Spinal Cord and Autonomic Nervous System

Spinal Cord and Autonomic Nervous System

Gross Anatomy of the Spinal Cord

The spinal cord extends from the foramen magnum to the conus medullaris at L1/L2. Below the conus, the cauda equina (horse's tail) — a bundle of lumbar, sacral, and coccygeal nerve roots — descends within the vertebral canal to exit at their respective intervertebral foramina. The filum terminale is a thin strand of pia mater extending from the conus to the coccyx, anchoring the cord. Denticulate ligaments (lateral pia mater extensions) attach the cord to the dural sheath bilaterally, stabilising it.

The meninges from outside in: dura mater (outermost, tough), arachnoid mater (middle, avascular), pia mater (innermost, adherent to cord). Spaces: epidural space (between dura and bone, contains fat and venous plexus); subdural space (potential space between dura and arachnoid); subarachnoid space (between arachnoid and pia, contains CSF).

Spinal Cord Blood Supply

Two-thirds of cord (anterior horns, lateral columns) supplied by the single anterior spinal artery (from vertebral arteries). One-third (posterior columns) supplied by two posterior spinal arteries. Anterior spinal artery syndrome: bilateral loss of motor function and pain/temperature sensation below lesion, with preserved proprioception and vibration (posterior columns intact).

Grey and White Matter Organisation

Grey matter: butterfly/H-shaped in cross section. Anterior (ventral) horns: lower motor neuron cell bodies (motor output). Posterior (dorsal) horns: sensory relay (incoming sensory fibres synapse here). Lateral horns: only present at T1–L2 (sympathetic preganglionic cell bodies) and S2–S4 (parasympathetic preganglionic = sacral outflow).

Autonomic Nervous System

Sympathetic (thoracolumbar outflow, T1–L2): preganglionic neurons in lateral horns synapse in paravertebral chain ganglia (sympathetic trunk, bilateral) or prevertebral ganglia (coeliac, superior/inferior mesenteric). Short preganglionic, long postganglionic fibres. White rami communicantes carry myelinated preganglionic fibres to sympathetic chain; grey rami communicantes carry unmyelinated postganglionic fibres back to spinal nerves. Sympathetic pathways: (1) up/down chain then grey ramus; (2) synapse in chain then leave on named nerves; (3) through chain → splanchnic nerves → prevertebral ganglia (abdominal organs); (4) through to adrenal medulla (no postganglionic — ACh directly stimulates chromaffin cells).

Parasympathetic (craniosacral outflow): preganglionic from CN III, VII, IX, X (vagus) and S2–S4. Long preganglionic fibres synapse in ganglia close to or within the target organ. Short postganglionic fibres.

Neurotransmitters: All preganglionic fibres are cholinergic (ACh → nicotinic receptors). Sympathetic postganglionic: noradrenaline (NA) → α/β adrenoceptors (except sweat glands: ACh → muscarinic). Parasympathetic postganglionic: ACh → muscarinic receptors.