Central Nervous System (CNS)

Organization of the Nervous System

• The nervous system comprises two main divisions:
  • Central Nervous System (CNS): Brain and spinal cord.
  • Peripheral Nervous System (PNS): Connects the CNS to the rest of the body.
  • Divided into:
  • Sensory Division (Afferent): Brings information to the CNS from sensory receptors.
  • Efferent Division: Takes information from the CNS to target cells through:
    • Somatic Motor Neurons: Control skeletal muscles.
    • Autonomic Neurons: Control smooth muscles, cardiac muscles, and glands.

Components of the CNS

• Brain:
  • Rests in the cranium, composed of different regions including:
  • Cerebrum
  • Cerebellum
  • Brainstem (medulla oblongata, pons)
• Spinal Cord:
  • Runs down the back within the vertebral column.
• Protection:
  • Surrounded by bones (skull for brain, vertebrae for spinal cord), meninges, and cerebrospinal fluid (CSF).

Protective Structures of CNS

• Meninges:
  • Dura mater: Outer layer adjacent to bone.
  • Arachnoid membrane: Middle layer, contains cerebrospinal fluid (CSF).
  • Pia mater: Inner layer adhering to the brain and spinal cord.
• Cerebrospinal Fluid (CSF):
  • Produced by the choroid plexus, provides cushioning and chemical protection.
• Blood-Brain Barrier:
  • Specialized capillaries that protect the brain from toxins while allowing nutrients to pass through.

Neuronal Structure and Function

• Neuron Functionality:
  • Comprised of:
  • Dendrites: Receive signals.
  • Cell body: Integrates incoming signals.
  • Axon: Transmits signals away from the body to other neurons or muscles.
• Types of Neurons:
  • Afferent (Sensory) Neurons: Carry signals to the CNS.
  • Efferent (Motor) Neurons: Transmit signals from the CNS to effector cells.

Membrane Potential

• Resting Membrane Potential:
  • Electrical potential difference across the membrane at rest (approximately -70 mV).
• Ion Composition:
  • Extracellular Fluid: High in sodium (Na+), chloride (Cl-).
  • Intracellular Fluid: High in potassium (K+), phosphate ions, negatively charged proteins.

Gated Ion Channels

• Types:
  • Voltage-gated channels: Open in response to membrane potential changes.
  • Ligand-gated channels: Open with the binding of specific ligands (e.g., neurotransmitters).
  • Mechanically-gated channels: Respond to physical distortion (vibration/stretch).

Action Potentials

• All-or-None responses that occur if the stimulus reaches a threshold.
• Characteristics:
  • Frequency represents stimulus strength.
  • Do not diminish with distance.
  • Involve a refractory period preventing summation.

Myelination and Conduction of Nerve Signals

• Myelination:
  • Increases conduction velocity (e.g., myelinated axon conducts at 120 m/sec compared to 2 m/sec for unmyelinated).
• Nodes of Ranvier:
  • Gaps in myelin sheath enhance signal conduction via saltatory conduction.

Regions of the Cerebrum

• Functional Areas:
  • Frontal Lobe: Involved in movement, reasoning, and problem-solving.
  • Parietal Lobe: Processes sensory information.
  • Temporal Lobe: Associated with auditory information and memory.
  • Occipital Lobe: Responsible for visual processing.

Blood-Brain Barrier (BBB)

• Composed of tightly connected capillaries that restrict passage of substances.
• Maintains homeostasis by regulating ion transport and protecting against toxins.

Important Imaging Techniques for CNS

• Electroencephalography (EEG): Measures electrical activity of the brain.
• Positron Emission Tomography (PET): Visualizes metabolic activity using radioactively tagged glucose.
• Functional Magnetic Resonance Imaging (fMRI): Measures brain activity based on blood flow.