Central Nervous System

Ventricles of the Brain

The brain contains four interconnected, fluid-filled chambers called ventricles. These ventricles are essential for the production, circulation, and removal of cerebrospinal fluid (CSF), which cushions and nourishes the brain.

• Lateral Ventricles: Paired, C-shaped chambers located in each cerebral hemisphere.

• Third Ventricle: Located in the diencephalon, connected to the lateral ventricles via the interventricular foramen.

• Fourth Ventricle: Located between the brainstem and cerebellum, connected to the third ventricle via the cerebral aqueduct.

• Central Canal: Extends from the fourth ventricle into the spinal cord.

Cerebrospinal Fluid (CSF)

CSF is a clear, colorless liquid that fills the ventricles and surrounds the brain and spinal cord. It serves several critical functions:

• Bouyancy: Reduces the effective weight of the brain by 97%, preventing damage from its own mass.

• Protection: Cushions the brain from mechanical shocks and jolts.

• Chemical Stability: Nourishes neural tissue and removes waste, while carrying chemical signals.

Choroid Plexus

The choroid plexus is a cluster of capillaries enclosed by pia mater and surrounded by ependymal cells. It is the primary site of CSF production in each ventricle.

CSF Circulation

CSF is produced in the choroid plexus of each ventricle, circulates through the ventricles, and enters the subarachnoid space via the median and lateral apertures. It is absorbed into the dural venous sinuses through arachnoid granulations.

• Obstruction: Blockage of CSF circulation or drainage can lead to hydrocephalus, a condition characterized by abnormal accumulation of CSF.

• Hydrocephalus in Newborns: Due to unfused skull bones, the head enlarges. In adults, increased intracranial pressure occurs due to rigid skull.

Meninges

The meninges are three protective membranes that surround the brain and spinal cord:

• Dura Mater: Tough, outermost layer with periosteal and meningeal layers; forms dural sinuses for venous drainage.

• Arachnoid Mater: Middle layer; contains the subarachnoid space filled with CSF and arachnoid granulations for CSF absorption.

• Pia Mater: Delicate, innermost layer; adheres to the brain and spinal cord, containing tiny blood vessels.

Functional Brain Systems

Limbic System

The limbic system is a network of neurons involved in emotion, memory, and motivation. It spans the diencephalon and cerebral structures.

• Amygdaloid Body (Amygdala): Processes fear and aggression.

• Cingulate Gyrus: Coordinates emotional gestures and resolves mental conflicts.

• Hippocampus: Essential for memory formation.

• Olfactory Bulb: Links odors to emotional responses.

Limbic System & Hypothalamus

The limbic system interacts with the hypothalamus, influencing autonomic functions and emotional responses. Psychosomatic illnesses can arise when emotional stress manifests as physical symptoms (e.g., high blood pressure, heartburn).

Reticular Formation

The reticular formation is a network of nuclei in the brainstem that connects with the hypothalamus, thalamus, cerebral cortex, cerebellum, and spinal cord.

• Reticular Activating System (RAS): Maintains consciousness and alertness by relaying sensory stimuli to the cerebrum.

• Filtering: Filters out repetitive or weak stimuli; only 1% of sensory input reaches consciousness.

• Inhibition: Can be inhibited, leading to coma.

Higher Mental Functions

Language

Language processing occurs primarily in the association cortex of the left hemisphere.

• Broca’s Area: Responsible for speech production.

• Wernicke’s Area: Responsible for language comprehension.

• Aphasia: Language deficits due to lesions in these areas.

  • Nonfluent Aphasia: Lesion in Broca’s area; slow, labored speech.

  • Fluent Aphasia: Lesion in Wernicke’s area; speech is normal but nonsensical, with impaired comprehension.

Memory

Memory involves the storage and retrieval of information.

• Declarative Memory: Facts and events.

• Procedural Memory: Skills and procedures.

• Motor Memory: Motor skills.

• Emotional Memory: Emotional experiences.

Stages of Memory

• Short-Term Memory (STM): Temporary storage, primarily in the hippocampus.

• Long-Term Memory (LTM): Permanent storage, primarily in the cerebral cortex.

• Factors Influencing Transfer: Excitement, rehearsal, association, and automatic memory.

Amnesia

• Retrograde Amnesia: Loss of memories formed before the event.

• Anterograde Amnesia: Inability to form new memories after the event.

Brain Waves

Brain waves reflect the electrical activity of the brain and are measured by an electroencephalogram (EEG).

• Uses: Diagnosis of epilepsy, sleep disorders, brain lesions, and brain death.

• Seizures: Abnormal electrical discharges can prevent normal brain function.

Consciousness

Consciousness encompasses perception, voluntary movement, cognition, memory, logic, and judgment.

• Phineas Gage: Famous case illustrating the effects of brain injury on personality and higher mental functions.

Sleep and Sleep-Wake Cycles

Sleep is regulated by the reticular activating system and hypothalamic suprachiasmatic nuclei.

• NREM (Non-rapid Eye Movement): Four stages, from light to deep sleep.

• REM (Rapid Eye Movement): Occurs about 90 minutes into sleep; characterized by increased vital signs and dreaming.

• Circadian Rhythms: Governed by the hypothalamus; disruptions can cause sleep disorders such as narcolepsy and insomnia.

• Importance: Sleep is restorative, helps analyze daily events, and eliminates unneeded synapses.

Spinal Cord

Gross Structure

The spinal cord begins at the foramen magnum and ends at the L1 or L2 vertebra.

• Conus Medullaris: Tapered end of the spinal cord.

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

• Cervical and Lumbosacral Enlargements: Regions with increased neural input/output for limbs.

• Function: Two-way communication between brain and body; reflex center.

Spinal Cord Meninges

The spinal cord is protected by three meninges: dura mater, arachnoid mater, and pia mater.

• Epidural Space: Contains fat; located outside the dura mater.

• Subdural Space: Between dura and arachnoid mater.

• Subarachnoid Space: Contains CSF; between arachnoid and pia mater.

Spina Bifida

Spina bifida is a congenital defect where vertebrae fail to form a complete arch, exposing the spinal cord.

• Spina Bifida Occulata: Mild form; small gap in vertebrae.

• Spina Bifida Cystica: Severe form; large opening with protruding sac.

• Prevention: Folic acid supplementation reduces risk.

Spinal Cord – Gray Matter

The spinal cord's gray matter is organized into horns:

• Dorsal Horn: Contains interneurons receiving sensory input.

• Ventral Horn: Contains motor neurons.

• Lateral Horn: Contains autonomic motor neurons (in thoracic and upper lumbar regions).

Spinal Cord – White Matter

White matter consists of myelinated axons organized into columns (funiculi): dorsal, ventral, and lateral.

Spinal Cord – Pathways and Tracts

• Ascending Tracts: Carry sensory information to the brain.

• Descending Tracts: Carry motor commands from the brain to the body.

• Key Features: Decussation (cross-over), relay (multiple neurons), somatotopy (spatial mapping), and symmetry (paired pathways).

Ascending Pathways

• Medial Lemniscal Pathway: Discriminative touch, vibration, proprioception; includes fasciculus cuneatus (upper body) and fasciculus gracilis (lower body).

• Spinothalamic Pathway: Pain, temperature, crude touch, and pressure.

• Spinocerebellar Tracts: Proprioception; coordinates muscle activity.

Descending Pathways

• Direct (Corticospinal) Pathways: Originate in the cerebral cortex; control fast and fine movements.

• Indirect Pathways: Originate in the brainstem; control axial muscles, balance, posture, and head/eye movements.

• Upper Motor Neurons: Begin in the primary motor cortex or brainstem.

• Lower Motor Neurons: Begin in the ventral horn; innervate skeletal muscles.

Summary Table: Major Ascending and Descending Pathways