Central Nervous System (CNS) Overview

Central Nervous System (CNS)

• Comprises the brain and spinal cord.

• Structure:

  • CNS = Brain + Spinal Cord

Organization of the CNS

White Matter

• Definition: Myelinated axons which are grouped into tracts.

• Functions:

  • Connects different brain regions.

  • Ascends and descends the spinal cord.

  • Carries sensory or motor impulses.

Gray Matter

• Definition: Contains cell bodies and dendrites (nuclei) along with unmyelinated axons and neuroglia.

CNS Structure Variations

• Figure 12.2: Illustrates the arrangement of white and gray matter in the CNS.

• Brain Structure:

  • Gray matter is surrounded by white matter which is then surrounded again by gray matter.

• Spinal Cord Structure:

  • The gray matter is centrally located and is surrounded by white matter.

Brain

Overview

• Consists of several regions:

  • Diencephalon

  • Cerebrum

  • Cerebellum

  • Brain stem (including midbrain, pons, medulla oblongata)

• Contains billions of neurons and neuroglial cells.

• Different regions specialized for varying functions and often work in conjunction.

Components of the Brain

Cerebrum

• Percentage of brain mass: 83%.

• Divided into:

  • 2 hemispheres

  • 5 lobes

  • Contains both white and gray regions:

  • Inner Gray: Basal nuclei, which are islands of gray matter.

  • Middle White Matter: Myelinated axon tracts.

  • Outer Gray: Cerebral cortex containing functional areas.

Gross Anatomical Features

• Terms:

  • Gyrus: An elevation.

  • Sulcus: A groove.

  • Fissure: A deep groove.

• Cerebrum Lobes:

  • Frontal

  • Parietal

  • Temporal

  • Occipital

  • Insula (located beneath the temporal lobe)

Structure of Lobes

• Cerebrum Lobes Overview:

  • Frontal: Involved in cognitive functions and voluntary movement.

  • Parietal: Processes sensory information.

  • Temporal: Processes auditory information and memory.

  • Occipital: Responsible for visual processing.

Sulci and Fissures of the Cerebrum

• Key Structures:

  • Precentral gyrus: Motor control area.

  • Postcentral gyrus: Sensory area.

  • Central sulcus: Divides frontal and parietal lobes.

  • Parieto-occipital sulcus: Separates parietal and occipital lobes.

  • Lateral fissure: Separates the temporal lobe from the frontal and parietal lobes.

Basal Nuclei

• Components:

  • Caudate nucleus

  • Putamen

  • Globus pallidus

• Function: Movement regulation.

• Relationship to other structures: Lies deep within the cerebral cortex, contributing to motor control.

White Matter of the Cerebrum

• Components:

  • Commissural fibers: Connect corresponding gray areas of both hemispheres (example: corpus callosum).

  • Association fibers: Connect different parts of the same hemisphere.

  • Projection fibers: Connect lower brain areas and the spinal cord to the cerebral hemispheres.

Functional Areas of the Cerebral Cortex

1. Motor Areas:

   • Controls voluntary muscle movements.

   • Includes the primary motor cortex and premotor cortex.

2. Sensory Areas:

   • Awareness and processing of sensory information.

   • Examples include the primary somatosensory cortex and primary visual cortex.

3. Association Areas:

   • Involved in complex cortical functions such as memory, emotions, reasoning, and problem-solving.

Properties of the Cerebrum

• Contralateral Processing: Each hemisphere processes sensory information from the opposite side of the body.

• Lateralization of Function:

  • Left Hemisphere: Controls language, math, and logic-related tasks.

  • Right Hemisphere: Associated with visual-spatial skills, emotions, and artistic expression.

Sensory and Motor Maps

• Motor Cortex:

  • More brain tissue corresponds to body regions that require precise motor control (e.g. face, hands).

• Sensory Cortex:

  • Greater sensitivity in body regions with a higher density of sensory receptors.

Cerebral Cortex Structures

Motor Areas

• Primary Motor Cortex: Controls conscious movements of skeletal muscle.

• Premotor Cortex: Manages learned motor skills and patterns.

• Frontal Eye Field: Coordinates eye movements.

• Broca’s Area: Motor area involved in speech production.

Sensory Areas

• Primary Somatosensory Cortex: Processes information from somatic and proprioceptors.

• Function: Determines spatial discrimination and origin of sensations.

• Special Senses: Includes areas for visual, auditory, olfactory, gustatory, and vestibular functions.

Association Areas of the Cerebral Cortex

• Functions:

  • Involvement in memory, emotions, reasoning, and decision-making.

  • Examples of Key Areas:

  • Prefrontal cortex: Higher cognitive functions, planning, personality.

  • Somatosensory association cortex: Integrates sensory stimuli.

  • Visual and Auditory Association Areas: Processing and interpretation of visual and auditory information.

The Cerebrum - Gray Matter

Table of Motor, Sensory, and Association Areas of the Cerebral Cortex

Cerebellum

• Comprises 11% of brain mass.

• Contains both gray and white areas:

  • Gray Matter: Cerebellar cortex (characterized by folds, or folia).

  • White Matter: Arbor vitae, resembling a tree structure.

• Function: Regulates posture and balance and coordinates complex movements.

• Receives input from proprioceptors, visual receptors, equilibrium receptors, and motor information from the primary motor cortex.

Diencephalon

Regions

• Thalamus: Acts as a relay station, synapsing afferent (sensory) impulses to the cerebral cortex and efferent (motor) impulses from the cerebral cortex to the spinal cord.

• Hypothalamus: Regulates ANS, emotional responses, body temperature, sleeping patterns, food intake, water balance, and controls the endocrine system via the pituitary gland.

• Epithalamus: Houses the pineal gland, which secretes melatonin influencing sleep-wake cycles.

Nuclei of the Diencephalon

• Thalamic Nuclei: Various specific nuclei involved in sensory processing, motor relay, and limbic functions.

• Hypothalamic Nuclei: Nuclei affecting numerous autonomic functions and maintaining homeostasis.

Brain Stem

Structure

• Divided into:

  • Midbrain

  • Pons

  • Medulla Oblongata

• Functions include control of vital homeostatic functions, reflex responses, and regulating alertness.

Midbrain

• Contains superior and inferior colliculi (responsible for visual and auditory reflexes) and substantia nigra (dopaminergic neurons).

Pons

• Houses pontine nuclei regulating various functions connected to movement and sleep.

Medulla Oblongata

• Contains the reticular formation for consciousness and regulates essential autonomic functions (like heart rate and breathing).

• Notably, the crossing of fiber tracts occurs here for motor control.

Protection of the CNS

Structures

• Bones of the Skull: Provide structural protection.

• Meninges: Three connective tissue membranes:

  • Dura Mater: Outermost, tough layer.

  • Arachnoid Mater: Middle layer with a web-like structure.

  • Pia Mater: Innermost, delicate layer directly covering the CNS.

• Cerebrospinal Fluid (CSF): Found in the ventricles and subarachnoid space, providing a liquid cushion and optimal chemical environment.

• Blood-Brain Barrier: A selective barrier preventing potentially harmful substances in the bloodstream from entering the brain and spinal cord tissues.

Meningitis

• Inflammation of the meninges which can be life-threatening, caused by various infectious agents (bacterial or viral).

Cerebrospinal Fluid (CSF)

Formation and Flow

• Choroid Plexuses: Structures that produce CSF.

• Function:

  • Acts as a shock absorber for the brain.

  • Maintains optimal chemical environment.

  • Circulates nutrients and removes waste products.

Hydrocephalus

• Condition caused by an imbalance in CSF production and absorption, leading to increased intracranial pressure. Treatment often requires a shunt.

Blood Supply to the Brain

• High blood flow is essential due to the brain's high oxygen and glucose demands.

• Lack of glucose can lead to confusion, dizziness, or loss of consciousness; lack of oxygen can rapidly cause unconsciousness and potential cell damage within a matter of minutes.

Spinal Cord

Overview

• Extends from the base of the skull to the conus medullaris.

Anatomy

• Regions: Cervical, thoracic, lumbar, sacral, and coccygeal.

• Conus Medullaris: Cone-shaped termination of the spinal cord.

• Cauda Equina: Bundle of spinal nerves extending beyond the conus medullaris.

Spinal Cord Organization

Gray Matter

• Composed of unmyelinated cell bodies, arranged into horns (dorsal, lateral, and ventral).

White Matter

• Composed of myelinated axon tracts arranged into funiculi/columns (dorsal, lateral, and ventral).

Protection of the Spinal Cord

• Similar protective features as the brain including vertebrae, meninges, and CSF.

Conclusion

• CNS Functionality: The brain and spinal cord work in tandem as central components coordinating sensory input, motor output, and integrating complex behaviors and functions crucial for survival.

• Importance of Protection: The structures and fluids surrounding the CNS are vital for maintaining its integrity and function against physical and pathological challenges.