The Central Nervous System: Structure and Function

The Central Nervous System (CNS)

Overview and Major Functions

The central nervous system (CNS) consists of the brain and spinal cord. It is responsible for integrating sensory information, coordinating movement, maintaining homeostasis, and supporting higher mental functions such as language, learning, and memory.

• Motor functions: Initiate muscle contraction and gland secretion (in conjunction with the peripheral nervous system, PNS).

• Sensory functions: Interpret sensations from inside and outside the body (PNS input, CNS processing).

• Integrative functions: Decision-making, planning, and monitoring movement, as well as maintaining homeostasis and higher mental activities.

Basic Structure of the Brain and Spinal Cord

Brain Divisions and Their Functions

The brain is a soft, whitish-gray organ located in the cranial cavity and is continuous with the spinal cord. It is composed mostly of nervous tissue, with some epithelial and connective tissue. The brain is divided into four main regions, each with specialized functions:

Brain Division	Function
Cerebrum	Performs higher mental functions, interprets sensory stimuli, plans and initiates movement
Diencephalon	Processes, integrates, and relays information; maintains homeostasis; regulates biological rhythms
Cerebellum	Monitors and coordinates movement
Brainstem	Maintains homeostasis, controls certain reflexes, monitors movement, integrates and relays information

White and Gray Matter

Both the brain and spinal cord contain white and gray matter:

• White matter: Composed of myelinated axons; forms tracts that carry information.

• Gray matter: Contains cell bodies, dendrites, and unmyelinated axons; forms nuclei and the cerebral cortex.

The Cerebrum

Surface Features: Gyri and Sulci

The surface of the cerebrum is highly folded, increasing its surface area. The folds are called gyri (ridges) and sulci (shallow grooves).

Lobes and Major Landmarks

Each cerebral hemisphere is divided into five lobes:

• Frontal lobe: Motor functions, complex mental functions

• Parietal lobe: Sensory processing

• Temporal lobe: Hearing

• Occipital lobe: Vision

• Insula: Taste and visceral sensation

Major fissures and gyri include the longitudinal fissure (separates hemispheres), central sulcus, precentral gyrus (primary motor cortex), and postcentral gyrus (primary somatosensory cortex).

Cerebral Cortex: Functional Areas

The cerebral cortex is the outer layer of gray matter covering the cerebral hemispheres. It is responsible for conscious processes such as planning movement, interpreting sensory information, and higher cognitive functions. The cortex is divided into three main functional areas:

• Motor areas: Control voluntary movement (e.g., primary motor cortex in the precentral gyrus).

• Sensory areas: Receive and process sensory input (e.g., primary somatosensory cortex in the postcentral gyrus, visual cortex in the occipital lobe).

• Association areas: Integrate information for complex functions (e.g., prefrontal cortex for behavior and personality, Broca's and Wernicke's areas for language).

Area Name	Type of Cortex	Location	Function
Primary motor cortex	Motor	Precentral gyrus of frontal lobe	Plans and executes movement
Premotor cortex	Unimodal association	Widespread throughout lateral and medial frontal lobe	Plans and executes complex movement
Frontal eye fields	Unimodal association	Anterior to the premotor cortex	Back-and-forth eye movements
Primary somatosensory cortex	Sensory	Postcentral gyrus of parietal lobe	Interprets incoming somatic sensory information
Somatosensory association cortex	Unimodal association	Posterior to the primary somatosensory cortex in the parietal lobe	Integrates somatic sensory information
Primary visual cortex	Sensory	Occipital lobe	Interprets and processes visual stimuli
Visual association area	Unimodal association	Occipital lobe	Processes visual stimuli
Primary auditory cortex	Sensory	Superior temporal lobe	Processes auditory stimuli
Auditory association cortex	Unimodal association	Superior temporal lobe	Processes auditory stimuli
Vestibular areas	Sensory	Parietal and temporal lobes	Processes stimuli relating to equilibrium and balance
Olfactory cortex	Sensory	Temporal lobe	Processes smell stimuli
Broca's area	Multimodal association	Superolateral frontal lobe	Language production
Wernicke's area	Multimodal association	Superolateral temporal lobe	Language comprehension
Prefrontal cortex	Multimodal association	Anterior frontal lobe	Planning, personality, higher cognitive functions
Parietal association cortex	Multimodal association	Widespread in the cerebral lobe	Spatial awareness and attention
Temporal association cortex	Multimodal association	Widespread in the temporal lobe	Recognition and association

Basal Nuclei and Limbic System

Basal nuclei are masses of gray matter deep within each hemisphere, involved in movement regulation and inhibition of involuntary movements. The limbic system includes the limbic lobe, hippocampus, and amygdala, and is involved in memory, learning, emotion, and behavior.

The Diencephalon

Major Components and Functions

The diencephalon is located centrally in the brain, between the hemispheres and above the brainstem. It consists of four main parts:

• Thalamus: Gateway for sensory information to the cerebral cortex (except smell).

• Hypothalamus: Regulates the autonomic nervous system, sleep/wake cycle, thirst, hunger, and body temperature; secretes hormones that regulate the pituitary gland.

• Epithalamus: Contains the pineal gland, which secretes melatonin for sleep/wake regulation.

• Subthalamus: Functionally connected with basal nuclei to control movement.

The Cerebellum

Structure and Function

The cerebellum is located inferior to the occipital lobe and is responsible for coordinating voluntary movement and maintaining balance. Its white matter forms the arbor vitae, a tree-like structure visible in section.

The Brainstem

Major Regions and Functions

The brainstem is one of the oldest parts of the brain and is vital for survival. It includes the midbrain, pons, and medulla oblongata, and is responsible for basic involuntary processes such as breathing, heart rate, and reflexes.

• Midbrain: Involved in visual and auditory reflexes; contains the substantia nigra, which produces dopamine and works with basal nuclei to control movement.

• Pons: Regulates movement, breathing, reflexes, and sleep/arousal functions.

• Medulla oblongata: Controls vital functions such as breathing and heart rate.

Protection of the Brain and Spinal Cord

Meninges, CSF, and Blood-Brain Barrier

The CNS is protected by three main features:

• Cranial meninges: Three layers of membranes (dura mater, arachnoid mater, pia mater) surround the brain and spinal cord.

• Cerebrospinal fluid (CSF): Bathes the brain and spinal cord, cushions them, and helps maintain temperature and remove wastes.

• Blood-brain barrier: Prevents many substances from entering the brain from the blood.

Ventricles and CSF Circulation

The brain contains four ventricles (lateral, third, and fourth) filled with CSF, which is produced by the choroid plexuses and reabsorbed by arachnoid villi. CSF circulates through the ventricles and central canal of the spinal cord.

Spinal Cord Structure and Protection

Spinal Meninges and Spaces

The spinal cord is protected by meninges similar to those of the brain. Key spaces include:

• Epidural space: Between dura mater and vertebral wall; contains veins and adipose tissue.

• Subarachnoid space: Between arachnoid and pia mater; filled with CSF.

Summary Table: Major Brain Structures and Functions

Division	Main Function
Cerebellum	Coordinates ongoing voluntary movement to reduce motor error
Brainstem: Midbrain	Processes and routes visual and auditory stimuli to the thalamus; carries motor fibers from the cerebral cortex; monitors movement with the basal nuclei; mediates reflexes
Brainstem: Pons	Regulates movement, breathing, reflexes, and sleep/wake cycle
Brainstem: Medulla oblongata	Regulates many autonomic functions
Brainstem: Reticular formation	Involved in sleep and arousal, pain transmission, and mood regulation
All regions	Contain tracts of white matter involved in movement and sensation

Additional info: The CNS is essential for integrating sensory input, coordinating voluntary and involuntary actions, and supporting cognition, emotion, and memory. Damage to any part of the CNS can result in significant functional deficits, highlighting the importance of its protection and maintenance.