1. REFLEX

2. OBJECTIVES
To understand the functional components of a reflex arc ,reflex action
To explain how reflexes maintain homeostasis.

3. Motor control- ‘Reflexes’
Organization of motor system;
Types of movements
Reflex & components
Stretch reflex
Flexion withdrawal reflex
Properties of reflexes

6. REFLEX
A reflex is a fast, SPONTANEOUS automatic, unplanned sequence of actions that occurs in response to a particular stimulus.
Some reflexes are inborn, such as pulling your hand away from a hot surface before you even feel that it is hot.
Other reflexes are learned or acquired

7. REFLEX (Contd)
When integration takes place in the spinal cord gray matter, the reflex is a spinal reflex. Eg: knee jerk
If integration occurs in the brain stem rather than the spinal cord, the reflex is called a cranial reflex.
Eg: tracking movements of your eyes as you read this sentence.
Other terminology:
somatic reflexes, which involve contraction of skeletal muscles.
autonomic (visceral) reflexes, which generally are not consciously perceived. They involve responses of smooth muscle, cardiac muscle, and glands

8. Types of movements and hierarchy of motor control
Reflex movements that are totally involuntary and occurring at the spinal cord level. e.g., withdrawal reflex, stretch reflex.
Neural circuits located in the spinal cord itself
Under profound influence of supraspinal centers
Supraspinal influences mainly descend from the brainstem areas and also from other higher brain areas of motor control.

9. Types of movements and hierarchy of motor control
ii) Movements which have become automatic such as rhythmic responses like swallowing, chewing, scratching and walking
Performed at an involuntary level but subject to voluntary control.
Basic circuits for some of these responses (e.g., walking) are located in the spinal cord itself but are modified by higher brain centers

10. Types of movements and hierarchy of motor control
iii) Movements which are totally under voluntary control – controlled by cerebral cortex and assisted by cerebellum and basal ganglia.
Called volitional movements
Some are fine, skilled movements, requiring high degree of concentration
Motor skills are learnt and subsequently performed faster

11. Organization of the spinal cord for the motor functions
The spinal cord gray matter is the integrative area for the cord reflexes.
Sensory signals from the periphery (skin muscles etc,) enter the spinal cord through the dorsal roots.
After entering the cord, every sensory signal travels in two separate destinations.
One branch of the sensory nerve fiber terminates in the gray matter and elicits local cord reflexes and
The other branch transmits signals to higher levels of nervous system

12. Organization of the spinal cord for the motor functions (Contd)
The spinal cord has two principal functions in maintaining homeostasis: nerve impulse propagation and integration of information.
The white matter tracts in the spinal cord are highways for nerve impulse propagation.
The gray matter of the spinal cord receives and integrates incoming and outgoing information.

13. Organization of the spinal cord for the motor functions (Contd)
One of the ways the spinal cord promotes
homeostasis is by conducting nerve impulses along tracts.
Often, the name of a tract indicates its position in the white matter and where it begins and ends.
The second way the spinal cord promotes homeostasis is by serving as an integrating center for some reflexes.

14. REFLEX (Reflex action)
A reflex (reflex action) is a simple, involuntary, relatively stereotyped motor response to a specific type of stimulus.
Reflex arc is the neural circuit responsible for a particular reflex, consisting of:
sensory neurons
set of interneurons (absent in stretch reflex) and
motor neurons

15. REFLEX ARC

16. Bell-Magendie law
The principle that in the spinal cord the dorsal roots are sensory and the ventral roots are motor is known as the Bell-Magendie law

17. Muscle Stretch (myotatic) reflex
Muscle Stretch reflex is the basis of muscle tone.
It is a monosynaptic reflex.
The receptors are the muscle spindles.
When a muscle with an intact nerve supply is subjected to stretch, there is reflex contraction of that muscle.
Controlled by brain stem, cortical and subcortical centers through the influences on alpha and gamma motor neurons.

18. Components of the Stretch Reflex pathway
Stimulus: Stretch of the muscle
Receptors: Muscle spindles (intrafusal fibers)
Afferents: Fast conducting Ia afferents
Center: Spinal cord/Brain stem
Efferents: A- alpha fibers
Effector structure: Extrafusal (regular contractile) fibers of the muscle that is stretched
Response: Contraction of the muscle
Example: Knee jerk

21. FINAL COMMON PATHWAY
The motor neurons that supply the extrafusal fibers in skeletal muscles are the efferent side of many reflex arcs.
All neural influences affecting muscular contraction ultimately funnel through them to the muscles, and they are therefore called the final common paths.
Numerous inputs converge on them. Indeed, the surface of the average motor neuron and its dendrites accommodates about 10,000 synaptic knobs.
There are at least five inputs from the same spinal segment to a typical spinal motor neuron.
In addition to these, there are excitatory and inhibitory inputs, generally relayed via interneurons, from other levels of the spinal cord and multiple long descending tracts from the brain.
All of these pathways converge on and determine the activity in the final common paths.

22. Applied
If any one of these component is damaged the reflex action cannot take place.
Eg: in leprosy, in finger the receptors and afferent nerve fibers are damaged.
Damage to receptor is loss of reflex action.
Tabesdorsalis due to syphiles the posterior nerve root damage.
In center :- damage to spinal cord.
Damage to anterior horn cell :- occurs in poliomylites.
Myasthemia gravis – effector organ weak-affected

23. CLASSIFICATION OF REFLEXES
Spinal Reflexes:-
-Superficial reflexes
-Deep Reflexes
-Visceral reflexes
CLASSIFICATION BASED ON THE NUMBER OF SYNAPES:-
Monosynaptic reflex
(eg:- stretch reflex, one synapse)
Polysynaptic reflex
(eg:- withdrawal reflex, more than one synapse)

24. 1) SUPERFICIAL REFLEXES
Superficial reflexes result from stimulation of the receptors present in the skin.
They are:-

25. 1a) Planter reflex
Scratching the outer edge of the sole of the foot with a key results in planter flexion of the toes.

26. 1b) Abdominal reflex
Scratching the skin of the abdomen from the umbilicus laterally, causes reflex contraction of the underlying abdominal muscles.

27. 1c) Cremasteric reflex
Scratching the skin of the upper medial aspect of the thigh results in reflex contraction of the cremasteric muscle and elevation of the testicle.

28. 1d) Flexion Withdrawal reflex
It is a protective reflex.
This reflex is evoked by a strong noxious stimulus arising in a cutaneous receptor and results in a coordinated muscle contraction in a group of muscles that removes the part (usually a limb) away from the noxious stimulus.
Protects the limb from further damage
Considerable divergence of the primary afferent and internuncial pathways occurs in flexion reflex.
All the major joints of a limb (e.g. hip, knee, ankle) may be involved in a strong flexor withdrawal reflex

29. Flexor withdrawal reflex

30. 1e) Crossed extensor reflex
When an injurious stimulus is applied to a limb, it will result in a reflex flexion.
Withdrawal reflex of the stimulated limb, and reflex extension of the opposite limb - crossed extensor reflex

31. Crossed extensor reflex

32. 1f) Scratch reflex It is initiated by the itch and tickle sensation.
It consists of to and fro scratching movements, due to recicropal inhibition and rebound.

33. 2) DEEP REFLEXES
Deep reflexes result from stimulation of the receptors present in muscles and tendons.
They are :-

34. 2a) Stretch reflex
Sudden passive stretch of any skeletal muscle will lead to reflex contraction of the stretched muscle.
It is a monosynaptic reflex responsible for muscle tone and tendon jerks (deep reflexes)

35. 3) Visceral reflex The autonomic spinal reflexes include:
Micturition reflex:-
Distension of urinary bladder results in reflex contraction of the bladder wall and relaxation of urethral sphincters.
Defaecation reflex:-
Distension of rectum results in reflex contraction of the rectal wall and relaxation of anal sphincters

36. What is a muscle spindle?
Muscle spindle is a fusiform structure consisting of about
10 muscle fibers called as intrafusal fibers enclosed in a
Connective tissue capsule.
Intrafusal fiber characteristics:
Intrafusal fibers act as the receptor organ for mediating stretch reflex.
IF fibers are oriented parallel to extrafusal fibers
When muscle is stretched,IF fibers are also stretched.
There are two types of IF fibers in each muscle spindle
Nuclear bag fibers
Nuclear chain fibers.

39. Innervation of spindle
I. Sensory innervation
There are two kinds of sensory endings in each spindle
Primary (annulospiral) endings are the terminations of rapidly conducting group Ia fibers wrapping around the center of both nuclear bag and nuclear chain fibers.
Secondary (flower spray) endings are terminations of group II sensory fibers located nearer the ends of only nuclear chain fibers.
II. Motor innervation
A gamma group ( efferents). These fibers innervate the contractile ends of the intrafusal fibers

41. Spindle response to stretch
Two types of responses are observed from
muscle spindle afferents:
Dynamic response- Spindle afferent discharge
increases during stretch
(sensing velocity of change in muscle length)
2.Static response- Spindle afferent discharge
increases when the muscle has changed
its length
(sensing steady-state change in muscle length)

42. Inverse Stretch Reflex
Strong contraction of the muscle (due to extreme stretch) causes muscle to relax by the stimulation of Golgi tendon organs in the tendon of muscle.
These receptors are in series with the muscle fibers
N. fibers from these organs are the Ib sensory fibers.
Ib fibers end in the spinal cord on inhibitory interneurons that, in turn terminate on the motor neurons supplying the same muscle.
Stimulation of these Ib fibers leads to the production of IPSPs on the motor neurons resulting in inhibition of muscle contraction (example of postsynaptic inhibition).

43. Inverse stretch reflex

44. Properties of reflexes
Spatial & temporal summation
Local sign
Fractionation
Occlusion
Subliminal fringe
Recruitment
After-discharge
Adequate stimulus
Final common path
Reverberating circuits
Prepotent
Reflex delay
Reciprocal inhibition
Crossed extensor response
Irradiation

45. Adequate stimulus
The stimulus for a particular reflex should be of sufficient strength in order to elicit a response

46. Reverberating circuits
Polysynaptic reflexes involve pathways that are complex due to extensive branching of afferent neurons and involvement of several interneurons and synapses.
Some of the pathways turn back on themselves forming reverberating circuits
Impulse traveling in the reverbrating circuits causes prolonged and repeated bombardment of the motor neurons from a single stimulus

47. Prepotent
Withdrawal reflexes are prepotent i.e., any other reflex activity taking place at that moment is suppressed.

48. Reflex delay
The response observed is slightly delayed because the property of synaptic delay at each of the synapses.
Larger the number of synapses, greater is the delay

49. Reciprocal inhibition
Flexion withdrawal reflex not only involves the contraction of flexors but also relaxation of extensor muscles
Neural circuit is present in spinal cord

50. Crossed extensor response
When a strong noxoius stimulus is applied to a limb, the response includes not only flexion and withdrawal of that limb but also extension of the opposite limb.
The extension of the opposite limb supports the body.

52. Irradiation
Strong noxious stimuli in experimental animals generate activity in the interneuron pool that spreads to all four extremities.
This is due to the spread of excitatory impulses up and down the spinal cord to more and more motor neurons.
E.g., Mass reflex in a paraplegic patient

53. Spatial and temporal summations
As the strength of the stimulus is increased, spatial and temporal summations occur at the synapses in the polysynaptic pathways
Results from
activation of more sensory fibers from receptor
more number of action potentials per unit time

54. Local sign
The exact pattern of flexor response of the withdrawal reflex in a limb varies with the part of the limb that is stimulated so that the resultant tension developed by a specific group of muscles effectively removes the limb from the noxious stimulus

55. Fractionation
Supramaximal stimulation of any of the sensory nerves from a limb never produces as strong a contraction of the flexor muscles as that elicited by direct stimulation of the muscles themselves. This is because the afferent inputs fractionate the motor neuron pool i.e., each input goes only to part of the motor neuron pool for the flexor muscles

56. Occlusion
When two afferent excitatory nerve fibers (a and b) – each of which capable of evoking a flexor withdrawal response – are simultaneously stimulated, it is some times found that the tension developed by the flexor muscle under observation is less than the sum of the tension produced by each afferent stimulated separately. This is because the various afferent inputs share some of the motor neurons

58. Subliminal fringe
Sometimes, the tension developed by a and b (in the above case) together is greater than the sum of the two reflex responses taken singly (see figure below). This results because each afferent while fully activating a certain number of motor neurons (discharge zone) acts also on other motor neurons subliminally. Some of these neurons which are in the subliminal fringe (of excitation) are common to a and b. When both a and b afferents are together stimulated, the fringe motor neurons in the common pool are excited due to spatial summation (facilitation zone) and the resultant tension developed is greater

59. Subliminal fringe

60. Recruitment
Recruitment of motor neurons in withdrawal reflex occurs gradually on continued afferent stimulation resulting in relatively gradual increase in muscle tension to reach maximum

61. After-discharge
On discontinuation of afferent stimulation, the tension may remain unaltered for several seconds.
Muscle relaxation proceeds more gradually because motor neurons stop discharging successively.
After-discharge is due to the continued discharge of motor neurons after afferent stimulation has ceased, and is attributed to persistent activation in the internuncial neuronal pool since impulses go on wandering in these paths for varying periods and continue to bombard the motor neurons (reverberating circuits)

62. OUTCOME Define the Bell-Magendie law.
Describe the mechanism of the monosynaptic reflex [stretch reflex]
Describe structure and innervation of muscle spindle
Represent diagrammaticlly structural features of muscle spindle and its innervation
Discuss the role of the muscle spindle in regulation of muscle length and force.
Explain the mechanism of tone in skeletal muscles
Explain the mechanism, significance and properties of the withdrawal reflex
Explain the final common path for skeletal muscle contraction
Explain the role of stretch reflex in the regulation of posture
Explain the clinically elicited reflexes and their significances