Describe the process of generation and conduction of nerve impulses in humans. Add a note on saltatory conduction.

Generation of Nerve Impulses (Action Potential)

The generation of a nerve impulse involves a rapid, transient change in the electrical potential across the neuronal membrane, driven by the movement of ions. This process can be divided into several stages:

1. Resting Membrane Potential:
   • In its resting state, a neuron maintains a potential difference across its membrane, typically around -70 mV (inside negative relative to outside). This is known as the resting membrane potential.
   • This potential is established and maintained primarily by the sodium-potassium pump (Na+/K+ pump) and differential membrane permeability to ions. The pump actively transports three Na+ ions out of the cell for every two K+ ions pumped in, consuming ATP.
   • The neuronal membrane is more permeable to K+ ions than to Na+ ions at rest due to the presence of more K+ leak channels. This allows K+ to diffuse out of the cell down its concentration gradient, contributing to the negative charge inside.
   • Large, negatively charged protein molecules and other organic anions are also present inside the cell and cannot leave, further contributing to the negative intracellular environment.
2. Depolarization (Rising Phase):
   • When a stimulus of sufficient strength (reaching a threshold potential, typically around -55 mV) is applied to the neuron, voltage-gated Na+ channels in the membrane open rapidly.
   • This causes a massive influx of positively charged Na+ ions into the cell, driven by both the concentration gradient and the electrical gradient.
   • The influx of positive charge causes the inside of the membrane to become less negative and then positive (reversing polarity), reaching a peak potential of approximately +30 mV to +40 mV. This rapid reversal of membrane potential is the depolarization phase.
3. Repolarization (Falling Phase):
   • Shortly after the peak of depolarization, the voltage-gated Na+ channels inactivate (close and become unresponsive for a brief period), preventing further Na+ influx.
   • Simultaneously, voltage-gated K+ channels open, allowing K+ ions to rapidly diffuse out of the cell, driven by their electrochemical gradient.
   • The efflux of positive K+ ions restores the negative charge inside the cell, bringing the membrane potential back towards the resting state.
4. Hyperpolarization (Undershoot):
   • The voltage-gated K+ channels often remain open for a brief period after the membrane potential has returned to the resting level, causing an excessive efflux of K+ ions.
   • This leads to a transient period where the membrane potential becomes even more negative than the resting potential (e.g., -90 mV), known as hyperpolarization or the undershoot.
   • The Na+/K+ pump then works to restore the precise ion concentrations, bringing the membrane back to its stable resting potential.

Conduction of Nerve Impulses

Once an action potential is generated, it must be propagated along the axon to transmit the signal to the next neuron or effector cell. The mechanism of conduction differs based on whether the axon is myelinated or unmyelinated.

1. Continuous Conduction (Unmyelinated Axons)

• In unmyelinated axons, the entire length of the axonal membrane is exposed to the extracellular fluid.
• The action potential propagates continuously along the membrane as a wave of depolarization.
• As one segment of the membrane depolarizes, local current flows to the adjacent, resting segment, bringing it to threshold and triggering a new action potential.
• This process is sequential and relatively slow, with conduction velocities typically ranging from 0.5 to 10 m/s.

2. Saltatory Conduction (Myelinated Axons)

Saltatory conduction is a specialized, highly efficient mode of nerve impulse propagation that occurs in myelinated axons.

Mechanism of Saltatory Conduction:

• Most axons in the human nervous system are covered by a myelin sheath, a fatty, insulating layer formed by Schwann cells (in the peripheral nervous system) or oligodendrocytes (in the central nervous system).
• The myelin sheath is not continuous; it has periodic gaps called Nodes of Ranvier (approximately 1-2 micrometers long).
• Voltage-gated Na+ and K+ channels are highly concentrated only at these Nodes of Ranvier, while the internodal regions (myelinated segments) have very few ion channels and are electrically insulated.
• When an action potential is generated at one Node of Ranvier, the depolarization causes a strong local current to flow rapidly through the low-resistance axoplasm of the myelinated internode to the next Node of Ranvier.
• The myelin sheath prevents ion leakage across the internodal membrane, effectively "jumping" the electrical signal over the insulated segment.
• Upon reaching the next node, this rapid passive spread of current depolarizes the membrane to threshold, triggering a new action potential at that node.
• This "jumping" or "leaping" of the action potential from one node to the next is what defines saltatory conduction.

Advantages of Saltatory Conduction:

Feature	Description
Increased Conduction Velocity	Action potentials travel significantly faster in myelinated axons (up to 120 m/s or even 150 m/s in humans) compared to unmyelinated axons. The passive current flow between nodes is much faster than the active regeneration of the action potential at every point along the membrane.
Energy Efficiency	The Na+/K+ pumps only need to work extensively at the Nodes of Ranvier to restore ion gradients, rather than along the entire axon. This substantially reduces the metabolic energy (ATP) required for nerve impulse transmission.
Space Saving	Myelinated axons can achieve high conduction velocities with smaller diameters compared to unmyelinated axons, allowing for more neurons to be packed into the nervous system.