Q4 LONG ANSWER 10M ★ High-yield

Describe the structure of a neurone and classify neurones. Describe the neuroglia and add a note on nerve degeneration and regeneration.

Anatomy · 1st Professional — Pre-Clinical

The neurone is the structural and functional unit of the nervous system, specialised for the reception, conduction and transmission of impulses.

Part	Features
Cell body (soma, perikaryon)	Large vesicular nucleus with a prominent nucleolus; NISSL bodies (rough endoplasmic reticulum and free ribosomes); Golgi, mitochondria, neurofibrils, lipofuscin
Dendrites	Multiple, short, branching, tapering; contain Nissl bodies; receive impulses; bear dendritic spines
Axon	Single, long, uniform diameter; arises from the axon hillock, which has NO Nissl substance; conducts impulses away
Axon terminals	End as boutons containing synaptic vesicles

Nissl bodies are absent from the axon and the axon hillock — a reliable way of identifying the axon in a section
The neurone does not divide after birth, apart from limited neurogenesis in the hippocampus and olfactory bulb

Type	Processes	Example
Unipolar	One process	Only in the embryo
Bipolar	One dendrite and one axon	Retina, olfactory epithelium, cochlear and vestibular ganglia
Pseudounipolar	A single process dividing into two	Dorsal root ganglion and sensory ganglia of cranial nerves
Multipolar	Many dendrites, one axon	The commonest — motor neurones, Purkinje cells, pyramidal cells

Cell	Location	Function
Astrocyte	Protoplasmic in grey matter, fibrous in white	Support; forms the blood–brain barrier with its foot processes; potassium buffering; glial scar formation
Oligodendrocyte	CNS	Myelination — one cell myelinates several axons
Microglia	CNS	The macrophage of the CNS; derived from monocytes (mesodermal); the only glial cell not of neuroectodermal origin
Ependyma	Lines the ventricles and central canal	Ciliated; forms the choroid plexus and circulates CSF
Schwann cell	PNS	Myelinates one internode of one axon; essential for regeneration
Satellite cell	Around ganglion cell bodies	Support and regulation

Myelinated fibres — the Schwann cell membrane wraps the axon repeatedly; nodes OF ranvier lie between adjacent Schwann cells
Saltatory conduction — the impulse jumps from node to node, so conduction is faster and uses less energy
Unmyelinated fibres — several axons are enclosed in the invaginations of a single Schwann cell, without wrapping
Conduction velocity rises with fibre diameter and with myelination
Coverings of a peripheral nerve — epineurium around the whole nerve, perineurium around each fascicle (and forming the blood–nerve barrier), endoneurium around each fibre

Neurapraxia — a conduction block with the axon intact; recovers fully in days to weeks
Axonotmesis — the axon is divided but the endoneurial tube survives; good recovery
Neurotmesis — the whole nerve is divided; recovery needs surgical repair and is never complete

Type of synapse	Description
Axodendritic	The commonest — axon to dendrite
Axosomatic	Axon to cell body
Axoaxonic	Axon to axon; the basis of presynaptic inhibition
Chemical synapse	Synaptic cleft 20–30 nm; vesicles; unidirectional; a synaptic delay
Electrical synapse	Gap junctions; bidirectional; no delay; rare in man

Sensory ganglia (dorsal root, cranial) — pseudounipolar cells arranged in groups, with satellite cells; NO synapses
Autonomic ganglia — multipolar cells scattered irregularly, with eccentric nuclei; synapses present
That difference — synapses or not, and the arrangement of the cells — is how the two are told apart in a section

Multiple sclerosis — demyelination in the CNS from loss of oligodendrocytes; disseminated in time and space, with characteristic optic neuritis and internuclear ophthalmoplegia
Guillain–Barré syndrome — acute demyelination of peripheral nerves; ascending flaccid paralysis with albumino-cytological dissociation in the CSF
Leprosy — still important in India; Mycobacterium leprae invades Schwann cells, producing thickened nerves with anaesthesia; the ulnar, common peroneal and great auricular nerves are typically affected
Nerve conduction studies distinguish demyelination (slow conduction) from axonal loss (reduced amplitude)
Central axons do not regenerate, which is why spinal cord injury is permanent and why so much research is directed at overcoming the glial scar
Neuroma — a tangle of regenerating axons and fibrous tissue at the end of a cut nerve, which is painful and troublesome after amputation
Nerve grafting uses a sensory nerve such as the sural, relying on the Schwann cell tubes of the graft to guide regeneration
Rabies virus travels along axons to the CNS, and its Negri bodies in the hippocampus are diagnostic
Tumours of glia (gliomas) are far commoner than tumours of neurones, because glial cells retain the ability to divide and neurones do not
Astrocytes form the glial scar that seals a CNS injury but also blocks regeneration — protective in the short term and obstructive in the long

All questions in General Histology