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Reflex Action

Image of Reflex Action

Image of Reflex Action

Image of Doctor With Patient Clipart

Image of Doctor with Patient Clipart

Reflex Action

  • Definition: It is an involuntary response to a peripheral stimulation

  • Sensory impulse is automatically converted into a motor effect

  • It forms the functional unit of nervous system

  • It depends on integrity of reflex arc

Reflex Arc

Following components

  • An Afferent from a receptor

  • Synapse, may be one or many

  • An Efferent to effector organ

Image of Reflex Arc

Image of Reflex Arc


A: Clinical classification

  • Superficial

  • Deep or Tendon reflexes

  • Visceral: at least one part of reflex arc is formed by autonomic nerve eg. Pupillary reflex, carotid sinus reflex

  • Pathological eg. Babinski’s sign

B: Anatomical cl.

  • Segmental Reflexes: end of afferent & beginning of efferent neuron are in the same seg. of spinal cord

  • Intersegment Reflex: here the end & beginning are in different seg.

  • Suprasegmental Reflex: centre for such reflex lies above the spinal cord

C: Inborn or Acquired

  • Conditioned or acquired: are acquired after learning or training eg. Reflex salivation

  • Unconditioned or Inborn: present since birth eg. Salivation when an object is placed in mouth.

D: depending upon no. of Synapses

  • Monosynaptic

  • Bisynaptic

  • Polysynaptic

E: physiological classification

  • Flexor reflex

  • Extensor re.

Monosynaptic Reflex

Stretch Reflex or Myotatic Reflex

Reflex Arc

  • Stimulus—Stretch to muscle

  • Receptor– Muscle spindle (gpIa & II fib)

  • Central Conn.—on alpha motor neuron

  • Response—Contraction of same muscle

  • Central delay— 0.5 ms. (one synapse)

Image of Monosynaptic Reflex

Image of Monosynaptic Reflex

Structure of Receptor

  • Muscles have two types of fibers

    • Extrafusal fib. ----contractile fib.

    • Intrafusal fib. ----form muscle spindle

Muscle Spindles: Receptors

  • Intrafusal fib. are more embryonal

  • Muscle Spindles are parallel to extrafusal fib.

  • Only ends of the Intrafusal fibers are contractile

  • 2 to 12 Intrafusal fib. are enclosed in a connective tissue capsule

Image of Structure of Receptor

Image of Structure of Receptor

Image of Types of Fibers And Types of Ending

Image of Types of Fibers and Types of Ending

Image of Neurons And Fibers

Image of Neurons and Fibers

Characteristic Features

  • Do not undergo rapid adaptation

  • Do not spread to other muscles

  • Examples: biceps, triceps, knee jerk

  • Best developed in antigravity muscles

  • Chief mechanism for the production of muscle tone & Posture regulation

  • Neurotransmitter----Glutamate


Have two types of fib.

  • Nuclear bag fib.

  • Nuclear chain fib.


Sensory / Afferent

  • Group Ia fibers form Annulospiral or primary sensory endings

    • Carry sensation from nuclear bag & nuclear chain fib.

    • Diameter about 17 microns

    • Conduction Velocity 70 to 120 m/sec

  • Gr.II fib. Also called flower spray or sec. endings

  • Carry sensation mainly from nuclear chain fib.

  • Diameter about 8 microns

  • Conduction velocity app. 30 to 70 m/sec

Motor Supply

  • Gamma –D (dynamic)

    • Supply striated poles of nuclear bag fib.

    • Control the dynamic response

  • Gamma –S (static)

    • Supply the striated poles of nuclear chain fib.

    • Control the static response

Mech. Of Stimulation

M. spindle can be stimulated by

  • Stretching the entire muscle

  • Stimulating the Gamma motor neuron

Dynamic Versus Static Reflex

Dynamic reflex

  • The primary nerve endings supplying the nuclear bag fibers discharge most rapidly while the muscle is being stretched

  • It causes instantaneous strong contraction of the same muscle

  • Contraction is over with in fraction of second

  • As soon the muscle contracts stimulation is lost & discharge decreases

  • Eg. Various jerks

Static Response:

  • If the muscle is stretched slowly & kept stretched, signals are sent continuously through primary & secondary nerve endings supplying the nuclear chain fibers

  • Muscle contracts, as long it is stretched

  • Important in Antigravity muscles.

Α-Γ Co-Activations

  • During voluntary muscle contraction there is increased γ discharge along with the increased α discharge and movement goes on smoothly & continuously

  • It is also called Follow up Servo Mechanism


  • Maintenance of tone

  • Regulation of posture

  • Control of voluntary mov.

Muscle Tone

  • Definition: is tension present in resting muscles due to low frequency & asynchronous discharge of Gamma M.N.

  • It is a state of partial tetanus or partial muscle contraction

  • Hypotonia: tone is less, muscle becomes flaccid

  • Hypertonia: an increase in tone

Hypotonia: Causes

  • Destruction of reflex arc

  • Damage to efferent fib. Eg. Injury or polio.

  • Destruction of dorsal column eg. Tabes dorsalis

  • Stimulation of inhibitory area

  • Destruction of facilitatory area

  • Drugs. Barbiturates, tranquilizers

  • Sleep

Hypertonia: Causes

  • Stimulation of facilitatory areas

  • Destruction of inhibitory area

Whenever Gamma M.N. discharge is more tone increases

  • UMN lesions produce Spasticity

  • Pathology of Basal Ganglia produces Rigidity

Imp. Terms

  • Spasticity: hypertonia is confined to one group of muscle, either agonist or antagonist eg. Upper Motor Neuron Lesion (UMNL)

  • Spastic mus. show

    • Clasp – knife type of hypertonia

    • Lengthening reaction present eg. Clonus

Lengthening Reaction

  • Seen when tone is high

  • Also called clasp knife effect (because it resembles closing of pocket knife) and the muscle is spastic muscle

  • Is due to operation of stretch reflex and inverse stretch reflex.

Image of Lengthening Reaction For Reflex Action

Image of Lengthening Reaction for Reflex Action


  • Is characterized by repetitive muscular contractions produced if foot is dorsiflexed suddenly & pressure is maintained to keep the foot dorsiflexed

  • It is also due to operation of stretch reflex & inverse stretch reflex eg. Ankle clonus


  • Tone increases in both groups of muscles i.e. agonist & antagonist

  • Lesions of basal Ganglia leads to it.

  • Hypertonia is described as

    • Lead-pipe Rigidity

    • Cog-Wheel Rigidity

Bi Synaptic Reflex

Two examples

  • Reciprocal Innervation: when agonist muscles contract & antagonist muscles are relaxed

  • Inverse Stretch Reflex: mediated by Golgi Tendon Organ

Reciprocal Innervation

Image of Reciprocal Innervation

Image of Reciprocal Innervation

Inverse Stretch Reflex /Golgi Tendon Reflex

Operates as follows:

  • Stimulus—hard stretching of muscle beyond certain point

  • Receptor—Golgi Tendon Organ

  • Central connection--- in spinal cord on inhibitory interneuron which terminates on the concerned motor neuron

  • Response—relaxation of muscle

Structure of GTO or Neurotendinous Organ

  • Present at musculotendon junction

  • They are in series with the muscles

  • There are 3 to 25 muscle fibers per tendon organ


  • Regulates tension during normal muscle activity

  • Results in autogenic inhibition

  • Protective Reflex: Prevents tearing of muscles. A very strong contraction can be damaging

Image of Structure of GTO or Neurotendinous organ

Image of Structure of GTO or Neurotendinous Organ

Image of Group Ib Fibers, Tendon And Muscle

Image of Group Ib Fibers, Tendon and Muscle

Image of Bone, Golgi Tendon Organ, Ib afferent, Inhibitory I …

Bone, Golgi Tendon Organ, Inhibitory Interneuron, Alpha Moto …

Poly Synaptic Reflex / Flexor Reflex

Withdrawal Reflex or Crossed Extensor Reflex

Definition: Is a protective reflex. It is prepotent & immediate attention is paid blocking all other activities

  • Stimulus—Nociceptive (pain)

  • Receptors—Free nerve endings

  • Central Pathway: In spinal cord fibers synapse on many interneurons

    • Convey information to CNS

    • Form several reflex pathways

    • Irradiation of the stimulus up and down if stimulus is strong

    • Form reverberating circuits responsible for after discharge

  • Effector organ —skeletal muscles

Image of Poly Synaptic Reflex And Flexor Reflex

Image of Poly Synaptic Reflex and Flexor Reflex

  • Response—Various types (depends on strength of stimulus)

1. Local sign (one limb response)

  • The stimulated limb is withdrawn

2. Crossed Extensor Response (two limb response)

  • Ipsilateral limb—flexion & withdrawal

  • Opposite limb—extension

Image of Image of Ipsilateral Limb And Contralateral Limb

Image of Image of Ipsilateral Limb and Contralateral Limb

3. Shifting Reaction— 4 limb response (seen in spinal animal only)

  • Ipsilateral hind limb-----Flexion

  • Contralateral hind limb—Extension

  • Ipsilateral fore limb-------Extension

  • Contralateral fore limb---Flexion

4. Widespread Withdrawal– If stimulus is very strong whole body moves away.

Properties of Reflex Action

  • Adequate stimulus

  • Delay

  • Summation: spatial & temporal

  • Occlusion

  • Subliminal fringe

  • Irradiation

  • Final common pathway

  • Facilitation

  • Inhibition

  • After discharge

  • Fatigue

  • Fractionation

  • Habituation & sensitization

Image of Subliminal Fringe

Image of Subliminal Fringe

Image of Different Types of Tract

Image of Different Types of Tract

Functions Sub Served at Spinal Level

Neuronal circuits in spinal cord can cause (Spinal Animal)

  • Withdrawal reflex—to take part of body away from the damaging object

  • +ve supporting reaction to support the body against gravity

  • Stepping and walking movements Mark Time Reflex

  • Galloping Reflex

  • Reflexes that control local blood vessels

  • Micturition Reflex

Image of Functions Sub Served At Spinal Level

Image of Functions Sub Served at Spinal Level

Somato Sensory System


  • Receptors

  • Pathway (tracts): A bundle of nerve fibers in spinal cord or brain that makes an Anatomical & Functional unit

  • Role of thalamus

  • Role of cerebral cortex

Tracts in the Spinal Cord

Two types

  • Ascending Tracts

  • Descending tracts

Ascending tracts / Sensory Tract

  • Tracts of post. or dorsal column

    • Fasciculus Gracilis

    • Fasciculus Cuneatus

In Lateral Column

  • Lateral Spin thalamic Tract

  • Dorsal (posterior) Spin cerebellar Tract

  • Ventral (Anterior) Spin cerebellar Tract

In Ventral (ant.) column

  • Anterior (ventral) spin thalamic Tract

Sensation Carried by Various Tracts

  • Fasciculus gracilis & F. Cuneatus

    • Fine touch, Tactile Localization & 2 point discrimination

    • Pressure

    • Vibration, Stereognosis

    • Sense of position & sense of movement

  • Ant. Spin thalamic Tract:

    • Tactile sensation & Crude touch

  • Lateral Spin thalamic Tract :

    • Pain

    • Temperature

  • All Sensory fibers enter through Dorsal Nerve Root

    • Dorsal root divides into two parts

  • Medial part contains

    • Proprioceptive fibers from muscles

    • Sensory fib. Conveying touch, pressure & vibration sense

  • Lateral part

    • Slow Pain & Fast Pain

    • Temperature

    • Visceral Pain

Image of Descending Tracts And Ascending Tracts

Image of Descending Tracts and Ascending Tracts

Post. Column Tracts: Tracts of Gall & Burdach/ Medial Lemniscal Pathway

  • I order neurons: are axons of post. Root ganglia they enter in dorsal column. Here they have definite arrangement of fib.

  • Fasciculus Gracilis-- is located medially and fib. Are from sacral & lumbar region arranged from medial to lateral side

  • Fasciculus Cuneatus—is lateral & fibers are from thoracic and cervical region

  • Fib. Ascend up to medulla

Image of Fasciculus Gracillis And Fasciculus Cuneatu

Image of Fasciculus Gracillis and Fasciculus Cuneatu

II Order Neuron

  • I order neurons end in Nucleus Gracilis & Cuneatus

  • II order neurons begin from here

  • They cross to opposite side (sensory decussation)

  • Ascend up through medulla, pons & mid brain as medial lemniscal fibers

  • Terminate in Ventro Post. Lateral nucleus of thalamus

III Order Neuron

  • Begins from VPL of thalamus

  • Fibers pass through Internal capsule & Corona radiata

  • End in Somato Sensory area of cerebral cortex

Sensory Tracts Have Total Three Neurons

Image of II Order Neuron And III Order Neuron

Image of II Order Neuron and III Order Neuron

Ant. / Ventral Spin Thalamic Tract

  • I order neurons are central processes of dorsal root ganglia

  • Fibers end on the cells situated in the medial part of dorsal horn (I relay station) from here II order neurons arise

  • II order neurons cross in ant. White commissure to opposite side of same segment

  • Few fibers are uncrossed

  • Fibers ascend up to end in VPL nucleus of thalamus

  • III order neurons end in Somato sensory area

Image of Somatosensory Cortex (Postcentral Gyrus)

Image of Somatosensory Cortex (Postcentral Gyrus)

Lateral Spin Thalamic Tract

  • I Order neurons: begin from axons of post. Root ganglia the fibers ascend or descend for one or two segments in Tract of Lissauer.

  • A δ fibers terminate mainly on neurons in lamina I of spinal cord

  • Type C fibers on neurons in lamina II & III

Image of Lateral Spin Thalamic Tract

Image of Lateral Spin Thalamic Tract

Image of Fast Sharp Pain Fibers And Slow-Chronic Pain Fibers

Fast Sharp Pain Fibers Slow-Chronic Pain Fibers

  • II Order neurons– some neurons before crossing run up & down for few seg.

  • Majority of neurons cross in white commissure, obliquely to opposite side

  • They ascend in Lateral Spin thalamic Tr

  • Fibers end in the Thalamus (Ventro posterior lateral Nucleus)

Image of Lateral Spinothalamic Tract

Image of Lateral Spinothalamic Tract

  • III Order Neurons: Starts from Thalamus

  • Fibers pass from Post. Limb of Internal Capsule

  • End in Sensory Cortex (post central gyrus)

Image of Somatosensory Cortex

Image of Somatosensory Cortex

Neospinothalamic Tract

  • Fibers are type Aδ, Fast Pain

  • In Brain Stem---

    • Most fibers to Thalamus

    • Few terminate in Reticular formation

  • Thalamus—Fibers end in ventro postero lateral nucleus

  • Cerebral cortex-somatosensory area

  • Functions: fast pain appears within 0.1 ms. after the application of stimulus

  • Well localized (simultaneous stimulation of touch receptors help)

  • Meaningful interpretation

  • Elicits withdrawal Reflex

  • Produces Symp. Response like ↑BP, tachycardia etc.

Paleospinothalamic tract

  • Fibers are type C

  • Brain stem

  • Mainly in Reticular Formation

  • Tectal area

  • Periaqueductal Grey

  • Thalamus

  • Intralaminar Nuclei

  • Cerebral Cortex– All parts of brain

  • Some fibers go to Hypothalamus also


  • Slow pain begins a second or more after the application of stimulus but it increases slowly and lasts for a longer time

  • Difficulty in sleeping

  • Activating or alerting response

  • Emotional disturbances make pain unbearable, intense unpleasantness

  • Associated with nausea, vomiting, lowering of BP.

Image of Somatosensory Areas

Image of Somatosensory Areas

Image of Somatosensory Area I And Somatosensory Area II

Image of Somatosensory Area I and Somatosensory Area II

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