Nervous Tissue Overview and Neuron Structure

Nervous Tissue Overview

• Nervous tissue is one of the four basic tissues in the body
• It comprises two cell types: neurons and neuroglia
• It forms the basis of the nervous system

Nervous System Division

• Divided into two main parts:
  • Central nervous system (CNS): Brain and spinal cord
  • Peripheral nervous system (PNS): Cranial nerves, spinal nerves, and ganglia
• Functionally divided into:
  • Sensory (afferent)
  • Motor (efferent)
  • Interneurons

Neuron Structure

• The structural and functional unit of the nervous system
• Composed of:
  • Cell body (perikaryon, soma, cyton)
  • Processes: Axon and dendrites

Neuron Cell Body (LM & EM)

• LM: Large, rounded, central nucleus; basophilic cytoplasm (Nissl granules)
• EM: Ribosomes, mitochondria, RER, Golgi apparatus, prominent nucleolus; vesicular, numerous polyribosomes (Nissl granules)-related to protein synthesis, especially neurotransmitters synthesis.

Neuron Processes

• Axon:
  • Single, arising from the axon hillock (no Nissl granules)
  • Long and thin, constant diameter, smooth contour
  • Contains neurofibrils, SER, mitochondria; no Nissl granules
  • Terminal arborization
  • Axolemma (membrane) and axoplasm (cytoplasm)
• Dendrites:
  • Multiple, short, branching
  • Irregular contour (thick origin, thin apex)
  • Contains Nissl granules, mitochondria, SER, neurofibrils
  • Arise from the cell body

Neuronal Classification

• Based on polarity (number of processes):
  • Unipolar (rare in humans)
  • Pseudounipolar (sensory neurons)
  • Bipolar (retina)
  • Multipolar (most common; e.g., polygonal, stellate, pyramidal)
• Based on axon length:
  • Golgi type I (long axon)
  • Golgi type II (short axon)
• Based on axonal diameter and myelin sheath thickness:
  • Type A fibers (large diameter, heavily myelinated)
  • Type B fibers (medium diameter, moderately myelinated)
  • Type C fibers (small diameter, unmyelinated)

Myelin Sheath

• Formed by lipids (from cell membrane)
• Wraps around axon, interrupted at nodes of Ranvier
• Protective insulation, crucial for rapid signal transmission
• Not stained by H&E; stained by osmic acid (black circles)

Myelin Formation (Myelination)

• Schwann cells (PNS) wrap axons repeatedly, squeezing cytoplasm between layers; major role in myelination in the PNS
• Oligodendrocytes (CNS) form myelin in the CNS

Peripheral Nerve Trunk

• Varying number of nerve fibers grouped into bundles
• Nerve fibers are acidophilic (stained by H&E)
• CT investment of peripheral nerves:
  • Endoneurium (thin layer of loose CT around each fiber)
  • Perineurium (dense CT around each bundle)
  • Epineurium (thicker dense CT covering the whole trunk)
• Function of CT: nourish, support, protect

Neuroglial Cells (CT of the NS)

• Astrocytes (types A & B):
  • Protoplasmic: Large, branching cells, (perivascular feet)
  • Fibrous: Longer, thinner processes, important in supporting framework, maintaining blood brain barrier
  • Both contain intermediate filaments (GFAP)
• Oligodendrocytes: Small, oval nucleus, few processes; myelin production in CNS
• Microglial cells: Small, oval nucleus, thin cytoplasmic processes with spines;Mononuclear phagocytic cells (brain macrophage)

Synapses

• Junctions between neurons, where nerve impulses are transmitted

• Comprises: presynaptic, postsynaptic membranes & synaptic cleft

• Morphology:

  • Axo-dendritic
  • Axo-somatic
  • Axo-axonic
  • Dendro-dendritic

• Type of transmission:

  • Chemical (neurotransmitters)
  • Electrical (ions flow through gap junctions)

Ganglia

• Nodular structures of neuronal cell bodies in PNS, covered by CT capsule
• Types include sympathetic ganglia (mostly small, multipolar neurons with eccentric nuclei, numerous scattered neurons) and spinal ganglia (large, primarily pseudounipolar neurons with centrally located nuclei, fewer neurons, arranged in bundles)

Spinal & Sympathetic Ganglia

• Spinal ganglia generally associated with the spinal nerves; contain a higher proportion of pseudounipolar neurons than other types
• Sympathetic ganglia are usually part of the sympathetic chain, with smaller, multipolar neurons and a more scattered organization