عصبية

• Nervous tissue


  The main tissue component of the nervous system, composed of two types of cells: neurons and glial cells
• Neurons


  • The primary type of cell in the nervous system, responsible for computation and communication
  • They are electrically active and release chemical signals to target cells
• Glial cells


  • Cells of the central and peripheral nervous system that do not produce electrical impulses, but play a supporting role for nervous tissue
  • Provide nutrients, assist the propagation of the nerve impulse, and support and protect neurons
• Neurons are the cells considered to be the basis of nervous tissue
• Neurons are responsible for the electrical signals that communicate information about sensations, produce movements in response to stimuli, and induce thought processes within the brain
• Structure of nervous tissue

  • Made of nerve cells or neurons, all of which consist of an axon
  • Axons are long stem-like projections emerging out of the cell, responsible for communicating with other cells called the Target cells, thereby passing impulses
• Neuron


  Made up of three parts: 1. Nerve cell body, 2. Dendrite, 3. Axon
• Dendrite


  Short processes of a neuron, responsible for receiving information from other neurons and synapses
• Axon


  Longer process of a nerve cell, arises from the axon hillock and extends for a long distance away from the nerve cell body, transmits impulses away from the nerve cell body
• Nerve cell body


  • Also known as soma, irregular in shape, constituted by a mass of cytoplasm called neuroplasm, covered by a cell membrane, contains a large nucleus, Nissl bodies, neurofibrils, mitochondria and Golgi apparatus
• Nucleus


  Each neuron has one centrally placed nucleus, without a centrosome, so the nerve cell cannot multiply like other cells
• Nissl bodies


  Small basophilic granules found in the cytoplasm of neurons, present in soma and dendrite but not in axon and axon hillock
• Mitochondria


  Present in soma and axon, form the powerhouse of the nerve cell, where ATP is produced
• Golgi apparatus


  Concerned with processing and packing of proteins into granules
• Parts of a neuron and their functions

  • Dendrites - Receive signals from other cells
  • Cell body - Organizes and keeps the cell functional
  • Cell membrane - Protects the cell
  • Axon hillock - Generates impulse in the neuron
  • Nucleus - Controls the entire neuron
  • Axon - Transfers signals to other cells and organs
  • Myelin sheath - Increases the speed of the signal
  • Axon terminal - Forms junctions with other cells
  • Node of Ranvier - Allow diffusion of ions
  • Schwann cell - Produces the myelin sheath
• The nervous system regulates and controls bodily functions and activity and it consists of two parts: the central nervous system (CNS) comprising the brain and spinal cord, and the peripheral nervous system (PNS) comprising the branching peripheral nerves
• Neuron does not have centrosome, so it cannot undergo division
• Neuron functions


  • Receiving
  • Transmitting
  • Processing information
• Neuron transmission is one way: from dendrites to axon
• Methods of classifying neurons

  • Depending upon the number of poles
  • Depending upon the function
  • Depending upon the length of axon
• Unipolar neurons


  Neurons that have only one pole, from which both axon and dendrite arise, present only in the embryonic stage in humans
• Bipolar neurons


  Neurons with two poles, with axon arising from one pole and dendrites from the other pole
• Multipolar neurons


  Neurons with many poles, one pole giving rise to the axon and all other poles giving rise to dendrites
• Motor or efferent neurons


  Neurons that carry motor impulses from the central nervous system to peripheral effector organs like muscles, glands, blood vessels
• Sensory or afferent neurons


  Neurons that carry sensory impulses from the periphery to the central nervous system
• Golgi type I neurons


  Neurons with long axons, with cell bodies in different parts of the central nervous system and axons reaching remote peripheral organs
• Golgi type II neurons


  Neurons with short axons, present in the cerebral cortex and spinal cord
• Axon


  The long extension of the neuron that passes nerve impulses away from the cell body
• Dendrites


  The short extensions that pass nerve impulses towards the cell body
• Synapses


  Specialized junctions between neurons that facilitate transmission of impulses from one neuron to another neuron, also occur between axons and effector cells (muscle cells, gland cells)