The Central Nervous System (CNS)

Overview and Major Anatomical Features

The CNS consists of the brain and spinal cord, serving as the main control center for the body. It processes sensory information, coordinates responses, and is responsible for higher functions such as thought and emotion.

• Major anatomical features: White matter (myelinated axons), gray matter (neuronal cell bodies, dendrites, unmyelinated axons), central cavity (ventricles in the brain, central canal in the spinal cord), and cerebrospinal fluid (CSF) spaces.

• Meninges: Three connective tissue membranes (dura mater, arachnoid mater, pia mater) that cover and protect the CNS.

Example: The cerebral cortex is composed of gray matter, while the underlying white matter contains axonal tracts connecting different brain regions.

Meninges: Structure and Function

• Dura mater: Tough, outermost layer; provides protection and support.

• Arachnoid mater: Middle, web-like layer; contains the subarachnoid space filled with CSF.

• Pia mater: Thin, innermost layer; closely adheres to the brain and spinal cord surface.

• Functions: Protect CNS, contain CSF, form partitions within the skull.

Additional info: The meninges also help anchor the CNS within the skull and vertebral canal.

Cerebrospinal Fluid (CSF): Production and Function

• Produced by: Choroid plexuses in the brain's ventricles.

• Functions: Cushions the brain, provides nutrients, removes waste, maintains chemical stability.

• Circulation: Flows through ventricles, central canal, and subarachnoid space; reabsorbed into venous blood via arachnoid granulations.

• CSF sample: Obtained via lumbar puncture (spinal tap).

Blood-Brain Barrier (BBB)

Structure and Function

• Formed by: Tight junctions between endothelial cells of CNS capillaries, astrocyte end-feet, and pericytes.

• Function: Regulates passage of substances from blood to brain, protecting neural tissue from toxins and pathogens.

• Molecules blocked: Most large or hydrophilic molecules (e.g., proteins, many drugs).

• Molecules allowed: Gases (O2, CO2), glucose, some amino acids, lipid-soluble substances.

• Breaks in BBB: Occur in areas like the hypothalamus and vomiting center, allowing monitoring of blood composition.

Example: The BBB prevents many antibiotics from entering the brain, complicating treatment of CNS infections.

Brain Structures and Functional Regions

Major Brain Divisions

• Hindbrain: Includes medulla oblongata, pons, cerebellum; controls vital functions (breathing, heart rate, coordination).

• Midbrain: Involved in vision, hearing, motor control, sleep/wake cycles.

• Forebrain: Includes diencephalon (thalamus, hypothalamus) and cerebrum; responsible for sensory processing, voluntary movement, cognition, emotion.

Matching brain structures to functions:

• Frontal lobe: Voluntary movement, planning, reasoning, speech.

• Parietal lobe: Sensory perception, spatial awareness.

• Temporal lobe: Hearing, memory, language.

• Occipital lobe: Vision.

Specialized Functions of the Right and Left Hemispheres

• Left hemisphere: Language, logic, analytical tasks.

• Right hemisphere: Spatial abilities, creativity, facial recognition.

• Communication: Corpus callosum allows information exchange between hemispheres.

Functional Integration and Neural Circuits

• Neural circuits: Networks of neurons that process specific types of information.

• Types of processing: Immediate (reflexes), short-term (working memory), long-term (learning, memory consolidation).

• Mechanisms: Synaptic plasticity, long-term potentiation (LTP), neurotransmitter release.

Additional info: Concepts such as reverberating circuits and facilitation are important for understanding memory and learning.

The Spinal Cord

General Functions

• Conduction: Transmits sensory and motor information between body and brain.

• Reflexes: Mediates simple, automatic responses to stimuli.

• Integration: Processes some information locally (e.g., withdrawal reflex).

Spinal Cord Anatomy

• Dorsal (posterior) horns: Contain sensory neurons; receive input from peripheral sensory receptors.

• Ventral (anterior) horns: Contain motor neurons; send output to skeletal muscles.

• Gray matter: Central region; contains neuron cell bodies.

• White matter: Surrounds gray matter; contains ascending (sensory) and descending (motor) tracts.

• Spinal nerves: Connect via dorsal and ventral roots.

Ascending and Descending Tracts

• Ascending tracts: Carry sensory information to the brain (e.g., spinothalamic tract for pain and temperature).

• Descending tracts: Carry motor commands from the brain to the body (e.g., corticospinal tract for voluntary movement).

Spinal Cord Injury and Paralysis

Types of Paralysis

• Flaccid paralysis: Damage to lower motor neurons; muscles lose tone and cannot contract.

• Spastic paralysis: Damage to upper motor neurons; muscles remain contracted due to loss of inhibitory input.

Types of spinal cord injury:

• Paraplegia: Paralysis of lower limbs (injury below T1).

• Quadriplegia: Paralysis of all four limbs (injury above C5).

• Spinal shock: Temporary loss of all reflex activity below the injury level.

Table: Comparison of CNS Structures and Functions

Key Equations and Concepts

• Nernst equation (for membrane potential):

• Ohm's Law (for neural conduction):

Additional info: These equations are fundamental for understanding how neurons generate and transmit electrical signals.