2Axons and Nerve Impulses Axons end in axonal terminalsAxonal terminals contain vesicles that contain chemicals called neurotransmittersWhen the impulses reach the axonal terminals, they stimulate the release of neurotransmitters into the extracellular space.Axonal terminals are separated from the next neuron by a gapSynaptic cleft – gap between adjacent neuronsSynapse – junction between nerves
4MyelinMost long nerve fibers are covered with a whitish, fatty material called myelinHas a waxy appearanceProtects and insulates the fibersIncreases the transmission rate of nerve impulsesMyelin sheaths – a tight coil of wrapped membranes that encloses the axon
5Nerve Fiber CoveringsSchwann cells – produce myelin sheaths in jelly-roll like fashion in axons outside the CNSNodes of Ranvier – gaps in myelin sheath along the axon
6Multiple Sclerosis (MS) The myelin sheaths around the fibers are gradually destroyed and converted to hardened sheaths called sclerosis.As this happens, the circuit is short-circuited.The affected person loses the ability to control his or her muscles and becomes increasingly disabled.Is an autoimmune disease in which a protein component of the sheath is attacked.
7Neuron Cell Body Location Most are found in the CNSNuclei – clusters of cell bodies within the white matter of the CNSWell-protected location within the bony skull or vertebral column is essentialNeurons do not routinely undergo cell division after birthIf it is damaged the cell dies and is not replacedSome are found outside the CNSGanglia – collections of cell bodies outside the CNS
8Nerve FibersTracts – bundles of nerve fibers (neuron processes) running through the CNSNerves – bundles of nerve fibers (neuron processes) running through the PNSWhite matter – consists of dense collections of myelinated fibers (tracts)Gray matter – contains mostly unmyelinated fibers and cell bodies
9Functional Classification of Neurons Sensory (afferent) neuronsCarry impulses from the sensory receptors to the CNSKeep us informed about what is happening both inside and outside the bodyThe dendrite endings of the sensory neuron are usually associated with specialized receptors.Cutaneous sense organs – found in the skinProprioceptors – detect stretch or tension in the muscles and tendons
10Functional Classification of Neurons Motor (efferent) neuronsCarry impulses from the CNS to the muscles and glandsInterneurons (association neurons)Found in neural pathways in the CNSConnect sensory and motor neurons
12Structural Classification of Neurons Multipolar neurons – many extensions from the cell bodyMost common type: all motor and association neurons are multipolar
13Structural Classification of Neurons Bipolar neurons – Neurons with two processes - one axon and one dendriteRare in adultsFound only in some special sense organs such as the ear and the eye, where they act as sensory receptor cells
14Structural Classification of Neurons Unipolar neurons – have a short single process leaving the cell bodyThe single process is short and divides almost immediately into central and peripheral fibers.In this case, the axon conducts nerve impulses both toward and away from the cell body.
15Functional Properties of Neurons Neurons have two major functional properties:Irritability – ability to respond to stimuliConductivity – ability to transmit an impulse
16Plasma Membrane of a Resting Neuron The plasma membrane at rest is polarized.Fewer positive ions are inside the cell than outside the cell.The major positive ions inside the cell are potassium (K+).The major positive ions outside the cell are sodium (Na+).As long as the inside remains more negative as compared to the outside, the neuron will stay inactive.
17StimulusMany different stimuli excite neurons to become active and generate an impulse.Light excites the eye receptors, sound excites some of the ear receptors, and pressure excites some cutaneous receptors of the skin.Most neurons in the body are excited by neurotransmitters released by other neurons.Regardless of what the stimulus is, the result is always the same – the “sodium gates” in the membrane open allowing an inward rush of sodium ions.
18Starting a Nerve Impulse A stimulus depolarizes the neuron’s membrane.Depolarization – the loss of a negative charge inside the plasma membrane.A depolarized membrane allows sodium (Na+) to flow inside the membrane.The inside becomes more positive.The exchange of ions initiates an action potential in the neuron.
19The Action PotentialIf the action potential (nerve impulse) starts, it is propagated over the entire axon.
20RepolarizationAlmost immediately after the Na+ rush into the neuron, the membrane permeability changes again: It becomes impermeable to Na+, but permeable to K+.K+ rush out of the neuron, which repolarizes the membraneRepolarization - the outflow of positive ions, which restores the electrical conditions at the membrane to the resting state.Until repolarization occurs, a neuron cannot conduct another impulse
21RepolarizationUntil repolarization occurs, a neuron cannot conduct another impulseRefractory Period – Period of repolarization of the neuron during which it cannot respond to a second stimulus
22The Sodium Potassium Pump The sodium-potassium pump restores the original configuration of sodium and potassium ions inside and outside the neuron.This action requires ATP.
23Nerve Impulse Propagation The impulse continues to move toward the cell bodyImpulses travel faster when fibers have a myelin sheathNerve impulses literally jumps or leaps from node to node along the fiber.No current can flow across the axonal membrane where there is fatty myelin insulation.
24Blocking Nerve Impulse Conduction It is possible to block nerve impulses by reducing membrane permeability to sodium ions.No sodium entry = no action potentialAlcohol, sedatives, and anesthetics all do this.It is also possible to hinder impulse conduction by interrupting blood circulation (interrupt the delivery of oxygen and nutrients).Examples:Cold – fingers get numb when you hold an ice cube.Continuous pressure – when you sit on your foot, it “goes to sleep”When you warm the fingers or remove the pressure from the foot, the impulses begin to be transmitted again, leading to an unpleasant prickly feeling.
25Continuation of the Nerve Impulse between Neurons Impulses are able to cross the synapse to another nerveNeurotransmitter is released from a nerve’s axon terminalThe dendrite of the next neuron has receptors that are stimulated by the neurotransmitterAn action potential is started in the dendrite