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Reflexes - Physiology - Lecture Notes, Study notes of Physiology

Dhirubhai Ambani Institute of Information and Communication TechnologyPhysiology

Very well accomplished lecture for Physiology students. This lecture handout includes: Reflexes, Receptors, Electromagnetic Wave Receptors, Qualitative Law, Law of Weber and Fechner, Law of Specific Energy, Adaptation of Receptors, Tonic Receptors, Phasic Receptors

Typology: Study notes

2013/2014

Uploaded on 01/31/2014

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Reflexes:
Reflexisaninvoluntaryandunconsciousresponsetoastimulusappliedonareceptor.Inthis
reactionthenervoussystemisinvolved.Theanatomicalsupportofthereflexisthereflexarch.Itis
madeupof5elements:thereceptor(afreenervousendingoraspecializednervousending);
Afferentpathway(theyarenervousfiberswhichhavetheirbodyinspinalgangliaorgangliaattached
tocranialnerves);Nervouscenter(Arelocalizedisspinalcordorinbrain);Efferentpathway(whichis
madeofsomaticorautonomicnervousfibers);Effector(allcellsofthebodycanbeeffectors,butin
physiologyitisglandsormuscles).
RECEPTORS:
Theyhavetheroletoconvertdifferentformofenergyfromtheinternalenvironment,orthe
externalenvironment.Classificationofreceptors:
‐Accordingtotheirlocalization,theyareclassifiedin
Exteroreceptor:arestimulatedbystimulicomingfromoutsideofthebody,extero
receptorsareclassifiedintelereceptors(rodcellsfromeye).ContactReceptors,direct
contactbetweenstimulusandreceptor(tactilereceptorsorheatreceptors)
Interoreceptors:arestimulatedbystimulicomingfrominsidetebody,theyareclassifiedin
proprioreceptorsandvisceroreceptors.Proprioreceptorsarefoundinmuscles,tendons,
joints,vestibularapparatus(ear).Visceroreceptorsarefoundininternalorgans.
‐Accordingtothenatureofstimuli,theyareclassifiedin
mechanoreceptors
baroreceptors
thermoreceptors
chemoreceptors
painreceptors
electromagneticwavereceptors.
Receptorsarethefirstcomponentofthereflex,theyhavetheroletotransformdifferentformof
energyfromtheenvironment,innervousimpulses.Attheirleveltakesplacetheencodingof
information.Thereareseveraltypesofstimuli,butallofthemaretransformedinnervousimpulses.
Oneoftherolesofthereceptorsistoencodetheinformation.Receptorscanbefreenervous
endings,orspecializednervousendings.Thefunctioningofreceptorsisdoneaccordingtotwo
characteristics:
Qualitativelaw:lawofspecificenergy:Thecentralnervoussystemispermanentlyinformed
aboutthenatureofstimulibymeansofspecificityofreceptors,andalsoaccordingtothe
frequencyorrayofimpulsestransmittedthroughthenervousfiber.Stimulationofreceptors
producesaspecificsensation.Bystimulationoftouchreceptorsthesensationoftouchingis
obtained,bystimulationofheatreceptors,thesensationofwarmifobtained.Everyreceptor
hastheroletosenseacertaintypeofreceptors,fordifferentkindofstimulithereare
differentreceptors.Thesamesensationisobtainedbystimulationofthepathwayfromthe
receptoruntilthecortex.Evenifthecortexisstimulatedwefeelthesamesensation.The
formofenergyatwhichareceptor’sresponseisoptimalisnamedadequatestimulus.In
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Reflexes :

Reflex is an involuntary and unconscious response to a stimulus applied on a receptor. In this reaction the nervous system is involved. The anatomical support of the reflex is the reflex arch. It is made up of 5 elements: the receptor (a free nervous ending or a specialized nervous ending); Afferent pathway (they are nervous fibers which have their body in spinal ganglia or ganglia attached to cranial nerves); Nervous center (Are localized is spinal cord or in brain); Efferent pathway (which is made of somatic or autonomic nervous fibers); Effector (all cells of the body can be effectors, but in physiology it is glands or muscles).

RECEPTORS :

They have the role to convert different form of energy from the internal environment, or the external environment. Classification of receptors: ‐According to their localization, they are classified in  Extero receptor : are stimulated by stimuli coming from outside of the body, extero receptors are classified in telereceptors (rod cells from eye). Contact Receptors, direct contact between stimulus and receptor (tactile receptors or heat receptors)  Intero receptors : are stimulated by stimuli coming from inside te body, they are classified in proprioreceptors and visceroreceptors. Proprioreceptors are found in muscles, tendons, joints, vestibular apparatus (ear). Viscero receptors are found in internal organs. ‐According to the nature of stimuli, they are classified in  mechano receptors  baroreceptors  thermoreceptors  chemoreceptors  pain receptors  electromagnetic wave receptors. Receptors are the first component of the reflex, they have the role to transform different form of energy from the environment, in nervous impulses. At their level takes place the encoding of information. There are several types of stimuli, but all of them are transformed in nervous impulses. One of the roles of the receptors is to encode the information. Receptors can be free nervous endings, or specialized nervous endings. The functioning of receptors is done according to two characteristics :  Qualitative law: law of specific energy: The central nervous system is permanently informed about the nature of stimuli by means of specificity of receptors, and also according to the frequency or ray of impulses transmitted through the nervous fiber. Stimulation of receptors produces a specific sensation. By stimulation of touch receptors the sensation of touching is obtained, by stimulation of heat receptors, the sensation of warm if obtained. Every receptor has the role to sense a certain type of receptors, for different kind of stimuli there are different receptors. The same sensation is obtained by stimulation of the pathway from the receptor until the cortex. Even if the cortex is stimulated we feel the same sensation. The form of energy at which a receptor’s response is optimal is named adequate stimulus. In

some unusual conditions, receptors can respond to other type of energy, but the sensation is always the one corresponding to adequate stimulus. For instance, retina is stimulated by light, but if eye balls are pressed then also the sensation is of light.  Quantitative law: Law of Weber and Fechner: By this law it is established the proportionality between the intensity of stimulation of a receptor and the amplitude of response. Weber performed some experiments on human beings, whose eyes were closed, and he tried to establish what should be the intensity of stimulus for the people to find a change. In the palm of the person were put different weight, and he wanted to know when the person feel the difference of weight between the two palms. He discovered that from one hundred gram the person starts to feel a difference. They established a formula to calculate the relationship between the stimuli and response. S=K.logW (S= sensation; K=constant; W= weight) When the stimulus increases with 10%, then there is a sensation that the stimuli are different, and they also established that when the weight is increased by 10%, the number of action potential for the nerves is twice more frequent. The nervous system evaluates the importance of a stimulus according to the number of action potential created by the stimulus. The functioning of receptors was established using micro‐electrodes in different receptors. One receptor very well studied was Vater_Paccini corpuscule, receptor for touch and was used in experiments for studying function of receptors. It is made up of three nervous ending having the diameter of 2 micrometers, and this nervous ending is covered by several layers of connective tissue, and just before leaving the corpuscule the nervous ending is myelinated. The first noeud of ranvier is inside the corpuscule, and it s aspect is like and oignon. The receptor was stimulated by compression, so by applying a weight, the receptor was stimulated, and by introducing electrode inside of this corpuscule, the action potential, or the change of potential was measured. It was established that after the stimulation of this receptor, the depolatosation f the nervous ending occurs and ths change of potential is named : generator potential, or receptor potential. The value of resting membrane potential of the nervous ending is ‐70mV; but under the action the stimulus, sodium channels are opened, sodium penetrates inside this nervous ending and depolarize the structure, but only when the change of potential becomes higher than 10mV, only then an action potential takes place and is transmitted. So it’s important that enough sodium penetrates. The receptor potential doesn’t obey to the law “all or none”, in other words, by increasing the pressure exerted on receptors the amplitude of depolarization will increase. In case of action potential, the action potential obey to the law of all or nothing. The intensity of stimulus is established in nervous system according to the frequency of action potentials which are transmitted. A weak stimulus will generate rare potentials in nervous fibers, while a stranger stimulus will generate more action potential.

ADAPTATION OF RECEPTORS :

When a receptor is stimulated, at the beginning the number of impulses is higher, but after a certain period of time, the number of impulses decreases, and at a certain moment, action potentials, or impulses will not be transmitted anymore even if the stimulus are continued. According to the capacity of receptors to adapt there are two types of receptors: