Chapter 12: The Central Nervous System (CNS) – Structure and Functional Organization

The Central Nervous System (CNS)

General Organization of the CNS

The central nervous system (CNS) consists of the brain and spinal cord, serving as the main control center for processing and integrating information throughout the body.

• Brain: Responsible for higher functions such as sensation, communication, memory, and voluntary movement.

• Spinal Cord: Conducts sensory and motor information between the brain and the rest of the body.

Tissue Organization of the CNS

The CNS is organized into distinct regions of gray and white matter, each with specific roles in neural processing.

• Spinal Cord:

  • Central cavity rimmed by a core of gray matter (cell bodies & dendrites).

  • External layer of white matter (myelinated nerve axons).

• Brain:

  • Central cavity expands to form ventricles.

  • Gray matter surrounds ventricles and forms the cortex on the outer surface of the cerebrum and cerebellum.

  • Nuclei (clusters of neuron cell bodies) are scattered within the white matter.

Ventricles of the Brain

The brain contains interconnected cavities called ventricles, filled with cerebrospinal fluid (CSF) that cushions and nourishes neural tissue.

• Lateral ventricles: Located in each cerebral hemisphere.

• Third ventricle: Located in the diencephalon.

• Fourth ventricle: Located between the brainstem and cerebellum.

Major Anatomic Regions of the Brain

Cerebrum

The cerebrum is the largest part of the brain and serves as the main integrative center for sensation, communication, memory, understanding, and voluntary movements.

• Gyri: Elevated ridges on the surface of the cerebrum.

• Sulci & Fissures: Grooves that divide the cerebrum into lobes.

• Lobes of the Cerebrum:

  • Frontal lobe: Motor control, problem-solving, speech production.

  • Parietal lobe: Sensory perception, spatial awareness.

  • Occipital lobe: Visual processing.

  • Temporal lobe: Auditory processing, memory.

  • Insula: Taste, visceral sensation, equilibrium.

Diencephalon

The diencephalon is located deep within the brain and includes:

• Thalamus: Relay station for sensory information.

• Hypothalamus: Homeostasis, endocrine regulation.

• Epithalamus: Contains the pineal gland, regulates circadian rhythms.

Brainstem

The brainstem connects the brain to the spinal cord and controls vital functions.

• Midbrain: Visual and auditory reflexes.

• Pons: Relays information, regulates breathing.

• Medulla oblongata: Controls heart rate, respiration, and other autonomic functions.

Cerebellum

The cerebellum coordinates voluntary movements, balance, and posture.

Cerebral Cortex and Hemispheres

Cerebral Cortex

The cerebral cortex is the superficial layer of gray matter covering the cerebrum, responsible for conscious thought and processing.

• Contains specialized areas for motor, sensory, and association functions.

Cerebral Hemispheres

Each hemisphere of the cerebrum controls the opposite side of the body (contralateral control), and they are not functionally identical (hemispheric lateralization).

• No functional area acts alone; conscious behavior involves the entire cortex.

• Some functions are localized, but most require cooperation among multiple areas.

Functional Areas of the Cerebral Cortex

Motor Areas

Motor areas control voluntary movement and are primarily located in the frontal lobe.

• Primary motor cortex: Located in the precentral gyrus; controls voluntary muscle movement.

• Pyramidal cells: Neurons whose axons form the corticospinal (pyramidal) tract for contralateral control.

• Motor homunculus: Map showing the body regions controlled by specific areas of the motor cortex.

• Premotor cortex: Plans complex learned motor skills and coordinates sequential actions.

• Broca's area: Motor speech area, controls muscles of the tongue; usually in the left hemisphere.

Sensory Areas

Sensory areas provide conscious awareness of sensation and are located in various lobes.

• Primary somatosensory cortex: Located in the postcentral gyrus of the parietal lobe; receives general sensory input and proprioception.

• Somatosensory association cortex: Integrates sensory information to determine size, texture, and relationship of parts; forms comprehensive understanding of stimuli.

• Primary visual cortex: Located at the posterior tip of the occipital lobe; receives visual information from the retinas.

• Visual association area: Surrounds the primary visual cortex; interprets visual stimuli (color, form, movement).

• Primary auditory cortex: Located at the superior margin of the temporal lobe; perceives sound (pitch, rhythm, loudness).

• Auditory association area: Posterior to the primary auditory cortex; stores memories of sounds and permits identification.

• Other sensory areas:

  • Olfactory cortex: Smell (medial side of temporal lobe).

  • Gustatory cortex: Taste (insula).

  • Vestibular cortex: Equilibrium/balance (insula).

  • Visceral sensory area: Visceral sensation (insula).

Association Areas

Association areas integrate diverse information, interpret incoming data, and coordinate motor responses. They are involved in intellect, emotion, memory, and higher cognitive functions.

• Specific sensory association areas: Interpret sensory input for each modality.

• Multimodal association areas: Receive widespread sensory input and influence multiple cortical regions.

Multimodal Association Areas

Overview

Multimodal association areas (integrative centers) receive input from multiple senses and send output to various regions, integrating sensory input to give meaning, store memory, and generate appropriate responses.

• Conscious awareness of emotions and thoughts (perception) occurs here.

• Three main regions:

  • Anterior Association Area (Prefrontal cortex): Abstract intellectual functions, judgement, planning, reasoning, personality traits; linked to the limbic system for emotional responses.

  • Posterior Association Area: Includes temporal, parietal, and occipital lobes; facial and pattern recognition, binding sensory inputs, language comprehension (Wernicke's area), spatial localization.

  • Limbic Association Area: Medial aspect of cerebral hemispheres and diencephalon; includes cingulate gyrus, parahippocampal gyrus, hippocampus, thalamus, hypothalamus; produces emotional responses and links emotions to memories.

Language Areas

Language areas are located in a large region surrounding the lateral sulcus in the language-dominant hemisphere and require integration of multiple brain regions.

• Wernicke's area: Sounding out unfamiliar words; integrates visual and auditory memories.

• Broca's area: Speech preparation and production; controls muscles of the larynx, pharynx, and diaphragm.

• Lateral prefrontal cortex: Language comprehension and word analysis.

• Lateral and ventral temporal lobe: Coordinate auditory and visual aspects of language.

Table: Major Brain Regions and Their Functions

Region	Main Function	Key Structures
Cerebrum	Integration, sensation, voluntary movement, cognition	Frontal, parietal, occipital, temporal lobes, insula
Diencephalon	Sensory relay, homeostasis, circadian rhythms	Thalamus, hypothalamus, epithalamus
Brainstem	Autonomic functions, reflexes	Midbrain, pons, medulla oblongata
Cerebellum	Coordination, balance, posture	Cerebellar hemispheres, cortex

Key Terms and Concepts

• Gray matter: Contains neuron cell bodies and dendrites; site of synaptic integration.

• White matter: Composed of myelinated axons; transmits signals between regions.

• Contralateral control: Each hemisphere controls the opposite side of the body.

• Hemispheric lateralization: Functional differences between the left and right hemispheres.

• Homunculus: Map of body regions represented in the cortex.

Example: Motor Homunculus

The motor homunculus is a visual representation of the body parts controlled by the primary motor cortex. Larger areas are devoted to regions requiring fine motor control, such as the hands and face.

Additional info:

• The organization of the CNS is essential for understanding neurological diseases and injuries, as damage to specific regions leads to predictable deficits.

• Functional imaging (e.g., fMRI) is used to study activity in different cortical areas during various tasks.