7. Nervous System

• Functions of the Nervous System


  • Sensory input - gathering information
  • To monitor changes occurring inside and outside the body
  • Integration - to process and interpret sensory input and decide if action is needed
  • Motor output - a response to integrated stimuli

  
• Central Nervous System (CNS)


  • Brain and Spinal cord
  • Acts as integrating and command center - interpret incoming sensory information and issue instructions based on past experiences and current conditions

  
• Peripheral Nervous System (PNS)


  • Nerves outside the brain and spinal cord
  • Link all parts of the body by carrying impulses to the CNS and back
• Functional Classification of the Peripheral Nervous System


  • Sensory (afferent) division - nerve fibers that carry information to the central nervous system
  • Motor (efferent) division - nerve fibers that carry impulses away from the central nervous system
• Motor (efferent) division of the Peripheral Nervous System


  • Somatic nervous system = voluntary nervous system
  • Autonomic nervous system = involuntary nervous system

  
• Astrocytes


  • Abundant, star-shaped cells
  • Brace neurons
  • Form barrier between capillaries and neurons and make exchanges between the two
  • Control the chemical environment of the brain by capturing ions and neurotransmitters

  
• Microglia


  • Spider-like phagocytes
  • Dispose of debris - dead cells and bacteria
• Ependymal cells


  • Line cavities of the brain and spinal cord
  • Circulate cerebrospinal fluid with cilia

  
• Oligodendrocytes


  • Wrap their flat extensions tightly around the nerve fibers
  • Produce myelin sheath around nerve fibers in the central nervous system

  
• Satellite cells


  • Protect neuron cell bodies
• Schwann cells


  • Form myelin sheath in the peripheral nervous system

  
• Neurons


  Nerve cells specialized to transmit messages

  
• Major regions of neurons


  • Cell body - nucleus and metabolic center of the cell
  • Processes - fibers that extend from the cell body
• Cell body


  • Nissl substance - specialized rough endoplasmic reticulum
  • Neurofibrils - intermediate cytoskeleton that maintains cell shape
  • Nucleus
  • Large nucleolus
• Dendrites


  Conduct impulses toward the cell body
• Axons


  Conduct impulses away from the cell body
• Axonal terminals


  Contain vesicles with neurotransmitters
• Synaptic cleft


  Gap between adjacent neurons
• Synapse


  Junction between nerves
• Schwann cells


  • Produce myelin sheaths in jelly-roll
• Nodes of Ranvier


  Gaps in myelin sheath along the axon

  
• Gray matter


  Cell bodies and unmyelinated fibers
• White matter


  Myelinated fibers
• Ganglia


  Collections of cell bodies outside the central nervous system
• Functional Classification of Neurons


  • Sensory (afferent) neurons - carry impulses from the sensory receptors to CNS
  • Motor (efferent) neurons - carry impulses from the central nervous system
  • Interneurons (association neurons) - found in neural pathways in the central nervous system, connect sensory and motor neurons

  
• Structural Classification of Neurons


  • Multipolar neurons - many extensions from the cell body
  • Bipolar neurons - one axon and one dendrite
  • Unipolar neurons - have a short, single process leaving the cell body
• Irritability


  Ability to respond to stimuli
• Conductivity


  Ability to transmit an impulse
• Depolarization


  A stimulus depolarizes the neuron's membrane, allowing sodium (Na+) to flow inside the membrane, initiating an action potential (nerve impulse)
• Action potential


  Nerve impulse that is propagated over the entire axon - all-or-none response

  
• Repolarization


  Potassium ions rush out of the neuron after sodium ions rush in, restoring the original configuration
• Sodium-potassium pump


  Restores the original configuration, requires ATP
• Nerve impulse propagation


  Impulses travel faster when fibers have a myelin sheath, literally jumping from node to node because it cannot cross myelin insulation

  
• Neurotransmitter


  Released from a nerve's axon terminal, stimulates receptors on the dendrite of the next neuron to start an action potential
• Reflex


  Rapid, predictable, and involuntary responses to stimuli
• Reflex arc


  Direct route from a sensory neuron, to an interneuron, to an effector
• Types of Reflexes and Regulation


  • Autonomic reflexes - smooth muscle regulation, size of eye pupils, heart and blood pressure regulation, regulation of glands and sweating, digestive system and elimination regulation
  • Somatic reflexes - activation of skeletal muscles
• Elements of Reflex Arcs

  • A sensory receptor - reacts to stimuli
  • An effector receptor - muscle or gland stimulated
  • Afferent and efferent neurons connecting the two
  • The CNS integration center

  
• Neural tube


  Develops into the brain and spinal cord, opening becomes the ventricles
• Regions of the Brain

  • Cerebral hemispheres
  • Diencephalon
  • Brain stem
  • Cerebellum