Reflex Action YouTube Lecture Handouts Medical Science
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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
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. BabinskiΥs 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 β 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)
Structure of Receptor
- Muscles have two types of fibers
- Extrafusal fib β¦ contractile fib.
- Intrafusal fib β¦ 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
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 β¦ Glutamate
M. S β¦
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 β D (dynamic)
- Supply striated poles of nuclear bag fib.
- Control the dynamic response
- Gamma β 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 β 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.
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
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 β¦ 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
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
- 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
3. Shifting Reaction β 4 limb response (seen in spinal animal only)
- Ipsilateral hind limb β¦ Flexion
- Contralateral hind limb β¦ Extension
- Ipsilateral fore limb β¦ Extension
- Contralateral fore limb β¦ Flexion
4. Widespread Withdrawal β 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
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
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
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 β¦ 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
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
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
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
- II Order neurons β 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)
- III Order Neurons: Starts from Thalamus
- Fibers pass from Post. Limb of Internal Capsule
- End in Sensory Cortex (post central gyrus)
Neospinothalamic Tract
- Fibers are type AΞ΄, Fast Pain
- In Brain Stem β¦
- 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 βBP, tachycardia etc.
Paleospinothalamic tract
- Fibers are type C
- Brain stem
- Mainly in Reticular Formation
- Tectal area
- Periaqueductal Grey
- Thalamus
- Intralaminar Nuclei
- Cerebral Cortex β 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.