NERVOUS

• Generalized functions of the nervous system

  Describe the generalized functions of the nervous system as a whole
• Major types of cells in the nervous system

  • Neurons
  • Glia (neuroglia)

  
• Neuron structure

  • Cell body of neuron
  • Dendrites: Branching projections that conduct impulses to cell body of neuron
  • Axon: Elongated projection that conducts impulses away from cell body of neuron

  
• Types of neurons

  • Sensory (afferent) neurons: Conduct impulses to the spinal cord and brain
  • Motor (efferent) neurons: Conduct impulses away from brain and spinal cord to muscles and glands
  • Interneurons: Conduct impulses from sensory neurons to motor neurons or among a network of interneurons; also known as central or connecting neurons

  
• Glial cells of the CNS


  • Astrocytes: Star-shaped cells that anchor small blood vessels to neurons
  • Microglia: Small cells that move in inflamed brain tissue carrying on phagocytosis
  • Oligodendrocytes: Form myelin sheaths on axons in the CNS

  
• Schwann cells


  Form myelin sheaths on axons of the PNS
• Nerve

  Bundle of peripheral axons

  
• Tract

  Bundle of central axons
• White matter

  Tissue composed primarily of myelinated axons (nerves or tracts)

  
• Gray matter

  Tissue composed primarily of cell bodies and unmyelinated fibers
• Nerve coverings

  • Endoneurium: Surrounds individual fibers within a nerve
  • Perineurium: Surrounds a group (fascicle) of nerve fibers
  • Epineurium: Surrounds the entire nerve
• Reflex arc

  Nerve impulses are conducted from receptors to effectors over neuron pathways or reflex arcs; conduction by a reflex arc results in a reflex (i.e., contraction by a muscle or secretion by a gland)

  
• Types of reflex arcs

  • Two-neuron arcs: Sensory neurons synapsing in the spinal cord with motor neurons
  • Three-neuron arcs: Sensory neurons synapsing in the spinal cord with interneurons that synapse with motor neurons

  
• Nerve impulse

  Self-propagating wave of electrical disturbance that travels along the surface of a neuron membrane (also called action potential)

  
• Mechanism of a nerve impulse

  1. At rest, the neuron's membrane is slightly positive on the outside—polarized—from a slight excess of sodium ions (Na+) on the outside

  2. A stimulus triggers the opening of Na+ channels in the plasma membrane of the neuron

  3. Inward movement of Na+ depolarizes the membrane by making the inside more positive than the outside at the stimulated point; this depolarization is a nerve impulse (action potential)

  4. The stimulated section of membrane immediately repolarizes, but by that time, the depolarization has already triggered the next section of membrane to depolarize, thus propagating a wave of electrical disturbances (depolarizations) all the way down the membrane

  
• Synapse

  The place where impulses are transmitted from one neuron to another (the postsynaptic neuron)

  
• Components of a synapse

  • Synaptic knob
  • Synaptic cleft
  • Plasma membrane
• Neurotransmitters


  Chemicals by which neurons communicate
• Examples of neurotransmitters

  • Acetylcholine (ACh)
  • Catecholamines: Norepinephrine, dopamine, and serotonin
  • Endorphins
  • Enkephalins
  • Nitric oxide (NO)
  • Other compounds
• Organs and divisions of the nervous system

  • Central nervous system (CNS)
  • Autonomic nervous system (ANS)


  * Peripheral nervous system (PNS)