REFLEX Reflex is an involuntary response to stimulus REFLEX ARC Reflex Arc is the basic unit of integrated reflex activity It consists of Sense organ an afferent neuron one or more synapses that are generally in a central integrating area an efferent neuron an effector organ ID: 935006
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Slide1
PNS; THE REFLEX ARC AND GENERAL PROPERTIES OF SPINAL REFLEXES
REFLEX
Reflex is an involuntary response to stimulus.
REFLEX ARC
Reflex Arc is the basic unit of integrated reflex activity
It consists of ; Sense organ, an afferent neuron, one or more synapses that are generally in a central integrating area, an efferent neuron, an effector organ.
Connection between afferent and efferent somatic neurons is generally in the brain or spinal cord (Integrating center).
Afferent neurons enter through dorsal or cranial nerves and have their cell bodies in the dorsal root ganglia or homologous ganglia on the cranial nerves.
Slide2Efferent fibers leave via the ventral roots or corresponding motor cranial nerves
BELL-MAGENDIE LAW
In the spinal cord - dorsal roots are sensory
- ventral roots are motor.
Slide3STRETCH REFLEX (MYOTATIC REFLEX)
Monosynaptic reflex
Stretch of skeletal muscle with intact nerve fiber
Response : contraction (stretch reflex)
Sense organ: Muscle spindle
Impulses originating in the spindle are conducted in the CNS by fast sensory fibers that pass directly to the motor neurons supplying the same muscle.
Neurotransmitter - glutamate
Slide4CLINICAL EXAMPLES
Knee Jerk – Tapping of the patella tendon elicits knee jerk.
Tap on the tendon stretches the quadriceps
femoris
.
Manual stretch of the muscle produces same response
Tapping on the triceps
brachii
causes extensor response at the elbow
Tapping on the Achilles tendon causes an ankle jerk
Tapping on the side of the face causes a stretch reflex in the masseter
Slide5MUSCLE SPINDLES
The receptors which detect stretch in muscle are known as muscle spindles.
Each muscle spindle consists of 3 to 12 small muscle fibres called
intrafusal
muscle fibres.
Each
intrafusal
fibre is between 1- 5mm in length and it is striated.
The fibres of the main muscle in which the muscle spindle lies are known as
extrafusal
fibres.
There are two types of
intrafusal
muscle fibres:
Nuclear bag fibres: commonly 2 per spindle; they are longer and thicker than the other and have a group of nuclei clustered around the centre of the fibre.
Nuclear chain fibres: which are shorter and thinner and have fewer nuclei arranged in a chain along the centre of the fibre; there are commonly 4 to 5
per spindle.
Slide6STRUCTURE OF MUSCLE SPINDLES
REPRESENTATION OF THE MAIN COMPONENTS OF MAMMALIAN MUSCLE SPINDLE
Slide7Afferents from spindle:
Ia
– from the nuclear bag fibers (
annulospiral
endings) – responds to change in length (static) and rate of change in length (dynamic)
II – from the nuclear chain fibers (flower spray endings) – static response only.
Afferents synapse directly on the anterior horn cell or
motoneurone which innervates the muscle group in which the spindle is situated. Spindle afferents stimulate the motorneurone resulting in muscle contraction.
Slide8GAMMA (
γ
) –EFFERENT
Motor nerve supply to the contractile ends of the
musle
spindle.
Originate from smaller
motoneurone
, the γ-motoneurones in the anterior horn of the cord.Stimulation cause contraction of the spindle resulting in stretch of the nuclear bag
leading to increase afferent impulses from the muscle spindle
causing an increase in muscle tone
Slide9γ
–efferent activity is under the control of the extra-pyramidal system particularly the
Reticulo
-spinal and
Vestibulo
-spinal tracts.
Increased
γ
–efferent activity is responsible for the increased tone seen in decerebrate animals (decerebrate rigidity).Clinical uses of Myotatic reflexHyperreflexia occurs in anxiety
May be a sign of damage to descending inhibitory pathways,
commomly
associated with upper motor neuron lesion.
Slide10Absent reflexes
may be due to emotional depression
Damage to afferents (
tabes
dorsalis
) or to
α
-motor neuron (poliomyelitis) or damage to both afferent and efferent nerve fibers as in peripheral neuropathies lead to absent tendon reflexes and poor muscle tone.Abnormal tendon reflexes may take the form of a pendular knee jerk as in cerebellar dysfunction or a prolongation of the contraction and relaxation time as in hypothyroidism.
Slide11GENERAL FUNCTIONS OF THE MUSCLE SPINDLE
Monitors and regulates muscle length, especially when the muscle is stretched
Contributes towards
propioception
.
Some collaterals from spindle afferents make connections with pathways to the cerebellum (
spinocerebellar
tracts) and cerebral sensory cortex (dorsal column
lemniscal pathways) and thus contribute to proprioception.
Slide12INVERSE STRETCH REFLEX (Autogenic Inhibition).
When muscle is overstretched:
T
he
reflex contraction suddenly gives way and the muscle
relaxes.
Receptor
for this reflex is the
Golgi tendon organA high threshold receptor and responds to increase in stretchAfferents are Group Ib fibers which synapse on inhibitory interneurone This is the cause of the “clasp-knife or lengthening reaction seen in decerebrate
preparations.
Slide13FLEXOR/WITHDRAWAL REFLX
Spinal reflex in response to a
norciceptive
/noxious stimulus
There is reflex flexion of the limb resulting in its withdrawal from the noxious stimulus.
CROSS EXTENSOR REFLEX
When nociceptive stimulus is strong:
Affected limb undergoes flexor/withdrawal
Irradiation of impulse to contraleteral limb induces extension Reflex helps maintain upright posture while the other leg is withdrawn.
Slide14EXPERIMENTAL PREPARATIONS FOR STUDYING CORD REFLEXES.
SPINAL ANIMAL
Spinal cord is transected in the neck so that most of the cord still remains intact: Spinal animal results
In toad, cut the head behind the tympanic membrane
or single pith to destroy the brain
Slide15Results in syndrome of Spinal Shock
Complete loss of reflex activity (days in cats, months in humans)
When reflexes return they are no longer under the influence of brain stem and do not follow normal patterns
eg
. A light touch on the foot can cause activation of all flexor muscles of body.
Slide16Excitability
of
motoneurons
ultimately becomes greater than normal, causing particular groups of muscles to contract continuously
.
2
. DECEREBRATE
ANIMAL
Brain is transected above the pontine reticular formation and results in DECEREBRATE RIGIDITY (Spasticity).In frog, cut the head behind the eyesInhibitory inputs from cortex and cerebellum to reticular formation are removed
α
&
γ
- antigravity
motoneurons
are subjected to intense excitation
.
Excessive firing of these
motoneurons
causes the antigravity muscles (leg extensors and arm flexors) to contract
continuosly
.
Firing of
γ
motoneurons
activates the gamma loop, causing increased discharge of the 1a afferent fibers.
Slide173.
DECORTICATE
/ THALAMIC
PREPARATION
Due to removal of Cerebral
hemispheres
Thalamus is intact
Can regulate body temperature (because hypothalamus is intact).
Movement occurs but poorly. Initiated mostly by extrapyramidal pathways. Sensations are crudely appreciated, especially pain and temperature. Decorticate rigidity – moderate rigidity due to loss of frontal inhibitory areas.
Slide18Exhibits sham rage – Destruction of cortical association areas enhance primitive emotions (which are usually suppressed).
Animal exhibits primitive emotions when noxious stimulus is applied (struggles and fights).
Slide19REFLEXES OF POSTURE AND LOCOMOTION
Positve
Supportive Reaction,
Pressure on the footpad of a
decerebrate
animal causes
Extension of the limb against the pressure
Reflex can support animal in a rigid position – the positive supportive reaction.Cord Righting reflexesWhen a spinal cat is laid on its side It will make uncoordinated movements that indicate that It is trying to raise itself to standing position Illustrates that complicated reflexes are associated with posture.
Slide20PROPERTIES
OF SPINAL REFLEXES
1. INHIBITION
Pre-synaptic: Inhibitory neuron acts on pre-synaptic fiber and reduces the magnitude of action potential, causing release of less transmitter
Post-synaptic: Inhibitory neuron connects with post-synaptic membrane, releases inhibitory transmitter which causes hyperpolarization.
Inhibitory neuron
Presynaptic
(a) and Post-synaptic (b) Inhibition
(a)
(b)
Slide212. SUMMATION
Spatial: Sub threshold stimuli in 2 or more afferents add to cause excitation of post-synaptic membrane
.
(
a
)
(b)(b) Temporal: Sub threshold stimuli in the same afferent coming in close succession (time and temporal summation) leading to summation and excitation of post-synaptic membrane.
Spatial (a) and Temporal (b) Summation
Slide223. FACILITATION
Fringe of a
Overlap of subliminal fringes
Fringe of b
2 neurons stimulated separately:
Independent responses are obtained
An area of sub-threshold excitation, the subliminal fringe, is created
b
a
Slide23If the 2 neurons are stimulated simultaneously,
2 subliminal fringes will overlap in the middle area
Additional neurons in that area will reach threshold due to spatial summation
So response will exceed the sum of individual responses.
Slide244. OCCLUSION
Occlusion in the CNS. (Sharing of neurons)
Occlusion occurs when 2 afferents are stimulated simultaneously and the response is less than the sum of the individual responses.
Due to sharing of the same efferent
neurone
by 2 or more afferents.
Slide255. AFTER- DISCHARGE
Persistence of the response after the stimulus is over
Due to
reverberatory
circuits
On stimulation, the primary path elicits an initial response but due to
reverberatory
circuits 2,3&4, which are longer, the excitation continues and the response persists.
2
3
4
1
After-discharge
Effector
Slide266. POST-TETENIC FACILLITATION
Increased
response of post-synaptic membrane following brief but rapid repetitive (tetanic) series of impulses. This is due to excessive release of calcium ions during the period of stimulation.
7
. FATIGUE
Due
to excessive and continuous stimulation at a rapid rate. This causes the transmitter substance to be exhausted.