Nerve Fibers PDF

## Akson med skede(r) - Akson (7), aksecylinder, fungerer her som neurit og leder impulser bort fra cellelegemet. - Aksonen afgår med en udspringskegle (8) fra cellelegemet og består af cytoplasma, der her kaldes aksoplasma. - Contains neurofibriller, mikrotubuli og mitochondrier, but not Nissl's s...

## Akson med skede(r) - Akson (7), aksecylinder, fungerer her som neurit og leder impulser bort fra cellelegemet. - Aksonen afgår med en udspringskegle (8) fra cellelegemet og består af cytoplasma, der her kaldes aksoplasma. - Contains neurofibriller, mikrotubuli og mitochondrier, but not Nissl's substans or Golgiapparat. - **Fig. 53 Hi.Ku.** Shows a strong dendrite (3) upwards. - The axon hillock downards is marked (4), Nissl's substance is missing, so this part is just part of the axon. This is visible at (5). ### En nervetråd består af en akson (1) with sheaths (2) and (3) - **Fig. 40 b** is an important drawing, it shows a longitudinal section through a piece of nerve fiber, an axon (1) with 2 sheaths (2) and (3). - **Fig. 40 a** shows a nerve fiber where only one sheath (2) is around the axon (1). ### Long axons in both the central nervous system and peripheral nerves are surrounded by a myelin sheath (2). - They consist of short tubes that surround the axon, resembling 'long pearls on a string'. - The spaces between the individual tubes are called Ranvier's nodes (4). ### **Fig. 40 c** shows 3 nerve bundles. - Each bundle contains dots that are axons surrounded by a myelin sheath (white). - **Fig. 40 d** shows a cross section through an axon with myelin sheath. - Shows axon (1) in the center, myelin sheath (2) as a thick sheath around it. - The outermost circle shows the next sheath (3), which will be discussed later. ### In peripheral nerves, axon and myelin sheath are effectively surrounded by Schwann's sheath, or neurilemma (3). - The sheath consists of tubular cells, and the small space between the cells is located at Ranvier's nodes (4). - Spaces between 2 myelin segments are also located at Ranvier's nodes. - Myelin sheath is formed by Schwann's cells. - Schwann's cell membrane is wrapped many times around the axion cylinder. ### **Fig. 47 A II** shows an axon (1) embedded in a Schwann's cell (2). - The nucleus is up. - A mesaxon (3) is formed at the location of the axon embedding. - It consists of two layers of cell membrane (arrows). - It grows and forms a spiral (4) around the axon. - In the beginning, there is cytoplasm between the «windings», but it is pressed together until the membrane spirals lie close together and then fuse together in the myelin sheath (5). - Longitudinal section (V) shows Ranvier's nodes (6). There is space between Schwann's cells (2) and between myelin segments (5) and some cytoplasm is also preserved in the spirals (7). ### The axon is isolated from the environment by the myelin sheath. - However, as the myelin sheath is missing at Ranvier's nodes (4), the axon's surface membrane is exposed at this point. - This means that the changes in the axon membrane that occur during nerve conduction can only occur here, where there is contact with the environment. - Therefore, the impulse must jump from one Ranvier's node to the next. The conduction of the impulse is called saltatory. - In bare nerve fibers, without myelin sheath, conduction is uniform. Membrane changes must occur along the entire length of the axon, as there is no insulating myelin sheath. - The advantage of saltatory conduction is that much higher conduction speed can be achieved with a relatively thin axon. - The length of the myelin segments varies. The longest segments occur around the thickest axons. - The section between two Ranvier's nodes is called an internodal segment. ### Axons without myelin sheath are embedded in Schwann's cells (VI). Each Schwann's cell (2) contains many axons (1). ### The thickest nerve fibers (type A) have a conduction speed from 15-100 meters per second. - Within this group, different subgroups are distinguished, and among the motor fibers are alpha fibers, which are the thickest and conduct the fastest. They go to «normal», striated (extrafusal) muscle cells, while gamma fibers (2) go to the thin muscle cells (4) (intrafusal) in muscle spindles. - Gamma fibers are thin and conduct slower. ### The thickest pressure and touch sensations are conducted by the thickest sensory fibers. - The second group of fibers (type B) is thinner and conducts with a rate of 3-15 meters per second, while the thinnest (C-fibers) have a conduction speed of only 0.5-2 meters per second. Pain and temperature stimuli are conducted via thin nerve fibers. - The «mnemonic» for conduction speeds in the three nerve fiber types are A: approx. 100 meters per second, B: about 15 meters per second and C: about 1.5 meters per second. Dividing by 5 gives the approximate diameter in μm, 20, 3 and 0.3. - The speeds are not impressive. Converted to km/h, the maximum is approximately 400 and the minimum is approximately 2 km/h. When someone steps hard on your toes, you first notice a 'step', as pressure and touch stimuli are conducted by thick, fast-conducting nerve fibers. - Later, it starts to hurt. This is because the pain stimuli are conducted by slow-conducting thin nerve fibers. ### Schwanns sheaths are essential for nerve fiber regeneration in peripheral nerves, as new nerve fibers grow out through the «old» Schwann's sheaths.