Transcript Reflex Action

REFLEX ACTION
Definition
Classification
Properties of Reflex Action
Reflex Action
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Definition
Automatic involuntary motor response to a sensory stimulus
based on a reflex arc is called reflex action
The arc consists of an afferent part – receptor ( superficial or
deep )and an afferent neurone
Center – usually in the spinal cord or brain stem
Efferent part – efferent nerve and an effector organ – usually
a skeletal muscle , may be a muscle – ( smooth, cardiac or
skeletal ) or gland ( exo or endocrine ). Usually it is skeletal
muscle response which is considered.
Example
Classification
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1.Based on the number of synapses involved –
Monosynaptic – Deep or Stretch reflex –Knee jerk
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Disynaptic – negative stretch reflex, reciprocal
innervation
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Polysynaptic - Flexor or withdrawal reflex
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Asynaptic – local axon reflex
2.Based on development – phylogeny
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Unconditioned
Present at Birth
Number Few
Due to Existing Reflex arc
Abolished with difficulty, permanent
Function protective, Instinctive
example touching a hot stove
species specific
Predictable
Training not required
2.Based on development – phylogeny
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Conditioned reflex
Acquired
Many superimposed easily
some good some bad
Salivation at thought of food
Based on past Experience
on existing arcs
individual specific
Learned
Habits and learning
Unpredictable
Classification
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3.Based on clinical importance and site of presence
of receptors –
Superficial – flexor reflex – Plantar reflex
Babinski’s response , conjunctival, corneal and
abdominal
Deep – stretch reflex
Visceral – Urinary bladder actions
Classification
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4.Segments of spinal cord involved
segmental – one segment of same side involved
Intersegmental – segments of both sides or more
than one segment involved
Suprasegmental – higher segments involved
Classification
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5.Depending on the purpose – functional
a.Protective – flexor reflex
b.Postural – antigravity – extensor reflex
c.Righting reflex
d.Conditioned reflex
e.Sexual reflex
f.Locomotor
g.Systemic – CVS, respiratory etc.
Classification
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6.Response confined to one or more muscle groups
a.Simple –
b.coordinated
c.Convulsive
Classification
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7.Depending on site of centre
a.Spinal
b.Bulbar
c.Mesencephalic
d.Diencephalic
e.Cortical
Properties of reflex action
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Reflex action combines the properties of both
synapse and receptors – demonstrated in spinal
and better in decerebrate animal. Most of the
properties apply to flexor or withdrawal reflexes
as there are multisynaptic.
Properties of reflex action
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1.Specificity – A sensory stimulus always give rise to
a specific response – eg. Showing light in to the eye
gives rise to constriction of the eye etc .Specificity
of the reflex action makes the predictability of the
response .However, super imposed conditioned
reflexes may mask them
Properties of reflex action
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2.Synaptic delay – reflex time is the time between
the stimulus and response and is dependant on
number of synapses and the delay of conducting
the impulses across it .
Properties of reflex action
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3.Unidirectional conduction – Impulses are
transmitted in one direction only – from the
receptors towards to the center in the sensory and
from the centre towards periphery in motor nerves .
4.Excitation – some reflexes are excitatory
5.Inhibition- some reflexes are inhibitory in nature
Properties of reflex action
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6.Reciprocal Innervation – Muscle contracts reflexly with the contraction of
agonist and relaxation of an antagonists . In knee jerk – contraction of
extensor of knee is accompanied by inhibition of flexor of the knee.
Basis - due to an inter neuron placed in the reflex arc
branch of the afferent connects to the excitator
(develops EPSP) to the agonist
and another to a Golgi bottle neuron or Renshaw cell neurone which it
stimulates .
This in turn inhibits the antagonist by forming IPSP due to release of an
inhibitory transmitter to the neuron supplying the antagonists .
This ensures the smooth contraction of the agonist
Diagram
Properties of reflex action
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7.Irradiation A feeble stimulus gives rise to weak
contraction and a strong stimulus gives rise to a
strong contraction due to inclusion of adjacent
motor neurone .A small pin prick – only finger
withdrawn. A bold cut , entire hand is withdrawn.
Properties of reflex action
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8.Recruitement and after discharge – When a muscle
contracts to direct stimulation there is a quick simple
muscle twitch.
When it is reflexly contracting instead of quick and
short lived contraction, it builds momentum and is
sustained with a slow decline.
This is due to recruitment and after discharge .
Basis- Parallel circuits in the reflex arc of varying
length.
Diagram
Properties of reflex action
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9.Rebound – For a short time following the
elicitation of a reflex action ,the excitability of the
agonist is reduced and that of antagonist is
increased.
Properties of reflex action
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10.Summation – both Spatial and Temporal
summations are seen in the reflex actions. When two
relax actions give the same effect , elicitation of
both of the simultaneously gives a greater
response – spatial summation .
When the stimuli are given repeatedly also the
response is more. – temporal summation
Properties of reflex action
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11.Due to convergence and divergence in the reflex
pathways subliminal fringe and occlusion are seen
similar to synapse )
12.Facilitation
13.Fractionation
14.Fatigue – Repeated elicitation of reflexes leads
to reduced response due to fatigue .
Functions
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1.Major functions of CNS and ANS are reflex in nature. For
eg. posture, equilibrium, locomotion, visceral activities.
2.In CNS itself , learned activities may become patterned for
reflex performance .For eg.walking – learned becomes a
locomotor reflex only command for start and stop are actually
voluntary.
Human beings are given the capacity to convert reflex to
delibration- intelligence .The response can be stopped, altered
, augmented. Brain development ensures that more and more
actions come under delibration so that there is coordination
and the organism is elevated from lower to higher status (
wisdom )
Functions
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3.Economy of energy as spinal cord itself takes
care of most of the actions
4.Homeostasis –
5.Clinical application for diagnosis – When elicited
with properly- absence indicates LMN lesion and
exaggeration UMN Lesion
Role of higher centres . Release phenomenon .
Site of lesion can be located.
Abnormal plantar – Babinski’s – Pyramidal lesion