LECT. 1, Reflex action by dr sadia zafar

7/30/2019 LECT. 1, Reflex action by dr sadia zafar

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Reflex action.Classification of Reflexes.

Muscle spindle &Muscle tone.


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Dermatome:

Area of skin supplied by a spinal nerve or a segment ofspinal cord.

Trunk horizontal band (T2T12)

Limbs specific pattern There is overlapping of adjacent dermatomes.

We can assess the level of damage from dermatomes.

There are 31 pairs of spinal nerves.

Region-wise: 8 cevical, 12 thoracic, 5 lumbar, 5 sacral,

1 coccygeal.


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Reflex action:

An automatic response to a stimulus, withoutthe involvement of will or consciousness, e.g.,

light is thrown on eye pupil constricts.

Advantage: protective value.

Purpose: quick response, save time, preventsdamage to body tissues.


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Reflex arc:

Reflex action involves specific organizationcalled reflex arc.

Reflex arc has 5 components:

Receptor

Sensory / afferent neuron

Centre

Motor / efferent neuron

Effector / target cell


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Components of a reflex arc:

Receptors: respond to stimuli by producing impulses.

Impulses from receptors are carried by sensory neurons to thecentre.

Centre is the integrating part & is always in CNS

In the centre there is 1 or more than 1 synapse between afferent& efferent neurons.

Efferent neuron carries impulses from centre to target cells.

Target cells are responding cells (skeletal, cardiac, smooth

muscles & glands). Integrity of reflex arc is essential for reflex action.

If arc is broken at any point no reflex action.


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Classification of reflex:Depending upon number of synapses in reflex arc: Monosynaptic (e.g: stretch reflex like knee jerk)

Disynaptic (e.g: golgi tendon organ reflex) Polysynaptic / multisynaptic (e.g: withdrawal reflex)Depending upon conditioned / unconditioned or acquired / inborn:

Inborn / unconditioned: present at birth. (e.g: light reflex, salivationon introduction of something in mouth, defecation reflex withouthigher control).

Acquired / conditioned: develop in response to a conditionedstimulus (ringing of bell) with the help of an unconditioned stimulus

(sight of food, smell of food). On sight / smell of food salivation(unconditioned). On ringing of bell salivation (conditioned).

PAVLOVS EXPERIMENTS: Animal is kept in cage. First bell isrung & then animal is served food. Practiced many times. Bell isrung without serving food salivation (acquired reflex in response

of a conditioned stimulus with the help of an unconditionedstimulus.


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Superficial, deep, visceral & pathological reflexes:

Superficial reflexes: receptors are in superficial

parts of body. (mucus, cutaneous)

Corneal

Conjunctival

Superficial abdominal

Plantar

Cremasteric


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Deep reflexes / tendon jerks: receptors arepresent in deeper tissues.

Biceps jerk

Triceps jerk

Knee jerk

Ankle jerk

Supinator reflex

Scapular reflex


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Visceral reflexes: receptors are in viscera.

Light reflex

Micturition reflex Defecation reflex

Oculo-cardiac reflex

Baro-receptor reflex Bain-bridge reflex


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Pathological reflexes: in diseased states.

Babinski sign /plantar reflex

Ankle clonus Knee clonus


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Babinski sign / plantar reflex


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Monosynaptic reflexes:

Example: Stretch reflex / myotatic reflex:

Whenever a skeletal muscle is stretched, itcontracts.

Stimulus: is stretch.

Response: is contraction.

Receptors: are muscle spindle.


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Ends of muscle spindle are fused with sides ofmuscle fibers. So when a change in muscle sizeoccurs, they are stretched. Inside each spindle

are modified muscle fibers called intrafusalfibers.

Number of muscle fibers vary. Muscle fibersinvolved in fine skilled movements (handmuscles) have increased number of musclespindles.


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2 types of fibers in muscle spindles:

Nuclear bag fibers:

1-3 in number in eachspindle.

Nuclei in central dilatedportion.

Nuclear chain fibers:

3-9 in number.

Ends are attached with

sides of nuclear bagfibers.

Nuclei in central dilated portion

Nuclei are present in the form of a chain

throughout the length of fiberBAG CHAIN


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The end portion of intrafusal fibers containcontractile components (actin & myosinfilaments).

Central portion of fibers is without actin-myosin filaments. It is called receptor portion.


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Sensory nerve supply of intrafusal fibers:

2 types of sensory nerve endings:

Primary or annulospiral:

Present around centralportion of both nuclear

bag & nuclear chainfibers.

Endings of type IAfibers with conduction

velocity: 70-120 m/sec.

Secondary or flower spray:

Endings of type II nervefibers.

Velocity is 30-70 m/sec.

Only around nuclearchain fibers.

NUCLEAR CHAIN FIBERS ARE SUPPLIED BY BOTH TYPES OF NERVE ENDINGS


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Motor nerve supply of intrafusal

fibers:

Gamma efferents supply end portion ofintrafusalfibers & end portions are contractile.

Conduction velocity: 15-30 m/sec.

Forms 30% of nerve fibers in the ventral root ofspinal nerve (motor root).

*Alpha motor neuron supply the extrafusal fibers.


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Muscle spindle is stimulated when its central

portion is stretched by either of 2 ways:

When muscle is stretched,along with that, musclespindle is also stretched, sogets stimulated.

Internal stretching: i-e.,muscle spindle gets stretched

without stretching of muscle,through gamma efferents

which supply end portions ofintrafusal fibers whichcontract so central portion is

stretched stimulation.

This is the mechanism ofmuscle tone.


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Stretching of muscle can be

Static/slow or Rapid/dynamic

STATIC / SLOW:

When a static stretch is

applied to a muscleincrease in muscle length

increased discharge ofimpulses from both primary& secondary nerve endings &remains increased as long as

muscle is stretched. Nuclear chain fibers are

involved.

RAPID / DYNAMIC:

When a rapid stretch is

appliedrapid increase inlength of muscleprimarynerve endings dischargeincreases this dischargedecreases when rapidincrease in length stops.

Nuclear bag fibers areinvolved in dynamic response(e.g., knee jerk)

WHEN CENTRE PORTION OF A MUSCLE SPINDLE IS STRETCHED,DISCHARGE OF IMPULSES FROM MUSCLE SPINDLE INCREASE & VICE VERSA


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FUNCTIONS OF MUSCLE

SPINDLE: 1) Helps to regulate length of muscle:

& prevents length of muscle to go beyond limits. When a muscle isstretched, it contracts (stretch reflex). So increase in muscle lengthbeyond limit is prevented & when muscle contracts & shortensdischarge of impulses from muscle spindle decreases musclerelaxation so too much muscle shortening is also prevented.

2) Involved in muscle tone mechanism: 3)Are receptors in tendon jerks: which are clinically important.All

tendon jerks are monosynaptic stretch reflexes & receptors aremuscle spindles.

4) Stretch reflex helps in lifting of load / weight: muscle is stretched& it contracts, so helps in lifting.

5) For voluntary smooth movements: There is coactivation of alpha& gamma motor neurons. Whenever there is voluntary movement,impulses from motor cortex simultaneously go to alpha & gammamotor neurons smooth voluntary contraction


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When muscle contractsonly alpha efferentscarrying impulses decreased discharge ofimpulses from muscle spindle relaxation (no

voluntary smooth movement because at 1 endmuscle is contracting & at other end it isrelaxing).

MUSCLE CONTRACTS


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When impulses come alongalpha & gamma

efferents simultaneously smooth voluntarymovements muscle contracts but intrafusal

fibers lengthenfiring rate remainsconstant, discharge is not decreased becauseof gamma neuron co-activation.


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Monosynaptic reflex / knee jerk /

typical stretch reflex: Stimulus quadriceps stretched spinal cord alpha motor

neuron muscle (quadriceps contracts effect: leg jerksforward. (L3, L4).

In UMN disease, tendon jerks become aggravated. In LMN

disease (below the level of alpha motor neuron), they aredecreased or absent.

In UMN lesion (at or above the level of alpha motor neuron)inhibitory effect gone

gamma efferents facilitated stretch reflex facilitated tendonjerks aggravated.

Normally when alpha motor neuron was intact, contraction isfollowed by relaxation (normal stretch reflex).


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STIMULUS:

Blow to tendon

RECEPTOR:

Muscle spindle

AFFERENT

PATH:

Action potential

INTEGRATING CENTRE:

Spinal cord L2, L3

onto

Efferent path 1: Somatic motor neuron

Effector 1: Quadriceps muscle

Response: Quadriceps contracts

Efferent path 2: Interneuron inhibiting

somatic motor neuron

Effector 2: Hamstring muscle

Response: Hamstring stays relaxed(reciprocal inhibition)

Human Physiology by Silverthorn


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Knee jerk & Ankle jerk


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CLONUS: (In UMN lesion)

Regular, rhythmic contraction of muscle whensubjected to sudden maintained stretch.

Seen in facilitation of stretch reflex / facilitationof gamma neurons.

PALM OF HAND SUSTAINED PRESSURE

FOOT OF PATIENT


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Muscle tone:

Continuous state of partial contraction in muscle

Involvement of stretch reflex & muscle spindle, also calledmyotatic reflex.

There is some continuous discharge of impulses from gamma

motor neuron. Efferents go to end portions of intrafusal fibers & they contract,

so central portion is stretched muscle spindle excitationinternal stretching of muscle spindle.

From muscle spindle impulses come along sensory nerve fiber tospinal cord & excite alpha motor neuron impulses go toskeletal muscle partial contraction.

If we cut dorsal or ventral nerve root of spinal cord reflex arcis broken in either case & muscle tone disappears. Integrity of

reflex arc is required.


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SPINALCORD

SKELETAL MUSCLE

GAMMAMOTORNEURON

CONTINUOUS

DISCHARGE

ALPHA EFFERENTS

EXCITATION OF SPINDLE

GAMMA EFFERENTS CONTRACTION OF ENDS

GAMMA EFFERENTS CONTRACTION OF ENDS

STRETCH OF SPINDLE


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Regulation of muscle tone:

Discharge from gamma motor neuron is controlled primarily byBULBO-RETICULAR FACILITATORY AREA in Reticularformation of Pons & Medulla.

Muscle tone is influenced bycerebral cortex, basal ganglia &

cerebellum. Normal influence is inhibitory. In their lesionhypertonia / rigidity.

Normal influence of cerebellum on stertch reflex & muscle tone excitatory. In cerebellum lesion hypotonia. Also dampingfunction is lost pendular knee jerk.

Muscle tone is lost in LMN lesion.

In UMN lesion hypertonia.

Di i fl / l i d fl /


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Disynaptic reflex / golgi tendon organ reflex /

inverse stretch reflex / lengthening reaction /

autogenic inhibition:

When a strong stretch is applied to a muscleit undergoes relaxation (lengthening reaction /autogenic inhibition).

When a stretch is applied to a muscle normally it contracts.

When we increase the intensity of stretch

force of contraction increases till muscle tensionbecomes very high stimulation of golgitendon organs muscle relaxation.

So this reflex regulates tension in muscle &

doesn't allow tension to become very high.


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LIFTING LOAD WITH THE HELP OF

STRETCH REFLEX

ARM

LOAD

TENSION CAN DAMAGE

THE MUSCLE

GOLGI TENDON REFLEX

INHIBITING INTERNEURON

INVOLVED

Fig:1

Fig: 2

Fig: 3


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Golgi tendon organs are present at junction

between muscle fibers & the tendon:

I b nerve fiber

70 m/sec

Spinal

cord

inhibitory interneuron

DISYNAPTIC REFLEX

SKELETAL MUSCLE


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These receptors regulate muscle tension & dont

allow tension to become very high.

When these receptors are stimulated impulsego alongI b nerve fibers with velocity70 m/secto spinal cord, where they first synapse with aninterneuron which in turn synapse with motor

neuron supply the muscle. (DISYNAPTICREFLEX).


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Synaptic delay along1 synapse: 0.5 milli sec.

In disynaptic reflex: 0.5 + 0.5 = 1 milli sec.


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CLASP KNIFE RIGIDITY:

In UMN lesion.

On passive flexion of arm at elbow joint Initialresistance than arm is flexed rapidly.

Initial resistance is due to STRETCH REFLEX. It willresist flexion (stretching of Triceps).

In UMN lesion, resistance is more than normal becausestretch reflex is facilitated.

Rapid flexion was due to golgi tendon organ reflex.Very increased muscle tension in triceps

inverse stretch reflex triceps muscle relaxed.


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Polysynaptic reflex / withdrawal reflex/

flexor reflex / multisynaptic reflex:

When a noxious / harmful stimulus is applied toa part of body part becomes flexed &withdrawn from source of noxious stimulus.

Response is graded & proportional to intensityof stimulus.

Ifintensity is less foot withdrawn.

Ifintensity is more

leg withdrawn. Ifintensity is very strong whole body

withdrawn.


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In flexor reflex there is crossed extensor reflex& is a part of flexor reflex to support the body.

Receptors here are nociceptors / pain

receptors/ free nerve endings.

This is a poly/multi synaptic reflex. So synapticdelay will be more than 2 milli sec.

A number of neuronal circuits are involved inthis reflex.


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Reciprocal innervation & inhibition:

Is the cause of extension & contraction ofagonists & antagonists on same side.

For a smooth reflex action,when agonist

contract, antagonist must relax, throughreciprocal innervation & inhibition.

Other examples of multisynaptic reflexes are

cremasteric reflex & superficial abdominalreflex.


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LECT. 1, Reflex action by dr sadia zafar