Reflex Action YouTube Lecture Handouts Medical Science

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Reflex Action

Illustration: Reflex Action
Illustration: Reflex Action

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
Illustration: Reflex Arc

Classification

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 β€” Stretch to muscle
  • Receptor – Muscle spindle (gpIa & II fib)
  • Central Conn. β€” on alpha motor neuron
  • Response β€” Contraction of same muscle
  • Central delay β€” 0.5 ms. (one synapse)
Illustration: 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
Illustration: Muscle Spindles: Receptors
Illustration: Muscle Spindles: Receptors
Illustration: Muscle Spindles: Receptors

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

M. S …

Have two types of fib.

  • Nuclear bag fib.
  • Nuclear chain fib.

Innervation

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.

Ξ‘-Ξ“ Co-Activations

  • During voluntary muscle contraction there is increased Ξ³ discharge along with the increased Ξ± discharge and movement goes on smoothly & continuously
  • It is also called Follow up Servo Mechanism

Functions

  • 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.
Illustration: Lengthening Reaction

Clonus

  • 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

Rigidity

  • 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

Illustration: Reciprocal Innervation

Inverse Stretch Reflex/Golgi Tendon Reflex

Operates as follows:

  • Stimulus β€” hard stretching of muscle beyond certain point
  • Receptor β€” Golgi Tendon Organ
  • Central connection … in spinal cord on inhibitory interneuron which terminates on the concerned motor neuron
  • Response β€” 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

Functions

  • Regulates tension during normal muscle activity
  • Results in autogenic inhibition
  • Protective Reflex: Prevents tearing of muscles. A very strong contraction can be damaging
Illustration: Functions
Illustration: Functions
Illustration: Functions

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 β€” Nociceptive (pain)
  • Receptors β€” 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 β€” skeletal muscles
Illustration: Poly Synaptic Reflex/Flexor Reflex
  • Response β€” 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 β€” flexion & withdrawal
  • Opposite limb β€” extension
Illustration: Poly Synaptic Reflex/Flexor Reflex

3. Shifting Reaction β€” 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
Illustration: Properties of Reflex Action
Illustration: Properties of Reflex Action

Functions Sub Served at Spinal Level

Neuronal circuits in spinal cord can cause (Spinal Animal)

  • Withdrawal reflex β€” 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
Illustration: Functions Sub Served at Spinal Level

Somato Sensory System

Includes

  • 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
Illustration: Sensation Carried by Various 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 β€” is lateral & fibers are from thoracic and cervical region
  • Fib. Ascend up to medulla
Illustration: Post. Column Tracts: Tracts of Gall & Burdach/Medial Lemniscal Pathway

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

Illustration: Sensory Tracts Have Total Three Neurons

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
Illustration: Ant. / Ventral Spin Thalamic Tract

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 Ξ΄ fibers terminate mainly on neurons in lamina I of spinal cord
  • Type C fibers on neurons in lamina II & III
Illustration: Lateral Spin Thalamic Tract
Illustration: Lateral Spin Thalamic Tract
  • 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)
Illustration: Lateral Spin Thalamic Tract
  • III Order Neurons: Starts from Thalamus
  • Fibers pass from Post. Limb of Internal Capsule
  • End in Sensory Cortex (post central gyrus)
Illustration: Lateral Spin Thalamic Tract

Neospinothalamic Tract

  • Fibers are type AΞ΄, Fast Pain
  • In Brain Stem …
    • Most fibers to Thalamus
    • Few terminate in Reticular formation
  • Thalamus β€” 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

Functions

  • 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.
Illustration: Functions
Illustration: Functions