Central Nervous System: Structure and Function (Chapter 12, Part A)

The Central Nervous System

Overview and Organization

The central nervous system (CNS) consists of the brain and spinal cord, serving as the primary control center for the body. It integrates sensory information, coordinates motor output, and is responsible for higher mental functions.

• Cephalization: Evolutionary process resulting in increased complexity and neuron number in the anterior CNS (head region).

• Main regions of the adult brain:

  1. Cerebral hemispheres

  2. Diencephalon

  3. Brain stem (midbrain, pons, medulla oblongata)

  4. Cerebellum

Brain Regions and Their Functions

Cerebral Hemispheres

The cerebral hemispheres are the largest part of the brain, responsible for conscious thought, voluntary movement, and sensory perception.

• Cerebral cortex: The outer layer, known as the "executive suite" of the brain, is the site of awareness, memory, communication, and understanding.

• Gray matter: Contains neuron cell bodies, dendrites, and glial cells; responsible for processing information.

• White matter: Composed of myelinated axons; facilitates communication between different brain regions.

Brain Lobes and Their Functions

The brain is divided into distinct lobes, each with specialized functions.

• Frontal lobe: Thinking, problem solving, voluntary movement, behavioral control, decision making.

• Parietal lobe: Sensory perception, object classification, spelling, spatial processing.

• Temporal lobe: Hearing, language, memory.

• Occipital lobe: Vision, color identification.

• Cerebellum: Balance and coordination.

• Brain stem: Involuntary functions such as breathing, heart rate, and swallowing.

Surface Markings of the Cerebral Hemispheres

• Gyri: Ridges on the brain surface.

• Sulci: Shallow grooves between gyri.

• Fissures: Deep grooves separating major brain regions.

• Longitudinal fissure: Separates the two hemispheres.

• Transverse cerebral fissure: Separates cerebrum from cerebellum.

Ventricles of the Brain

Ventricles are fluid-filled chambers within the brain, continuous with the central canal of the spinal cord.

• Filled with cerebrospinal fluid (CSF), which cushions and nourishes the CNS.

• Lined by ependymal cells (a type of neuroglia).

Cerebral Cortex: Structure and Function

General Considerations

The cerebral cortex is a thin (2–4 mm) layer of gray matter covering the cerebral hemispheres, accounting for 40% of brain mass.

• Contains three types of functional areas:

  • Motor areas: Control voluntary movement.

  • Sensory areas: Conscious awareness of sensation.

  • Association areas: Integrate diverse information.

• Each hemisphere controls the contralateral (opposite) side of the body.

• Lateralization: Specialization of cortical functions in one hemisphere.

• Conscious behavior involves the entire cortex.

Motor Areas

• Primary motor cortex: Planning and executing voluntary movements.

• Premotor cortex: Controls learned, repetitive, or patterned motor skills.

• Broca's area: Speech and language production.

• Frontal eye field: Controls voluntary eye movements.

Sensory Areas

• Primary somatosensory cortex: Receives information from general sensory receptors and proprioceptors.

• Somatosensory association cortex: Integrates sensory inputs to produce understanding of objects (size, texture, relationship of parts).

• Visual areas: Receives visual information from the retina.

• Auditory areas: Interprets impulses from the ear as pitch, loudness, and location.

• Vestibular cortex: Conscious awareness of balance.

• Olfactory cortex: Conscious awareness of odors.

• Gustatory cortex: Perceives taste stimuli.

• Visceral sensory area: Conscious perception of visceral sensations (e.g., upset stomach, full bladder).

Multimodal Association Areas

These areas allow us to give meaning to information, store it in memory, relate it to previous experience, and decide on actions.

Cerebral Lateralization

Functional specialization occurs between the left and right hemispheres.

• Left hemisphere: Language, math, logic.

• Right hemisphere: Visual-spatial skills, intuition, emotion, artistic, musical abilities.

Diencephalon

Structure and Components

The diencephalon consists of three paired gray-matter structures: thalamus, hypothalamus, and epithalamus. All three enclose the third ventricle.

Region	Function
Cerebral Hemispheres	Cortical gray matter: Localizes and interprets sensory inputs, controls voluntary movement, and is involved in intellectual and emotional processing. Basal nuclei: Subcortical motor centers, help control skeletal muscle movements.
Thalamus	Relay station for sensory impulses to cerebral cortex. Sorts, edits, and relays ascending input (emotion, motor control, memory). Mediates sensation, motor activities, cortical arousal, learning, and memory.
Hypothalamus	Main visceral control center, vital to homeostasis. Controls autonomic nervous system (blood pressure, heart rate, digestive tract, pupil size). Initiates physical responses to emotions (pleasure, fear, rage, biological rhythms, drives). Regulates body temperature, hunger, satiety, water balance, thirst, sleep-wake cycles. Controls endocrine system (pituitary gland secretions).
Epithalamus	Forms roof of third ventricle. Contains pineal gland, which secretes melatonin to regulate sleep-wake cycles.
Limbic System	Functional system including cerebral and diencephalon structures. Puts emotional responses to odors, involved in memory processing.

Brain Stem

Structure and Function

The brain stem consists of the midbrain, pons, and medulla oblongata. It controls automatic behaviors necessary for survival and connects higher and lower neural centers.

• Midbrain: Contains nuclei and fiber tracts.

• Pons: Located between midbrain and medulla; some nuclei help maintain normal rhythm of breathing.

• Medulla oblongata: Autonomic reflex center; overlaps with hypothalamus functions.

Medulla Oblongata Functional Centers

• Cardiovascular center:

  • Cardiac center: Adjusts force and rate of heart contraction.

  • Vasomotor center: Adjusts blood vessel diameter for blood pressure regulation.

• Respiratory centers: Generate respiratory rhythm, control rate and depth of breathing (with pontine centers).

• Other centers: Regulate vomiting, hiccupping, swallowing, coughing, sneezing.

Cerebellum

Structure and Function

The cerebellum is located dorsal to the pons and medulla. It processes input from the cortex, brain stem, and sensory receptors to provide precise, coordinated movements of skeletal muscles and plays a major role in balance.

The Limbic System

Function and Connections

The limbic system is a functional system that puts emotional responses to odors and is involved in memory processing. Most output is relayed via the hypothalamus, which plays a role in psychosomatic illnesses. The limbic system interacts with prefrontal lobes, allowing emotional reactions to conscious events.

Higher Mental Functions

Language

• Broca's area: Involved in speech production. Lesions cause inability to speak, though comprehension remains.

• Wernicke's area: Involved in understanding spoken and written words. Lesions cause nonsensical speech, though speaking ability remains.

Memory

• Declarative memory: Facts (names, faces, words, dates).

• Procedural memory: Skills (e.g., playing piano).

• Motor memory: Motor skills (e.g., riding a bike).

• Emotional memory: Experiences linked to emotion (e.g., heart pounding when hearing a rattlesnake).

Types of Memory

• Short-term memory (STM): Temporary holding of information; limited capacity.

• Long-term memory (LTM): Limitless capacity for information storage.

Consciousness

• Involves perception of sensation, voluntary initiation and control of movement, and higher mental processing (memory, logic, judgment).

• Clinically defined on a continuum: alertness, drowsiness (lethargy), stupor, and coma.

Pattern of Distribution of Gray and White Matter in the CNS

Gray matter consists of short, nonmyelinated axons and neuron cell bodies, while white matter consists of myelinated axons with some nonmyelinated axons, primarily in fiber tracts. Myelin gives white matter its color.

Summary Table: Major Brain Regions and Functions

Region	Main Function
Cerebral Cortex	Conscious thought, voluntary movement, sensory perception
Diencephalon	Relay and integration (thalamus), homeostasis (hypothalamus), sleep-wake regulation (epithalamus)
Brain Stem	Autonomic functions, cranial nerve nuclei, survival behaviors
Cerebellum	Coordination of movement, balance

Key Terms and Definitions

• Cephalization: Evolutionary development of a head region with concentrated nervous tissue.

• Gyri: Elevated ridges on the brain surface.

• Sulci: Shallow grooves between gyri.

• Fissures: Deep grooves separating major brain regions.

• Gray matter: Neuron cell bodies and dendrites; site of processing.

• White matter: Myelinated axons; site of communication.

• Ventricles: Fluid-filled chambers in the brain.

• Contralateral: Opposite side control (e.g., left hemisphere controls right side of body).

• Lateralization: Specialization of function in one hemisphere.

• Association areas: Regions integrating information from multiple sources.

• Homeostasis: Maintenance of stable internal environment.

• Melatonin: Hormone regulating sleep-wake cycles.

Additional info: These notes expand on the provided slides by integrating definitions, examples, and summary tables for clarity and completeness.