reflexes

68
PHYSIOLOGY OF REFLEXES

Upload: drnaveent

Post on 07-May-2015

8.706 views

Category:

Documents


6 download

TRANSCRIPT

Page 1: Reflexes

PHYSIOLOGY OF REFLEXES

Page 2: Reflexes

Definition

Classification

Characteristics of reflexes

Monosynaptic reflexes

Polysynaptic reflexes

Others

Page 3: Reflexes

Definition

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

Page 4: Reflexes

• 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.

Page 5: Reflexes
Page 6: Reflexes

Definition

Classification

Characteristics of reflexes

Monosynaptic reflexes

Polysynaptic reflexes

Others

Page 7: Reflexes

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

Page 8: Reflexes

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

Page 9: Reflexes

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)

Page 10: Reflexes

Number of synapses

• Asynaptic axon reflex

• Monosynaptic stretch reflex

• Bisynaptic reciprocal innervation

• Polysynaptic superficial reflex

Page 11: Reflexes

Functional classification

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

Page 12: 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’.

Page 13: Reflexes

Definition

Classification

Characteristics of reflexes

Monosynaptic reflexes

Polysynaptic reflexes

Others

Page 14: Reflexes

MONOSYNAPTIC REFLEXES

• Stretch reflex

• Inverse stretch reflex

• Reciprocal innervation

• Clonus

Page 15: Reflexes

Stretch reflex

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

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

Page 16: Reflexes
Page 17: Reflexes

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

Page 18: Reflexes
Page 19: Reflexes

• 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

Page 20: Reflexes
Page 21: Reflexes

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.

Page 22: Reflexes

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 .

Page 23: Reflexes

(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.

Page 24: Reflexes

• 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)

Page 25: Reflexes

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.

Page 26: Reflexes
Page 27: Reflexes

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)

Page 28: Reflexes
Page 29: Reflexes

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

reflex or H – reflex,

Page 30: Reflexes
Page 31: Reflexes

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.

Page 32: Reflexes
Page 33: Reflexes
Page 34: Reflexes

• 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

Page 35: Reflexes
Page 36: Reflexes

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

Page 37: Reflexes

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

Page 38: Reflexes

• 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

Page 39: Reflexes

Definition

Classification

Characteristics of reflexes

Monosynaptic reflexes

Polysynaptic reflexes

Others

Page 40: Reflexes

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

Page 41: Reflexes

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.

Page 42: Reflexes
Page 43: Reflexes
Page 44: Reflexes

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

Page 45: Reflexes

• 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)

Page 46: Reflexes
Page 47: Reflexes

Definition

Classification

Characteristics of reflexes

Monosynaptic reflexes

Polysynaptic reflexes

Others

Page 48: Reflexes

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

Page 49: Reflexes

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

Also due to prolonged effects of sympathetic mediators.

Page 50: Reflexes

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.

Page 51: Reflexes

• Mechanism –

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

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

Page 52: Reflexes

Definition

Classification

Characteristics of reflexes

Monosynaptic reflexes

Polysynaptic reflexes

Others

Page 53: Reflexes

Characteristics of reflexes

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

• After discharge• Fatigue • Fractionation • Reciprocal

innervation• Rebound

phenomenon

Page 54: Reflexes

Irradiation

• Strong stimulus• Spread to neighboring neurons producing a

wider response.• Mechanism – collaterals

Withdrawal responseCrossed extensor responseMass reflex Reinforcement

Page 55: Reflexes

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

Page 56: Reflexes

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.

Page 57: Reflexes

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.

Page 58: Reflexes

Tt 1 + t 2

Page 59: Reflexes

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.

Page 60: Reflexes

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)

Page 61: Reflexes

Recruitment

Direct motor nerve stimulation – tension quickly rises to maximum.

Sensory afferent stimulation – gradual rise

Mechanism – inteneuron ( ?? )

Page 62: Reflexes

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.

Page 63: Reflexes

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.

Page 64: Reflexes

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

Page 65: Reflexes

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

Page 66: Reflexes
Page 67: Reflexes

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 - ??

Page 68: Reflexes

THANK YOU