8.1 Nervous System Anatomy

8.1 Nervous System Anatomy

Quick Answer: The nervous system has a central nervous system (CNS)—brain and spinal cord—and a peripheral nervous system (PNS)—cranial nerves, spinal nerves, and ganglia. Know brain regions (cerebrum lobes, cerebellum, brainstem parts), meninges, neuron parts, the 12 cranial nerves at overview level, spinal nerves/plexuses, and somatic vs autonomic (sympathetic/parasympathetic) as anatomical divisions, with a brief enteric mention. Signal physiology is deferred; structure comes first for NEX Human Anatomy (~20% of scored Science).

The nervous system coordinates sensation, movement, and rapid internal regulation. For nursing entrance exams, anatomy stems ask which structure sits where, which nerve leaves which foramen, or whether a pathway is CNS or PNS. Build a hierarchy: divisions → major organs → neuron as the cellular unit → functional subdivisions of the PNS.

CNS vs PNS

Ganglia are clusters of neuron cell bodies outside the CNS (e.g., dorsal root ganglia, autonomic ganglia). Nuclei are clusters of cell bodies inside the CNS.

Sensory (afferent) fibers carry information toward the CNS. Motor (efferent) fibers carry commands away from the CNS. Most named nerves are mixed (contain both), but some cranial nerves are purely sensory or primarily motor.

Brain Overview and Major Regions

The adult brain includes the cerebrum, diencephalon (thalamus, hypothalamus, epithalamus), brainstem, and cerebellum. The cerebrum is the largest mass and occupies most of the cranial cavity.

Cerebrum and Lobes

The cerebrum has two cerebral hemispheres connected by the corpus callosum (a major white-matter commissure). Surface gray matter is the cerebral cortex; deeper white matter contains myelinated tracts. Raised folds are gyri; grooves are sulci; deep grooves are fissures (e.g., longitudinal fissure between hemispheres).

Insula (deep to lateral sulcus) is sometimes listed as a fifth lobe; intro exams mainly test the four named surface lobes.

Central sulcus separates frontal from parietal lobes. Lateral (Sylvian) sulcus separates temporal lobe from frontal/parietal.

Diencephalon (Brief Anchor)

Cerebellum

The cerebellum sits posterior to the brainstem under the occipital lobes, in the posterior cranial fossa. It has two hemispheres and a midline vermis, with a highly folded cortex. Anatomically it coordinates balance, posture, and fine motor timing—know its location relative to the brainstem and its role label without diving into circuit physiology.

Brainstem: Midbrain, Pons, Medulla

The brainstem is the stalk connecting cerebrum/diencephalon to the spinal cord. Cranial nerves III–XII attach along its length (I and II are associated with forebrain structures).

Foramen magnum is where the medulla becomes the spinal cord.

Spinal Cord

The spinal cord runs in the vertebral canal from the foramen magnum to about the L1–L2 vertebral level in adults (ends as the conus medullaris). Inferior nerve roots form the cauda equina (“horse’s tail”).

Spinal nerves exit via intervertebral foramina. There are 31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.

Meninges and CSF Spaces

Three connective tissue membranes—the meninges—cover brain and spinal cord:

Epidural space (outside dura) is clinically important in the spinal canal (epidural anesthesia). Subdural potential space lies between dura and arachnoid. CSF is produced mainly by choroid plexuses in the ventricles and cushions the CNS—anatomy-level fact for NEX; detailed CSF circulation physiology can wait.

Cranial Nerves Overview (Names and Numbers)

Twelve pairs of cranial nerves (CN I–XII) arise from the brain/brainstem and mainly supply the head and neck (vagus also reaches thorax and abdomen).

Mnemonic starter: “Oh Once One Takes The Anatomy Final, Very Good Vacations Are Heavenly”—use any reliable mnemonic, then lock the number–name pairs. Exam items often ask which CN is purely sensory (I, II, VIII) or which moves the tongue (XII).

Spinal Nerves and Plexuses

After exiting the vertebral column, many spinal nerve ventral rami form plexuses (networks) that redistribute fibers into named peripheral nerves.

Thoracic nerves (T2–T12) typically remain segmental as intercostal nerves without a large plexus. Knowing that the phrenic nerve innervates the diaphragm and that the sciatic nerve is the largest nerve of the lower limb covers many intro stems.

Neuron Anatomy

The neuron is the signaling cell of the nervous system.

Support cells (neuroglia) include astrocytes, microglia, ependymal cells, oligodendrocytes (CNS), and Schwann cells and satellite cells (PNS)—know that myelin producers differ by division (oligodendrocytes CNS vs Schwann cells PNS).

Multipolar neurons (many dendrites, one axon) are the most common motor and CNS type. Unipolar (pseudounipolar) sensory neurons have cell bodies in dorsal root ganglia. Bipolar neurons appear in special senses (retina, olfactory epithelium).

Somatic vs Autonomic Divisions (and Enteric)

The PNS motor outflow is divided anatomically by target:

Autonomic: Sympathetic and Parasympathetic

Both divisions typically use a two-neuron chain: preganglionic neuron (CNS → ganglion) and postganglionic neuron (ganglion → effector). Somatic motor pathways use a single neuron from CNS to skeletal muscle.

Enteric Nervous System (Brief)

The enteric nervous system is a large network of neurons in the walls of the gastrointestinal tract (myenteric and submucosal plexuses). It can operate with considerable local autonomy while still receiving sympathetic and parasympathetic modulation. For NEX anatomy, remember it as the “gut’s own nervous system,” not a third major body region like CNS/PNS—but a recognized division of visceral neural control.

Clinical and Nursing Anchors

• Stroke and trauma localize by lobe or brainstem signs; knowing frontal vs occipital function labels guides assessment vocabulary.
• Spinal cord injury level maps to which spinal segments and plexuses are affected (e.g., phrenic roots C3–C5 and breathing).
• Lumbar puncture samples CSF from the lumbar cistern below the conus (typically L3–L4 or L4–L5)—safe because the cord has ended higher.
• Epidural anesthesia uses the epidural space outside the dura.
• Cranial nerve exams (pupil, eye movement, facial symmetry, gag, shrug, tongue) are bedside anatomy in action.
• Peripheral neuropathies involve named PNS nerves (e.g., median nerve at the carpal tunnel).

Exam Traps

• CNS vs PNS: spinal nerves are PNS; the spinal cord is CNS.
• Dorsal root = sensory; ventral root = motor — mixed only after they join as the spinal nerve.
• Adult cord ends ~L1–L2, not at the coccyx; cauda equina fills the lower canal.
• Cranial nerve numbers: optic is II (vision), oculomotor is III (eye muscles)—easy swap.
• Sympathetic = thoracolumbar; parasympathetic = craniosacral.
• Myelin cells: Schwann (PNS) vs oligodendrocytes (CNS).
• Cerebellum ≠ cerebrum: cerebellum is the smaller posterior coordinator, not the lobed hemispheres.

Study Map for NEX

1. Draw CNS vs PNS boxes and place brain, cord, cranial nerves, and spinal nerves.
2. Recite the four lobes and brainstem order: midbrain → pons → medulla.
3. List CN I–XII names in order once daily until automatic.
4. Name the four plexuses and one signature nerve each (phrenic, median/ulnar/radial, femoral, sciatic).
5. Sketch a neuron and label dendrite, soma, axon, myelin, synapse; add somatic vs autonomic target table.

With that map, later physiology (action potentials, reflexes, autonomic effects) attaches to structures you already recognize—efficient preparation for the Anatomy share of NEX Science and for clinical courses that assume neuroanatomical vocabulary.