Anatomy of the Nervous System: Central and Peripheral Components

Central Nervous System (CNS)

The Brain

The brain is the primary organ of the nervous system, responsible for processing sensory information, coordinating movement, and regulating behavior and homeostasis. It is divided into four major regions: the cerebrum, diencephalon, brain stem, and cerebellum.

• Cerebrum: The largest part of the brain, responsible for higher cognitive functions, voluntary movement, and sensory perception.

• Diencephalon: Located beneath the cerebrum, it includes the thalamus and hypothalamus, serving as a relay and regulatory center.

• Brain Stem: Composed of the midbrain, pons, and medulla oblongata, it connects the brain to the spinal cord and regulates vital functions.

• Cerebellum: Involved in balance, posture, and coordination of movement.

Cerebrum

• Cerebral Cortex: The wrinkled, outer layer of the cerebrum, composed of grey matter (regions with many neuron cell bodies and dendrites). The folds (gyri) and grooves (sulci) increase surface area for higher cognitive functions.

• Gyri (singular: gyrus): Ridges on the surface of the cerebral cortex.

• Sulci: Depressions between gyri.

• Longitudinal Fissure: Deep groove dividing the cerebrum into left and right hemispheres.

• Corpus Callosum: A bundle of white matter (myelinated axons) connecting the two hemispheres, allowing communication between them. Found only in placental mammals.

Lobes of the Cerebral Cortex

• Frontal Lobe: Associated with higher cognitive functions (decision making, problem solving), personality, and voluntary movement (primary motor cortex in the precentral gyrus).

• Parietal Lobe: Processes tactile senses (touch, pressure, pain, itch, vibration), proprioception (body position and movement), and visual perception.

• Occipital Lobe: Main visual processing region.

• Temporal Lobe: Processes auditory information and language comprehension.

Subcortical Structures

• Limbic System: Center of emotional and behavioral expression. Includes the amygdala (fear/anxiety, long-term memory), hippocampus (long-term memory), and hypothalamus (memory, emotion, homeostasis).

Diencephalon

The diencephalon is a central relay and regulatory region beneath the cerebrum, composed mainly of the thalamus and hypothalamus.

• Thalamus: Principal relay center for all sensory information (except olfaction), processing and sending it to the cerebrum.

• Hypothalamus: Regulates homeostasis (body temperature, circadian rhythm, food and fluid intake, autonomic nervous system), controls the endocrine system via the anterior pituitary gland, and is involved in emotional responses.

• Olfactory System Exception: The sense of smell bypasses the diencephalon, connecting directly to the cerebrum.

Brain Stem

The brain stem connects the cerebrum and spinal cord, containing major ascending (sensory) and descending (motor) tracts.

• Midbrain: Processes auditory (inferior colliculi) and visual (superior colliculi) information; involved in reflexes. Houses nuclei for cranial nerves III (oculomotor) and IV (trochlear).

• Pons: Main bridge between cerebellum and brain stem; houses nuclei for cranial nerves V (trigeminal), VI (abducens), VII (facial), and VIII (vestibulocochlear).

• Medulla Oblongata: Regulates heart rate, blood pressure, and breathing; houses nuclei for cranial nerves IX (glossopharyngeal), X (vagus), XI (accessory), and XII (hypoglossal).

Cerebellum

The cerebellum, or "little brain," has gyri and sulci like the cerebrum. It integrates motor commands from the cerebrum with proprioceptive and vestibular information to maintain balance and posture.

• Compares intended movement with actual movement and sends corrective signals as needed.

• Alcohol impairs cerebellar function, affecting balance.

Spinal Cord

The spinal cord is the main nerve cord within the vertebral column, composed of grey and white matter.

• Grey Matter: Divided into horns:

  • Anterior Horns: Contain cell bodies of lower motor neurons (skeletal muscle contraction).

  • Posterior Horns: Receive sensory information from muscles, skin, and organs.

  • Lateral Horns: Present only in thoracic and upper lumbar regions; contain visceral motor neuron cell bodies (autonomic nervous system).

• White Matter: Divided into columns:

  • Posterior Columns: Ascending tracts (sensory info to brain).

  • Anterior and Lateral Columns: Both ascending and descending tracts (motor and sensory info).

Meninges and Ventricular System

The meninges are protective connective tissue membranes covering the brain and spinal cord. The ventricular system produces and circulates cerebrospinal fluid (CSF).

• Dura Mater: Tough, outermost layer attached to the inner skull.

• Arachnoid Mater: Web-like middle layer; forms a sac around the CNS. The subarachnoid space contains CSF.

• Pia Mater: Thin, delicate innermost layer lining the brain's surface and sulci.

• Ventricles: Open spaces in the brain where CSF is produced and circulates. Lined with ependymal cells.

• CSF Circulation:

  • Produced by ependymal cells in the choroid plexus via blood filtration.

  • Pathway: Lateral ventricles → third ventricle (via interventricular foramina) → cerebral aqueduct → fourth ventricle → central canal of spinal cord and subarachnoid space (via median and lateral apertures).

  • CSF is reabsorbed into blood via arachnoid granulations.

Peripheral Nervous System (PNS)

Ganglia and Nerves

• Ganglion: A group of neuron cell bodies in the PNS. Types include sensory ganglia (e.g., dorsal root ganglion) and autonomic ganglia.

• Dorsal Root Ganglion: Contains cell bodies of sensory (pseudo-unipolar) neurons transmitting information from the periphery to the CNS.

• Spinal Nerves: 31 pairs, named for their region of emergence from the spinal cord. Each splits into a dorsal (sensory) and ventral (motor) root.

• Nerves: Bundles of axons in the PNS, surrounded by connective tissue layers:

  • Epineurium: Surrounds the entire nerve.

  • Perineurium: Surrounds bundles (fascicles) of axons.

  • Endoneurium: Surrounds individual axons.

Cranial Nerves

There are 12 pairs of cranial nerves (CN I–XII), each with specific sensory and/or motor functions, primarily serving the head and neck.

Number	Name	Type	Main Function(s)
CN I	Olfactory	Sensory	Smell
CN II	Optic	Sensory	Vision
CN III	Oculomotor	Motor	Eye movement, eyelid elevation, pupil constriction
CN IV	Trochlear	Motor	Eye movement
CN V	Trigeminal	Both	Mastication, facial sensation
CN VI	Abducens	Motor	Eye movement
CN VII	Facial	Both	Facial expression, taste, saliva production
CN VIII	Vestibulocochlear	Sensory	Hearing, balance
CN IX	Glossopharyngeal	Both	Swallowing, taste, saliva production
CN X	Vagus	Both	Swallowing, voice, autonomic control of thoracic/abdominal organs
CN XI	Accessory	Motor	Swallowing, head/neck/shoulder movement
CN XII	Hypoglossal	Motor	Tongue movement, speech, swallowing

Key Definitions and Concepts

• Grey Matter: Brain regions with many neuron cell bodies and dendrites.

• White Matter: Brain regions with many myelinated axons (myelin gives the white appearance).

• Proprioception: The sense of body position and movement.

• Contralateral Organization: Many sensory and motor pathways cross over, so the right side of the body is controlled by the left brain and vice versa.

Sample Pathways and Functions

• Ascending Tracts: Carry sensory information from the PNS to the brain.

• Descending Tracts: Carry motor information from the brain to the PNS.

• Corticospinal Tract: Major descending tract for voluntary motor control; upper motor neurons in the brain send signals to lower motor neurons in the spinal cord, which then activate skeletal muscles.

Summary Table: Major Brain Regions and Functions

Region	Main Function(s)
Cerebrum	Higher cognitive functions, voluntary movement, sensory perception
Diencephalon	Sensory relay, homeostasis, endocrine regulation
Brain Stem	Vital functions, cranial nerve nuclei, sensory/motor pathway relay
Cerebellum	Balance, posture, coordination

Additional info:

• The choroid plexus is a network of capillaries in the ventricles that, along with ependymal cells, produces CSF.

• CSF provides cushioning, removes waste, and maintains a stable environment for the CNS.

• Damage to the cerebellum can result in ataxia (loss of coordination).