Anatomy

• Nervous system


  Includes various organs: brain, spinal cord, receptors of sense organs, nerves that connect to other systems
• Nervous tissue


  Contains two kinds of cells: neurons for intercellular communication, neuroglia (glial cells) essential to survival and function of neurons
• Anatomical divisions of the nervous system


  • Central nervous system
  • Peripheral nervous system
• Central nervous system (CNS)


  • Brain and spinal cord
  • Consists of nervous tissue, connective tissue, and blood vessels
  • Functions to process and coordinate sensory data from inside and outside body
  • Motor commands control activities of peripheral organs
  • Higher functions include intelligence, memory, learning, and emotion
• Peripheral nervous system (PNS)


  • Includes all nervous tissue outside CNS and enteric nervous system
  • Delivers sensory information to the CNS
  • Carries motor commands to peripheral tissues
  • Nerves (peripheral nerves) carry sensory information and motor commands
• Functional divisions of the PNS

  • Afferent division (carries sensory information)
  • Efferent division (carries motor commands)
• Efferent division of PNS

  • Somatic nervous system (controls skeletal muscle contractions)
  • Autonomic nervous system (controls subconscious actions, contractions of smooth and cardiac muscle, and glandular secretions)
• Autonomic nervous system (ANS)


  • Sympathetic division has a stimulating effect
  • Parasympathetic division has a relaxing effect
• Enteric nervous system (ENS)


  • 100 million neurons in walls of digestive tract
  • Uses the same neurotransmitters as the brain
  • Initiates and coordinates visceral reflexes locally without instructions from CNS
  • Can be influenced by ANS
• Neurons


  • Basic functional units of the nervous system
  • Send and receive signals
  • Function in communication, information processing, and control
• Neuron structure


  • Cell body (soma)
  • Dendrites
  • Axon
• Axon


  Single, long cytoplasmic process that propagates electrical signals (action potentials)
• Structures of the axon

  • Axolemma (plasma membrane)
  • Initial segment
  • Axon hillock
  • Collaterals (branches)
  • Telodendria (fine extensions)
  • Axon terminals (synaptic terminals)
• Axonal (axoplasmic) transport


  Movement of materials between cell body and axon terminals along neurotubules, powered by mitochondria, kinesin, and dynein
• Structural classification of neurons

  • Anaxonic
  • Bipolar
  • Unipolar
  • Multipolar
• Ganglia


  Groups of nerve cell bodies located outside of the central nervous system (CNS) in the peripheral nervous system (PNS)
• Functional classifications of neurons

  • Sensory neurons
  • Motor neurons
  • Interneurons
• Sensory neurons


  • Unipolar
  • Cell bodies grouped in sensory ganglia
  • Processes (afferent fibers) extend from sensory receptors to CNS
  • Somatic sensory neurons monitor external environment
  • Visceral sensory neurons monitor internal environment
• Types of sensory receptors

  • Interoceptors (monitor internal systems)
  • Exteroceptors (monitor external environment)
  • Proprioceptors (monitor position and movement of skeletal muscles and joints)
• Motor neurons


  • Carry instructions from CNS to peripheral effectors via efferent fibers (axons)
  • Somatic motor neurons innervate skeletal muscles
  • Visceral motor neurons innervate all other peripheral effectors (smooth and cardiac muscle, glands, adipose tissue)
• Interneurons


  • Located between sensory and motor neurons
  • Responsible for distribution of sensory information, coordination of motor activity, and higher functions like memory, planning, and learning
• Types of neuroglia in the CNS

  • Astrocytes
  • Ependymal cells
  • Oligodendrocytes
  • Microglia
• Astrocytes


  • Have large cell bodies with many processes
  • Maintain blood brain barrier
  • Create three-dimensional framework for CNS
  • Repair damaged nervous tissue
  • Guide neuron development
  • Control interstitial environment
• Oligodendrocytes


  • Have small cell bodies with few processes
  • Form myelin sheath that insulates myelinated axons, increasing speed of action potentials
• Internodes and nodes of Ranvier


  Myelinated segments of axon separated by nodes where axons may branch
• White matter and gray matter

  White matter contains many myelinated axons, gray matter contains unmyelinated axons, neuron cell bodies, and dendrites
• Ependymal cells


  • Form epithelium lining central canal of spinal cord and ventricles of brain
  • Produce and monitor cerebrospinal fluid (CSF)
  • Have cilia that help circulate CSF
• Neuron cell bodies


  • Dendrites
• Central canal


  Fluid-filled passageway within the brain and spinal cord
• Gray matter


  Contains neuron cell bodies and dendrites
• White matter


  Contains myelinated axons
• Neuroglia in the CNS

  • Ependymal cells
  • Microglia
• Ependymal cells


  Simple cuboidal epithelial cells that line fluid-filled passageways within the brain and spinal cord
• Microglia


  Phagocytes that move through nervous tissue removing unwanted substances
• Neuroglia of the PNS


  • Satellite cells
  • Schwann cells (neurolemmocytes)
• Satellite cells


  Surround ganglia (clusters of neuronal cell bodies); regulate O2, CO2, nutrient, and neurotransmitter levels around neurons in ganglia
• Schwann cells


  Form myelin sheath or indented folds of plasma membrane around axons; responsible for myelination of peripheral axons; participate in repair process after injury
• Formation of myelin sheath by Schwann cell

  1. Schwann cell surrounds portion of axon

  2. Schwann cell rotates around axon

  3. Myelin is wound around axon in multiple layers
• Wallerian degeneration


  • Axon distal to injury degenerates
  • Schwann cells form path for new growth and wrap around new axon
• Nerve regeneration in CNS

  • Limited by astrocytes, which produce scar tissue and release chemicals that block regrowth