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‫االسراء اية ‪58‬‬
Reflex Action
By
Dr. Abdel Aziz M. Hussein
Lecturer of Physiology
Member of American Society of Physiology
Def:
• It is an automatic (involuntary) specific response of an
organ caused by an adequate sensory stimulus
Pathway of Reflex Action
(1) Stimulus
(2) Receptor
sensory neuron
(3) CNS (spinal cord)
motor neuron
(4) Effector
(5) Response
4
Pathway of Reflex Arc
a) Physiological importance:
• Differs according the type of reflex.
• Some reflexes are protective such as flexion withdrawal
reflex , and others are supportive such as stretch reflex,
so its importance differs according to the type of reflex.
a) Clinical importance:
 They test the integrity of the pathway (afferent , center in
CNS, and efferent)
Local
Reflexes
Systemic or
General
Reflexes
• Their centers outside CNS
• Their centers inside CNS
•
•
•
a)
Their centers are present outside CNS
Examples:
Their centers may be present in;
Dorsal root ganglia (DRG) as those responsible for the flare
and allodynia (1ry hyperalgesia)
b) Enteric nervous system as those regulating the GIT
activities
c) Prevertebral sympathetic ganglia as those responsible for
inhibitory enterogastric reflexes
Local axon reflex
Conditioned
Reflexes
Unconditioned
Reflexes
• Their centers present in
cerebral cortex
• Their centers present in CNS
outside cerebral cortex
• Are acquired reflexes i.e. need previous experience
or training
• Need intact cerebral cortex → their centers are
present in cerebral cortex.
• Initiated by visual, olfactory, auditory stimuli or even
thinking of the stimulus
• Play a role in regulation of G.I.T., respiratory and
cardiovascular functions
• They are inherent (inborn) reflexes
• Don’t need intact cerebral cortex → their centers are
not present in cerebral cortex
• Don’t need training.
• According to the site of their centers, they include;
1. Hypothalamic reflexes
2. Midbrain reflexes
3. Pontine reflexes
4. Medullary reflexes
5. Spinal reflexes
Hypothalamic
Midbrain
Pontine
Medullary
Spinal
1. Hypothalamic reflexes include reflexes of regulation of food
intake, body temperature, etc…
2. Midbrain reflexes include pupillary light reflex and righting
reflexes
3. Medullary reflexes include reflexes of regulation of CVS function
e.g. baroreceptor reflex and GIT functions such as mastication,
vomiting and GIT secretions and motility
1) Def.,
• Are reflexes their centers are present in the spinal
cord.
2) Classification:
• Are classified into 3 types;
A) Superficial spinal reflexes:
 Their receptors are present on body surface (skin).
• Examples;
a. Scratch reflex
b. Flexion withdrawal reflex
c. Abdominal reflex
d. Cremasteric reflex.
e. Plantar reflex.
2) Classification:
B) Deep spinal reflexes:
• Their receptors are present in deep structures e.g.
ms, bone and joints.
• e.g. Ms Stretch reflex
C) Visceral spinal reflexes:
• Their receptors are present in visceral organs.
Examples;
• i) Micturition reflex: → is initiated by distension of
urinary bladder.
• ii) Defecation reflex: → is initiated by distension of
rectum.
A) Somatic reflexes:
•Mediated by somatic efferent supplying skeletal ms
B) Autonomic reflexes:
•Mediated by autonomic efferent supplying blood vessels,
glands and visceral organs
•
1.
2.
3.
Its pathway consists of 3 main components;
Afferents
Interneurons
Efferents
• Are monopolar neurons which present in the DRG.
•The axon of each neuron divides into 2 branches;
1. Peripheral branch → terminates in sensory receptors
2. Central branch → enters into the spinal cord.
•The central branch divides into several terminals which
ends on;
a. Gray matter of spinal cord e.g. posterior horn cell as
MSN,SGR ,etc…,interneurons and anterior horn cells
b. Ascend or descend to higher or lower segments
respectively
Functions:
a)Conduction of the sensory signals to the spinal centers
to excite the subsequent neurons in the reflex pathways.
Functions:
b)Divergence of the incoming sensory signals into wider
areas in the NS.
•Are small highly excitable neurons
•Are located in the gray matter between afferent
neurons and the efferent neurons.
•All reflex arcs include interneurons except the stretch
reflex i.e. monosynaptic
• Some of them are excitatory and the others are inhibitory.
• Interneurons form different types of circuits that
perform the following functions;
1. Divergence
2. Convergence
3. After-discharge
4. Inhibitory circuits
•A single input (afferent)
neuron or (more
commonly) interneuron
divide to give several
collaterals and reach a
larger number of efferent
neurons
Significance :
•Help in spread of a single
afferent signal to a large
No. of postsynaptic
neurons in the spinal cord .
• May be on the same side and may be on the
opposite side.
•Multiple collaterals from
input (afferent) neurons or
more commonly interneurons
converge (collect) on a
relatively fewer number of
output (efferent) neurons
Significance:
•This help spatial summation
of EPSPs → discharge of
impulses on the postsynaptic
neurons.
•May come from single or multiple sources
•Def.
•It is prolonged (continuous) discharge from an
efferent neuron even after stoppage of stimulation of
the afferent nerves
•Mechanism:
•Because the afferent impulses do not reach efferent
directly but through interneurons circuits
• There are 2 types of these circuits;
1. Open interneuron circuits
2. Closed interneuron circuits.
A) Parallel ( Open ) Chain Circuits:
•In this circuit an afferent neuron stimulates an efferent
neuron both directly and indirectly through an
interneuron which is anatomically arranged in parallel
with the afferent neuron.
A) Parallel ( Open ) Chain Circuits:
•Impulses from inputs not reach to output at the same
time due to delay 0.5 ms at each synapse
•Duration of discharge depends upon the No. of
interneurons
B) Closed-chain (Reverberating) Circuits:
Principal Interneurons
Efferent
Afferent
1
2
3
Collaterals
B) Closed-chain (Reverberating) Circuits:
•The activity of these circuits stop by either;
1. Fatigue of synapse
2. Inhibitory interneurons
B) Closed-chain (Reverberating) Circuits:
Examples : Reticular activating system (RAS)
•Wakefulness depends upon the activity of RAS (contains
many reverberating circuits)
•Single sensory stimulation causes activation of RAS for
long time (16-18 hours).
•RAS, in turn stimulate the cerebral cortex which by its turn
re-stimulate it & so on.
•RAS activity continues till fatigue of the synaptic
transmission occurs and then sleep occurs.
•In this circuit, an excitatory input is converted into an
inhibitory output.
•Types:
•2 types of these circuits;
1. Lateral inhibitory circuits
2. Recurrent inhibitory circuits
A) Lateral inhibitory circuits:
Inhibitory
interneuron
Collateral
less active More active
neurons
neurons
A) Lateral inhibitory circuits:
•In this circuit the afferent neuron activates an inhibitory
interneuron which in turn inhibit the adjacent efferent
neurons.
Site:
•Ascending sensory pathways
Importance:
•These circuits are important in focusing on or
sharpening of the most important sensation.
B) Recurrent inhibitory circuit:
•Nerve fiber gives a collateral branch which excites (via
Ach) an inhibitory neuron which in turn, inhibits (via
glycine) the original neuron as well as the surrounding
neurons.
Glycine
Ach
B) Recurrent inhibitory circuit:
Renshaw cells :
•Small inhibitory interneurons present in AHCs of the
spinal cord
•Stimulated A.H.C. gives a recurrent collateral branch
which stimulates the Renshaw cells.
•Renshaw cells in turn inhibits either: stimulated or
surrounding A.H.C.
Importance:
a)Focus the motor activity to the most facilitated motor
neurons
b)Stops the activity of the stimulated motor neurons.
THANKS