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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)

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)

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