General Characteristics of the Central Nervous System (CNS)

Overview

The CNS acts as the main processing center for the entire nervous system, functioning like an information funnel that receives, integrates, and sends out neural signals.

• Groups of cell bodies in CNS: Called nuclei.

• Bundles of axons in CNS: Called tracts.

• Types of tissue:

  • White matter: Myelinated axons.

  • Gray matter: Unmyelinated axons and cell bodies.

Brain: Structure and Complexity

Key Features

• The brain is the largest and most complex part of the nervous system.

• Contains approximately 86 billion multipolar neurons.

• Each neuron can form up to 10,000 synaptic connections with other neurons, resulting in up to 1,000 trillion synapses.

• Male brains are about 10% larger than female brains, but this is due to body size, not intelligence.

• The brain accounts for about 2% of total body weight (~3 lbs in adults).

Development of the Brain

Embryological Origins

• The CNS develops from the neural plate, which folds to form the neural groove and then the neural tube.

• The lumen (interior) of the neural tube is called the neurocoel.

Primary Brain Vesicles

• Three primary vesicles form:

  • Prosencephalon – "forebrain"

  • Mesencephalon – "midbrain"

  • Rhombencephalon – "hindbrain" (continuous with spinal cord)

Secondary Brain Vesicles

• Prosencephalon divides into:

  • Telencephalon

  • Diencephalon

• Mesencephalon remains undivided.

• Rhombencephalon divides into:

  • Metencephalon

  • Myelencephalon

Formation of Ventricles and Brain Regions

• The five cavities formed become the ventricles of the brain and the tubes that connect them.

• Cells surrounding these spaces differentiate into major brain regions:

  • Telencephalon → Cerebrum (eventually enlarges and covers other regions)

  • Diencephalon → Major processing center for information heading to cerebrum

  • Mesencephalon → Midbrain

  • Metencephalon → Pons and cerebellum

  • Myelencephalon → Medulla oblongata

• Brain stem is formed by the midbrain, pons, and medulla oblongata, connecting the brain to the spinal cord.

Major Regions of the Brain

• Cerebrum

• Diencephalon

• Mesencephalon (Midbrain)

• Pons

• Medulla oblongata

• Cerebellum

Cerebrum

Structure

• Largest part of the brain, divided into two hemispheres.

• Cerebral cortex: Thin layer of gray matter covering the cerebrum.

• Underlying white matter: Myelinated axons connecting cell bodies of cortex to other parts of the nervous system.

• Contains subcortical nuclei.

Landmarks

• Fissures: Deep grooves (e.g., longitudinal, lateral, transverse fissures).

• Gyri: Folds that increase surface area ("bumps" or "hills").

  • Examples: precentral and postcentral gyri

• Sulci: Shallow depressions that separate the gyri.

  • Examples: central sulcus, parieto-occipital sulcus

Lobes of the Cerebrum

• Frontal lobe: Contains somatic motor association area.

• Parietal lobe: Contains primary sensory cortex.

• Temporal lobe: Contains primary auditory cortex and auditory association area.

• Occipital lobe: Contains primary visual cortex and visual association areas.

• Insula: Lobe deep to the lateral sulcus of each hemisphere ("island of cortex").

Cerebral Cortex

Functions

• Responsible for higher brain functions:

  • Conscious thought

  • Memory storage and processing

  • Sensory processing

  • Regulation of skeletal muscle contractions

Lobe Functions

General Principles

• Lobes have specific functions, with primary cortices and association areas.

• Boundaries between lobes are not precise.

• Normal individuals use all portions of the brain.

• There are important differences between hemispheres.

• Integration areas coordinate information from multiple sources.

Frontal Lobe

• Primary motor cortex: Generates neural impulses that control the execution of movement.

• Somatic motor association area: Coordinates learned movement.

• Executive function areas:

  • Higher intellectual processes (concentration, planning, problem solving).

  • Prefrontal cortex: Controls emotional behavior, produces awareness of consequences.

  • Broca's area: Speech production.

Parietal Lobe

• Primary sensory cortex: Receives input from touch, pressure, pain, vibration, and temperature receptors.

• Somatic sensory association area: Monitors activity in the primary sensory cortex and brings certain sensations to awareness.

• Additional association areas:

  • Aid in understanding speech and choosing words to express thoughts and feelings.

  • Awareness of the form of objects.

Temporal Lobe

• Primary auditory cortex: Receives auditory information.

• Primary olfactory cortex: Receives olfactory (smell) information.

• Auditory association area: Monitors activity and recognizes patterns, allowing recognition of words and understanding of speech.

  • Contains part of Wernicke's area (other part in parietal lobe).

• Stores memories of visual scenes, music, and other complex sensory patterns.

Occipital Lobe

• Primary visual cortex: Receives information from the lateral geniculate nucleus (visual information).

• Visual association area: Recognizes patterns of activity and interprets results, allowing us to see the "whole picture" and recognize what it means. Also involved in facial recognition.

Integrative Centers

• Prefrontal cortex: Coordinates information relayed from other association areas.

• Frontal eye field: Controls learned eye movements (e.g., side-to-side movements for reading a line of text).

General Interpretive Area

• Wernicke's area:

  • Receives information from auditory association areas.

  • Usually present in only one hemisphere (typically the left).

  • Allows recognition and understanding of speech.

  • Enables selection of appropriate words.

  • Damage leads to Wernicke's aphasia: person can speak but cannot control the content of speech.

Speech Center

• Broca's area:

  • Motor speech area, usually in the same hemisphere as Wernicke's area.

  • Involved in speech production.

Additional info:

• The lateralization of function means that certain cognitive processes are more dominant in one hemisphere than the other (e.g., language in the left hemisphere for most people).

• Association areas integrate sensory and motor information, contributing to complex functions such as learning, memory, and reasoning.