Anatomy of the Nervous System: Central and Peripheral Components

The Central Nervous System (CNS)

Overview of the CNS

The central nervous system is composed of the brain and spinal cord, serving as the main control center for processing and integrating information in the body.

• Brain: Divided into four major regions: cerebrum, diencephalon, brain stem, and cerebellum.

• Spinal Cord: The main nerve cord within the vertebral column, responsible for transmitting signals between the brain and the peripheral nervous system.

The Brain

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

• Cerebral Cortex: The wrinkled, outer layer composed of grey matter (neuron cell bodies and dendrites).

• Gyri: Ridges on the surface of the cerebral cortex, increasing surface area for processing.

• Sulci: Depressions between gyri.

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

• Corpus Callosum: White matter tract connecting the two hemispheres, allowing communication between them.

Lobes of the Cerebral Cortex

• Frontal Lobe: Associated with higher cognitive functions (decision making, problem solving), personality, intelligence, and voluntary movement (primary motor cortex in 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 for 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 located beneath the cerebrum and is primarily composed of the thalamus and hypothalamus.

• Thalamus: Principal relay center for all sensory information (except olfaction) 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: Olfactory (smell) signals connect directly to the cerebrum, bypassing the diencephalon.

Brain Stem

The brain stem connects the cerebrum with the spinal cord and is composed of the midbrain, pons, and medulla oblongata.

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

• Pons: Main bridge between the 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 and is involved in maintaining balance and posture. It integrates motor commands from the cerebrum with proprioceptive and vestibular information to coordinate movement.

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

• Clinical Note: Alcohol impairs cerebellar function, affecting balance.

Spinal Cord

• Grey Matter: Divided into horns:

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

  • Posterior Horns: Receive sensory information from the body.

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

• White Matter: Divided into columns:

  • Posterior Columns: Ascending sensory tracts.

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

Meninges and Ventricular System

• Meninges: Three connective tissue membranes protecting the CNS:

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

  • Arachnoid Mater: Web-like middle layer; encloses the subarachnoid space filled with cerebrospinal fluid (CSF).

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

• Ventricular System: Network of cavities (ventricles) where CSF is produced and circulates.

  • Two lateral ventricles (one in each hemisphere) connect to the third ventricle (in the diencephalon) via interventricular foramina.

  • The third ventricle connects to the fourth ventricle (posterior to pons/medulla, anterior to cerebellum) via the cerebral aqueduct.

  • CSF is produced by ependymal cells lining the ventricles and circulates through the ventricular system and subarachnoid space.

  • Arachnoid granulations absorb CSF into the blood, removing metabolic waste.

The Peripheral Nervous System (PNS)

Overview of the PNS

The peripheral nervous system includes all neural structures outside the CNS, such as nerves and ganglia. It connects the CNS to limbs and organs.

• Ganglion: Group of neuron cell bodies in the PNS. Types include sensory ganglia (e.g., dorsal root ganglia) 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 the region of the spinal cord from which they emerge. Each splits into a dorsal (sensory) and ventral (motor) root near the spinal cord.

• 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), primarily responsible for sensory and motor functions of 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, speech, 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: Sensing body position and movement.

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

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

• 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 Questions and Answers

• What composes the nervous system? The brain and spinal cord (CNS), and the peripheral nervous system (PNS).

• What is the primary pathway for communication between the right and left cerebral hemispheres? The corpus callosum.

• Name the four lobes of the cerebral cortex: Frontal, parietal, occipital, temporal.

• What is proprioception? Sensing body position and movement.

• List two functions of the hypothalamus: Regulates homeostasis (e.g., body temperature, circadian rhythm), controls the endocrine system via the anterior pituitary gland.

• What is the one exception to signals passing through the diencephalon to reach the cerebrum? Olfactory (smell) signals connect directly to the cerebrum.

Additional info:

• The corticospinal tract is a major descending pathway for voluntary motor control, with most fibers crossing to the contralateral side in the medulla (pyramidal decussation).

• Cerebrospinal fluid (CSF) provides cushioning, removes waste, and circulates nutrients throughout the CNS.