Reflex

 Definition

Reflex is automatic motor response to a sensory impulse through the involvement of center (CNS).

 

Reflex Arc

Pathway for any reflex action is known as reflex arc. It consists of:

 1) Receptor- Is end organ which receive stimulus and when stimulated generates impulses in afferent nerve

 2) Afferent nerve- afferent or sensory nerve transmits impulses from receptor to the center.

 3) Centre- This is the part of CNS where afferent limb ends and either synapses directly with efferent motor neuron or establish connection with the efferent neuron via internuntial or intercalated neurons.

 4) Efferent nerve- efferent or motor nerve transmits impulses from center to effector organ.

 5) Effector organ- It may be muscle or gland which show response to the stimulus

  

Significance

Protection, various GIT, CVS, RS, CNS functions are due to reflex activities

Classification of reflexes

1. Clinical classification

i) Superficial reflexes- These are the reflexes which are initiated by stimulating appropriate receptors of skin or mucous membrane. They are usually multisynaptic. e.g. plantar response, abdominal and cremasteric reflexes, corneal and conjunctival reflexes.

 ii) Deep reflexes- These are elicited on stroking the tendon. They are basically stretch reflexes and are also called deep tendon reflexes, e.g knee jerk, ankle jerk, etc.

 iii) Visceral reflexes- These are the reflexes where at least one part of the reflex arc is formed by autonomic nerve, e.g. pupillary reflex, carotid sinus reflex.

 iv) Pathological reflexes- These are the reflexes which are not found normally and their presence indicates pathological condition within the body, e.g. Babinski's sign.

 

2. Anatomical classification of reflexes

i) Segmental reflexes- In these, end of afferent neuron and the beginning of efferent neuron are in the same segment of the spinal cord.

 ii) Intersegmental reflexes- In these reflexes, end of afferent neuron and the beginning of efferent neuron are in the spinal cord but in different segments 

iii) Suprasegmental reflexes-The centre for such reflex lies above the spinal cord,

 

3. Physiological classification of reflexes

i) Flexor reflexes- They are produced when nociceptive (pain) stimulus is applied. Such stimuli cause flexion of the joint, e.g. thorn prick to the sole, causes flexion of knee, hip joints. These reflexes are protective in nature

 ii) Extensor reflexes- Stretch reflexes are extensor reflexes which are the basis of tone and posture

 4. Depending upon inborn or acquired reflexes

 i)Unconditioned reflexes- They are inborn reflexes, as they are present since birth

 ii) Conditioned reflexes- These are the reflexes which develop after birth. Their appearance depends on previous experience,

 How studied?

1) Spinal preparation- Most of the reflexes are studied in spinal animal, i.e. animal where the spinal cord is transected at cervical region and respiration is maintained by respiratory pump So properties of reflexes such studied are mainly the properties of spinal reflexes

 If spinal cord is transected in the thoracic region, diaphragmatic breathing continues, artificial respiration is not required,

 2) Decerebrate preparation- Reflexes can also be studied in such a preparation. In this, transaction is taken in the brain stem between superior and inferior colliculi.

 Properties of Reflex

1. Adequate stimulus

Stimulus needed to elicit the reflex is usually precise and is called adequate stimulus, e.g scratch reflex in a dog is initiated only by multiple linear touch stimuli. If multiple touch stimuli are widely separated, reflex is not initiated.

2. Delay

There is some interval between application of stimulus and starting of the response which is called delay. It is mainly due to delay at synapses and partly due to time required for passage of impulse along the nerves. It is least in monosynaptic reflex

 3. Spatial and Temporal summation

Temporal summation Application of subminimal (subthreshold) stimulus does not elicit response. But if second subminimal stimulus is applied sufficiently quickly (taking care of refractory period of the nerve), response does occur though individual stimulus is subthreshold. Therefore two or more subthreshold stimuli applied in succession, their effects are summated and evoke response because of summation of two EPSPs.

 Spatial summation Two or more subthreshold stimuli when applied simultaneously at different but closely situated spots, their effects are summated and can evoke response though each individual stimulus is subthreshold.

4. Occlusion and Subliminal fringe

Occlusion- Stimulation of two or more neighboring nerves simultaneously evokes lesser response than sum total of the responses obtained when each afferent nerve is separately stimulated, e.g. there are two afferent nerves A and B. Stimulation of A with strong electric shock causes development of tension (T1) in the muscle. Stimulation of the nerve B with a strong electric shock causes development of tension (T2) in the muscle,

 When both A and B is simultaneously stimulated with strong stimulus, tension developed is lesser than T1 + T2'. Nerves A and B stimulate some common neurons, therefore their simultaneous stimulation excites lesser number of neurons than the sum total of neurons stimulated when they are separately stimulated.

 Subliminal fringe- When two or more neighboring nerves are stimulated simultaneously with weak shock, the tension developed in the muscle is more than sum total of the tensions developed when the nerves are separately stimulated with weak shocks. This can be explained as follows:

 Each afferent nerve on entering the spinal cord stimulates two groups of neurons. One group is stimulates two groups of neurons. One group is stimulated adequately and second group subminimally. Each weak stimulus therefore produces action potential in the nerves of group one neurons. The neurons belonging to second group are also excited but only subminimally (so this group is common). When both A and B are simultaneously stimulated, the action potential also develops in second group of neurons and therefore response obtained is greater.

 

5.Irradiation

When sensory stimulus is too strong, impulse spreads too many neighboring neurons in the centre and produces wider response. It is due to transmission of impulse through large number of collaterals of afferents and their interneurons.

 

6. Final common path

Efferent pathway of the reflex arc is formed by motor neurons that supply the extrafusal muscle fibers. All neuronal influences affecting muscular contraction ultimately funnel through the motor neurons therefore they are called common final paths. Numerous inputs converge on them. On an average about 10,000 synapses are present on the motor neuron. All these converge and determine the activity in the final common path.

 7. Facilitation

If a reflex is elicited repeatedly at proper intervals the response becomes progressively higher for first few occasions, i.e. each subsequent stimulus exerts a better effect than the previous one. This is due to facilitation occurring at synapse

 

8. Inhibition

Impulses through sensory fibers from protagonist muscles inhibit the action of antagonist muscles, e.g. when flexors muscles of the joint are stimulated extensor muscles are inhibited. Such a reciprocal inhibitory effect SỊ due to inhibitory activity exerted by interneurons.

9. After discharg

Continuation of response even after stimulus is over is called after discharge. This is due to discharge from the centre even after stoppage of stimulation, i.e. motor neurons are stimulated through multiple interneuntial pathways. Some of them take longer time to reach motor neurons. Therefore, even after cessation of stimulation impulses travel to motor neurons for a certain period of time. This causes after discharge

 10. Fatigue

If a particular reflex is elicited repeatedly at frequent intervals, response becomes progressively feebler and then disappears all together. This is called fatigue. The first site of fatigue is synapse, then the motor endings and lastly the muscle.

11. Fractionation

When a stimulus is directly applied to the motor nerve of a muscle, the amount of contraction becomes much higher than when the muscle is stimulated reflexly through a sensory nerve. This is due to phenomenon of occlusion of the motor neurons when sensory nerve is stimulated. Because of occlusion, number of motor neurons stimulated is lesser..

 12. Habituation and sensitization

When a non-injurious stimulus is applied repeatedly, intensity of response becomes lesser, declines and even ceases. This phenomenon is known as habituation

 On the other hand, when the injurious stimulus is repeatedly applied there is intensification of response. This is known as sensitization

1. Local sign and recruitment

2. Irradiation

3. Central excitatory and inhibitory state

4. Final common pathway (alpha motor neuron)

 5. Fractionation