Chapter 24: Nervous System Overview & Orientation

The system is fundamentally divided into the Central Nervous System (CNS), comprising the brain and spinal cord, and the Peripheral Nervous System (PNS), which includes cranial and spinal nerves, peripheral ganglia, and their ramifications. Within the CNS, neuronal cell bodies are aggregated into grey matter (often forming specialized collections called nuclei where synaptic interactions occur), while myelinated axons are grouped into defined pathways called tracts that constitute the white matter. Information processing frequently involves decussation, where pathways cross the midline. The brain is segmented phylogenetically and ontogenetically into the rhombencephalon (hindbrain, including the cerebellum and brainstem), mesencephalon (midbrain), and prosencephalon (forebrain, including the diencephalon and the massive cerebral hemispheres/telencephalon). The brain maintains its environment through a protective blood-brain barrier and contains the interconnected ventricles, which are remnants of the neural tube lumen and house cerebrospinal fluid secreted by the choroid plexuses. Sensory input is organized into special senses (e.g., vision, taste) and general senses (e.g., pain, touch, proprioception), which typically ascend to the contralateral somatosensory cortex via a three-neurone sequence. Pain and temperature travel via the spinothalamic tract, while discriminative touch and proprioception ascend first through the dorsal columns before crossing the midline in the medulla as the medial lemniscus. Motor control originates from pathways like the corticospinal (pyramidal) tract, which descends through the internal capsule, crosses the midline in the medulla, and controls fine, fractionated movements. Clinically, motor pathways are differentiated into upper motor neurones (descending pathways, damage causing spasticity) and lower motor neurones (peripheral alpha motor neurones, damage causing muscle atrophy and reduced reflexes). Movement is also modulated by the basal ganglia, involved in movement selection, and the cerebellum, essential for coordination. The PNS connects the CNS to the body via 31 pairs of segmentally arranged spinal nerves and 12 pairs of cranial nerves. The Autonomic Nervous System (ANS), responsible for regulating the internal environment, uses a two-neurone chain (preganglionic and postganglionic) divided into sympathetic (energy mobilizing, originating T1–L2/3) and parasympathetic (energy conserving, originating in the brainstem and S2–S4) components. The Enteric Nervous System (ENS) maintains local reflex activity within the gastrointestinal tract. Visceral afferents convey sensation, including referred pain, which is felt somatically due to afferents sharing spinal cord segments. Finally, surface anatomy landmarks are detailed for approximating crucial intracranial structures like the central and lateral sulci and the dural venous sinuses.