Nervous Tissue Histology
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Direct answer
Nervous tissue consists of excitable neurons and supporting neuroglia. A neuron has a cell body (perikaryon) containing Nissl substance, dendrites conducting towards the soma, and a single axon conducting away; neurons are multipolar, bipolar or (pseudo)unipolar. The glia outnumber neurons: astrocytes, oligodendrocytes, ependymal cells and microglia in the central nervous system, with Schwann and satellite cells in the peripheral; oligodendrocytes myelinate several central internodes while each Schwann cell wraps a single peripheral internode and permits regeneration.
What you must remember
- Perikaryon: nucleus with prominent nucleolus, coarse Nissl substance (rough endoplasmic reticulum) absent from the axon hillock and axon, neurofibrils, and lipofuscin with age; Nissl disperses peripherally (chromatolysis) after axon injury.
- Axoplasmic transport: fast anterograde by kinesin carrying vesicles and transmitters; slow anterograde flow of cytoskeleton; fast retrograde by dynein returning debris — and carrying rabies and herpes viruses to the cell body.
- Neuron types: multipolar in brain, cord and autonomic ganglia; bipolar in retina, olfactory epithelium and vestibulocochlear ganglia; unipolar (pseudounipolar) in dorsal root and trigeminal ganglia with a T-shaped junction.
- Synapse: presynaptic terminal with vesicles, a cleft and a postsynaptic membrane; mostly chemical and commonly axo-dendritic or axo-somatic.
- Astrocytes: the largest glia, protoplasmic in grey and fibrous in white matter; perivascular feet help form the blood-brain barrier, glial fibrillary acidic protein is their marker, and they form the glial scar after injury.
- Oligodendrocytes: small, dark nuclei; one cell myelinates internodes of several axons — central myelin is the target in multiple sclerosis.
- Microglia: the smallest cells, of mesodermal monocyte origin — the brain's resident macrophages, activated after injury; ependyma lines the ventricles and the choroid plexus epithelium secretes cerebrospinal fluid; satellite cells capsule ganglion neurons.
Common confusion
Nissl substance is absent from axon and axon hillock, which distinguishes dendrites from axons on a slide. The two myelinating cells are swapped at will: oligodendrocytes serve the central nervous system, many internodes each, with no neurilemma; Schwann cells serve peripheral axons, one internode each, with a neurilemma — the basis of peripheral nerve regeneration and central failure. Microglia alone are mesodermal, not neuroectodermal.
Exam-focused takeaway
Histology practicals identify neurons, Nissl substance, myelinated fibres with nodes of Ranvier in a teased nerve, and glia on silver-stained sections. Theory questions ask for neuron classification, perikaryon ultrastructure, types and functions of neuroglia, and central versus peripheral myelin. In the viva, explain axoplasmic transport, chromatolysis and Wallerian degeneration.
Frequently asked questions
Where is Nissl substance found and where is it absent?
It fills the cell body and dendrites of neurons; it is absent from the axon hillock and axon — the key to telling them apart on a slide.
Which cell myelinates peripheral axons?
The Schwann cell, one cell per internode, the outer lamella forming the neurilemma; gaps between internodes are the nodes of Ranvier, allowing saltatory conduction.
What do astrocytes do?
They contribute perivascular feet to the blood-brain barrier, buffer potassium and transmitters, store glycogen and replace dead tissue with a glial scar; their marker is glial fibrillary acidic protein.
How do microglia differ from other neuroglia?
They arise from mesodermal monocyte precursors rather than neuroectoderm and act as resident macrophages, enlarging and phagocytosing debris after injury.
What are the types of axoplasmic transport?
Fast anterograde by kinesin, slow anterograde flow of cytoskeletal proteins, and fast retrograde by dynein — the route by which certain viruses reach the cell body.