Nervous Tissue

Neurons are the functional units of the nervous system. They typically comprise a cell body (soma), dendrites (receiving inputs), and a single axon (transmitting outputs). The soma contains Nissl substance (rough ER) for high protein synthesis. Glial cells outnumber neurons and differ between the CNS and PNS. CNS glia include astrocytes (blood-brain barrier, metabolic support), oligodendrocytes (myelination), microglia (immune defence), and ependymal cells (ventricular lining). PNS glia include Schwann cells (myelination) and satellite cells (support). Myelin is a lipid-rich sheath that allows saltatory conduction via Nodes of Ranvier, significantly increasing impulse speed. Peripheral nerves are bundled into fascicles by connective tissue layers: endoneurium (around individual fibres), perineurium (around fascicles), and epineurium (outermost layer). Synapses are the gaps across which neurons communicate via neurotransmitters. Gray matter in the CNS contains cell bodies and dendrites, while white matter consists primarily of myelinated axons.

Nervous tissue relies on transmembrane electrochemical gradients. The resting membrane potential is maintained by Na+/K+ ATPase. Action potentials are generated when the threshold is reached at the axon hillock. The BBB is formed by endothelial tight junctions, basement membrane, and astrocyte foot processes, limiting the entry of toxins and polar molecules into the brain. In the cerebellum, histology shows three distinct layers: molecular, Purkinje (large flask-shaped cells), and granular. In the spinal cord, the gray matter is central (H-shaped) and white matter is peripheral, the reverse of the cerebral cortex.

Multiple Sclerosis (MS) is an autoimmune demyelinating disease of the CNS, whereas Guillain-Barré Syndrome (GBS) affects the PNS. Parkinson's disease involves the loss of dopaminergic neurons in the substantia nigra. Gliomas are the most common primary brain tumours, originating from glial cells. Wallerian degeneration occurs when an axon is severed, leading to the breakdown of the distal segment. Neurofibromatosis involves tumours of the Schwann cells and fibroblasts. Knowledge of myelin-forming cells is critical for distinguishing CNS vs PNS pathology.

MRI is the imaging modality of choice for nervous tissue structure. Lumbar puncture for CSF analysis. Nerve Conduction Studies (NCS) assess the integrity of PNS myelination and axonal health.

Not directly applicable to this basic-science topic; management focuses on neuroprotection and rehabilitation. See Multiple Sclerosis or Stroke topics.