded potentials and action potentials. Graded potentials are local, variable changes in voltage that travel only short distances. If a graded potential is strong enough to reach a specific threshold potential, it triggers an action potential—an "all-or-nothing" electrical impulse.

The action potential follows a predictable sequence of permeability changes:

1. Depolarization: Upon reaching threshold, voltage-gated $\text{Na}^+$ channels open rapidly, allowing $\text{Na}^+$ to rush into the cell.
2. Absolute Refractory Period: During the peak of the action potential, $\text{Na}^+$ channels become inactivated, meaning no new signal can be generated regardless of stimulus strength.
3. Repolarization and Hyperpolarization: $\text{K}^+$ channels open, allowing $\text{K}^+$ to exit the cell. This often causes the voltage to drop below the resting potential (hyperpolarization).
4. Relative Refractory Period: During hyperpolarization, the cell is more difficult to excite, but a very strong stimulus might trigger a new action potential as $\text{Na}^+$ channels begin to recover.

Synaptic Communication

When the action potential reaches the axon terminal, it must cross a gap to communicate with the next cell. This occurs at a synapse:

• Electrical Synapses: Allow for extremely rapid, direct transmission between cells.
• Chemical Synapses: The more common type, where an action potential triggers the influx of $\text{Ca}^{2+}$ into the terminal. This calcium influx causes vesicles to fuse with the membrane and release neurotransmitters into the synaptic cleft.

Neurotransmitters can be categorized by their effect:

• Excitatory Postsynaptic Potentials (EPSPs): Depolarize the target cell, bringing it closer to the threshold.
• Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarize the cell (often via $\text{K}^+$ efflux), making it harder to fire.

Common neurotransmitters include Acetylcholine (crucial at neuromuscular junctions), Dopamine (involved in reward and motor control), and Serotonin (regulating mood and sleep). While neurotransmitters trigger immediate EPSPs or IPSPs, neuromodulators can induce more complex, long-lasting metabolic changes via second messenger systems.

Systemic Organization: Somatic and Autonomic Systems

The nervous system is organized into functional divisions to manage both voluntary and involuntary actions.

The Somatic Nervous System

This division governs voluntary movements by controlling skeletal muscles. It is primarily excitatory; for example, it uses acetylcholine to stimulate muscle contraction, but relies on the cessation of signals for muscles to relax.

The Autonomic Nervous System (ANS)

The ANS manages involuntary functions, such as heart rate, digestion, and gland secretion. It is divided into two opposing branches that provide "dual control," allowing for precise physiological regulation:

• Sympathetic Division: The "fight-or-flight" system. It increases heart rate, redirects blood to muscles, and suppresses digestion to prepare the body for action.
• Parasympathetic Division: The "rest-and-digest" system. It lowers heart rate and enhances digestive processes.

Finally, higher-order functions like memory and emotion are processed in areas such as the limbic system. This explains why emotionally charged experiences—such as meeting someone you are strongly attracted to—are more easily encoded into declarative memory than neutral ones.