Unit 15 — Nervous System Introduction · Question Bank

(a) Classification by number of processes

• Unipolar neuron: Single process arising from the cell body that divides into a central and peripheral branch. True unipolar neurons are rare in humans; exist transiently in the embryo. Sometimes listed for completeness.
• Pseudounipolar neuron: Appears unipolar but embryologically derived from bipolar. Single process divides in T-shape into a peripheral branch (to receptor) and a central branch (to spinal cord). Location: dorsal root ganglia (somatic sensory) and equivalent cranial nerve sensory ganglia. Function: primary sensory neuron.
• Bipolar neuron: One dendrite and one axon. Location: special sensory organs — retina (rod/cone bipolar cells), olfactory epithelium (olfactory receptor neurons), cochlear (spiral) ganglion, vestibular ganglion. Function: sensory transduction and relay in special senses.
• Multipolar neuron: One axon and two or more dendrites. The most common type in the CNS. Examples: anterior horn motor neurons, pyramidal cells of cortex, Purkinje cells of cerebellum, all interneurons. Function: motor output and integration.

(b) Classification by function

• Afferent (sensory) neuron: Carries impulses from receptors to the CNS. Structurally pseudounipolar (cell body in DRG). Carries pain, temperature, touch, proprioception. Enter spinal cord via dorsal root.
• Efferent (motor) neuron: Carries impulses from CNS to effectors (skeletal muscle, smooth muscle, glands). Structurally multipolar. Cell body in anterior horn (LMN) or cortex (UMN). Exit spinal cord via ventral root.
• Interneuron (association/intermediate neuron): Located entirely within the CNS. Connect afferent and efferent neurons. Account for >99% of all CNS neurons. Responsible for integration, learning, memory, and all higher CNS functions. Structurally multipolar.

CNS neuroglia (4 types)

• Astrocytes (fibrous & protoplasmic): Most numerous CNS glia. Star-shaped cells. Functions: (1) Structural support for neurons; (2) Metabolic support (supply glucose/lactate); (3) K&sup+; buffering; (4) Glutamate reuptake from synapses; (5) Blood-brain barrier — end-feet induce endothelial tight junctions; (6) Gliosis (scar formation) after CNS injury. Clinical: Glioblastoma multiforme (Grade IV astrocytoma) = most common malignant primary brain tumour; astrogliosis in neurodegenerative diseases.
• Oligodendrocytes: Produce and maintain CNS myelin; each wraps multiple axons (up to 50 internodes). Compact myelin → saltatory conduction → fast impulse transmission. Clinical: Multiple sclerosis (MS) — autoimmune destruction of oligodendrocytes and CNS myelin → demyelinating plaques → conduction failure. MRI shows white matter lesions. No effective myelin regeneration in CNS.
• Microglia: Smallest CNS glia. Derived from mesodermal monocyte precursors (not neuroectoderm), resident macrophages of the CNS. Resting state: ramified morphology. Activated state: amoeboid → phagocytosis of pathogens, dead cells, synaptic pruning. Clinical: neuroinflammation in Alzheimer's (amyloid phagocytosis), HIV encephalitis, stroke.
• Ependymal cells: Ciliated simple columnar epithelium lining ventricles and central canal of spinal cord. Specialised choroidal ependyma forms outer layer of choroid plexus and contributes to CSF production. Clinical: Ependymoma — tumour of ependymal cells, common in 4th ventricle (children) and spinal cord (adults).

PNS neuroglia (2 types)

• Schwann cells: Produce and maintain PNS myelin. Each Schwann cell myelinates ONE internodal segment of ONE axon. Outer cytoplasm = neurilemma (sheath of Schwann). After PNS injury, Schwann cells form Bands of Büngner that guide axon regeneration (~1 mm/day). Clinical: Schwannoma (benign); in NF2. Guillain–Barré syndrome = autoimmune demyelination of PNS Schwann cells → ascending flaccid paralysis.
• Satellite cells: Surround and support neuron cell bodies within sensory and autonomic ganglia (dorsal root ganglia, sympathetic ganglia). Regulate the microenvironment of ganglionic neurons. No myelin production.

Origin (CNS outflow)

• Sympathetic (thoracolumbar): T1–L2 intermediolateral cell column (lateral horn) of the spinal cord.
• Parasympathetic (craniosacral): Brain (CN III, VII, IX, X) and sacral spinal cord (S2–S4 lateral horn).

Ganglia location

• Sympathetic: Paravertebral chain ganglia (near vertebral column) OR prevertebral ganglia (coeliac, superior mesenteric, inferior mesenteric) → short preganglionic / long postganglionic fibres.
• Parasympathetic: Ganglia located near or within the effector organ (ciliary, pterygopalatine, submandibular, otic, intramural ganglia in organ walls) → long preganglionic / short postganglionic fibres.

Neurotransmitters

• Sympathetic: Preganglionic = ACh (nicotinic receptor). Postganglionic = noradrenaline (adrenergic receptors α⊂1;/β⊂1;/β⊂2;) at most targets. Exception: sweat glands (eccrine) = ACh (muscarinic). Adrenal medulla = modified sympathetic ganglion; releases adrenaline + noradrenaline into blood.
• Parasympathetic: ACh at BOTH synapses (preganglionic = nicotinic; postganglionic = muscarinic). No exceptions.

Effects on target organs

• Heart: Sympathetic → increases heart rate and contractility (β⊂1;); noradrenaline. Parasympathetic (CN X) → decreases heart rate; ACh (muscarinic M⊂2;).
• Pupil: Sympathetic → mydriasis (pupil dilation) via dilator pupillae muscle (α⊂1;). Parasympathetic (CN III, Edinger-Westphal nucleus → ciliary ganglion) → miosis (pupil constriction) via sphincter pupillae (muscarinic). Horner's syndrome = loss of sympathetic = miosis + ptosis + anhidrosis.
• GI tract: Sympathetic → decreases peristalsis and secretion; contracts sphincters (“fight or flight” → digestion inhibited). Parasympathetic (CN X + S2–S4) → increases peristalsis and secretion; relaxes sphincters (“rest and digest”).

Definition

A reflex arc is the neural pathway that mediates a stereotyped, involuntary response to a stimulus. It bypasses higher cortical control (though the cortex monitors it via corticospinal tracts). Reflexes are reproducible and used clinically to test the integrity of specific spinal cord segments.

5 components

• 1. Receptor: Transduces the stimulus. In the stretch reflex = muscle spindle (annulospiral endings on intrafusal fibres) in the quadriceps femoris. Detects sudden stretch of the muscle.
• 2. Afferent neuron: Ia sensory fibre from the muscle spindle. Pseudounipolar, cell body in L3–L4 dorsal root ganglion. Enters spinal cord via the dorsal root.
• 3. Integration centre: In the monosynaptic stretch reflex, the Ia afferent synapses directly on the alpha motor neuron in the L3–L4 anterior horn — no interneuron. This is the only truly monosynaptic reflex arc in the body. Simultaneously, inhibitory interneurons (Ia inhibitory interneurons) inhibit antagonist muscle motor neurons (reciprocal inhibition).
• 4. Efferent neuron: Alpha lower motor neuron, cell body in L3–L4 anterior horn. Axon exits via the ventral root → femoral nerve → quadriceps femoris.
• 5. Effector: Quadriceps femoris muscle contracts → knee extends (the visible “jerk”).

Clinical significance

• Tests L3–L4 integrity and the femoral nerve. Absent/reduced = LMN lesion (L3–L4 root, femoral nerve, anterior horn) or severe upper motor neurone lesion in the acute phase.
• Exaggerated/hyperreflexia = UMN lesion above L3–L4 (corticospinal tract damage removes descending inhibition).
• Clonus (rhythmic contractions) = severe UMN lesion.
• Other tendon reflexes: Biceps C5–C6, Triceps C7–C8, Ankle jerk S1–S2.

Production

CSF is produced primarily by the choroid plexuses located in the lateral ventricles (temporal horns), the third ventricle (roof), and the fourth ventricle (roof). The choroid plexus consists of specialised ependymal cells overlying a vascular core; it actively secretes CSF by ultrafiltration and active transport of Na&sup+;. Total volume ~150 mL; produced at ~500 mL/day (therefore turns over ~3×/day). Functions of CSF: mechanical cushioning of the brain against trauma, metabolic support, removal of waste products (recent research: glymphatic clearance during sleep), maintenance of intracranial pressure (~5–15 cmH⊂2;O).

Circulation route

• Lateral ventricles → (via interventricular foramina of Monro) → Third ventricle → (via cerebral aqueduct of Sylvius) → Fourth ventricle →
• Exits fourth ventricle through: (1) Foramen of Magendie (median, in roof of 4th ventricle) and (2) paired Foramina of Luschka (lateral recesses) →
• Subarachnoid space surrounding the entire brain and spinal cord. Collects in cisterns (enlarged subarachnoid spaces): pontine cistern, cisterna magna (between cerebellum and medulla), lumbar cistern (below conus medullaris at L1–L2 to S2).

Drainage (absorption)

CSF is absorbed into the dural venous sinuses via arachnoid granulations (Pacchionian granulations) that project into the superior sagittal sinus and other dural sinuses. Absorption is pressure-dependent (bulk flow into venous blood).

Clinical relevance

• Lumbar puncture (LP): Needle inserted at L3–L4 interspace (below the conus medullaris which ends at L1–L2 in adults) into the lumbar cistern of the subarachnoid space. Layers traversed: skin → supraspinous ligament → interspinous ligament → ligamentum flavum → epidural space → dura mater → subdural space → arachnoid → subarachnoid space. Normal CSF: clear, colourless; protein <45 mg/dL; glucose ~60% plasma glucose; <5 WBC/mm³. In bacterial meningitis: turbid, low glucose, high protein, many neutrophils.
• Hydrocephalus: Excessive accumulation of CSF → raised intracranial pressure → headache, papilloedema, vomiting, altered consciousness. Types: (1) Non-communicating (obstructive) — blockage within the ventricular system (e.g. aqueductal stenosis, tumour at 4th ventricle blocking foramina); (2) Communicating — impaired absorption at arachnoid granulations (e.g. post-meningitic scarring, subarachnoid haemorrhage). Treatment: ventriculoperitoneal (VP) shunt.