Part 5: Nervous System & Neuroanatomy

Central Nervous System

The central nervous system (CNS) — comprising the brain and spinal cord — is the command centre of the entire body. It receives sensory input, integrates and processes information, and generates motor commands. The brain alone contains roughly 86 billion neurons, each forming up to 10,000 synaptic connections, creating a network of staggering complexity. Understanding CNS anatomy is fundamental to neurology, neurosurgery, psychiatry, and rehabilitation medicine.

Cerebrum Lobes

The cerebrum is the largest part of the brain, divided into two hemispheres connected by the corpus callosum. Each hemisphere has four lobes, named after the overlying skull bones:

Cerebellum

The cerebellum ("little brain") sits posterior to the brainstem, connected by three pairs of cerebellar peduncles. Despite being only 10% of brain volume, it contains over 50% of all neurons. The cerebellum has three functional divisions:

• Vestibulocerebellum (flocculonodular lobe) — balance and eye movements; receives input from vestibular nuclei
• Spinocerebellum (vermis + paravermis) — posture, limb coordination, and gait; receives proprioceptive input from the spinal cord
• Cerebrocerebellum (lateral hemispheres) — planning and timing of complex movements, motor learning; receives input from the cerebral cortex

Brainstem

The brainstem connects the cerebrum to the spinal cord and houses critical nuclei for vital functions. It consists of three parts (superior to inferior):

Running through the core of the brainstem is the reticular formation — a diffuse network of neurons responsible for arousal, consciousness, sleep-wake cycles, and pain modulation. The ascending reticular activating system (ARAS) projects to the thalamus and cortex to maintain wakefulness. Damage to the ARAS (as in severe brainstem stroke) can cause coma.

Spinal Cord Tracts

The spinal cord extends from the medulla oblongata to approximately the L1-L2 vertebral level (the conus medullaris), below which the cauda equina (nerve roots) continues. In cross-section, the cord shows a butterfly-shaped grey matter (cell bodies) surrounded by white matter (myelinated tracts).

The major ascending (sensory) and descending (motor) tracts are organised in specific columns:

• Dorsal columns (gracile + cuneate fasciculi) — carry fine touch, proprioception, and vibration; ascend ipsilaterally, cross in the medulla
• Lateral spinothalamic tract — carries pain and temperature; crosses within 1-2 segments of entry, ascends contralaterally
• Lateral corticospinal tract — primary voluntary motor pathway; crosses at the pyramidal decussation in the medulla, descends contralaterally
• Anterior corticospinal tract — minor motor pathway; descends ipsilaterally, crosses near the target level

Peripheral Nervous System

The peripheral nervous system (PNS) includes all neural structures outside the brain and spinal cord — 12 pairs of cranial nerves, 31 pairs of spinal nerves, and their branches. The PNS serves as the communication highway between the CNS and the rest of the body.

Cranial Nerves

The twelve cranial nerves emerge from the brain (mainly the brainstem) and innervate structures primarily in the head and neck (except CN X, the vagus, which extends to the thorax and abdomen). The classic mnemonic is: "Oh, Oh, Oh, To Touch And Feel Very Good Velvet, AH!"

Spinal Nerves & Plexuses

The 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal) exit through intervertebral foramina. Each spinal nerve carries both motor and sensory fibres and supplies a specific dermatome (skin area) and myotome (muscle group).

In the cervical, lumbar, and sacral regions, ventral rami intermingle to form nerve plexuses:

Autonomic System

The autonomic nervous system (ANS) regulates involuntary functions — heart rate, digestion, respiratory rate, pupil dilation, glandular secretion, and more. It operates largely below conscious awareness, maintaining homeostasis through two opposing divisions.

Sympathetic vs Parasympathetic

Enteric Nervous System

The enteric nervous system (ENS) is sometimes called the "second brain" — it contains approximately 500 million neurons embedded in the wall of the gastrointestinal tract, from oesophagus to rectum. It can function independently of the CNS (though it is modulated by sympathetic and parasympathetic input). Two major plexuses comprise the ENS:

• Myenteric (Auerbach's) plexus — between the longitudinal and circular muscle layers; primarily controls gut motility (peristalsis)
• Submucosal (Meissner's) plexus — in the submucosa; primarily controls glandular secretion and blood flow to the mucosa

Functional Systems

Motor and sensory pathways are the highways of the nervous system — carrying commands from brain to muscles and sensory information from the periphery to the cortex. Understanding these pathways is essential for localising neurological lesions.

Motor Pathways

The primary voluntary motor pathway is the corticospinal (pyramidal) tract:

1. Upper motor neuron — cell body in the primary motor cortex (precentral gyrus) → descends through the internal capsule → cerebral peduncle → pons → medullary pyramid
2. Decussation — 85-90% of fibres cross at the pyramidal decussation in the lower medulla → form the lateral corticospinal tract
3. Lower motor neuron — synapses in the anterior horn of the spinal cord → exits via ventral root → peripheral nerve → neuromuscular junction → muscle

Sensory Pathways

The two major ascending sensory pathways carry different modalities and cross at different levels:

Reflex Arcs

A reflex arc is the simplest functional unit of the nervous system — a rapid, involuntary response to a stimulus that bypasses conscious processing. The basic components are:

1. Receptor — sensory receptor detects stimulus (e.g., muscle spindle detects stretch)
2. Afferent neuron — sensory nerve carries signal to spinal cord
3. Integration centre — in the spinal cord (may include interneurons)
4. Efferent neuron — motor nerve carries command to effector
5. Effector — muscle contracts (or gland secretes)

Clinically important reflexes include the knee-jerk (patellar) reflex (L3-L4, tests femoral nerve), ankle jerk (Achilles) reflex (S1-S2, tests tibial nerve), biceps reflex (C5-C6, tests musculocutaneous nerve), and triceps reflex (C7-C8, tests radial nerve). Hyperactive reflexes suggest UMN lesion; absent reflexes suggest LMN lesion.

Advanced Neuroanatomy

Beyond the cortex and pathways, several deep brain structures play crucial roles in movement regulation, emotion, memory, and fluid homeostasis.

Basal Ganglia

The basal ganglia are a group of subcortical nuclei that modulate movement — they do not initiate movement but fine-tune and facilitate desired movements while suppressing unwanted ones. Key structures include:

• Caudate nucleus + Putamen = Striatum (input centre — receives cortical input)
• Globus pallidus (internal + external) — output centre
• Subthalamic nucleus — excitatory component of the indirect pathway
• Substantia nigra — pars compacta: dopamine-producing neurons; pars reticulata: output pathway

The basal ganglia work through two parallel circuits: the direct pathway (facilitates movement — "go" signal) and the indirect pathway (inhibits movement — "stop" signal). Dopamine from the substantia nigra tips the balance in favour of the direct pathway, promoting movement. Loss of dopaminergic neurons → Parkinson's disease (difficulty initiating movement).

Limbic System

The limbic system is a ring of structures surrounding the corpus callosum that governs emotion, memory, motivation, and autonomic regulation. Key components:

Ventricles & CSF Flow

The brain contains four interconnected ventricles that produce and circulate cerebrospinal fluid (CSF) — a clear, cushioning fluid that bathes the brain and spinal cord, providing buoyancy, protection, and metabolic waste removal.

CSF flow path: Lateral ventricles (in cerebral hemispheres) → interventricular foramen (of Monro) → Third ventricle (midline, surrounded by thalamus) → cerebral aqueduct (of Sylvius) → Fourth ventricle (between cerebellum and brainstem) → exits via median aperture (of Magendie) and lateral apertures (of Luschka) → subarachnoid space → reabsorbed into venous sinuses via arachnoid granulations.

Clinical Links

The nervous system is vulnerable to a wide range of pathologies — vascular, traumatic, degenerative, and autoimmune. Understanding neuroanatomy allows precise clinical localisation of lesions based on the pattern of deficits.

Stroke Localization

Because different brain regions have specific functions and specific blood supplies, the pattern of neurological deficits after a stroke allows clinicians to identify which vessel is occluded:

• MCA stroke → contralateral face/arm weakness > leg, aphasia (dominant hemisphere), neglect (non-dominant)
• ACA stroke → contralateral leg weakness > arm, personality changes, urinary incontinence
• PCA stroke → contralateral visual field loss, memory problems (hippocampal involvement)
• Basilar artery stroke → cranial nerve palsies, cerebellar signs, potentially "locked-in syndrome"
• Lateral medullary syndrome (Wallenberg) → ipsilateral facial numbness, Horner's, ataxia + contralateral body pain/temperature loss (PICA territory)

Nerve Injuries

Peripheral nerve injuries produce characteristic patterns based on which nerve is damaged:

Parkinson's, MS & Neuropathies

Practice & Tools

Practice Exercises

Applied Code Example

This Python script visualises the dermatome map — the relationship between spinal nerve levels and the skin regions they innervate — a fundamental concept for neurological examination.

import numpy as np
import matplotlib.pyplot as plt

# Key dermatomes and their spinal nerve levels
dermatomes = {
    'C2': 'Occiput (back of head)',
    'C3': 'Neck',
    'C4': 'Shoulder',
    'C5': 'Lateral arm (deltoid)',
    'C6': 'Lateral forearm, thumb',
    'C7': 'Middle finger',
    'C8': 'Medial forearm, ring/little finger',
    'T1': 'Medial arm',
    'T4': 'Nipple line',
    'T10': 'Umbilicus',
    'L1': 'Inguinal region',
    'L3': 'Anterior thigh, knee',
    'L4': 'Medial leg',
    'L5': 'Dorsum of foot, great toe',
    'S1': 'Lateral foot, sole',
    'S3-S5': 'Perineum (saddle area)'
}

# Create visualisation
fig, ax = plt.subplots(figsize=(10, 8))
y_positions = np.arange(len(dermatomes))
levels = list(dermatomes.keys())
regions = list(dermatomes.values())

# Colour code by region (cervical, thoracic, lumbar, sacral)
colors = []
for level in levels:
    if level.startswith('C'):
        colors.append('#3B9797')  # Teal for cervical
    elif level.startswith('T'):
        colors.append('#16476A')  # Blue for thoracic
    elif level.startswith('L'):
        colors.append('#BF092F')  # Crimson for lumbar
    else:
        colors.append('#132440')  # Navy for sacral

bars = ax.barh(y_positions, [1]*len(levels), color=colors, edgecolor='white', height=0.7)

for i, (level, region) in enumerate(zip(levels, regions)):
    ax.text(0.05, i, f'{level}: {region}', va='center', fontsize=10,
            fontweight='bold', color='white')

ax.set_xlim(0, 1)
ax.set_yticks([])
ax.set_xticks([])
ax.invert_yaxis()
ax.set_title('Key Dermatome Reference Map', fontsize=14, fontweight='bold', pad=15)
ax.spines['top'].set_visible(False)
ax.spines['right'].set_visible(False)
ax.spines['bottom'].set_visible(False)
ax.spines['left'].set_visible(False)

# Add legend
from matplotlib.patches import Patch
legend_elements = [
    Patch(facecolor='#3B9797', label='Cervical'),
    Patch(facecolor='#16476A', label='Thoracic'),
    Patch(facecolor='#BF092F', label='Lumbar'),
    Patch(facecolor='#132440', label='Sacral')
]
ax.legend(handles=legend_elements, loc='lower right', fontsize=9)
plt.tight_layout()
plt.show()

Neuroanatomy Pathway Mapper

Use this tool to document neural pathways, cranial nerve findings, and lesion localisations. Generate a downloadable report in Word, Excel, or PDF format.

Conclusion & Next Steps

The nervous system is the most intricate organ system in the human body — from the 86 billion neurons of the cerebral cortex to the autonomic reflexes that maintain homeostasis without conscious effort. We've covered the architecture of the CNS (cerebral lobes, cerebellum, brainstem, spinal cord), the PNS (cranial nerves, spinal nerve plexuses), the autonomic system, functional motor and sensory pathways, and advanced structures like the basal ganglia and limbic system. Understanding these anatomical foundations enables precise clinical localisation of neurological lesions.

The key concepts to carry forward include the distinction between UMN and LMN signs, the crossing patterns of major tracts, cranial nerve testing, and the clinical correlations of basal ganglia and limbic system pathology. In the next instalment, we'll explore the visceral anatomy of the thorax and abdomen — the organs that the nervous system innervates and regulates.