Central Nervous System Overview

Introduction to the Central Nervous System (CNS)

• The human brain offers immense complexity and remarkable abilities despite its unimpressive appearance.
• The average adult human brain weighs about 1500 g (3.3 lb) and has a wrinkled surface resembling a walnut.

Cephalization and Brain Development

• Cephalization: The evolutionary trend of increasing neuronal elaboration in the brain, particularly in the anterior (rostral) portion of the CNS.
• Overview of primary brain vesicles:
  • Prosencephalon (forebrain)
  • Mesencephalon (midbrain)
  • Rhombencephalon (hindbrain)
• The neural tube divides further into five secondary brain vesicles, which develop into major adult brain structures:
  • Telencephalon -> Cerebral hemispheres
  • Diencephalon -> Thalamus, hypothalamus, retina
  • Mesencephalon -> Midbrain
  • Metencephalon -> Pons and cerebellum
  • Myelencephalon -> Medulla oblongata

Brain Folding and Growth

• As the brain grows faster than the skull, it folds to occupy the available space, increasing its surface area through convolutions.
• Cerebral hemispheres grow posteriorly and laterally, covering the diencephalon and midbrain.

Major Regions of the Adult Brain

1. Cerebral Hemispheres
2. Diencephalon
3. Brain Stem (midbrain, pons, medulla oblongata)
4. Cerebellum

Gray and White Matter Organization

• Gray Matter: Composed of neuron cell bodies and unmyelinated fibers.
• White Matter: Contains myelinated axons, facilitates communication within the nervous system.
• The CNS pattern features a central cavity surrounded by gray matter, with white matter external to it, differing in structure between the spinal cord and various brain regions.

Ventricles

• The brain contains fluid-filled ventricles that are continuous with the central canal of the spinal cord, housing cerebrospinal fluid (CSF), which cushions the CNS.
• Major ventricles include:
  • Lateral Ventricles: C-shaped, located in each hemisphere
  • Third Ventricle: Located in the diencephalon
  • Fourth Ventricle: Located between the brainstem and cerebellum

The Cerebral Hemispheres

• Lobes of the Brain: Frontal, parietal, temporal, occipital, and insula.
• Cortex: The outer layer of the brain, involved in voluntary movement, sensory perception, and higher cognitive processes.

Functional Regions of Cerebral Cortex

Motor Areas

• Primary Motor Cortex: Located in the precentral gyrus, controls voluntary muscle movements.
• Premotor Cortex: Plans and sequences movements.
• Broca's Area: Involved in speech production.
• Frontal Eye Field: Controls voluntary eye movements.

Sensory Areas

• Primary Somatosensory Cortex: Receives sensory input from the body.
• Visual Cortex: Processes visual information.
• Auditory Cortex: Processes auditory information.

Association Areas

• Integrate sensory information and are involved in higher cognitive functions, such as recognition, memory, and language.

Limbic System and Emotional Response

• Limbic System: Associated with emotions and drives, includes structures like the amygdala and hippocampus.

The Brain Stem

• Consists of the midbrain, pons, and medulla oblongata, involved in basic life functions:
  • Controls respiration, heart rate, and reflex actions.
  • Contains cranial nerve nuclei that innervate the head and neck.

Spinal Cord

• Extends from the foramen magnum to the lumbar vertebrae, acting as a conduit for sensory and motor signals.
• Protected by bone, meninges, and cerebrospinal fluid; consists of gray and white matter similar to the brain.

Clinical Considerations

• Diseases and disorders affecting the CNS can lead to significant loss of functions, including:
  • Cerebrovascular Accidents (Strokes) or TIAs: Temporary symptoms due to reduced blood flow.
  • Degenerative Diseases: Such as Alzheimer's and Parkinson's disease, caused by neuron degeneration.
  • Traumatic Injuries: Head injuries or spinal cord injuries can lead to paralysis or long-term complications.

Summary of Neural Pathways

• Ascending Pathways: Carry sensory information (e.g., pain, temperature) to the brain.
• Descending Pathways: Carry motor commands from the brain to the body.
  • Includes direct (pyramidal) and indirect pathways involving multiple neurons.

Conclusion

• Understanding the complex interactions within the CNS is crucial for grasping how the body responds to various stimuli, maintains homeostasis, and performs higher functions associated with human experience.