Characteristic of signal propagation on nerve trunks

2021-06-04 03:49 PM

The propagation speed of action potentials in nerve fibres ranges from as small as 0.25 m/s in unmyelinated fibres to as large as 100 m/s (more than the length of a football field in 1 second) in large fibres. have myelin.

Myelinated and unmyelinated nerve fibres

 

Figure. Cross-section of a small nerve stem containing both myelinated and unmyelinated fibres.

The figure shows a cross-section of a typical small nerve, revealing many large nerve fibres occupying most of the cross-sectional area. However, a more cautious look shows that many smaller fibres lie between the large ones. Large fibres are covered by a myelin sheath, and small fibres are not. The median nerve trunks are composed of unmyelinated fibres twice as many as myelinated fibres.

Figure. The function of Schwann cells to insulate nerve fibres. A: Wraps the Schwann cell membrane around a large axon to form the myelin sheath of the myelinated nerve fibre. B: Partially encloses the membrane and cytoplasm of a Schwann cell around numerous unmyelinated nerve fibres (shown in cross-section).

The figure shows a typical myelinated fibre. The central core of the fibre is the axon, and the cell membrane of the axon is the membrane that conducts the action potential.

The axon is attached at its centre to the axonal substance, a sticky substance inside the intracellular fluid. Surrounding the nerve fibres is a myelin sheath that is usually much thicker than the axon itself. Approximately every 1-3 mm along the length of the myelin sheath is a Ranvier node.

Myelin sheaths are deposited around the axons of Schwann cells in the following manner: The membranes of a Schwann cell first enclose the nerve fibres.

The Schwann cells then rotate around the nerve axis several times, laying down multiple layers of Schwann cell membranes containing the lipid compound sphingomyelin. This substance is a very good insulator, reducing the flow of ions through the membrane about 5000 times. At the junction between each of the two successive Schwann cells along the axon, a small non-insulated area of ​​only 2-3 micrometres in length remains where ions can still flow easily through the middle axonal membranes. extracellular fluid and intracellular axonal fluids. This area is called Ranvier's node.

Conduction jumps from node to node

Figure. Conduction of salts along myelinated axons. The current from one node to another is illustrated by arrows.

Although almost no ions can flow through the thick myelinated membranes of myelinated nerves, they can flow easily through the Ranvier nodes. Therefore, action potentials occur only at the nodes. The action potential is conducted from one node to the next, as shown in the figure; This is known as Jump Conduction. That is, current flows around the extracellular fluid outside the myelin sheath, as well as through the inner axon from node to node, successfully stimulating the next node. Thus, nerve impulses jump along the axon, which is the origin of the term "jumping".

Jumping conductors are valuable for two reasons. First, by inducing depolarization to jump long distances along the axis of the nerve fibre, this mechanism increases the velocity of nerve conduction in the myelin sheath by as much as 5 to 50 times. Second, saving energy is maintained for the nerve fibres because there are only depolarizing nodes, allowing about 100 times less loss of ions if not needed, and thus requiring energy dissipation. little for re-establishing the sodium and potassium concentration differences across the membrane after a series of nerve impulses.

Conduction speed of nerve fibres

Conduction velocity in nerve fibres. The propagation speed of action potentials in nerve fibres ranges from as small as 0.25 m/s in unmyelinated fibres to as large as 100 m/s (more than the length of a football field in 1 second) in large fibres. have myelin.