Chapter 13: Nervous System: Neural Tissue

The nervous system is organized anatomically into the Central Nervous System (CNS), consisting of the brain and spinal cord, which serves as the core for integration, processing, and higher functions such as memory and emotion, and the Peripheral Nervous System (PNS), which encompasses all neural tissue outside the CNS. Functionally, the PNS is divided into the afferent division, which transmits sensory information collected by receptors (categorized as exteroceptors, proprioceptors, and interoceptors) toward the CNS, and the efferent division, which carries motor commands to peripheral effectors. The efferent division is further specialized into the Somatic Nervous System (SNS), which directs voluntary and involuntary control over skeletal muscles, and the Autonomic Nervous System (ANS), which handles involuntary visceral motor regulation of glands, smooth muscle, and cardiac muscle. Neural tissue is fundamentally composed of nerve cells, or neurons, which are specialized for information transfer and processing, and neuroglia, or glial cells, which provide critical supporting functions and structure. In the CNS, the four types of neuroglia are the largest and most numerous astrocytes (essential for maintaining the blood-brain barrier and providing structural support), oligodendrocytes (which form myelin sheaths around CNS axons, creating white matter separated by myelin sheath gaps or nodes of Ranvier), microglia (the phagocytic defense cells that remove debris and pathogens), and ependymal cells (which line the ventricles and central canal and aid in cerebrospinal fluid circulation). In the PNS, neuroglia include satellite cells (surrounding neuron cell bodies in ganglia) and Schwann cells (which cover peripheral axons and are responsible for PNS myelination). Neurons possess a cell body (soma), multiple dendrites that receive input, and a single axon that propagates the action potential (nerve impulse). Neurons are classified structurally as anaxonic, bipolar, pseudounipolar, or multipolar, and functionally as sensory neurons, motor neurons, or the most numerous interneurons, which reside entirely within the CNS for processing and coordination. The speed of nerve impulse conduction is significantly increased by both the large diameter of the axon and the presence of myelin. Communication occurs at a synapse, most often a vesicular (chemical) synapse where neurotransmitters like acetylcholine transmit signals unidirectionally across a synaptic cleft; less commonly, nonvesicular (electrical) synapses allow rapid, bidirectional ion passage. CNS processing involves the organization of interneurons into neuronal pools, utilizing neural circuits that display patterns such as divergence (spreading information widely), convergence (multiple inputs on a single neuron), serial processing (stepwise relay), parallel processing (simultaneous information handling), and reverberation (positive feedback loops). Following peripheral nerve injury, limited recovery can occur through Wallerian degeneration, a process involving Schwann cells, although the ability of neurons to regenerate is very limited and is severely restricted in the CNS by scar tissue produced by astrocytes.