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AN7.1-8 | Introduction to the nervous system — Part 2
Part 4: Structure of a Typical Spinal Nerve (AN7.4) & Muscle Innervation (AN7.5–7.6)
Structure of a Typical Spinal Nerve
A spinal nerve has two roots:
• Dorsal (posterior) root — sensory; contains a dorsal root ganglion (DRG) with pseudounipolar sensory neurons
• Ventral (anterior) root — motor; axons of anterior horn cells (LMNs) + preganglionic sympathetic fibres (T1–L2)
Figure: Structure of a Typical Spinal Nerve
After the two roots unite at the intervertebral foramen:
1. Spinal nerve (mixed, very short)
2. Divides into:
- Dorsal (posterior) ramus — skin and deep muscles of the back
- Ventral (anterior) ramus — anterior and lateral body wall, limbs (forms plexuses)
- Meningeal ramus — recurrent branch to meninges
- Rami communicantes (grey and white) — to sympathetic chain
Principles of Muscle Innervation (AN7.5)
• Each muscle is supplied by a specific myotome (segment of spinal cord)
• Most limb muscles are supplied by 2–3 segments (e.g., biceps: C5, C6; quadriceps: L2, L3, L4)
• Motor unit = one LMN + all muscle fibres it innervates
• Smaller motor units = finer movement (hand intrinsics); larger = powerful (gastrocnemius)
Loss of Innervation (AN7.6) — UMN vs LMN
| Feature | UMN Lesion | LMN Lesion |
|---|---|---|
| Site | Cortex to anterior horn cell | Anterior horn cell, root, nerve |
| Tone | Increased (spasticity) | Decreased (flaccidity) |
| Power | Reduced | Reduced |
| Reflexes | Brisk (hyperreflexia) | Absent (areflexia) |
| Babinski sign | Positive (extensor) | Absent |
| Wasting | Late, disuse atrophy | Early, severe (denervation atrophy) |
| Fasciculations | Absent | Present (dying LMN) |
| Example | Stroke (MCA infarct) | Radial nerve palsy, poliomyelitis |
Part 5: Synapses (AN7.7) & Ganglia (AN7.8)
Types of Synapse (AN7.7)
UMN vs LMN Lesion — Clinical Features
Figure: Types of Synapse (AN7.7)
| Feature | UMN Lesion | LMN Lesion |
|---|---|---|
| Paralysis type | Spastic | Flaccid |
| Tone | Hypertonia (clasp-knife) | Hypotonia |
| Reflexes | Hyperreflexia | Areflexia / hyporeflexia |
| Babinski sign | Positive (upgoing plantar) | Negative (downgoing plantar) |
| Muscle wasting | Absent (initially); disuse atrophy late | Present (denervation atrophy) |
| Fasciculations | Absent | Present |
| Example | Stroke (internal capsule infarct) | Radial nerve palsy (humerus fracture) |
Figure: Loss of Innervation (AN7.6) — UMN vs LMN
By mechanism:
• Chemical synapse — most common; uses neurotransmitters; unidirectional; has synaptic delay (0.5 ms)
• Electrical synapse (gap junction) — direct ionic coupling; bidirectional; no delay; in cardiac muscle, smooth muscle, CNS (retina)
By location (morphology):
| Type | Contact | Example |
|---|---|---|
| Axodendritic | Axon → dendrite | Most common excitatory synapse |
| Axosomatic | Axon → cell body | Common inhibitory synapse |
| Axoaxonic | Axon → axon | Presynaptic inhibition |
| Dendrodendritic | Dendrite → dendrite | Retina, olfactory bulb |
By function:
• Excitatory — depolarisation (EPSP); e.g., glutamate (CNS), ACh (NMJ)
• Inhibitory — hyperpolarisation (IPSP); e.g., GABA, glycine
Differences: Sympathetic Ganglion vs Spinal (DRG) Ganglion (AN7.8)
| Feature | Sympathetic Ganglion | Spinal (Dorsal Root) Ganglion |
|---|---|---|
| Type of neurons | Multipolar (motor — postganglionic) | Pseudounipolar (sensory) |
| Function | ANS efferent relay | Sensory afferent cell bodies |
| Capsule | Thin | Prominent |
| Satellite cells | Present | Present |
| Synapse present | Yes (pre→post ganglionic) | No synapse |
| Location | Paravertebral or collateral | Intervertebral foramen |
| Myelination | Pre: myelinated (B fibre); Post: unmyelinated (C fibre) | Central: myelinated; peripheral: varies |
SELF-CHECK — : Nervous System Foundations
Sympathetic Ganglion vs Spinal (DRG) Ganglion
| Feature | Spinal (DRG) Ganglion | Sympathetic Ganglion |
|---|---|---|
| Location | On dorsal root of spinal nerve | Paravertebral sympathetic chain (or collateral ganglia) |
| Neuron type | Pseudounipolar (sensory) | Multipolar (motor/autonomic) |
| Synapses within ganglion | Absent (relay station only) | Present (preganglionic → postganglionic synapse) |
| Satellite cells | Present (ring around each neuron) | Present |
| Function | Sensory (afferent) | Motor/autonomic (efferent) |
| Associated ramus | Dorsal root | White and grey rami communicantes |
Figure: Differences: Sympathetic Ganglion vs Spinal (DRG) Ganglion (AN7.8)
A 22-year-old presents with ascending flaccid paralysis 10 days after a diarrhoeal illness. NCS shows demyelination of peripheral nerves. Which glial cells are primarily damaged?
A. Oligodendrocytes
B. Schwann cells
C. Astrocytes
D. Microglia
Reveal Answer
Answer: B. Schwann cells
A primary sensory neuron in the dorsal root ganglion is best classified as:
A. Multipolar
B. Bipolar
C. Pseudounipolar
D. Unipolar
Reveal Answer
Answer: C. Pseudounipolar
In a patient with a C5–C6 root avulsion after a motorcycle accident, you would expect:
A. Flaccid paralysis of deltoid and biceps with absent biceps reflex
B. Spastic paralysis with brisk reflexes
C. Positive Babinski sign
D. Loss of sensation only
Reveal Answer
Answer: A. Flaccid paralysis of deltoid and biceps with absent biceps reflex
CLINICAL PEARL
Nerve Injury Classification — Seddon's System (Practical for Indian Orthopaedics)
| Grade | Term | Pathology | Recovery |
|---|---|---|---|
| 1 | Neuropraxia | Myelin injury, axon intact | Complete, spontaneous (weeks) |
| 2 | Axonotmesis | Axon cut, endoneurium intact | Spontaneous, 1 mm/day (months) |
| 3 | Neurotmesis | Complete nerve cut | Requires surgical repair |
Wallerian degeneration — after axon injury, the distal axon degrades within 24–48 hours (Schwann cells phagocytose myelin debris). Proximal stump shows chromatolysis.
Rate of nerve regeneration: ~1 mm/day or ~2.5 cm/month — used to predict recovery timeline after nerve repair in Indian orthopaedic and plastic surgery practice.