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PHYSIOLOGY OF

REFLEXES

Prof.Sangeeta SinghalDepartment of Physiology

JNMC ALIGARH

Definition

Reflex is an involuntary response to astimulus which depends on the integrityof a particular nervous pathway. i.e. thereflex arc

• 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

Classification of reflexesREFLEXES

Clinical classificationSuperficial Deep VisceralPathological

No. of synapsesMonosynaptic BisynapticPolysynaptic

Physiological classificationFlexor Extensor Righting Postural Withdrawal

Inborn/AcquiredConditioned Unconditioned

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

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

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).

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’.

Characteristics/Properties of Reflexes• Adequate stimulus• Irradiation • Habituation/Sensitization• Delay • Summation • Occlusion • Subliminal fringe • Facilitation • After discharge• Fatigue

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.

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

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.

Delay

Total reflex delay / reaction time

Time interval present between application of

stimulus and start of response.

Mechanism – due to synapse

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.

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.

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.

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)

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.

Fatigue

If a reflex is elicited repeatedly, it becomes feeblerand disappear.

Mechanism – seat of fatigue is CNS ( synapse)

Synapse > motor end plate > muscle.