The Nerve Fiber

 The Nerve Fiber 

The axon (AFG1) is surrounded by a sheath: in unmyelinated nerve fibers by the cyto-plasm of the sheath cells, and in myelinatednerve fibers by themyelin sheath(ABG2). Axon and sheath together are called the nerve fiber. The myelin sheath begins be-hind the initial segment of the axon and ends just before the terminal ramification. It consists of myelin, a lipoprotein produced by the sheath cells. The sheath cells in the CNS are oligodendrocytes; in the peripheralnerves they are Schwann cells, which origi-nate from the neural crest. The myelin sheath of fresh, unfixed nerve fibers appears highly refractile and without structure. Its lipid content makes it birefringent in polarized light. The lipids are removed upon fixation, and the denatured protein scaffold remains as a gridlike structure (neurokeratin) (D3).


At regular intervals (1 – 3 mm), the myelin sheath is interrupted by deep constrictions, the nodes of Ranvier (AB4 F). The segment be-tween two nodes of Ranvier in peripheral nerves, the internode or interannular segment (F), corresponds to the expansion of one sheath cell. The cell nucleus (ADF5) and perinuclear cytoplasm form a slight bulge on the myelin sheath in the middle of the in-ternode. Cytoplasm is also contained in ob-lique indentations, the Schmidt–Lantermanincisures (C, F6) (see also p. 40, A4). The mar-gins of the sheath cells define the node of Ranvier at which axon collaterals (E) may branch off or synapses may occur.

Comments

Post a Comment

Popular posts from this blog

Orbital cavity (eye socket) - Structure of the Eye

Image
  Orbit The orbital cavity (eye socket) is lined by the periosteum ( periorbita ) ( C26 ) and is filled with fatty tissue, the  orbital fat body  ( C27 ), in which the eyeball ( C28 ), the optic nerve ( C29 ), and the eye muscles ( C30 ) are embedded. At the anterior margin of the orbit, the fatty tissue is demarcated by the orbital septum ( BC31 ). The fatty tissue is separated from the eyeball by a connective tissue capsule, the  bulbar sheath  ( C32 ), which encloses the  sclera  ( C33 ). C34 Choroidea. C35 Osseous wall of the orbit.
Read more

Ciliary Body - Structure of the Eye

Image
  Ciliary Body The lens is suspended at the circular  ciliarybody  ( A19 ,  D ). The muscles of the ciliarybody control the curvature of the lens  and, hence, the visual acuity for close and distant vision. It consists of a radially folded surface, the  ciliary disk  ( D20 ), from where approximately 80  ciliaryprocesses  ( D21 ) protrude ( ciliary crown ). Theanterior part of the disk is occupied by the ciliary muscle; its muscle fibers, the  merid-ional fibers  ( A22 ), span between the oraser-rata and the reticulum trabeculare as well as Descemet’s membrane. From here, radial muscle fibers extend inward and bend to take a circular course ( circular fibers ) ( A23 ). The posterior surface of the ciliary body is covered by the  ciliary part of the retina ( A24 ). Delicate fibers, the  zonular fibers  ( AD25 ), extend from the ciliary body to the lens and form the  ciliary zone  ( D26 ). Many fibers ori...
Read more

Red Nucleus and Substantia Nigra

Image
  Red Nucleus and Substantia Nigra Lateral View of the Brain Stem (A) The two large nuclei extend far toward the diencephalon. The  substantia nigra  ( AB1 ) reaches from the oral part of the  pons  ( A2 ) to the  pallidum ( AB3 ) in the diencephalon. Both nuclei are important relay stations of the extrapyramidal system. Red Nucleus (A, B) The nucleus ( AB4 ) appears reddish in a fresh brain section (high iron content, p. 148, A). It consists of the parvocellular  neorubrum  and the magnocellular  paleoru-brum  situated ventrocaudally. Afferent connections The  dentatorubral fasciculus  ( B5 ) of the  dentate nucleus  ( B6 ) of the cerebellumruns in the  superior cerebellar peduncle  and terminates in the       contralateral red nucleus.   !     The  tectorubral tract  ( B7 ) of the superior colliculus terminates in the ipsilateral and contralateral...
Read more