Lecture 3.. Dr. Hameed


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Nerves .. Date:30/1/2013

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Lecture 3.. Dr. Hameed

  1. 1. CHAPTER 7• In lecture today:• Structure and function of neurons and other cells in the nervous system.• Blood-Brain Barrier.• Properties of Action Potentials• Conduction of nerve impulses• Synaptic transmission & neurotransmitters
  2. 2. Nervous System• The nervous system is divided into: 1- Central Nervous System (CNS) – Brain and Spinal cord 2- Peripheral Nervous System (PNS) – Cranial and Spinal nerves , ganglia
  3. 3. Neurons and Glial Cells• There are two types of cells in the nervous system: 1- Neurons: are the functional units of the nervous system. 2- Supporting cells: help functioning of the neurons.
  4. 4. Neuron• In general the structure of neuron is divided into: 1- cell body - contains nucleus and is the nutritional center of neuron. groups of cell bodies in the CNS = neuclei groups of cell bodies in the PNS = ganglia 2- dendrites- transmit electrical signals to the cell body. 3- axon- longer than dendrite and transmits signals away from cell body.
  5. 5. Specialized Transports in NeuronsSubstances synthesized in the cell body are transported through the axon by twomechanisms: 1- axoplasmic flow- wave like contraction that pushes cytoplasm away from the cell body. 2- axonal transport- more rapid and can occur in both directions.
  6. 6. Neurons
  7. 7. Classification of NeuronsNeurons maybe classified according to their function: 1- Sensory or afferent neurons 2- Motor or efferent neurons 3- Interneurons or association neurons • OR according to their structure: 1- Pseudounipolar- 2- Bipolar- 3- Multipolar-
  8. 8. Supporting CellsThere are 6 types of supporting cells in the nervous system:1- Schwann cells- form myelin sheath in PNS2- Oligodendrocytes- form myelin sheath in CNS3- Microglia- phagocytic cells in CNS4- Astrocytes- regulate ECF of CNS5- Ependymal cells- separate CNS from CSF6- Satellite cells- support cell bodies in PNS
  9. 9. Cells in Nervous System
  10. 10. Schwann Cells• Some axons in the PNS are surrounded by a myelin sheath formed by Schwann cells, this type of axons are called myelinated.• The insulation in the myelinated axons is by successive wrapping of the cell membrane of Schwann cells around the axon.• Unmyelinated is the type of axons that do not have myelin insulation.• Each Schwann cell wraps only a mm of length of axon leaving gaps (node of Ranviour) of exposed axon between adjacent Schwann cells.
  11. 11. Myelination
  12. 12. Myelination in the CNS• The process of forming a myelin sheath around axons in the CNS is carried out by oligodendrocytes. These cells form myelin sheath around several axons oligodendrocytes in the CNS.
  13. 13. Astrocytes• The most abundant type of supporting cells in the CNS are astrocytes.• Astrocytes support the function of neurons in by: 1- Regulating extracellular K+ concentration around neurons. 2- Regulating extracellular neurotransmitter concentration. Glutamic acid γ-aminobutyric acid (GABA) • Astrocytes also interact with brain capillaries. It is thought that they help in the formation of blood-brain barrier.
  14. 14. Blood-Brain Barrier Brain capillaries have tight junctions between their endothelial cells separating blood from brain tissue forming the blood-brain barrier (BBB). Tight junctions Brain Tissue Capillary endothelial Cells at the cerebral capillaries BLOOD Gaps Other Tissues Non-cerebral capillary endothelial cells BLOOD
  15. 15. Action potential in NeuronsNeurons communicate by producing action potential and propagatingit along their axon to other neurons or cells.
  16. 16. Conduction of Action Potentials- Electrical signals in the nerve axon are generated and transmitted in the form of action potential.- A nerve signal maybe initiated by a stimulus which can be: chemical, electrical, physical, or change in temperature.
  17. 17. Characteristics of Nerve Action Potential and its Conduction 1. Threshold: Is the minimum stimulus required for generation of action potential. 2. All-or None Law: The amplitude of the action potential is constant regardless of the strength of stimulus.
  18. 18. Characteristics of Nerve Action Potential and its Conduction Q- How the nerve can transmit different strengths of stimuli?
  19. 19. Characteristics of Nerve Action Potential and its Conduction 3. Refractory Period: During the time of action potential area of axon which the action potential is occurring can not produce another action potential.
  20. 20. Types of Nerve Fibers1- Unmyelinated Nerve Fibers:-most of the axon is exposed and without myelin covering. Transmits signals slower thanmyelinated fibers, e.g. used in slow response such as control of blood vessels.
  21. 21. 2- Myelinated Nerve Fibers (saltatory conduction):-Their axon is covered with myelin sheath, it transmits nerve signals very fast the larger thediameter of axon and myelin sheath around it the faster nerve conduction, e.g. fast sensorySignals from skin to brain or spinal cord.
  22. 22. SynapseSynapse is the functional connection between a neuron and a second cell. - Synapse can occur between: - Neurons (neuron-neuron) - A neuron and an effector cell in muscle/gland -Two types of synapses: - Electrical synapse - Chemical synapse
  23. 23. Electrical SynapseGap Junction
  24. 24. Chemical Synapse
  25. 25. Neuromuscular Junction
  26. 26. NeurotransmittersChemical synapses transfer information from neuron to the next cellacross the synapse by chemical compounds called neurotransmitters.There are many chemicals which act as neurotransmitters in the nervoussystem, here is a list of some of the most abundant ones:1- Amines, e.g. Acetylcholine (Ach)2- Cathecolamines, e.g Norepinephrine and Epinephrine3- Amino acids, e.g Glutamic acid, and GABA4- Polypeptides, e.g Glucagone, insulin5- Nitric oxide