reflexes

PHYSIOLOGY OF REFLEXES

Definition

Classification

Characteristics of reflexes

Monosynaptic reflexes

Polysynaptic reflexes

Others

Definition

Coordinated involuntary motor response initiated by stimulus, applied to peripheral receptors.

• Basic unit of integrated reflex activity is REFLEX ARC. it has 5 components :

1. Receptor - muscle spindle ( DTR)2. Afferent - 1a , II fibers3. Centre - spinal cord 4. Efferent - motor nerve 5. Effector - extrafusal fibers.

Definition

Classification




Others

Classification of reflexes

REFLEXES

Clinical classificationSuperficial

Deep Visceral

Pathological

Anatomic classification

Segmental Intersegmental Suprasegmental

No. of synapsesAsynaptic

Monosynaptic Bisynaptic

Polysynaptic

Functional classificationFlexor

Extensor Righting Postural

Withdrawal

Conditioned Unconditione

d

Clinical classification

• Superficial - stimulating superficial structures Examples• Deep – stimulating receptors deep in muscle. Examples• Visceral – stimulating receptors in viscera. Examples• Pathological – present only during abnormality

Anatomic classification

• Segmental – reflex arc pass thru one anatomic segment . Ex : knee jerk

• Intersegmental – involve > one segment Ex : crossed extensor response.

• Suprasegmental – involve interaction with suprasegmental components.

Ex: postural reflexes (head-limb)

Number of synapses

• Asynaptic axon reflex

• Monosynaptic stretch reflex

• Bisynaptic reciprocal innervation

• Polysynaptic superficial reflex

Functional classification

• Flexor reflexes• Extensor reflexes• Righting reflexes• Postural reflexes• Withdrawal reflexes

Others

• Unconditioned reflexes – inborn or inherent reflexes .

• Conditioned reflexes – acquired reflexes.

Secretion of saliva when food is kept in mouth is unconditioned reflex and,

secretion even with thought is ‘conditioned’.

Definition

Classification




Others

MONOSYNAPTIC REFLEXES

• Stretch reflex

• Inverse stretch reflex

• Reciprocal innervation

• Clonus

Stretch reflex

• When a skeletal muscle with intact nerve supply is stretched, it contracts – stretch reflex.

• Stimulus – stretch• Response – contraction • Sense organ – muscle spindle

Muscle spindle

• Proprioceptor present in skeletal muscle.

• 6-14 muscle fibers - Intrafusal fibers (IFF) - in a spindle shaped fibrous sheath.

NBF - nuclear bag fibersNCF – nuclear chain fibers

• Afferents : I a and II fibers

I a - from central part of all fibers (annulospiral ends)II – only from NCF ( flower spray ends )

• Efferent : gamma fibers to both NBF and NCF. alpha motor neurons supply extrafusal fibers

I a and gamma 1 – dynamic responseII and gamma 2 – static response

Functions of muscle spindle :

• Maintains length of muscle fibers (EFF) at rest or activity – gives feed back to higher centers for comparing with intended movement.

• Monitor velocity of muscle contraction.

Gamma efferent discharge• IFF are not strong enough or not plentiful enough

to cause shortening of muscle.

• It causes shortening thru reflex arc (indirectly) – shortening of IFF – stretch of NBF and 1a aff - reflex muscle contraction through alpha fibers .

• If muscle is stretched during the gamma stimulation, additional AP s are stimulated and rate of discharge of 1a afferents increases .

(gamma motor neurons increases sensitivity of muscle spindle to stretch)

Normally there is increased gamma discharge in

parallel to alpha. Due to this alpha-gamma linkage, spindle also shortens along with muscle.

Due to this continues response of spindle through out muscle contraction , the physiologic oscillation is reduced and muscle contraction is smoothened.

• Factors influencing gamma discharge –

noxious stimulus of skin

anxiety

Jendrassik’s maneuver : strong simultaneous motor act causes increased gamma discharges and thru irradiation influences other levels.

when voluntary act precedes by 0.5 sec, jerk is inhibited (negative reinforcement)

Higher centers also influence muscle tone through influencing gamma efferents – brain stem nuclei, cerebellum and motor cortex.

Cortex inhibits gamma neurons and cerebellum facilitates them.

Importance of stretch reflex• Production of muscle tone – “partially contracted

state of muscle”.• Maintenance of posture – in erect posture gravity

tends to flex hip/knee - this slight initial flexion stretches the extensor , causing its contraction reflexly.

due to this man remains standing.Mainly in anti-gravity musclesNon fatiguebleSimple reflex arc (short latency)

• DTR are monosynaptic reflexes.• Electrical analogue for DTR is the HOFFMAN’s

reflex or H – reflex,

INVERSE STRETCH REFLEX [Auto inhibition / lengthening reaction / clasp knife

reflex ]• When a limb is passively flexed, in spasticity, lot of

resistance occurs due to contraction of antagonist muscle. ( in this muscle, the spindle is activated by stretch which inturn causes contraction of the same thru 1a )

• When flexed with force it gives off (clasp knife) . This is due to activation of GTO of antagonist muscle.

• Impulses from GTO thru 1b afferents – reach motor neurons supplying same muscle thru an inhibitory interneuron in b/w ( golgi bottle organ) . Therefore it relaxes

• GTO detects force of contraction and helps in preventing tearing of muscle

RECIPROCAL INNERVATION

• Stimulation of afferent nerve results in contraction of agonist muscle with relaxation of antagonist muscle.

• This is the physiological basis for normal movements and Crossed extensor reflex

CLONUS

• Regular, rhythmic contractions of muscle, subjected to sudden maintained stretch.

• Mechanism – in UMN lesions, the inhibitory effect of CST on gamma neurons is lost. So gamma eff discharges are increased. So, spindles are already hyperactive

• When muscle is stretched passively, bursts of impulses from them discharges all the motor neurons at once .

The consequent contraction stops the spindle discharges, but due to sustained stretch the spindles are again stimulated and it continues

Definition

Classification




Others

POLYSYNAPTIC REFLEXES

Response with fewer synapses is earlier than with more synapses (due to synaptic delay)

Some activities reverberate until they become unable to cause a propagated transsynaptic response and dies out.

These are common in brain and SC

WITHDRAWL REFLEX • Mechanism is ‘reciprocal innervation’ . Severe pain leads to crossed extensor

response.

Sherrington pointed out the survival value of it.

flexion of stimulated limb- is to get away extn. Of other limb – to support the body.

SUPERFICIAL REFLEXES

• Stimulating areas of skin or mucosa causes contraction of corresponding muscles due to their surface origin.

• Corneal / conjunctival reflex• Abdominal reflex• Gluteal reflex• Anal reflex• Bulbocavernous reflex• Cremastric reflex• Plantar reflex

• Reflex arc for these reflexes is not definitely known. It seems to be long and complex – including many interneurons.

Afferent- carried thru posterior columns and STTEnd up - in mid brain, thalamus or fore brainEfferent – carried down thru CST, or extra

pyramidal tracts – to anterior horn cell.

• In UMN lesions , superficial reflexes lost ( path may be damaged) and DTR are exaggerated (release phenomenon)

Definition

Classification




Others

MASS REFLEX

• When central excitatory state is marked, the excitatory impulses irradiate not only to many somatic areas of SC , but also to autonomic areas.

• Ex : In chronic paraplegias, mild noxious stimulus cause – in addition to withdrawal response in all 4 limbs- urination, defecation, sweating and BP fluctuations

• Mechanism – when central inhibitions are lost, it leads to increased activity through reverberating circuits (irradiation)

Also due to prolonged effects of sympathetic mediators.

SPINAL SHOCK

• WHEN spinal cord transection is complete , there is a period immediately after the accident when all spinal reflexes below the level of transection are reduced or suppressed.

• Over wks-months , the reflexes gradually return and are exaggerated.

• Mechanism –

(of shock)- sudden withdrawal ,of tonic facilitatory influences from the brain.

(of recovery) – denervation super-sensitivity and increased post synaptic receptors.

Definition

Classification




Others


• Irradiation • Delay • Summation • Occlusion • Subliminal fringe • Facilitation • Recruitment

• After discharge• Fatigue • Fractionation • Reciprocal

innervation• Rebound

phenomenon

Irradiation

• Strong stimulus• Spread to neighboring neurons producing a

wider response.• Mechanism – collaterals

Withdrawal responseCrossed extensor responseMass reflex Reinforcement

Delay

• Total reflex delay / reaction time• Central delay

• Mechanism – due to synapse

Ex : KNEE JERK - reaction time – 19-24 msec central delay – 0.6-0.9 msec

Summation

• Subliminal stimuli – insufficient response

• Spatial summation – applied simultaneously• Temporal summation – in series

• Mechanism – persistence of excitability in the path of reflex arc, which summates with the next.

Occlusion

T : tension produced by simultaneous stimulation of 2 afferents.

t1 : by afferent 1 t2 : by afferent 2

T < t1 + t2

Mechanism : due to common motor nerves shared by both afferents.

Tt 1 + t 2

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 proper intervals, the response becomes progressively higher .

Mechanism : passage of 1st impulse facilitates the transmission of next one – by decreasing synaptic resistance , the next subliminal stimulus becomes liminal. (facilitation)

Recruitment

Direct motor nerve stimulation – tension quickly rises to maximum.

Sensory afferent stimulation – gradual rise

Mechanism – inteneuron ( ?? )

After discharge

After a reflex contraction , if stimulus discontinued – relaxes gradually ( not at once )

Mechanism – interneuron go on discharging . And also impulse takes longer time to reach muscle thru interneuron.

Fatigue

If a reflex is elicited repeatedly, it becomes feebler and disappear.

Mechanism – seat of fatigue is CNS (mainly synapse)

Synapse > motor end plate > muscle.

Fractionation

Direct motor N stimulation – higher amount of contraction , than reflexly thru afferent N

Mechanism – strength of impulse lost while crossing synapse, so only part of ( fraction of ) motor pool is stimulated

Reciprocal innervation

In a reflex , when protagonists contracts, antagonists relax to same degree.

Mechanism – bisynaptic – inhibitory interneuron present.

i afferent – at single joint. ii, iii, iv - at several joints

Rebound phenomenon

(just as muscle is excited , it can also be inhibited reflexly – reflex inhibition- tone decreased and muscle elongated)

Following it, if stimulus is stopped – tone is increased much more , instead of going to normal resting tone .

Mechanism - ??

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reflexes

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