Structure of nerve fibers
Neurites and dendrites have a similar cellular structure (Fig 7-51), which can be broken down as follows:
- The axon or axis cylinder emerges directly from the nerve cells and lies in the middle of a nerve fiber. The axon is made up of neurofibrils.
- The neurilemma (Schwann sheath) forms the covering around the axon. This outer membrane is enveloped around fatlike substances containing phosphorus, namely myelin. The outer covering has regular constrictions where the neurilemma has direct contact with the axon. These constrictions are known as nodes of Ranvier.
- The sheath of Henle (endoneurium) is the outer covering around a nerve fiber; it is made up of connective tissue fibers with blood vessels.
Reception of stimuli happens by means of the special senses: the sense of sight and hearing, the sense of smell and taste, the sense of balance and touch, and the sense of temperature and pain.The nerve impulse is the signal that is conducted by the nerve fiber. The conduction speed, depending on the diameter of the fiber, is between 2 m/s for a fiber thickness of 1 um and 120 m/s for a 20-um-thick fiber.
When a nerve cell is in a resting state, there is electrical tension (voltage) between the nerve fiber and the neurilemma. As a result of stimulation, the outer covering loses its positive charge compared with the nerve fiber and becomes slightly negatively charged. Sodium ions then flow into the inside of the cell and positively charge the cell briefly. From the stimulated site, the impulse spreads to both sides and jumps from one node of Ranvier to another.
A nerve can be stimulated mechanically, chemically, thermally, or electrically. The stimulus must have a specific strength in order to cross the stimulus threshold so that the nerve is excited to the maximum. Because nerves are always combined to form bundles, the strength of sensation depends on how many nerve fibers are stimulated and conducting an impulse. After an impulse has been conducted, there is a resting pause of about 0.4 thousandths of a second.The nerve impulse is conducted until it is passed on to another nerve cell via the synapse or is delivered to a motor end plate of a muscle cell or another organ cell.