Chapter 12: Nerve Tissue & Neural Organization

The comprehensive field of histology examines nervous tissue, which is organized anatomically into the central nervous system (CNS, comprising the brain and spinal cord) and the peripheral nervous system (PNS, including nerves, ganglia, and specialized receptors). Functionally, the somatic nervous system (SNS) manages conscious motor control, while the autonomic nervous system (ANS) controls involuntary functions of internal organs via sympathetic, parasympathetic, and enteric divisions. Nerve tissue fundamentally consists of functional units called neurons and supporting neuroglial cells. Neurons, which are specialized to conduct impulses and are generally non-dividing cells, feature a protein-synthesizing cell body (perikaryon) containing Nissl bodies, an impulse-generating axon that transmits signals away from the cell body (often originating at the axon initial segment, AIS), and dendrites that receive stimuli toward the cell body. Neuronal function relies heavily on fast and slow bidirectional transport systems (anterograde via kinesins, retrograde via dyneins) moving materials along microtubules. Neurons communicate through synapses, typically chemical junctions involving presynaptic neurotransmitter release (e.g., excitatory acetylcholine or glutamate; inhibitory GABA or glycine) into a synaptic cleft, binding postsynaptically to receptors (ionotropic or metabotropic) to propagate or inhibit an action potential; this action potential is conducted rapidly via saltatory conduction, where the electrical impulse jumps between the nodes of Ranvier along myelinated axons. Supporting cells, or neuroglia, differ between the systems: peripheral neuroglia include myelin-producing Schwann cells and satellite cells surrounding ganglia, while central neuroglia consist of oligodendrocytes (CNS myelin producers), astrocytes (providing physical scaffolding, metabolic support, potassium buffering, and contributing to the blood-brain barrier integrity), phagocytic microglia, and ependymal cells lining the CNS fluid cavities. The CNS is characterized by gray matter (containing cell bodies and synapses, forming the cerebral cortex and spinal cord horns) and white matter (composed primarily of myelinated axons or tracts). PNS nerves are encased in connective tissue layers: endoneurium, perineurium (forming the blood-nerve barrier), and epineurium. Following injury, PNS axons usually regenerate successfully due to the action of repair Schwann cells forming bands of Büngner to guide axonal sprouts; however, regeneration in the CNS is significantly inhibited by limited macrophage infiltration, inefficient microglial clearance of inhibitory myelin debris, and the formation of an astrocyte-derived glial scar (reactive gliosis).