Figure 15.1 Neuroglia. Capillary Neuron Astrocyte Astrocytes are the most abundant CNS neuroglia Myelin sheath Process of oligodendrocyte Neuron Microglial cell Nerve fibers Microglial cells are defensive cells in the CNS. Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers. Fluid-filled cavity Cilia Cell body of neuron Satellite cells Ependymal cells Schwann cells (forming myelin sheath) Brain or spinal cord tissue Nerve fiber Ependymal cells line cerebrospinal fluid-filled cavities. Satellite cells and Schwann cells (which form myelin) surround neurons in the PNS.
Figure 15.2 Structure of a typical motor neuron. Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) Nucleus of neuroglial cell Neurofibril Nucleus Nucleolus Dendrites Nucleus Chromatophilic substance Initial segment Nucleolus Axon (inpulse-generating and -conducting region) Impulse direction Myelin sheath gap (node of Ranvier) Chromatophilic substance (rough endoplasmic reticulum) Axon terminals (secretory region) Axon hillock Schwann cell Terminal branches Presynaptic neuron Direction of action potential Mitochondrion Synaptic cleft Axon terminal Synaptic vesicles Postsynaptic neuron
Figure 15.3 Myelination of a nerve fiber (axon) by Schwann cells. cell plasma membrane Schwann cell cytoplasm 1 A Schwann cell envelops an axon. Axon Schwann cell nucleus 2 The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers. Myelin sheath Outer collar of perinuclear cytoplasm (of Schwann cell) Axon 3 The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath. Myelin sheath Schwann cell cytoplasm 3
Myelin sheath gap Axon Myelin Schwann cell nucleus Figure 15.4 Photomicrograph of a small portion of a peripheral nerve in longitudinal section (40). Myelin sheath gap Axon Myelin Schwann cell nucleus 4
Figure 15.5 Classification of neurons according to structure. Multipolar Bipolar Unipolar (pseudounipolar) Cell body Cell body Peripheral process Cell body Central process Dendrite Axon Receptive endings Axon Dendrites Axon Multipolar Bipolar Unipolar Receptive endings Dendrites Dendrites Peripheral process (axon) Dendrite Cell body Cell body Cell body Axon Cell body Central process (axon) Cell body Axon Axon Axon Purkinje cell of cerebellum Pyramidal cell Olfactory cell Retinal cell Dorsal root ganglion cell 5
Figure 15.6 Photomicrographs of neurons. Dendrites Dendrites Cell body Cell body Nerve fibers Satellite cells Cell bodies 6
Figure 15.7 Classification of neurons on the basis of function. Peripheral process (axon) Ganglion Cell body Sensory neuron Central process (axon) Spinal nerve Afferent transmission White matter Interneuron Receptive endings Gray matter Efferent transmission Motor neuron Spinal cord (central nervous system) To effectors (muscles) 7
Figure 15.8 Structure of a nerve showing connective tissue wrappings. Axon Myelin sheath Endoneurium Perineurium Epineurium Myelin sheath Nonmyelinated axon Fascicle Blood vessels Endoneurium Heavily myelinated axons Perineurium Epineurium 8