The outer nucleated cytoplasmic layer of the Schwann cell, which encloses the myelin sheath, is called the neurolemma (sheath of Schwann) and is found only around axons in the PNS. The neurolemma aids in regeneration in an injured axon by forming a regeneration tube that guides and stimulates regrowth of the axon. The myelin sheath has gaps called neurofibril nodes or nodes of Ranvier along the axon. Oligodendrocytes form myelin sheaths for CNS axons. No neurolemma is formed. No regrowth after injury occurs. Clinical Connection: Demyelination events have adverse effects on signal transmission Collections of nervous tissue include several structures Clusters of cell bodies include: Ganglia in the PNS Nuclei in the CNS Nerves and tracts are groups of axons in the PNS and CNS, respectively. Gray and White Matter White matter is composed of aggregations of myelinated processes whereas gray matter contains nerve cell bodies, dendrites, and axon terminals or bundles of unmyelinated axons and neuroglia (Figure 12.9). In the spinal cord, gray matter forms an H-shaped inner core, surrounded by white matter; in the brain a thin outer shell of gray matter covers the cerebral hemispheres. ELECTRICAL SIGNALS IN NEURONS Excitable cells communicate with each other by action potentials or graded potentials. Action potentials allow communication over short and long distances whereas graded potentials allow communication over short distances only. Production of both types of potentials depend upon the existence of a resting membrane potential and the presence of certain types of ion channels. The membrane potential is an electrical voltage across the membrane. Graded and action potentials occur because of ion channels in the membrane that allow ion movement across the membrane that can change the membrane potential.
The outer nucleated cytoplasmic layer of the Schwann cell, which encloses the myelin sheath, is called the neurolemma (sheath of Schwann) and is found only around axons in the PNS. The neurolemma aids in regeneration in an injured axon by forming a regeneration tube that guides and stimulates regrowth of the axon. The myelin sheath has gaps called neurofibril nodes or nodes of Ranvier along the axon. Oligodendrocytes form myelin sheaths for CNS axons. No neurolemma is formed. No regrowth after injury occurs. Clinical Connection: Demyelination events have adverse effects on signal transmission Collections of nervous tissue include several structures Clusters of cell bodies include: Ganglia in the PNS Nuclei in the CNS Nerves and tracts are groups of axons in the PNS and CNS, respectively. Gray and White Matter White matter is composed of aggregations of myelinated processes whereas gray matter contains nerve cell bodies, dendrites, and axon terminals or bundles of unmyelinated axons and neuroglia (Figure 12.9). In the spinal cord, gray matter forms an H-shaped inner core, surrounded by white matter; in the brain a thin outer shell of gray matter covers the cerebral hemispheres. ELECTRICAL SIGNALS IN NEURONS Excitable cells communicate with each other by action potentials or graded potentials. Action potentials allow communication over short and long distances whereas graded potentials allow communication over short distances only. Production of both types of potentials depend upon the existence of a resting membrane potential and the presence of certain types of ion channels. The membrane potential is an electrical voltage across the membrane. Graded and action potentials occur because of ion channels in the membrane that allow ion movement across the membrane that can change the membrane potential.
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