The Central Nervous System (CNS) in Medicine

1. Overview and Definition

"The CNS consists of the brain and spinal cord. The three major parts of the brain are the cerebrum, the cerebellum, and the brain stem. The PNS consists of the nerves and nerve cells that lie outside the brain and spinal cord." — Neuroscience: Exploring the Brain, 5th Ed.

2. Embryological Development

3. Gross Anatomy

3.1 The Brain

Telencephalon (Cerebral Hemispheres)

• Contains the cerebral cortex (gray matter), white matter, and basal ganglia
• The cortical surface is folded into gyri (bumps) separated by sulci (grooves) and fissures
• Divided into four lobes: frontal, parietal, temporal, and occipital, each with distinct functional roles

Diencephalon

• Thalamus: major sensory relay station for nearly all sensory pathways to the cortex (except olfaction)
• Hypothalamus: regulates autonomic functions, temperature, hunger, thirst, endocrine axes, and circadian rhythms

Brainstem (most evolutionarily ancient)

• Midbrain: a narrow region connecting forebrain and hindbrain; contains nuclei for eye movement (CN III, IV), dopaminergic substantia nigra, and the periaqueductal gray (pain modulation)
• Pons: relay station; contains CN V, VI, VII, VIII nuclei; pontine respiratory group
• Medulla oblongata: controls vital functions — respiration, blood pressure, heart rate; contains CN IX, X, XI, XII nuclei

"The brainstem is the most evolutionarily ancient part of the human brain and controls many of the most basic bodily functions necessary for survival, such as respiration, blood pressure, and heart rate." — Neuroanatomy through Clinical Cases, 3rd Ed.

Cerebellum

• Coordinates movement, balance, and fine motor control
• Receives proprioceptive, vestibular, and cortical inputs; outputs via deep cerebellar nuclei to thalamus and motor cortex

3.2 The Spinal Cord

• Extends from the foramen magnum to the conus medullaris (~L1–L2 in adults)
• Gray matter (H-shaped on cross section): dorsal horns (sensory), ventral horns (motor), lateral horns (autonomic, T1–L2)
• White matter (surrounding gray matter): organized into ascending sensory tracts (dorsal columns, spinothalamic) and descending motor tracts (corticospinal, rubrospinal)
• 31 pairs of spinal nerves exit via intervertebral foramina

4. Functional Organization

5. Cellular Composition

5.1 Neurons

• Cell body (soma): contains nucleus and organelles (Nissl bodies = rough ER for protein synthesis)
• Dendrites: receive incoming signals
• Axon: conducts outgoing action potentials; can be myelinated or unmyelinated
• Synapse: specialized junction for chemical signal transmission between neurons via neurotransmitters

5.2 Glial Cells (Neuroglia)

"Functions of glial cells include physical support for neurons, insulation for nerve processes facilitating rapid impulse transmission, repair of neuronal injury, regulation of the internal fluid environment of the CNS, and clearance of neurotransmitters from synaptic clefts." — Histology: A Text and Atlas, 8th Ed.

6. Protective Structures

6.1 Meninges

• Pia mater: thin, closely adherent to brain/cord surface
• Arachnoid mater: forms the subarachnoid space through which CSF circulates
• Dura mater: tough outer layer; in the skull it has two layers; venous sinuses (e.g., superior sagittal sinus) lie within its folds

6.2 Cerebrospinal Fluid (CSF)

• Produced mainly by the choroid plexus in the lateral, third, and fourth ventricles
• Circulates: lateral ventricles → third ventricle → cerebral aqueduct → fourth ventricle → subarachnoid space → reabsorbed via arachnoid granulations into venous sinuses
• Functions: mechanical cushioning, waste removal, immune surveillance

6.3 Blood-Brain Barrier (BBB)

• Capillary endothelial cells joined by tight junctions (principal component)
• Basement membrane with pericytes
• Astrocytic perivascular foot processes

"The brain's vascular endothelial cells... are joined by tight junctions and generally lack the transport channels found elsewhere in the body. As a result, the blood-brain barrier prevents hydrophilic polar and large molecules in the blood from entering the brain. Glucose is the brain's sole source of energy and crosses via non-energy-dependent facilitated transport (Glut1)." — Goldman-Cecil Medicine

• Freely cross: O₂, CO₂, lipid-soluble drugs (e.g., general anaesthetics, opioids)
• Carrier-mediated: glucose (Glut1), amino acids
• Cannot cross: large proteins, most antibiotics, charged molecules
• Failure: occurs in ischemia, inflammation, trauma → vasogenic cerebral oedema

6.4 Neurovascular Unit

7. Major CNS Blood Supply

"Brain function is completely dependent on its supply of blood and oxygen. Clinical symptoms ensue when global or regional blood supply falls below the critical 50 mL per 100 g per minute." — Goldman-Cecil Medicine

8. Key CNS Pathologies (Clinical Relevance)

8.1 Cerebrovascular Disease

• Ischaemic stroke: focal infarction from thromboembolism or small vessel disease; MCA territory is most common
• Global ischaemia: cardiac arrest → selective neuronal necrosis (hippocampus, cerebellar Purkinje cells, neocortical layers III/V are most vulnerable)
• Haemorrhagic stroke: intracerebral or subarachnoid haemorrhage

8.2 Demyelinating Diseases

• Multiple sclerosis: autoimmune destruction of CNS myelin (oligodendrocytes); episodic neurological deficits with spatial and temporal dissemination; plaques in periventricular white matter, corpus callosum, optic nerves, spinal cord
• Reactive gliosis: astrocyte proliferation and scar formation following CNS injury (replaces function of peripheral Schwann cell regeneration, but is far more limited)

8.3 Neurodegenerative Diseases

• Parkinson disease: loss of dopaminergic neurons in the substantia nigra pars compacta → dopamine depletion in the striatum → bradykinesia, rigidity, rest tremor, postural instability; Lewy bodies (α-synuclein inclusions) are hallmark
• Alzheimer disease: amyloid-β plaques and neurofibrillary tangles (tau protein); progressive memory and cognitive decline
• ALS (Amyotrophic Lateral Sclerosis): combined upper and lower motor neuron degeneration

8.4 CNS Tumours

• Gliomas (from glial cells): glioblastoma multiforme (GBM) — most aggressive primary CNS tumour; astrocytomas, oligodendrogliomas
• Meningiomas: arise from arachnoid cells; usually benign and surgically resectable
• Metastases: most common brain tumours in adults; lung, breast, melanoma, renal, colorectal are leading primaries

8.5 CNS Infections

• Bacterial meningitis: infection of meninges/CSF; Streptococcus pneumoniae, Neisseria meningitidis most common
• Viral encephalitis: HSV-1 encephalitis (temporal lobe predilection); CSF PCR is diagnostic standard
• Brain abscess: ring-enhancing lesion on MRI; S. aureus, Streptococcus, anaerobes common organisms

8.6 Metabolic Encephalopathy

• Hypoglycaemia: causes acute encephalopathy with tremulousness, confusion, seizures; may mimic stroke; targets basal ganglia, cortex, substantia nigra, hippocampus
• Diabetic ketoacidosis (DKA): altered consciousness → coma; cerebral oedema may complicate treatment
• Hepatic encephalopathy: ammonia-driven astrocyte dysfunction; asterixis, confusion, coma

8.7 Epilepsy

• Abnormal, synchronous neuronal discharge causing transient neurological dysfunction (seizures)
• Focal vs. generalised; EEG is cornerstone of diagnosis; MRI identifies structural causes

9. CNS Pharmacology Highlights

• Lipid solubility and molecular size determine BBB penetration
• Opioid receptors are widely distributed in the CNS (medulla, locus coeruleus, periaqueductal gray, limbic system, cortex, spinal cord) — basis of analgesia and respiratory depression
• General anaesthetics enhance GABA-A inhibition and/or block NMDA receptors
• Antiepileptics act via Na⁺ channel stabilisation (phenytoin), GABA enhancement (benzodiazepines, valproate), or Ca²⁺ channel blockade
• Antidepressants/antipsychotics modulate monoamine systems (serotonin, dopamine, noradrenaline)

10. Summary Table

• Neuroanatomy through Clinical Cases, 3rd Ed. (Blumenfeld)
• Neuroscience: Exploring the Brain, 5th Ed. (Bear, Connors, Paradiso)
• Histology: A Text and Atlas, 8th Ed. (Ross & Pawlina)
• Gray's Anatomy for Students, 4th Ed.
• Goldman-Cecil Medicine, International Ed.
• Bradley and Daroff's Neurology in Clinical Practice
• Lippincott Illustrated Reviews: Pharmacology