OBJECTIVES By the end of this lecture you

OBJECTIVES By the end of this lecture you should be able to • Describe the functional structure of neuron – Cell body, Dendrites, Axon. • List the types of Nerve Conduction • List the factors that affecting nerve conduction • Illustrate the functional importance of is myelin sheath & Node of Ranvier • Discreminate between Contiguous Conduction and Saltatory Conduction 2

NEURON • Neuron (nerve cell) consists of Three Basic Parts : 1– Cell Body 2– Dendrites 3– Axon 3

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 – Signal toward the cell body Dendrite and cell body serve as the neurons input zone.

Neuron • Axon – Nerve fiber – Single, elongated tubular extension that conducts action potentials away from the cell body – Conducting zone of the neuron

Axon – Axon hillock • First portion of the axon plus the region of the cell body from which the axon leaves • Neuron’s trigger zone(where Action Potential are generated). – Axon terminals • Release chemical messengers that simultaneously influence other cells with which they come into close association • Output zone of the neuron 6

Neuron

NEURON (cont) ü REMEMBER • Dendrites & cell body – Input zone ( Receive) • Axon -- Conducting zone • Axon Terminals – Output zone • Action Potential are initiated at Axon Hillock (trigger zone) 8

Action Potentials are propagated from the Axon Hillock to the Axon terminal In cell body Graded Potentials (Local potentials) are produced in response to incoming signals. Action potentials are Initiated at the Axon Hillock, and conducted throughout a nerve fiber

Conduction of Action Potentials • Two types of propagation – Contiguous conduction • Conduction in unmyelinated fibers • Action potential spreads along every portion of the membrane – Saltatory conduction • Rapid conduction in myelinated fibers • Impulse jumps over sections of the fiber covered with insulating myelin –

Contiguous Conduction

Myelinated Fibers

Saltatory Conduction

Saltatory Conduction: Action Potential Propagation in a Myelinated Axon 14

Saltatory Conduction • Propagates action potential faster than contiguous conduction because action potential does not have to be regenerated at myelinated section • Myelinated fibers conduct impulses about 50 times faster than unmyelinated fibers of comparable size • Myelin – Primarily composed of lipids – Formed by oligodendrocytes in CNS – Formed by Schwann cells in PNS

CONDUCTION IN NERVE FIBERS • Conduction depends on Two factors: 1) Diameter of the nerve fiber 2) Axon (Nerve fiber) is Myelinated or Unmyelinated 16

CODUCTION IN NERVE FIBERS (cont) q. Diameter of Nerve fiber • Nerve fiber with small diameter – conducts slowly • Nerve fiber with large diameter – conducts fast 17

CONDUCTION IN NERVE FIBER (cont) q. Myelinated Nerve Fibers • Myelinated nerve fibers conduct FAST then Unmyelinated nerve fibers. Ø Why? • Myelinated nerve fibers have saltatory conduction. 18

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CONDUCTION IN NERVE FIBER (cont) ü SUMMARY • Small diameter fiber conduct SLOW • Large diameter fiber conduct FAST • Myelinated nerve fiber conduct VERY FAST. 20

Regeneration of Nerve Fibers • Regeneration of nerve fibers depends on its location • Schwann cells in PNS guide the regeneration of cut axons • Fibers in CNS myelinated by oligodendrocytes do not have regenerative ability – Oligodendrocytes inhibit regeneration of cut central axons

References • Human physiology by Lauralee Sherwood, seventh edition • Text book physiology by Guyton &Hall, 11 th edition • Text book of physiology by Linda. s contanzo, third edition