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Understanding Neuronal Structure and Function

Learn about neurons, glial cells, electrical activity, myelination, neurotransmission, and more in the nervous system.

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Understanding Neuronal Structure and Function

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  1. CHAPTER 7 • In lecture today: • Structure and function of neurons and other cells in the nervous system. • Electrical activity in axons. • Action potential • Conduction of nerve impulses • Synaptic transmission & neurotransmitters Dr. Hameed Al-Sarraf Dept. Physiology hameed@hsc.edu.kw

  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

  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. 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. Neuron Substances synthesized in the cell body are transported through the axon by two mechanisms: 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. Neurons

  7. Sensory Motor Classification of Neurons Neurons maybe classified according to theirfunction: 1- Sensory or afferent neurons 2- Motor or efferent neurons 3- Interneurons or association neurons • OR according to theirstructure: 1- Pseudounipolar- 2- Bipolar- 3- Multipolar-

  8. Extracellular Fluid Cerebrospinal Fluid Supporting Cells There are 6 types of supporting cells in the nervous system: 1- Schwann cells- form myelin sheath in PNS 2- Oligodendrocytes- form myelin sheath in CNS 3- Microglia- phagocytic cells in CNS 4- Astrocytes- regulate ECF of CNS 5- Ependymal cells- separate CNS from CSF 6- Satellite cells- support cell bodies in PNS

  9. 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.

  10. Myelination

  11. 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 in the CNS.

  12. Glutamic acid g-aminobutyric acid (GABA) Astrocytes • The most abundant type of supporting cells in the CNS are astrocytes. • Astrocytes support the function of neurons in two ways: 1- They regulate extracellular K+ concentration around neurons. 2- the also regulate extracellular neurotransmitterconcentration. • Astrocytes also interact with brain capillaries. It is thought that • they help in the formation of blood-brain barrier.

  13. Electrical Activity in Axons • All cells in the body show potential difference between their inside and outside. • The inside is negatively charged compared to the outside. Extracellular Intracellular - 65 mV This potential difference between inside and outside is called resting membrane potential (r.m.p.). In excitable cells (nerve and muscle) the r.m.p. can change in response to stimulation.

  14. Recording Membrane Potential Depolarization Hyperpolarization

  15. Depolarization Repolarization Action Potential

  16. Ion Gating in Axons • Channels in the cell membrane which regulate movement of ions in and out • of the cell are called ion gates. • Some channels have no gates and are always open. • Channels for Na and K are specific. • K channels are: • a) gated • b) non-gated • All Na channels are gated.

  17. Membrane potential • Membrane potential is produced by the action of Na/K pump, which transports • 3Na from inside to outside and at the same time it moves 2K+ from outside to • inside of the cell. _ _ _ protein K+ _ • Resting axon is very impermeable to Na+ • but permeable to K + . • The result is difference between concentrations • of Na+ and K+ inside and outside of the cells • which produces a potential difference across the • membrane. Na+ 2K+ K+ 3Na+ K+ 4 mM Na+ 142 mM 140 mM 14 mM - 70 mV

  18. Ionic Movements During Action potential 1) r.p.m. 2) depolarization 3) repolarization - + Na+ - K+ +

  19. = Na+ Undershoot Depolarization r.p.m. Repolarization = K+ = Na+ Channel = K+ Channel

  20. Ionic Movements During Action potential

  21. Conduction of Impulses • 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.

  22. Q- How the nerve will transmit different strength of stimuli? Characteristics of Nerve Impulse & Conduction • Threshold: • Is the minimum stimulus required for generation of action potential. • Refractory Period: • During the time of action potential area of axon which the action • potential is occuring can not produce another action potential. • All-or None Law: • The amplitude of the action potential is constant regardless of the strength of stimulus.

  23. 1- Unmyelinated Nerve Fibers: • most of the axon is exposed and without myelin covering. Transmits signals slower than • myelinated fibers, e.g. used in slow response such as control of blood vessels. Types of Nerve Fibers

  24. 2-Myelinated Nerve Fibers(saltatory conduction): • Their axon is covered with myelin sheath, it transmits nerve signals very fast the larger the • diameter of axon and myelin sheath around it the faster nerve conduction, e.g. fast sensory • Signals from skin to brain or spinal cord.

  25. Synapse Synapse 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

  26. Electrical Synapse Gap Junction

  27. Electrical Synapse

  28. Chemical Synapse

  29. Axon Chemical Synapse Action potential Ca2+ Ca2+ Synaptic Vesicle Ca2+ Ca 2+ Activates calmodulin Protein kinase (inactive) Protein kinase (active) Fusion & exocytosis Ca2+ Ca2+ Neurotransmitter released

  30. Neuromuscular Junction

  31. Neurotransmitters Chemical synapses transfer information from neuron to the next cell across the synapse by chemical compounds called neurotransmitters. There are many chemicals which act as neurotransmitters in the nervous system, here is a list of some of the most abundant ones: 1- Amines, e.g. Acetylcholine (Ach) 2- Cathecolamines, e.g Norepinephrine and Epinephrine 3- Amino acids, e.g Glutamic acid, and GABA 4- Polypeptides, e.g Glucagone, insulin 5- Nitric oxide

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