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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.