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AN7.1-8 | Introduction to the nervous system — Part 1
CLINICAL SCENARIO
A 55-year-old farmer from Tamil Nadu presents to a primary health centre with sudden weakness of the right arm and leg, slurring of speech, and deviation of the angle of the mouth to the left. He has hypertension. CT brain shows a left middle cerebral artery territory infarct.
Two weeks later, a 30-year-old labourer presents with flaccid paralysis of the right arm after a road traffic accident fracturing his humerus. Nerve conduction studies confirm radial nerve injury at the radial groove.
Both have "weakness" but the pattern is completely different. Why? What is the fundamental difference between an upper motor neuron (UMN) lesion and a lower motor neuron (LMN) lesion?
The answer lies in the architecture of the nervous system — and that is where we begin.
WHY THIS MATTERS
A solid foundation in nervous system organisation is clinically indispensable:
- UMN vs LMN distinction — the most tested concept in Medicine and Surgery finals; guides every neurological examination
- Dermatomes and myotomes — used daily in orthopaedics, neurosurgery, and anaesthesia for nerve block planning
- Nerve injury classification (Seddon/Sunderland) — determines prognosis and surgical decision-making in peripheral nerve injuries from RTAs (a major burden in India)
- Autonomic pharmacology — understanding sympathetic vs parasympathetic ganglia is the basis of drugs used in hypertension, asthma, and urological disorders
- NMC 2024 competency AN7.1–7.8 — foundational for the entire Neuroanatomy block
RECALL
Before we begin, recall from general anatomy:
- A neuron is the structural and functional unit of the nervous system
- Nervous tissue = neurons + neuroglia (supporting cells)
- The nervous system is embryologically derived from neuroectoderm (neural tube and neural crest)
- Reflex arc components: receptor → afferent neuron → interneuron → efferent neuron → effector
Part 1: General Plan of the Nervous System (AN7.1)
Organisational Hierarchy
The nervous system is divided into:
A. Central Nervous System (CNS)
• Brain (within cranium) + Spinal cord (within vertebral canal)
• Covered by meninges; bathed in CSF
• Contains upper motor neurons, interneurons, sensory relay nuclei
B. Peripheral Nervous System (PNS)
• All nervous tissue outside the CNS
• Includes: 12 pairs of cranial nerves + 31 pairs of spinal nerves + peripheral ganglia
• Contains lower motor neurons (anterior horn cells → motor end plate) and sensory neurons (DRG cells)
C. Autonomic Nervous System (ANS)
• Part of PNS; controls involuntary visceral functions
• Two divisions:
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| Origin | T1–L2 (thoracolumbar) | CN III, VII, IX, X + S2–S4 (craniosacral) |
| Pre-ganglionic fibre | Short | Long |
| Post-ganglionic fibre | Long | Short |
| Ganglia | Paravertebral chains + collateral ganglia | In/near target organ |
| Neurotransmitter (pre-gang) | Acetylcholine (nicotinic) | Acetylcholine (nicotinic) |
| Neurotransmitter (post-gang) | Noradrenaline (except sweat: ACh) | Acetylcholine (muscarinic) |
| General action | "Fight or flight" | "Rest and digest" |
Somatic vs Visceral components:
• Somatic afferent/efferent: body wall, limbs, skin — conscious sensation and voluntary movement
• Visceral afferent/efferent: organs — pain (often referred), autonomic control
Part 2: Components of Nervous Tissue (AN7.2)
Neurons (Nerve Cells)
• Structural and functional units; transmit electrochemical signals
• Cannot divide after birth (post-mitotic); highly metabolically active
• Components: cell body (soma/perikaryon) + processes (dendrites + axon)
Neuroglia (Supporting Cells) — 10× more numerous than neurons
| Cell | Location | Function |
|---|---|---|
| Astrocytes | CNS | Structural support, BBB maintenance, K⁺ buffering, scar formation |
| Oligodendrocytes | CNS | Myelination of CNS axons (1 cell → multiple axons) |
| Microglia | CNS | CNS macrophages (immune surveillance) |
| Ependymal cells | CNS (ventricle lining) | CSF production (choroid plexus) and circulation |
| Schwann cells | PNS | Myelination of PNS axons (1 cell → 1 axon segment) |
| Satellite cells | PNS ganglia | Support neuronal cell bodies in ganglia |
Clinical relevance:
• Multiple sclerosis — autoimmune demyelination of CNS oligodendrocytes → patchy neurological deficits
• Guillain-Barré syndrome — autoimmune demyelination of PNS Schwann cells → ascending flaccid paralysis (common in India after GI infections)
• Gliomas — tumours of astrocytes (astrocytoma, glioblastoma) are the most common primary brain tumours
Part 3: Neuron Structure and Classification (AN7.3)
Parts of a Neuron
1. Cell body (Soma/Perikaryon)
- Contains nucleus (large, pale, prominent nucleolus — active protein synthesis)
- Nissl bodies (rough ER + polyribosomes) — site of protein synthesis
- Absent in axon hillock and axon
- Chromatolysis — Nissl body dissolution after axon injury (indicates neuronal stress/degeneration)
2. Dendrites
- Multiple, short, branching processes
- Receive input; carry impulses TOWARD cell body
- Contain Nissl bodies
- Dendritic spines — tiny protrusions that increase synaptic surface area
3. Axon
- Single; arises from axon hillock (trigger zone for action potential)
- Carries impulses AWAY from cell body
- No Nissl bodies; has neurofilaments and microtubules
- May be myelinated or unmyelinated
- Ends in axon terminals (boutons) containing synaptic vesicles
Classification of Neurons
By number of processes:
| Type | Processes | Example |
|---|---|---|
| Multipolar | Many dendrites + 1 axon | Anterior horn motor neurons, Purkinje cells |
| Bipolar | 1 dendrite + 1 axon | Retinal bipolar cells, vestibular ganglion |
| Unipolar (pseudounipolar) | 1 process (T-shaped) | DRG cells (primary sensory neurons) |
By function:
• Sensory (afferent) — carry impulses to CNS
• Motor (efferent) — carry impulses from CNS to effectors
• Interneurons (association) — connect neurons within CNS (99% of all neurons)
By size:
• Golgi type I — long axon (projection neurons, e.g., pyramidal cells, Purkinje cells)
• Golgi type II — short axon (local circuit neurons, interneurons)