9a

Central Nervous System Overview

  • Definition: The CNS consists of the brain and spinal cord.

  • Functionality: Necessary for the maintenance of homeostasis.

  • Composition:

    • Contains approximately 101110^{11} (100 billion) neurons.

    • Contains approximately 101410^{14} (100 trillion) synapses.

  • Importance: Responsible for all human perception, actions, feelings, and thoughts.

  • Current Understanding: Despite its importance, the CNS remains the least understood organ system in the human body.

General Anatomy of the Central Nervous System

  • Glial (Neuroglial) Cells:

    • Comprise 75-90% of the CNS, are non-excitatory cells.

    • Types of glial cells include:

    • Astrocytes: Numerous functions.

    • Microglia: Act as phagocytes.

    • Ependymal Cells: Line the cavities of the CNS.

    • Oligodendrocytes: myleinate CNS

    • Schwann Cells: myelinate PNS

Types of Glial Cells: 90% of CNS

  • Astrocytes:

    • Star-shaped→ arms with hands

    • Functions:

    • Assist in neuron communication (astrocyte to astrocyte, astrocyte to neuron, astrocyte to non-neural cells).

    • Developmental functions supporting neuron development.

      • Support regenerative mechanisms for axons.

      • Contribute to forming the blood-brain barrier.

    • May modulate synaptic activity and protect neurons by:

    • protective function

      • Removing neurotransmitters (e.g., glutamate).

      • Protecting neurons from toxic substances and oxidative stress.

      • Maintaining normal electrolyte concentrations in CNS interstitial fluid (potassium levels).

      • Synthesizing glutamine and glycogen.

  • Microglia:

    • Characteristic size: Small.

    • protective Functions:

    • Protect neurons from toxic substances in coordination with astrocytes.

    • Conduct phagocytosis to engulf bacteria and dead or injured cells.

    • Protect the CNS from oxidative stress.

  • Ependymal Cells:

    • Location: Line the ventricles of the brain

    • Function: Synthesize cerebrospinal fluid (CSF).

Clinical Connections: Glial Cells in Neurodegenerative Diseases

  • Role of Glial Cells:

    • producing inflammatory agents

      • Multiple Sclerosis: An autoimmune disease characterized by the loss of myelin. (oligodenryctyes)

      • Alzheimer's Disease: Involves the loss of cholinergic neurons and production of inflammatory substances. (acetocholyn)

      • Parkinson's Disease: Features the loss of dopaminergic neurons and inflammatory chemical production. (Dopamine)

Physical Support of the CNS

  • Structural Support:

    • Bone:

    • Cranium (skull).

    • Vertebral column (spine).

  • Meninges: Protective membranes surrounding the CNS, including:

    • Dura Mater: "Tough mother" - the outermost layer.

    • Arachnoid Mater: "Spider" - the middle layer.

    • Pia Mater: "Tender mother" - the innermost layer.

Cerebrospinal Fluid (CSF)

  • Properties:

    • Clear, watery fluid bathing the CNS.

  • Production:

    • secreted by ependymal cells → ventricals of brain

    • reabsorbed by arachnoid vili

  • Total volume ranges from 125-150 mL, with 400-500 mL produced per day. 3x per day

  • Functions:

    • Provides neutral buoyancy for the CNA → floating in space

    • Maintains stable interstitial fluid environments.

Comparison of Plasma and CSF Composition

  • TABLE 9.1 Composition of Plasma and CSF:

    • Glucose:

    • Plasma: 80-100 mg/dL

    • CSF: 45-80 mg/dL

    • Proteins:

    • Plasma: 7000 mg/dL

    • CSF: 15-45 mg/dL

Blood Supply to the CNS

  • CNS Characteristics:

    • Accounts for about 2% of total body weight (3-4 pounds).

    • Receives approximately 15% of total blood supply.

    • Exhibits a high metabolic rate, utilizing:

    • 20% of the body's oxygen at rest.

    • 50% of the body's glucose at rest.

      • does brain have brain cell access to glucose easily? Glute 1→ independent of insulin

  • Dependence on Blood Flow:

    • Neurons rely on continuous blood flow and glucose due to:

      • Lack of anaerobic cellular respiration capabilities.

      • No significant glycogen stores (minimal in glial cells).

      • No fatty acid storage or hydrolysis,

        • ketones for energy

Types of Stroke and CNS Blood Supply Issues

  • Cerebrovascular Accident (CVA):

    • Also known as stroke, resulting from ischemia.

    • Types:

      • Occlusive Stroke:

        • atherosclerosis → clot

        • arteriosclerosis→ hardening of the artery

  • Treatment includes anticoagulants and tissue plasminogen activator (tPA).

  • Hemorrhagic Stroke: Occurs due to aneurysms or hypertension.

  • May require surgical intervention.

Blood-Brain Barrier (BBB)

    astrocytes

  • Structure:

    • Composed of capillaries featuring an endothelium and basement membrane regulating exchange between blood and interstitial fluid.

  • Functionality:

    • tight junctions formed by astrocytes and endothelial cells.

    • Hydrophobic Solutes: Such as alcohol.

    • Hydrophilic Solutes: Utilize specific transporters like GLUT-1 (glucose transporters, insulin-independent).

Central Nervous System Matter Types

  • Gray Matter:

    • Constitutes about 40% of the CNS.

    • Functions include synaptic transmission and neural integration.

  • White Matter:

    • mostly axons

      • Accounts for about 60% of the CNS.

      • Principal function is the transmission of information over long distances, primarily due to myelination.

White Matter Structures in the Brain

  • Projection Fibers:

    • Connection between the cerebral cortex and lower brain levels or spinal cord.

  • Association Fibers:

    • Connect regions within the same side of the cerebral cortex.

  • Corpus Callosum:

    • Composed of commissural fibers that connect similar cortical regions across both sides of the brain.