Biology - An Illustrated Guide to Science (794127), страница 31
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The cells do notactually join; there is always a smallgap between them.● The ends of neurons are oftenspecialized to encourage the passingof messages; for example, they canhave an increased surface area, or havea number of shorter processes calledden drites that link into the cellsaround them.●synaptic knobaxon of motor neuroncell body of motor neurondendriteschematic representation oftransmission across a synapseChemical messengersTransmission across a synapse: schematic representationtransmitter moleculesneurotransmitter receptorpostsynapticmembranedirection of impulse© Diagram Visual Information Ltd.synaptic knobsynaptic cleftsynaptic sacpresynaptic membraneEach impulse travels down the axonof the neuron to its end, which isswollen to form a synaptic knob.● The synaptic knob (see bottomdiagram) is filled with vesicles calledsynaptic sacs, which contain chemicalscalled n eu rotran sm itters.● When an impulse arrives at thesynaptic knob, chemicals trigger theejection of neurotransmitters fromsome of the vesicles, and theirneurotransmitter is released into thesynaptic cleft.● The neurotransmitter molecules bindto receptors on the postsynapticmembrane.● Some neurotransmitters stimulate animpulse in the next neuron.
Someinhibit the neuron, stopping theimpulse or blocking a pathway.● Neurotransmitter molecules arebroken down after a short time, sothat the synapse becomes open fornew impulses again.● Certain types of poisons (e.g., curare)affect these chemicals and theenzymes that regulate them. Thesepoisons kill the body by effectivelydestroying the functionality of thenervous system.●178HUMAN BIOLOGYKey wordsadrenalinautonomicnervous systemparasympatheticnervous systemspinal cordsympatheticnervous systemCoordination: autonomicnervous systemSympatheticParasympatheticIrisdilates pupilconstrictspupilBrainNo conscious controlThe au ton om ic n ervou s systemconsists of two systems that controlmany of the automatic responses inthe body.● They are not normally underconscious control and mainly deal withthe control of glands and the internalbody condition.●Salivary glandinhibits salivationcervicalgangliadecreases heart rateHeartSympathetic systemThe sym pathetic n ervou s systemprepares the body for activity and haseffects that are similar to the effects ofthe hormone adren alin .
This is thefight or flight response.● The sympathetic system arises fromthe spin al cord but with a ganglion(lump) of nervous tissue found nearthe root of the nerves.accelerates heartrate●Parasympathetic systemThe parasym pathetic n ervou s systemrelaxes the body. Its effects areantagonistic to the sympatheticsystem.● Parasympathetic nerves arise directlyfrom the spinal cord without anyganglia. The most importantparasympathetic nerve in the body isthe vagus nerve, which connects to awide range of organs in the chest andabdominal areas.stimulates salivationvagusnerveBronchidilates bronchiconstricts bronchistimulates activityStomach/ pancreasinhibits activity●glucose conversion toglycogenLiverglycogen conversionto glucoseinhibits release ofepinephrine andnorepinephrineAdrenalglandstimulatesrelease ofepinephrineandnorepinephrine© Diagram Visual Information Ltd.Intestineinhibits peristalsisstimulates peristalsisspinal cordcontracts bladderBladderchain gangliarelaxes bladder179Coordination: brainstructureHUMAN BIOLOGYKey wordscerebellumcerebralhemispheresmedullaoblongataBrainBrain from left sideparietal lobespinal cordfrontal lobeBrain structureThe brain is actually a tubularstructure that can be interpreted as aswelling of the end of the spinal cord.The central canal of the spinal cord iscontinuous with spaces at the centerof the brain.
However, because thetube is folded and expanded, thisstructure can be obscured.● Starting from the top of thespin al cord, the first swelling isthe m edu lla oblon gata. Growingout from this toward the back ofthe head is the cerebellu m .● Further along the structure are thetwo largest swellings—the cerebralhem ispheres. The cord has also bentthrough a right angle so that thehemispheres seem to sit on top ofthe cord.● Neurologists normally subdivide thecerebral cortex into four lobes: thefrontal lobe, the parietal lobe, theoccipital lobe, and the temporal lobe.cerebral hemisphere●occipitallobetemporallobecerebellummedulla oblongataspinal cordBrain from left side showing location of areassensory areamotor areapremotor areaKey areasDifferent parts of the brain havedifferent functions.
Broadly speaking,the further down the tube, the moreunconscious or automatic the activity.● The medulla oblongata coordinatesinvoluntary activities. The cerebellumcoordinates complex musclemovements and is involved inmaintaining balance.● Thinking and voluntary actionseem to be coordinated by thecerebral hemispheres.● The cerebral hemispheres havebeen extensively mapped toidentify areas concerned withsenses and movement (seebottom diagram).
Locating thearea that is concerned withconscious thought has beenmuch more difficult.visual areamotor speechareaauditory area© Diagram Visual Information Ltd.●180HUMAN BIOLOGYKey wordscerebellumcerebralhemispherehypothalamusspinal cordthalamusBrain stemThis is really an extension of thespin al cord and is a swollen area thatdeals with many automatic andmaintenance functions.● Control of breathing, heartbeat,dilation of pupils, vomiting, andcoughing are organized in this region.Lack of activity of the brain stem istaken as a conclusive sign of death—without these automatic functions, thebody cannot survive.●Coordination: brainfunctionBrain functionsCertain brain functions can be mapped toparticular areas of the cerebral cortex.● For example, a fold of the corteximmediately behind the frontal lobe, calledthe primary motor area, controls muscleactivity in various parts of the body.Hence, muscle activity in, say, the hand ortongue, can be pinpointed to a specificlocation in this area of cortex.● Similarly, the primary sensory area, lyingjust behind the motor area, receives sensoryinformation from specific parts of the bodyresponsible for the sensations of touch,pressure, temperature, and body position.●Brain from left side showing functions of areas1CerebellumThe cerebellu m consists oftwo paired hemispheresgrowing out of the back ofthe brain stem.● The decision to movea muscle is taken inthe higher centersof the cerebralhemispheres, butthe cerebellumcoordinates thefiring of thethousands ofnerve cells andcontraction ofindividual muscle fibersto achieve this movement.●Thalamus© Diagram Visual Information Ltd.●The thalam u s (not visible in thediagram) is located in the center ofthe brain beneath the centralhemispheres.
It connects the midbrainto the higher centers in the cerebralhemispheres. It has a more active rolein emotions, arousal, and somereflexes than the automatic systemsof the brain stem.Hypothalamus●The hypothalam u s (not visible in thediagram) lies beneath the thalamusand coordinates links between thehormone and nervous systems.Hearing, sight, taste, and smell, whichinvolve more complex signaling, are mappedto separate areas of the cortex.● The cerebral cortex is also responsible forconsciousness and emotions, and for mentalactivities such as language, learning, andmemory—functions that require theintegration of signals from various parts ofthe brain.● Several areas of the cortex have beenmapped as speech centers.
These areconfined to the left hemisphere in about 9out of 10 persons.1734567818291011121314151621192220cerebellumbrain stem1–8 Motor area functions1 Abdomen2 Thorax3 Arm4 Hand5 Finger6 Thumb7 Neck8 Tongue9 –16 Sensory area functions9 Abdomen10 Thorax11 Arm12 Hand13 Finger14 Thumb15 Neck16 Tongue171819202122Limb movementsSpeech controlHearingTaste and smellSpeech understandingVision181Coordination: tasteHUMAN BIOLOGYKey wordsTongueepitheliumpapillaTaste areasTongue structureThe tongue is a muscular organ thathelps to mix the food and saliva in themouth. The top surface is coveredwith specialized epithelial cells, calledpapillae, which hold the taste buds.● The taste buds are collections of nervecells that respond to a range ofchemicals. They line the papillae in thegaps between them.bittersourvertical sectionsweet●soursweetandsaltThe four tastesThe tongue responds to four basictastes: bitter, sweet, sour, and salty.● The different areas of the tonguerespond particularly well to one ofthese tastes.
The tip can detect bothsweet and salty flavors.● There appears to be limited structuraldifferences between the taste buds inthese different areas of the tongue,although the sensory nerves that arisefrom them run to slightly differentareas of the spinal cord.● Sweet tastes are the most difficult todetect, with bitter tastes a thousandtimes more dilute easily detected.●Vertical sectionpapillaepitheliumtaste budTaste budsensory fiber© Diagram Visual Information Ltd.sensory cells182HUMAN BIOLOGYKey wordsolfactory neuronCoordination: smellNoseSection through head to show nasal cavitySite of detectionThe sensory cells responsible for smellare found in the cavities behind thenose.● Smell is a chemical sense, and therelevant chemicals must dissolve in thelayer of mucus lining the nasal cavitiesbefore they can be detected.● If the nose is blocked, for example bymucus secreted during colds and flu,the sense of smell is degraded.
Sincesmell has a strong influence on taste,the sense of taste is alsocompromised.sensory fiber●smell sensory cellsnasal cavitySensory cellsChemicals that have dissolved in themucus lining the nasal cavities reactwith olfactory n eu ron s. These sendimpulses along to the olfactory bulbnear the base of the brain.● Smells have never been sorted intofour categories as tastes have been,and all olfactory neurons seem to bestructurally identical.● Olfactory neurons have a short lifespan (typically 1–2 months) and,uniquely for nerve cells, can beregenerated throughout life.● Smells appear to be able to evokeassociations much more readily thantastes.
A number of reflexive actionsare also mediated by smells, fromsneezing to feeling hungry when foodis smelled.© Diagram Visual Information Ltd.●Smell sensory cellssensory fibersensorycellsupportingcellfilm ofmoisturesensory “ hair”air currents183Coordination: earstructureHUMAN BIOLOGYKey wordsauditory ossiclecochleaeardrumeustachian tubesemi-circularcanalEarSection of the head to show internal structure of the earouter earmiddle earinner earThe outer ear● Thepinnasemicircular canalsouter ear consists of the pinna,which projects beyond the bone ofthe skull. It gathers sound wavesand funnels them toward the earcanal, which ends in the eardru m .● Sounds make the thin membraneof the eardrum vibrate.utriclesacculeossicles:stapes (stirrup)incus (anvil)auditorynervemalleus (hammer)The middle ear●ear canalcochleaThe middle ear consists of an air-filledcavity inside the eardrum.
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