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PHYSIOLOGY OF
REFLEXES
Prof.Sangeeta SinghalDepartment of Physiology
JNMC ALIGARH
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Definition
Reflex is an involuntary response to astimulus which depends on the integrityof a particular nervous pathway. i.e. thereflex arc
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• Basic unit of integrated reflex activity is REFLEX ARC. • It has 5 components :
1. Receptor - muscle spindle( Deep Tendon Reflex)2. Afferent - 1a , II fibers3. Centre - spinal cord 4. Efferent - motor nerve 5. Effector - extrafusal fibers.
COMPONENTS OF REFLEX ARC
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Classification of reflexesREFLEXES
Clinical classificationSuperficial Deep VisceralPathological
No. of synapsesMonosynaptic BisynapticPolysynaptic
Physiological classificationFlexor Extensor Righting Postural Withdrawal
Inborn/AcquiredConditioned Unconditioned
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Clinical classification Superficial – eliciting by stimulating superficial receptors of skin
or mucus membrane.Examples:- Plantar, Abdominal, Corneal etc.
Deep – basically stretch reflexes, by stimulating receptors deep inmuscle(tendon). Stretch of muscle results in muscle contraction.
Examples:- Knee jerk, Biceps Jerk, Ankle Jerk.
Visceral –stimulating receptors in viscera. A part of reflex isformed by autonomic nerve.
Examples;- Baroreceptors reflex, Pupillary.
Pathological – present only during abnormality.Example:- Babinski sign
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Based on Number of synapses
Monosynaptic- Only one synapse is present
between afferent and efferent. Eg stretch reflex
Bisynaptic- two synapses are seen. eg. Golgi
tendon and reciprocal innervation
Polysynaptic- Many interneurons are present
between afferent and efferent neurons. eg
withdrawal reflex
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Physiological ClassificationFlexor reflexes- produced when nociceptive
stimulus are applied cause flexion of joints.Eg withdrawal reflex(protective).
Extensor reflexes- stretch reflex( responsiblefor muscle tone and posture).
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Inborn/Acquired Reflexes
• Unconditioned reflexes – inborn or inherentreflexes, do not depend upon previous experience.
• Conditioned reflexes – acquired reflexes,developed after birth, appearance depends uponprevious experience.(social habits)
- Secretion of saliva when food is kept in mouth isunconditioned reflex and,
- secretion even with thought is ‘conditioned’.
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Characteristics/Properties of Reflexes• Adequate stimulus• Irradiation • Habituation/Sensitization• Delay • Summation • Occlusion • Subliminal fringe • Facilitation • After discharge• Fatigue
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Adequate stimulus• Reflex activity is stereotype and specific(in term
of both stimulus and response).
• Receptor responds maximally only whenappropriate stimulus is applied.
• So the stimulus that produces a reflex is veryprecise , know as adequate stimulus for aparticular reflex.
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Irradiation
• Too Strong stimulus• Spread to neighboring neurons in centre
producing a wider response.• Mechanism –Transmission thru many collaterals
Withdrawal responseCrossed extensor responseMass reflex Reinforcement
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Habituation/Sensitization
Reflexes Modified by experienceIf stimulus (non injurious) repeated at
frequent intervals response declines anddisappear habituation.If injurious stimulus applied next time
causes intensification of responsesensitization.Neurotransmitters is reduced in habituation
and augmented in sensitization.
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Delay
Total reflex delay / reaction time
Time interval present between application of
stimulus and start of response.
Mechanism – due to synapse
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Summation
Subliminal stimuli – insufficient response
Spatial summation – applied simultaneouslyTemporal summation – in series
Mechanism – persistence of excitability in the pathof reflex arc, which summates with the next.
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Occlusion
T : tension produced by simultaneous stimulationof 2 afferents is less.
t1 : by afferent 1t2 : by afferent 2
T < t1 + t2
Mechanism : due to common motor nerves shared byboth afferents.
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Subliminal fringe
Reverse of occlusion
T > t 1 + t 2
Mech – separate stimuli – inadequate for some motor neurons ( subliminal )
- simultaneous stimuli – these subliminal ones get summated.
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Facilitation
If reflex is elicted repeatedly at properintervals, the response becomes progressivelyhigher for first few occassions.
Mechanism : passage of 1st impulse facilitatesthe transmission of next one – by decreasingsynaptic resistance , the next subliminal stimulusbecomes liminal. (facilitation)
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After discharge
After a continous reflex contraction , ifstimulus discontinued – contraction continues forsome time . relaxes gradually ( not at once )
Mechanism – interneuron go on discharging . Andalso impulse takes longer time to reach musclethrough interneuron.
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Fatigue
If a reflex is elicited repeatedly, it becomes feeblerand disappear.
Mechanism – seat of fatigue is CNS ( synapse)
Synapse > motor end plate > muscle.