Nervous System

• Nervous System


  • Most complex system in the body
  • Formed by a network of many billion nerve cells (neurons)
  • Assisted by many more supporting cells (glial cells)
• Two major divisions of the Nervous System

  • Central Nervous System (CNS): brain and spinal cord
  • Peripheral Nervous System (PNS): composed of the cranial, spinal, and peripheral nerve conducting impulses to and from the CNS and ganglia
• Neurons


  • Typically have numerous long processes
  • Respond to environmental changes (stimuli) by altering the ionic gradient that exists across their plasma membrane
  • Excitable/Irritable: cells that can rapidly change this potential in response to stimuli
• Membrane depolarization


  Neurons reacting promptly to stimuli with a reversal of the ionic gradient that generally spreads from the place that received the stimulus and is propagated across the neuron's entire plasma membrane
• Action potential/depolarization wave/nerve impulse


  Capable of traveling long distances along neuronal processes, transmitting such signals to other neurons, muscles, and glands
• Glial Cells


  • Have short processes, that support and protect neurons, and participate in many neural activities, neural nutrition, and defense of cells in the CNS
• Sensory division (afferent)


  • Somatic: sensory input perceived consciously (e.g. from eyes, ears, skin, and musculoskeletal structures)
  • Visceral: sensory input not perceived consciously (e.g. from internal organs and cardiovascular structures)
• Motor division (efferent)


  • Somatic: motor output controlled consciously or voluntarily (e.g. by skeletal effectors)
  • Autonomic: motor output not controlled consciously (e.g. by heart or gland effectors)
• Autonomic division


  • Parasympathetic division: ganglia within or near the effector organs, maintains normal body homeostasis
  • Sympathetic division: ganglia close to the CNS and controls the body's responses during emergencies and excitement
• Nervous system development


  1. Develops from the outermost of the three early embryonic layers (ectoderm), beginning in the 3rd week of development

  2. Neural Plate: forms with the thickening of the ectoderm on the mid-dorsal side of the embryo with the signals from the underlying axial structure (notochord)

  3. Neural tube: sides of the neural plate fold upward and grow toward each other medially, fusing within a few days

  4. Neural crest: large population of developmentally important cells formed as folds fuse the neural tube separates from the now overlying ectoderm that will form epidermis
• Neurolemma


  For cell membrane
• Three main parts of most neurons

  • Cell body (AKA Perikaryon or Soma)
  • Dendrites
  • Axon
• Cell body


  • Contains the nucleus and most of the cell's organelles
  • Serves as the synthetic or trophic center for the entire neuron
• Dendrites


  • Numerous elongated processes extending from the perikaryon
  • Specialized to receive stimuli from other neurons at unique sites (synapses)
• Axon


  • A single long process ending at synapses specialized to generate and conduct nerve impulses to other cells (e.g. nerve, muscle, and gland cells)
  • May also receive information from other neurons, information that mainly modifies the transmission of action potentials to those neurons
• Neurons according to the number of processes extending from the cell body

  • Multipolar Neurons
  • Bipolar Neurons
  • Unipolar or Pseudounipolar Neurons
  • Anaxonic Neurons
• Nervous components based on functions

  • Sensory Neurons
  • Motor Neurons
  • Somatic Motor Neurons
  • Autonomic Motor Neurons
• Interneurons


  Establish relationships among other neurons, forming complex functional networks or circuits in the CNS
• Most neuronal perikarya occur in the gray matter, with their axons concentrated in the white matter
• Parkinson Disease


  • Slowly progressing disorder affecting muscular activity characterized by tremors, reduced activity of the facial muscles, loss of balance, and postural stiffness
  • Caused by gradual loss by apoptosis of dopamine-producing neurons whose cell bodies lie within the nuclei of the CNS substantia nigra
• 1. dopa (L-3,4-dihydroxyphenylalanine)


  Precursor of dopamine that augments the declining production of this neurotransmitter
• Cell body (Perikaryon or Soma)


  • Contains the nucleus and surrounding cytoplasm, exclusive of the cell processes
  • Acts as a trophic center, producing most cytoplasm for the processes
  • In contact with a great number of nerve endings conveying excitatory or inhibitory stimuli generated in other neurons
  • Typical neuron: unusually large, euchromatic nucleus with a prominent nucleolus, indicating intense synthetic activity
  • Cytoplasm: often contains numerous free polyribosomes and highly developed RER, indicating active production of both cytoskeletal proteins and proteins for transport and secretion
• Dendrites


  • Typically short, small processes, emerging and branching off the soma
  • Usually covered with many synapses and are the principal signal reception and processing sites of neurons
  • Its large number and extensive arborization allows a single neuron to receive and integrate signals from many other nerve cells
  • Become much thinner as they branch, with cytoskeletal elements predominating in these distal regions
  • In CNS, most of its synapses occur on dendritic spines, which are dynamic membrane protrusions along the small dendritic branches
• Axon


  • Most neurons have only one axon, typically longer than its dendrites
  • Axonal processes vary in length and diameter according to the type of neuron
  • Axons of motor neurons: innervate the foot muscles, have lengths of nearly a meter
  • Large cell bodies are required for maintenance, which contain most of such neuron's cytoplasm
  • Branch less profusely than dendrites but undergo terminal arborization
  • Axolemma: plasma membrane of axon
  • Axoplasm: contents of axolemma
• Nerve impulse


  Various excitatory and inhibitory stimuli impinging on the neuron are algebraically summed, resulting in the decision to propagate–or not
• Axons of interneurons and some motor neurons


  Have major branches (collaterals) that end at smaller branches with synapses influencing the activity of many other neurons
• Terminal bouton


  A dilation that contacts another neuron or non-nerve cell at a synapse to initiate an impulse in that cell
• Anterograde transport


  Movement of organelles and macromolecules synthesized in the cell body along axonal microtubules via kinesin from the perikaryon to the synaptic terminals
• Retrograde transport


  Opposite direction along microtubules via dynein carries certain other macromolecules
• Action potential


  • An electrochemical process initiated at the axon hillock when other impulses received at the cell body or dendrites meet a certain threshold
  • Travels along an axon like a spark moves along an explosive's fuse
  • Propagated along the axon as a wave of membrane depolarization produced by voltage-gated Na+ and K+ channels in the axolemma that allow diffusion of these ions into and out of the axoplasm
• Local Anesthetics


  • Low-molecular-weight molecules that bind to the voltage-gated sodium channels of the axolemma, interfering with sodium ion influx
  • Consequently, inhibiting the action potential responsible for the nerve impulse
• Synapse


  • Sites where nerve impulses are transmitted from one neuron to another, or from neurons and other effector cells
  • Converts an electrical signal (nerve impulse) from the presynaptic cell into a chemical signal that affects the postsynaptic cell
  • Most act by releasing neurotransmitters, which are usually small molecules that bind specific receptor proteins to either open or close ion channels or initiate second-messenger cascades
• Components of a synapse

  • Presynaptic axon terminal (terminal bouton)
  • Postsynaptic cell membrane
  • Synaptic cleft
• Presynaptic axon terminal (terminal bouton)


  Contains mitochondria and numerous synaptic vesicles from which neurotransmitter is release by exocytosis
• Postsynaptic cell membrane


  Contains receptors for the neurotransmitter, and ion channels or other mechanisms to initiate a new impulse
• Synaptic cleft


  20-30 nm wide intercellular space that separates these presynaptic and postsynaptic membranes
• Excitatory synapses


  Neurotransmitters cause postsynaptic Na+ channels to open, and the resulting Na+ influx initiates a depolarization wave in the postsynaptic neuron or effector cell
• Inhibitory synapses


  Neurotransmitters open Cl- or other anion channels, causing influx of anions and hyperpolarization of the postsynaptic cell, making its membrane potential more negative and more resistant to depolarization
• Major categories of neurotransmitters in the CNS

  • Certain amino acids
  • Acetylcholine
  • Other categories
• Neurotransmitter release at presynaptic region

  1. Nerve impulse briefly opens calcium channels

  2. Ca2+ influx triggers neurotransmitter release by exocytosis or similar mechanisms