Nervous system

Nervous Nervous SystemSystem

The nervous systemnervous system and the endocrineendocrine system together integrate oneone communication function of the human body

The nervous system is subdivided in a variety of ways according to its structure, the direction of information flow, and the control of effectors

Central nervous systemCentral nervous system

• Structural and functional center of the entire nervous system

• Consists of the brainbrain and spinal cordspinal cord• Integrates incoming informationincoming information from the senses,

evaluates the informationevaluates the information, and initiates an initiates an outgoing responseoutgoing response

Peripheral nervous systemPeripheral nervous system

• Consists of the nerve tissues that lie on the periphery or regions outside the CNS

• Cranial nervesCranial nerves – nerves that originate from the brain (or through the skull)

• Spinal nervesSpinal nerves – nerves that originate from the spinal cord

A-A-fferentfferent division division – consists of all incoming sensorysensory or afferent pathways

E-E-fferentfferent division division – consists of all outgoing motormotor or efferent pathways

Somatic nervous systemSomatic nervous system – carry information to the somatic effectors (skeletal muscles)

• Somatic motor divisionSomatic motor division – efferent pathways • Somatic sensory divisionSomatic sensory division – afferent

pathways• Integrating centersIntegrating centers – receive the

sensory information and generate the efferent response signal

Autonomic nervous systemAutonomic nervous system – carry information to the autonomic (visceral) effectors• Sympathetic divisionSympathetic division – pathways that exit

from the middle portions of the spinal cord; involved in preparing the body to deal with immediate threats: the fight-or-flight responsefight-or-flight response

• ParaPara--sympathetic divisionsympathetic division – exit from the brain or lower portions of the spinal cord and coordinate the body’s normal resting activities; “rest-and-repair” division

NeuronsNeurons consist of a cell body, one or more dendrites, and one axon

• Distal ends of dendritesDistal ends of dendrites of sensory neurons are called receptors because they receive the stimuli that initiate nerve signals• Axon hillockAxon hillock – tapered portion of the cell body;

“decides” whether to send the impulse any farther in the neuron• AxonsAxons with larger diameters conduct nervous impulses

faster than those with smaller diameters • Synaptic knobsSynaptic knobs – release neuroneuro--transmitterstransmitters

Classification of neuronsClassification of neurons – three distinct structural types of neurons

• MultiMulti--polarpolar neurons neurons – have only one axon but several

dendrites

• BiBi--polarpolar neurons neurons – have only one axon and also only one highly branched dendrite

• UniUni--polarpolar neurons neurons – sensory neurons with a single process extending from the cell body

Classification of neuronsClassification of neurons – according to the direction in which they conduct impulses

• AA-fferent -fferent (sensory) neurons(sensory) neurons – transmit nerve impulses to the spinal cord or brainto the spinal cord or brain

• EE-fferent -fferent (motor) neurons(motor) neurons – transmit nerve impulses away from the brain or spinal away from the brain or spinal cordcord to or toward muscles or glands

• InterInter-neurons-neurons – conduct impulses from afferent neurons to or toward motor neurons

Reflex arcReflex arc – automatic signal conduction route to and from the CNS• Most common form of reflex arc is the

three-neuron arc; consists of an: Afferent neuronInterneuronEfferent neuron

SynapseSynapse – junction between the synaptic knobs of one neuron and the dendrites (or cell body) of another neuron

Neur/o-Neur/o-gli/ogli/o - al- al (NeuroglialNeuroglial)– pertaining to the support cells, glial cells, of nerves.

Five basic types:• AstroAstro--cytescytes – largest and most numerous type of glia; help form the form the

blood-brain barrierblood-brain barrier (BBB)

• Microglia – serve a protective function when the brain is under attack by microorganisms

• Ependymal cellsEpendymal cells – produce the fluidfluid that fills the cavities in the brain and spinal cord

• Oligo-dendro-cytes – help hold nerve fibers together and also produce the vitally important myelin sheath in CNS

• Schwann cellsSchwann cells – support nerve fibers in the PNS and sometimes form a myelin sheath around them

Neur/o – al – (Neur/o – al – (NeuralNeural)) – Pertaining to nerves.NervesNerves - bundles of peripheral nerve fibers (axons) held together by several layers of connective tissues

• EndoEndo-neurium-neurium – delicate layer of fibrous connective tissue surrounding each nerve fiber

• PeriPeri-neurium-neurium – connective tissue layer surrounding each bundle of nerve fibers (fascicles)

• EpiEpi-neurium-neurium – fibrous coat surrounding numerous fascicles

TractsTracts – bundles of nerve fibers within the CNS

White matterWhite matter – bundles of myelinated fibersmyelinated fibers

Gray matterGray matter – cell bodiescell bodies and unmyelinated fibers

NucleusNucleus – distinct regions of gray matter within the CNS

GangliaGanglia – distinct regions of gray matter within the PNS

Mixed nervesMixed nerves – carry both sensorysensory (afferent) and motormotor (efferent) fibers

Sensory nervesSensory nerves – contain mostly afferent fibers

Motor nervesMotor nerves – contain mostly efferent fibers

Nerve fibers can sometimes be repaired if the damage is not extensive

Nerve impulseNerve impulse – wave of electrical energy that travels along the plasma membrane of the nerve

Cell membrane potentialCell membrane potential – difference in electrical charge across their plasma membranes• Resting membrane potential (RMP)Resting membrane potential (RMP) – mechanism that

maintains the potential voltage; when the neuron is not in an excited state, we say that the neuron is “at rest”; typically –70 mV

• The slight excess of positive ions on the outer surface is produced by ion transport mechanisms and the membrane’s permeability characteristics

• The membrane’s selective permeability characteristics help maintain a slight excess of positive ions on the outer surface of the membrane

SynapseSynapse – place where signals are transmitted from one neuron, called the prepre--synapticsynaptic neuronneuron, to another neuron, called thepostpost--synapticsynaptic neuronneuron

Types of synapsesTypes of synapses

• Electrical synapseElectrical synapse – where two cells are joined end-to-end by gap junctions; as a result, an action potential simply continues along the postsynaptic plasma membrane as if it belonged to the same cell

• Chemical synapsesChemical synapses – use a chemical neuro-transmitter to send the message to the postsynaptic cell:

• Synaptic knobSynaptic knob – contains many small sacs (vesicles) filled with neurotransmitter molecules

• Synaptic cleftSynaptic cleft – fluid-filled space (about one millionth of an inch in width) between a synaptic knob and the plasma membrane of a postsynaptic neuron

• Post-synaptic neuronPost-synaptic neuron – has protein molecules embedded in it, each facing toward the synaptic knob and its vesicles

NeuroNeuro--transmitterstransmitters – means by which neurons “talk” to one another; more than 50 compounds are now known to be neurotransmitters

NeuroNeuro--transmitterstransmitters are commonly classified by by their functiontheir function or by their chemical structure

• ExcitatoryExcitatory neurotransmitters • InhibitoryInhibitory neurotransmitters

•ExcitationExcitation – occurs when a stimulus triggers the opening of stimulus-gated Na+ channels

• InhibitionInhibition – stimulus triggers the opening of stimulus-gated K+ channels

Action potentialAction potential – an electrical signal that travels along the surface of a neuron’s plasma membrane• When an adequate stimulus is applied to a neuron, the

Na+ channels open at the point of stimulation. Na+ diffuses rapidly into the cell at the site of this local depolarization

• If the magnitude of the local depolarization exceeds a limit called the threshold potential (about –59 mV), then additional Na+ channels are opened

• As more Na+ rushes into the cell, the membrane moves rapidly toward 0 mV and then continues in a positive direction to a peak of +30 mV

• The action potential is an all-or-none response• Once the peak of the action potential is reached, the

membrane potential begins to move back toward the resting potential of –70 mV in a process called repolarization

• Because the K+ channels often remain open as the membrane reaches its resting potential, too much K+ may rush out of the cell; hyperpolarization

Absolute refractory Absolute refractory periodperiod – very brief period when a local area of an axon’s membrane resists restimulation

Conduction of the action potentialConduction of the action potential

• The action potential never moves never moves backwardbackward, restimulating the region from which it just came

• In myelinated fibers, the insulating properties of the thick myelin sheath resist ion myelin sheath resist ion movementmovement and the resulting flow of current

• The rate at which a nerve fiber conducts an impulse depends on its diameterdiameter and also on the presence or absence of a myelin presence or absence of a myelin sheathsheath

Severe psychic depressionpsychic depression occurs when a deficit of norepinephrinenorepinephrine, dopaminedopamine, serotoninserotonin, and other amines exists in certain brain synapses

• Anti-depressant drugsAnti-depressant drugs – some of these inactivate dopamine and serotonin; others called SSRIs (serotonin-specific reuptake inhibitors) produce antidepressant effects by inhibiting the uptake of serotonin

CocaineCocaine – produces a temporary feeling of well-being in cocaine abusers by blockingblocking the uptake of dopaminedopamine

AnestheticsAnesthetics – produce their effects by inhibitinginhibiting the opening of sodium channelssodium channels in the nerve cell membrane, thus blocking the initiation and conduction of nerve impulses