10.1 The Impact of Nutrition on Hematologic Wellness Across the Lifespan

The nervous system is the body's command center, orchestrating everything from conscious thoughts to involuntary reflexes. It's made up of neurons, specialized cells that communicate through electrical and chemical signals, forming complex networks that control our actions and reactions.

Understanding the nervous system's structure and function is crucial for grasping how drugs affect the body. From the brain's higher cognitive functions to the autonomic regulation of vital organs, this intricate system is the target of many medications and the source of numerous health conditions.

Structure and Function of the Nervous System

Structure and function of neurons

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• Neurons fundamental units of the nervous system
  • Cell body contains nucleus and other organelles essential for cellular functions
  • Dendrites branch-like extensions that receive signals from other neurons (synaptic input)
  • Axon long, thin fiber that conducts electrical impulses away from the cell body to other neurons or target cells
    • Axons are often covered by myelin, which enhances signal transmission speed
  • Axon terminals specialized structures at the end of the axon that release neurotransmitters to communicate with other neurons or target cells (synaptic output)
• Neurons communicate through electrical and chemical signaling
  • Electrical signaling occurs within the neuron through action potentials
    • Action potentials generated by changes in the membrane potential due to ion flow through voltage-gated channels (sodium and potassium)
  • Chemical signaling occurs between neurons at synapses
    • Neurotransmitters released from the presynaptic neuron's axon terminals into the synaptic cleft (space between neurons)
    • Neurotransmitters bind to receptors on the postsynaptic neuron or target cell, causing excitation (depolarization) or inhibition (hyperpolarization)

Components of nervous systems

• Central nervous system (CNS) consists of the brain and spinal cord
  • Brain divided into several regions with specific functions
    • Cerebrum responsible for higher cognitive functions (thinking, learning, memory), sensory processing (vision, hearing, touch), and voluntary movement
    • Cerebellum coordinates balance, posture, and fine motor control (smooth, coordinated movements)
    • Brainstem regulates vital functions such as breathing, heart rate, and consciousness (reticular formation)
  • Spinal cord serves as a conduit for sensory and motor information between the brain and the body
    • Conducts sensory and motor information between the brain and the body through ascending and descending pathways
    • Responsible for reflexes and central pattern generators (walking, swimming)
• Peripheral nervous system (PNS) consists of nerves and ganglia outside the CNS
  • Sensory (afferent) division carries information from sensory receptors to the CNS
    • Somatic sensory system detects stimuli from the external environment (touch, pressure, temperature) and the body's position and movement (proprioception)
    • Visceral sensory system monitors internal organs and tissues (blood pressure, gut distension)
  • Motor (efferent) division carries signals from the CNS to effector organs (muscles, glands)
    • Somatic motor system controls voluntary movement of skeletal muscles (walking, grasping)
    • Autonomic nervous system regulates involuntary functions of internal organs and glands (heart rate, digestion)
      • Sympathetic nervous system activates "fight or flight" response
      • Parasympathetic nervous system promotes "rest and digest" functions

Sympathetic vs parasympathetic regulation

• Sympathetic nervous system (SNS) activates the body's "fight or flight" response during stress or emergency situations
  • Increases heart rate and blood pressure to deliver more oxygen and glucose to tissues
  • Dilates bronchioles to improve oxygen intake and gas exchange
  • Stimulates glucose release from the liver to provide energy for the body
  • Diverts blood flow to skeletal muscles and away from digestive system to prioritize physical activity
  • Dilates pupils (mydriasis) and reduces tear and saliva production to enhance sensory awareness
• Parasympathetic nervous system (PNS) promotes "rest and digest" functions during relaxed states
  • Decreases heart rate and blood pressure to conserve energy
  • Constricts bronchioles to prevent overinflation of the lungs
  • Stimulates digestion and peristalsis (movement of food through the digestive tract)
  • Promotes secretion of digestive enzymes, tears (lacrimation), and saliva (salivation)
  • Constricts pupils (miosis) to protect the eyes from bright light
• SNS and PNS work together to maintain homeostasis by modulating the activity of various organs and systems
  • Often have opposing effects on target organs (antagonistic actions)
  • Balance between the two systems is crucial for proper functioning of the body and adaptation to changing environments

Additional Nervous System Components and Processes

• Glial cells support and protect neurons, maintaining the optimal environment for neural function
• The blood-brain barrier is a selective membrane that protects the CNS from potentially harmful substances in the bloodstream
• Neuroplasticity refers to the brain's ability to reorganize and form new neural connections throughout life
• Reflex arcs are rapid, involuntary responses to stimuli that bypass conscious control
• Neuromodulators are substances that can alter the effects of neurotransmitters, influencing neural communication and behavior