Nervous Tissue (PART 1)

• Nervous tissue


  The main tissue of the nervous system, serves as an integrated communications network which is responsible for coordinating and controlling many body activities
• Origin of nerve tissue cells

  CNS neurons and central glial cells except microglial cells are derived from neuroectodermal cells of the neural tube, PNS ganglion cells and peripheral glia are derived from the neural crest

  
• Major cell types of nervous tissue

  • Neurons
  • Neuroglia or glial cells

  
• Neurons


  • Arranged in a chain-like fashion as an integrated communication network, the structural and functional unit of the nervous system, receive, process and transmit information through electrical and chemical signals

  
• Neurons do not divide, however in some areas of the brain, neural stem cells are present and are able to differentiate and replace damaged nerve cells

  
• Three main parts of a neuron

  • Cell body
  • Axon
  • Dendrites

  
• Cell body


  Also called the perikaryon or soma, it is a protein producing cell, contains the euchromatic nucleus with large, prominent nucleolus and surrounding perinuclear cytoplasm with most of the cell's organelles
• Cell body


  • Nucleus is pale-staining that indicates the euchromatic nature of chromatin, contains organelles like rough endoplasmic reticulum, Golgi apparatus, mitochondria, cytoskeleton
• Dendrites


  Typically short, highly branched cytoplasmic extensions that are tapered from their bases at the neuron cell body to their tips, usually covered with many synapses, serve as extensions of the cell body to receive information from other neurons or from the external environment and carry or transmit the information towards the neuron cell body

  
• Dendrites


  • Form extensive arborization called dendritic trees, consist of dendritic spines
• Axon


  A single long cell process extending from the neuron cell body, conducts nerve impulses to target cells, each neuron has only one axon
• Axon


  • The plasma membrane of the axon is often called axolemma and its contents are known as axoplasm, it conveys information away from the cell body to another neuron or to an effector cell, connected to the cell body by the axon hillock which is the site where an action potential is generated
• Axon


  • Myelinated sheath is the regional insulation produced by specialized glial cells, nodes of Ranvier are the gaps between myelin coverings that facilitate rapid impulse conduction
• Axon terminal


  End of the axon that forms by multiple branches called terminal arborization, several branches terminating in synaptic end bulb, communication with target cells via synapses
• Classification of neurons based on structure

  • Unipolar
  • Pseudounipolar
  • Bipolar
  • Multipolar

  
• Classification of neurons based on function

  • Sensory neurons (afferent neurons)
  • Motor neurons (efferent neurons)
  • Interneurons (relay neurons)

  
• Synapse


  The point of contact between a neuron and another neuron or another cell located at the axon terminal, site of transmission of a nerve impulse and electrical or chemical signals from one neuron to another, allows neurons to communicate with each other or with effector (muscle and gland) cells and accomplish their integration and control functions

  
• Presynaptic neuron


  Transmits the signal toward a synapse, contains neurotransmitter

  
• Postsynaptic neuron


  Transmits the signal away from the synapse, contains receptor sites
• Types of synapses


  • Electrical synapse
  • Chemical synapse
• Electrical synapse


  Exist between some neurons in the brain stem, retina, and cerebral cortex, consist of gap junctions that enable neighboring neurons to communicate with each other by allowing adjacent cells to exchange molecules and small ions, only for excitatory

  
• Chemical synapse


  More common than electrical synapses, the nerve impulse is transmitted from one neuron to another cell by means of chemical substances called neurotransmitters

  
• Action potential/nerve impulse


  A gradual physicochemical change in a nerve fiber's membrane that occurs after stimulation, helps to send a record of sensation or a signal from a receptor, carries information along the neuron and throughout the nervous system

  
• Neurotransmitters

  A group of chemical agents released by neurons (nerve cells) to stimulate neighbouring neurons or muscle or gland cells, thus allowing impulses to be passed from one cell to the next throughout the nervous system

  
• Nerve impulses


  Send signals from the body to the brain, neurotransmitters send signals from the brain to the body

  
• Support cells


  Essential to the function and survival of nerve cells, the CNS and PNS each have their own specific types of support cells
• Neuroglial cells


  Supportive cells that provide physical support, insulation, and nutrients to neurons, fill spaces, provide structural framework, produce myelin and carry on phagocytosis, carry out various functions within the CNS and PNS to keep them functioning

  
• Types of neuroglial cells in the CNS

  • Ependymal cells
  • Oligodendrocytes
  • Astrocytes
  • Microglia/Microglial cells

  
• Ependymal cells


  Have cilia that beat in coordinated waves to circulate cerebrospinal fluid throughout the ventricles and spinal cord
• Oligodendrocytes


  The myelin-secreting cells of the CNS, a type of neuroglia found in the central nervous system of invertebrates and vertebrates

  
• Astrocytes


  Star-shaped cells that provide physical and metabolic support for nerve cells, bind blood vessels to nerve, form the blood-brain barrier
• Microglia/Microglial cells


  The phagocytes of the CNS and consequently play important roles in brain infections and inflammation
• Support cells in the PNS

  • Satellite cells
  • Schwann cells
• Satellite cells


  Surround the cell bodies of the neurons in ganglia

  
• Schwann cells


  Responsible for the myelination of axons in the PNS
• Nervous system


  Specialized for the rapid communication of information from one region of the body to another, divided anatomically into the central nervous system (CNS) - brain and spinal cord, and the peripheral nervous system (PNS) - peripheral nerves and ganglia

  
• How the nervous system works

  Involved in receiving information about the environment (sensation) and generating responses to that information (motor responses), integration and control functions are performed by collecting stimuli from the environment, transmitting nerve impulses to reception and correlation areas, and issuing orders to effector organs for appropriate responses
• Central nervous system (CNS)

  Consists of the brain and the spinal cord, has no connective tissue stroma so the nervous tissue is soft and jellylike, protected by bony structures (skull and vertebral column) and enveloping membranes called meninges

  
• Three meningeal layers


  • Dura mater
  • Arachnoid layer
  • Pia mater
• Dura mater


  The outermost of the meningeal layers, firm and made up of dense irregular collagenous connective tissue