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‫االسراء اية ‪58‬‬
Polysynaptic Reflexes
By
Dr. Abdel Aziz M. Hussein
Assist. Prof of Physiology
• Synaptic transmission is inhibited by all
the following, except :•
•
•
•
a- oxygen lack
b- alkalosis
c- acidosis
d- prolonged activity of synapse
• Post-tetanic Potentiation in synapses :•
a- is due to increased Ca ++ concentration in
postsynaptic neurons
• b- is due to increased Ca ++ influx into presynaptic
neurons
• c- results from slow prolonged stimulation of
synapse
• d- causes fatigue of the synapse
• Long-term potentiation of synaptic transmission :-
• a- is involved in pain control system
•
b- is caused by increased Ca ++ concentration in
presynaptic neurons
• c- results from fast repetitive stimulation of synapse
• d- is associated with decreased Ca ++ concentration in
postsynaptic neurons
• In spinal reflexes the efferent neurons are of 2 types;
Autonomic M. Neurons
Somatic M. Neurons
From LHCs
From AHCs
Mediate somatic reflexes
Mediate autonomic reflexes
Alpha MNs
Gamma MNs
Spinal Efferent Neurons
AHCs
Skeletal Ms
LHCs
Smooth Ms
Somatic Efferent Neurons
Somatic Efferent Neurons
Alpha motor
neuron
Gamma motor
neuron
Motor Unit
Motor Unit and motor pool
Def
 It is a polysynaptic reflex that moves the affected limb
away from the painfull stimulus;
Pathway
1. Stimulus → painful stimulus
2. Receptors → free nerve endings
3. Afferents→ A delta
4. Center → AHCs
5. Response
 Flexion of the stimulated limb
 Increased extension of the contralateral limb
Significance
1) Withdrawal reflex:
 Is of survival or protective value →help to immediately
remove the stimulated part of the body away from
sources of painful stimuli.
2) Crossed extensor reflex:
 Is supportive reflex i.e. supports the body weight which is
shifted to the opposite limb.
• Properties of polysynaptic spinal reflexes are due to
the presence of interneurons and synapses in the
reflex arc.
• They include;
• The impulse is conducted from receptors to afferents
to interneurons then to efferent i.e. one way direction
• At synapse, conduction only in one direction from the
presynaptic neuron to the post-synaptic neuron,
because the transmitter releasing vesicles present
only in the pre-synaptic membrane
Total reflex time:
Def.,
 Is the period or time that passes between application of
the stimulus and the onset of the reflex response.
Causes: ?
Central Delay :
• It is the time needed for transmission of impulses through
central synapses in the reflex pathway.
• It depends mainly on the number of synapses in reflex
pathway.
• It is a gradual decline in the reflex response when its
sensory nerve is stimulated for a relatively prolonged
period.
• It is due to fatigue of synapses in the reflex pathway .
The site (locality) of the stimulus determines the
nature of response e.g.:
In withdrawal reflex, the pattern of flexion that occurs
varies with the site of the stimulus.
a) Stimulus at the lateral aspect of the thigh leads to
adduction and medial rotation.
b) Stimulus at medial aspect of the thigh leads to
abduction and lateral rotation
Mechanism:
1. Pain fibers from the lateral aspect of the thigh
centrally connect with and excite the motor neuron
pool of the flexors and adductors
2. Pain fibers from medial side are connected with
flexors and abductors
Def.
 The extent of the response is determined by the strength
of stimulation, so;
1) Weak stimuli produce limited responses.
2) Stronger stimuli produce more widespread responses.
Example
 When applying painfull stimulus to
sole
1. Mild painful stimulus at the sole of the foot leads to
dorsiflexion of the ankle only.
2. Moderate painful stimulus at the sole of the foot leads
to dorsiflexion of the ankle and flexion of the knee.
3. Severe painful stimulus at the sole of the foot leads
to dorsiflexion of the ankle and flexion of both knee
and hip.
Mechanism:
 1) Strong stimuli cause stimulation of more receptors →
activate more afferents whose central terminals diverge
to activate many interneurons at various levels of CNS→
spreading of inputs in the CNS → excite greater numbers
of efferents.
 2) Divergence of central terminals of afferents and axons
of interneurons
Def.,
• Reflex contraction of a group of ms is usually associated
with inhibition of the antagonistic ms.
Mechanism:
Afferents give excitatory inputs to the MNs of a ms, activate at
the same time inhibitory interneurons ,which inhibit the MNs of
antagonistic ms.
Significance:
It provides a coordinated ms contraction around the joints
and reduces the resistance to movements.
Flexion withdrawal crossed extensor
reflex
• It is also found in other areas of the nervous system
other than spinal cord e.g.
A) In Medulla Oblongata: pressor area and depressor
area and inspiratory area and expiratory area
B) In the Hypothalamus: heat gain center and heat loss
center and feeding center and satiety center
Exception:
+ve supporting reaction in which all groups of ms of lower
limbs are contracted at the same time.
Deep pressure applied to the sole of foot by the body
weight during standing results in reflex contraction of both
flexors and extensors of the lower limb to support the body
against gravity
• 2 phenomena which are studied by comparing motor
and reflex tetanus
Tetanus
Contraction
Start
Relaxation
End
Tetanus
Contraction
Start
Relaxation
End
Contraction
Relaxation
Latent Period
Recruitment
Def.,
It is a gradual ↑ in the magnitude of the reflex response at the
onset of afferent stimulation until the response reaches its full
magnitude.
Cause:
• Motor neurons are said to be gradually recruited (activated),
one after the other i.e. not excited at the same time. Due to;
A. Different velocities of nerve impulses due to different
thickness of afferent neurons.
B. Presence of interneurons.
After-discharge
Def.,
It is the persistence of the reflex response for a period
of time after stoppage of sensory stimulation.
Cause:
• is explained by the repetitive discharge interneuron
circuits
Test yourself
A flexor withdrawal reflex shows all the following
properties, except :a- recruitment
b- prepotent
c- non-fatigable
d- irradiation
Test yourself
After-discharge of reflex responses :a- increase the magnitude of the reflex responses
b- delays the onset of fatigue of reflex responses
c- involves interneuron circuits
d- depends upon spatial summation
THANKS