Test: Physiology of Nerve- 1 - NEET PG MCQ

The Node of Ranvier is a small gap in the myelin sheath of a nerve fibre. It plays a crucial role in the way nerve impulses are transmitted.

Key points about the Node of Ranvier:

• Found along axons, which are the long projections of nerve cells.
• These nodes allow for faster communication between neurons.
• They facilitate the process of saltatory conduction, where nerve impulses jump from one node to the next.

This structure is essential for maintaining the efficiency of nerve signal transmission.

• A receptor or dendritic zone, where various local potentials produced by synaptic connections are combined;
• A location where propagated action potentials are initiated (specifically, the initial segment in spinal motor neurons and the initial node of Ranvier in cutaneous sensory neurons).

The initiation of an impulse occurs at the axon hillock, which is the part of the neuron where the axon begins. This region is crucial because it integrates signals from the dendrites and the cell body. If the combined signals exceed a certain threshold, an action potential is generated and travels down the axon.

Key points to remember:

• The axon hillock acts as a decision point for whether to send an impulse.
• This area is sensitive to the total incoming signals from the neuron.
• When enough stimulation occurs, it triggers the impulse to move along the axon.

Orthodromic conduction refers to the way an axon transmits electrical impulses. Here’s a breakdown of this concept:

• One Direction: An axon can only conduct impulses in one direction. This ensures that signals travel efficiently towards the target.
• Jumping Depolarisation: The process involves the jumping of depolarisation from one node to another, which speeds up the transmission of signals.

In summary, orthodromic conduction is crucial for the effective communication of signals within the nervous system.

Some sensory neurons belong to a subtype of bipolar cells known as pseudounipolar cells. During the cell's development, a single process divides into two, each acting as an axon—one extending to the skin or muscle and the other to the spinal cord.

Neurons in the ventral, lateral, and dorsal horns, as well as those in the sympathetic chain ganglia, are classified as multipolar. In contrast, neurons found in the dorsal root ganglia are categorised as pseudounipolar.

Nissl bodies are found within the perikaryon of a neuron. They are collections of rough endoplasmic reticulum and ribosomes, which are important for synthesising proteins. In contrast, smooth muscle, skeletal muscle, and cardiac muscle do not contain Nissl bodies.

When the axon thickens, it results in:

• Increased speed of conduction of nerve impulses.
• Better insulation, which helps in faster signal transmission.
• A thicker axon can support more myelin, enhancing efficiency.

Overall, a thicker axon improves the speed at which messages travel along the nerve. This is crucial for quick responses in the nervous system.

Action potentials are crucial for the functioning of nerve cells. Here are some key points to understand:

• Threshold stimulus is necessary to trigger an action potential.
• Once the threshold is reached, the action potential occurs in a full swing, which is referred to as the all-or-none phenomenon.
• During this process, potassium ions (K⁺) move from the extracellular fluid (ECF) into the intracellular fluid (ICF).
• It is important to note that action potentials are not decremental; they maintain their strength as they travel along the nerve.

When there is a non-diffusible ion located on one side of a membrane, the distribution of other ions that can pass through the membrane (the diffusible ions) is influenced in a consistent manner. This phenomenon is known as the Donnan effect.

The permeability of various ions across a synthetic biological membrane is as follows:

• Cl⁻ is significantly more permeable than K⁺
• K⁺ has greater permeability than Na⁺

In contrast, the permeability of different ions through a cell membrane is ranked:

• K⁺ is more permeable than Cl⁻
• Cl⁻ is more permeable than Na⁺

In general, all excitable cell membranes at rest are predominantly permeable to K⁺, which causes the resting membrane potential (RMP) to be near the equilibrium potential for K⁺ ions. The RMP of a neuron (–70 mV) is precisely equivalent to the equilibrium potential of Cl⁻ ions (–70 mV).

• Depolarisation of the membrane voltage indicates a reduction in the resting membrane potential (RMP).
• It is important to note that a decrease in the membrane potential (RMP) signifies that the voltage has shifted closer to ground (0 mV) or has become more positive.

• Nodes of Ranvier are gaps in the myelin sheath of a neurone.
• They play a crucial role in the rapid transmission of electrical impulses along the axon.
• These nodes allow the action potential to jump from one node to the next, a process known as saltatory conduction.
• This mechanism significantly increases the speed of nerve signal conduction compared to unmyelinated fibres.
• In essence, Nodes of Ranvier facilitate efficient communication within the nervous system.

Since the initial part of the axon contains the greatest concentration of voltage-sensitive Na⁺ channels, it consequently has the lowest threshold for triggering an action potential.

The minimum intensity or magnitude of a stimulus that can generate an action potential (AP) in excitable tissues, such as nerves or muscles, irrespective of the time it requires.

Rapid anterograde transport (400 mm/day) is facilitated by the kinesin molecular motor, while retrograde transport (200 mm/day) is carried out by the dynein molecular motor.

The maximum rate of axonal transport refers to the speed at which materials move along the axon of a neuron. This transport is crucial for maintaining neuronal function and facilitating communication between neurons.

In general:

• The speed of axonal transport can vary depending on several factors, including the type of materials being transported.
• It is typically measured in millimetres per day.

Among the options provided:

• 200 mm/day, 400 mm/day, and 600 mm/day are all feasible rates.
• However, the highest established rate of axonal transport is 400 mm/day.

This means that materials can be transported along the axon at a maximum speed of 400 millimetres per day, which is vital for effective neuronal communication.

The rate at which action potential spreads passively is inversely related to the product of Ra (axonal resistance) and Cm (capacitance).

• If this product is decreased, the rate of passive spread will increase.
• As a result, the action potential will propagate more rapidly.

• In the peripheral nervous system (PNS), unmyelinated axons are encompassed by Schwann cells (non-myelinating).
• On the other hand, myelinated axons are wrapped in multiple layers of Schwann cell membranes, similar to how oligodendroglia encase central axons.

Synaptic potential is measured by placing one microelectrode within the cell and another electrode outside the cell.

The maximum magnitude is defined by the equilibrium potential of the Na⁺ ion.

The connection between fiber diameter (thickness) and conduction velocity is:

• Linear for myelinated axons
• Parabolic for non-myelinated axons

Following axotomy, the nerve endings of the damaged neuron start to deteriorate (first). The distal axonal stump detaches from the original cell body, becoming irregular in shape, and experiences Wallerian degeneration (second).