L9 Neuron Physiology PDF

‫بسم الله الرحمن‬ ‫الرحيم‬ PHYSIOLOGY OF FOUNDATION BLOCK L9: NEURON  NEURON It is the structural unit of nervous system. Neuron (=nerve cell) consists of Three Basic Parts : 1– Cell Body 2– Dendrites 3– Axon...

‫بسم الله الرحمن‬ ‫الرحيم‬ PHYSIOLOGY OF FOUNDATION BLOCK L9: NEURON  NEURON It is the structural unit of nervous system. Neuron (=nerve cell) consists of Three Basic Parts : 1– Cell Body 2– Dendrites 3– Axon 2  NEURON Cell body Houses the nucleus and organelles. Dendrites Project from cell body and increase surface area available for receiving signals from other nerve cells Carry Signals toward the cell body Some neurons have up to 400,000 dendrites. Dendrite and cell body serve as the neurons input zone. 3  NEURON - In cell body, Graded Potentials (=Local potentials) are produced in response to incoming signals. - Action potentials are Initiated at the Axon Hillock (=Trigger zone), and conducted throughout a nerve fiber to the Axon terminals 4  AXON (= nerve fiber): Axon is a single, elongated, tubular extension from the cell body. Axon conducts Action Potentials away from cell body. It is the Conducting zone of the neuron. It terminates on other cells. Axons may be less than 1 mm or longer than 1 meter depending on the area they innervate. 5 Axon Hillock It is the first portion of Axon + Cell body from which axon leaves. It is the place where Action Potentials are generated (= trigger zone). Action potentials are then conducted from Axon Hillock along the axon to its branched endings called Axon Terminals. 6 Axon Terminals release chemical messengers or transmitters to other cells with which they come in contact. Axon Terminals are Output zone. 7  Dendrites & cell body (= Input or receptive zone). Action Potential are initiated at Axon Hillock (= Trigger zone) Axon (= Conducting zone) Axon Terminals (= Output zone) 8  Factors affecting conduction in nerve fibers Conduction depends on Two factors: 1) Myelinated or Unmyelinated. 2) Diameter of the nerve fiber. 9  1)What is Myelin Sheath ? It is formed by Schwann cells (= myelin forming cells in Peripheral Nervous System [PNS] that cover the axon.. It is a thick layer composed of lipoprotein. it is present at regular intervals along the length of nerve fiber (interrupted). In Central Nervous System (CNS) i.e Brain and Spinal cord, myelin is formed by oligodendrocytes. 10  Myelin Sheath It is an Insulator to electric currents. As water soluble ions (e.g. Na+), which carry current across the membrane, can not pass. Interrupted by nodes of Ranvier through which ions can pass. axonal membrane is not covered with myelin sheath and exposed to Extra-Cellular Fluid [ECF]. Increase the speed of conduction of impulses. 11  Myelinated Nerve Fibers have increased Nerve Conduction Velocity [NCV], Why ? As Myelinated fibers are covered with myelin sheath. In Myelinated Fibers, current can flow at Node Of Ranvier to produce Action Potential. Therefore, impulse jumps from node to node, skipping over the myelinated portion of axon “= saltatory conduction” [saltare means to jump]. Saltatory Conduction [in myelinated fibers] is faster than continuous conduction in unmyelinated fibers.  Unmyelinated Nerve Fibers In Unmyelinated Fibers, nerve impulse passes along the entire length of nerve fiber. It is slower conduction as compared to myelinated fibers. 13  TYPES OF CONDUCTION IN NEURON There are TWO types of conduction: 1- Contiguous or continuous conduction - Conduction in unmyelinated fibers - Action potential spreads along every portion of the membrane. 2- Saltatory conduction - Rapid conduction in myelinated fibers - Impulse jumps over sections of the fiber covered with insulating myelin 14 Contiguous Conduction 15  Saltatory Conduction Saltatory conduction Propagates action potential faster than contiguous conduction because action potential does not have to be regenerated at myelinated section. 16  2- Diameter of Nerve fiber Nerve fiber with small diameter conducts slowly Nerve fiber with large diameter conducts fast  CLINICAL APPLICATION In case of a cut axon in peripheral nerve, there is Regeneration (Re-growth) of axon due to Schwann cell. In case of CNS ( Brain and Spinal cord), axons which are myelinated by Oligodendrocytes, do not regenerate. 18  References Human physiology by Lauralee Sherwood, 9th edition Text Book Of Physiology by Guyton & Hall, 12th edition Review of Medical Physiology by Ganong, 24th edition