Properties of reflexes

PROPERTIES OF
REFLEXES
DR NILESH N KATE
PROFESSOR
DEPARTMENT OF PHYSIOLOGY
ESIC MEDICAL COLLEGE AND HOSPITAL,
SEDAM ROAD, GULBARGA.
Tuesday, February 4, 2020

Properties of reflexes
 Final common
pathway
 Recruitment
 Irradiation
 Summation
 Inhibition
 Rebound phenomenon
 Subliminal fringe
 Occlusion
 Facilitation
 Adequate stimulus
 Fractionation
 Fatigue
 Delay
 One way conduction
 Sensitization
 After discharge.

Final common pathway
 α-motor neurons that
supply the extrafusal
muscle fibres.
 If an α-motor neuron
 Stimulated, skeletal
muscle fibres contract
 If is not stimulated, the
skeletal muscle fibres
relax

Recruitment
 As excitatory nerve stimulated for long time
 Progressive motor neurons gets activated
 progressive increase in response of reflex
activity occurs.

Irradiation
 When sensory stimulus too strong – it spread
to neighbouring neurons
Produce wide response.

Summation
 Both temporal and
spatial – Facilitate the
responses during the
reflex activity

INHIBITION
 Impulses through
sensory fibres from
protagonist muscles
inhibit the action of
antagonist muscles.

Rebound phenomenon
 When reflex activity inhibited & the
inhibition is over reflex activity reappears &
becomes more powerful.

Subliminal fringe
 Simultaneous
stimulation of 2 nerves
with weak shock
evolkes greater
response than sum
total of response of
individual .

Occlusion
 Simultaneous
stimulation of 2 nerves
with weak shock
evolkes lesser response
than sum total of
response of individual .

Facilitation
 When reflex elicited repeatedly –
Response becomes progressively higher

Adequate stimulus
 Reflex response is obtained only
when a precise stimulus for a given reflex
activity is applied.

Fractionation
 The force of a muscle contraction is much
higher when it is stimulated directly through
motor nerves than sensory

Fatigue
 When a particular reflex is elicited
repeatedly at frequent intervals
the response is reduced
progressively and
then disappears all together

Delay
 Time interval between the application of
stimulus and starting of the response.
 Delay is minimum in a monosynaptic reflex.

One way conduction
 Impulses are
transmitted in only one
direction through the
reflex arc as per the
Bell–Magendie law.
 The impulses pass
from the receptors to
the centre and then
from the centre to
the effector organ.

Sensitization
 When an injurious stimulus is repeatedly
applied
Intensification of response.

After discharge.
 When a reflex action is elicited continuously
for some time, and then the stimulation is
stopped
the reflex response (contraction) may
continue for some time even after cessation
of the stimulus.

Spinal cord reflexes
 Depending upon origin, reflexes are 2 types
 Muscle reflex
 Stretch reflex
 Lengthening reaction or golgi tendon reflex.
 Cutaneous reflex
 Withdrawl reflex.

Stretch reflex.
 Myotactic
 The reflex contraction
of a muscle that is
stretched.
 Type – Monosynaptic
 Stimulus – Stretch
 Reaction time -19–24
ms.

Stretch reflex.
 Central delay - 0.6–0.9 ms.
 Well developed in
antigravity muscles, such as
extensor group of muscles
of legs and flexor groups of
muscles of arm
 Examples -- knee jerk,
ankle jerk, biceps jerk and
triceps jerk

Reflex arc of stretch reflex
 Afferent limb
 Receptor – Muscle spindle
 Afferent nerve – Ia & II
 Center – Ventral gery
horn, act on α Motor
neuron
 Efferent limb
 Efferent nerve – from αMN
 Effector organ - Muscles

Reciprocal innervation of
stretch reflex
 Excitation of one
group of muscles is
associated
with inhibition of the
antagonistic group of
muscles on the
same side.

Reciprocal innervation of
stretch reflex –Pathway.
 biphasic.
 A collateral from each Ia fibre passes in the
spinal cord to an inhibitory interneuron
(Golgi bottle neuron) that synapses directly
on one of the motor neurons supplying the
antagonist muscles.
 This is an example of Post-synaptic
inhibition

Significance of reciprocal
innervation
 In locomotion.
 It helps in the forward
movement of one limb
while causing the
Backward movement
of other limb

Dynamic vs static stretch
reflex
 Dynamic stretch
reflex
 Static stretch reflex

Dynamic stretch reflex
Muscle
stretched
suddenly
Muscle
spindle
length
increas
es
Stimulate
primary
nerve
endings
Shows
dynamic
response.
Discharg
e rapidly
Send strong
signals to
spinal cord
Causes
very
strong
reflex
contract
ion of
same
muscle.
Thus
oppose
sudden
change

Static stretch reflex
muscle is stretched slowly
and kept stretched
primary and secondary nerve endings from
nuclear chain fibres send signals continuously
and cause reflex contraction of the muscle
Causes muscle contraction as long as
the muscle is stretched.
Imp in posture, gravity causes continuous stretch
& causes antigravity muscles always contracted.

Role of gamma motor neurons.
 Role of γ efferent
discharge in
adjusting the spindle
sensitivity by
preventing
unloading.
 Role of co-activation
of α & γ motor
neurons.
 Role of gamma loop

Role of γ efferent discharge in adjusting
the spindle sensitivity by preventing
unloading.
 Muscle fibre stretched
 Ia fibre firing increases
 Α motor neuron activity
 Causes reflex contraction of muscle
 Makes the muscle slack
 firing rate of Ia fibre
 Decreased rate of firing of Ia afferent is called Unloading
of muscle spindle.
 Disadv- CNS stops receiving information about
the rate and extent of muscle shortening

γ-motor neurons- prevents
this unloading.
 The striated poles of intrafusal fibres
contracts along with shortening of
extrafusal fibres during muscle contraction.
 The central receptor region of the intrafusal
fibres remains stretched
 Unloading does not occur

Controls dynamic & static
response
 Dynamic γ-motor neurons primarily innervate the
striated poles of nuclear bag fibres
 When they are fired, only nuclear bag fibres
shorten.
 Since they are responsible for the phasic (i.e.
velocity sensitive) portion of Ia afferent response to
stretch, stimulation of the dynamic γ-fibres
increases phasic activity without affecting static
activity & same for static response.

Length Servomechanism
 This γ-motor neuron-mediated change in
length of intrafusal fibres.
 System of negative feedback that operates to
maintain muscle length during body
movements and thus helps in regulation of
posture

Role of co-activation of α- and
γ-motor neurons
 During lifting weights -- active shortening
of the extrafusal fibres – slacking of the
muscle spindles (i.e. unload the spindle)--
tend to decrease Ia discharge.

During voluntary contraction -
 α–γ co-activation -- increased
γ-discharge along with the increased α-
discharge-- maintains constant Ia discharge -
- constant level of Ia input to the CNS during
a voluntary movement indicates that motor
command is being carried out.
This is called Followup servomechanism

Role of γ-loop.
 CNS can initiate movements directly by stimulating
only γ-motor neurons, using a pathway called the γ-loop
 The loop begins with γ-motor neuron -- which
discharges to cause intrafusal muscle fibre contraction --
increase in Ia afferent fibre activity -- causes increased γ-
motor neuron discharge via a monosynaptic reflex causing
muscle contraction.
 But normally it does not happen.

Higher control of stretch
reflex.
 Brain areas that facilitate or inhibit the stretch reflex are
 Facilitatory reticular formation
 Inhibitory reticular formation
 Cerebral motor cortex and cerebellum stimulating the
inhibitory reticular formation

Other factors which influence γ-
efferent discharge:
 Anxiety causes an increased discharge- explains the hyperactive
tendon reflexes sometimes seen in the anxious patients.
 Stimulation of skin, by noxious agents, increases
γ-efferent discharge to ipsilateral flexor muscle spindles
and decreases that to extensors and produces the opposite
pattern in opposite limb.
 This fact is sometimes used as a reinforcement to elicit deep tendon
reflexes (such as knee jerk), which are not being elicited otherwise.
 Jendrassik’s manoeuvre.

Functions of stretch reflex
 Role in maintaining muscle tone
 Role in maintaining posture
 Role in controlling voluntary movements

Role in maintaining muscle
tone
 BRAIN – has 2 areas
 Fascilitatory – Pons
 Inhibitory – Lower medulla.

Role in maintaining muscle
tone
 Facilitatory area is intrinsically
active - continues to discharge
facilitatory impulses
causing constant activation of γ-
motor neurons --This causes
stretching of the muscle spindle
fibres resulting into reflex
slight contraction of the extrafusal
fibres of muscle under resting state
(producing muscle tone).
 Inhibitory area -- becomes active
only if it receives impulses from
the cerebellum or cerebral cortex.

Role in maintaining posture
 Static component of stretch reflex, the
fundamental posture control mechanism, is
especially prominent in the medial extensor
muscles and antigravity muscles

Role in controlling voluntary
movements
 During motor activity the group Ia fibres from the muscle
spindle inform the motor control system about the
changes in muscle length.
 The constant level of Ia input to the CNS during a
movement indicates that the motor command is being
carried out.
 An increase in activity of Ia indicates that motor
command is not being carried out.
 CNS uses this information to readjust its command to the
spinal cord.

Properties of reflexes

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