2.2 Central and peripheral nervous system organization

The nervous system is a complex network of cells that controls our body's functions. It's divided into the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves throughout the body). These systems work together to process information and control our actions.

Understanding the organization of the nervous system is crucial for developing neuroprosthetics. The brain's regions, spinal cord segments, and different nervous system divisions all play important roles in how we interact with our environment and control our bodies.

Central and Peripheral Nervous System Organization

Central vs peripheral nervous systems

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• Central Nervous System (CNS) consists of brain and spinal cord
  • Processes and integrates sensory information from the body (touch, vision, hearing)
  • Generates and coordinates motor commands to control movement and behavior
  • Responsible for higher cognitive functions like learning, memory, and decision making (problem-solving, language)
• Peripheral Nervous System (PNS) consists of nerves and ganglia outside the brain and spinal cord
  • Divided into somatic and autonomic nervous systems
  • Somatic nervous system relays sensory information from the body to the CNS (pain, temperature) and carries motor commands from the CNS to skeletal muscles (voluntary movements)
  • Autonomic nervous system regulates involuntary functions like heart rate, blood pressure, and digestion (breathing, sweating)

Brain and spinal cord regions

• Brain regions
  • Cerebral cortex divided into four lobes
    • Frontal lobe involved in planning, decision making, and motor control (prefrontal cortex)
    • Parietal lobe processes sensory information related to touch, pressure, and proprioception (somatosensory cortex)
    • Temporal lobe involved in hearing, language processing, and memory formation (hippocampus)
    • Occipital lobe processes visual information (primary visual cortex)
  • Basal ganglia involved in motor control, learning, and reward-based behavior (caudate nucleus, putamen)
  • Thalamus relays sensory and motor information between the cortex and other brain regions and regulates sleep-wake cycles
  • Hypothalamus regulates homeostatic functions like body temperature, hunger, and thirst and plays a role in emotional processing and stress responses
  • Brainstem consists of midbrain, pons, and medulla oblongata
    • Regulates vital functions like breathing, heart rate, and blood pressure
    • Relays information between the spinal cord and higher brain regions
• Spinal cord
  • Divided into cervical, thoracic, lumbar, and sacral segments based on vertebral levels
  • Relays sensory information from the body to the brain via ascending pathways (spinothalamic tract)
  • Carries motor commands from the brain to the muscles via descending pathways (corticospinal tract)
  • Contains neural circuits for simple reflexes (knee-jerk reflex) and central pattern generators (locomotion)

Somatic and Autonomic Nervous Systems

Somatic and autonomic nervous systems

• Somatic nervous system
  • Consists of sensory neurons that detect stimuli from the external and internal environment (touch, pain) and motor neurons that carry commands from the CNS to skeletal muscles
  • Enables voluntary control of movement (walking, grasping objects)
• Autonomic nervous system
  • Regulates involuntary functions of internal organs and glands (heart, lungs, digestive system)
  • Divided into sympathetic and parasympathetic divisions
  • Sympathetic division activates "fight or flight" responses
    1. Increases heart rate and blood pressure
    2. Raises blood glucose levels
    3. Diverts blood flow to skeletal muscles
  • Parasympathetic division promotes "rest and digest" functions
    1. Decreases heart rate and blood pressure
    2. Stimulates digestion and nutrient absorption
    3. Promotes relaxation and energy conservation

Blood-brain barrier structure and function

• Blood-brain barrier (BBB) is a selectively permeable barrier between the blood and the brain
  • Formed by tight junctions between endothelial cells of brain capillaries
  • Astrocytes and pericytes contribute to BBB formation and maintenance
• Functions of the BBB
  • Maintains homeostasis in the brain microenvironment by regulating ion and neurotransmitter concentrations
  • Protects the brain from toxins, pathogens, and immune system components (antibodies, cytokines)
  • Regulates the entry of nutrients, gases, and other molecules into the brain (glucose, amino acids, oxygen)
  • Allows selective transport of essential molecules via specialized transport proteins (GLUT1 for glucose)
• Relevance to neuroprosthetics
  • BBB can limit the delivery of drugs or bioactive molecules to the brain, requiring specialized delivery strategies (nanoparticles, ultrasound)
  • Neuroprosthetic devices may need to be designed to minimize BBB disruption and maintain its integrity
  • Strategies to temporarily and selectively permeabilize the BBB may be required for targeted drug delivery (focused ultrasound, osmotic agents)
  • Understanding BBB transport mechanisms can help develop novel neuroprosthetic technologies that exploit these pathways (receptor-mediated transcytosis)