Reflex activity

Reflex Activity

Reflex arc, classification, superficial and deep reflexes, pathological reflexes, properties and reciprocal innervation.

Definition Reflex Arc Classification Superficial Reflexes Deep Reflexes Visceral Reflexes Pathological Reflexes Properties Reciprocal Inhibition Motor Neuron Lesion

1 Definition and Significance of Reflexes

Reflex activity is an involuntary response to a stimulus. It is a type of protective mechanism and it protects the body from irreparable damages.

For example, when hand is placed on a hot object, it is withdrawn immediately.

When a bright light is thrown into the eyes, eyelids are closed and pupil is constricted to prevent the damage of retina by entrance of excess light into the eyes.

2 Reflex Arc

Reflex arc is the anatomical nervous pathway for a reflex action.

Simple reflex arc includes five components.

1. Receptor

Receptor is the end organ, which receives stimulus.

When receptor is stimulated, impulses are generated in afferent nerve.

2. Afferent Nerve

Afferent or sensory nerve transmits sensory impulses from receptor to the center.

3. Center

Center receives the sensory impulses via afferent nerve fibers and in turn, it generates appropriate motor impulses.

Center is situated in the brain or spinal cord.

In simple reflex arc, synapse between afferent nerve and efferent nerve forms the center.

4. Efferent Nerve

Efferent or motor nerve transmits motor impulses from center to the effector organ.

5. Effector Organ

Effector organ is a structure such as muscle or gland where the activity occurs in response to stimulus.

Afferent and efferent nerve fibers may be connected directly to the center.

In some places, one or more neurons are interposed between these nerve fibers and the center.

Such neurons are called connector neurons or internuncial neurons or interneurons.

3 Classification of Reflexes

Reflexes are classified by six different methods depending upon various factors.

I. Classification Depending Upon Whether Inborn or Acquired

1. Inborn Reflexes or Unconditioned Reflexes

Inborn reflexes or unconditioned reflexes are the natural reflexes which are present since the time of birth.

Such reflexes do not require previous learning, training or conditioning.

Example is the secretion of saliva when a drop of honey is kept in mouth of a newborn baby for the first time.

The baby does not know the taste of honey but still saliva is secreted.

2. Acquired Reflexes or Conditioned Reflexes

Acquired reflexes or conditioned reflexes are the reflexes that are developed after conditioning or training.

Such reflexes are not inborn but acquired after birth.

Acquired reflexes require previous learning, training or conditioning.

Example is secretion of saliva by the sight, smell, thought or hearing of a known edible substance.

II. Anatomical Classification: Classification Depending Upon Situation of Center

  1. Cerebellar reflexes.
  2. Cortical reflexes.
  3. Midbrain reflexes.
  4. Bulbar or medullary reflexes.
  5. Spinal reflexes.

III. Physiological Classification: Classification Depending Upon Purpose

1. Protective Reflexes or Flexor Reflexes

Protective reflexes protect the body from nociceptive (harmful) stimuli.

Protective reflexes are also called withdrawal reflexes or flexor reflexes.

Protective reflexes involve flexion at different joints hence the name flexor reflexes.

2. Antigravity Reflexes or Extensor Reflexes

Antigravity reflexes protect the body against gravitational force.

Antigravity are also called extensor reflexes because, extensor muscles contract during these reflexes resulting in extension at joints.

IV. Classification Depending Upon Number of Synapses

1. Monosynaptic Reflexes

Reflexes having only one synapse in the reflex arc are called monosynaptic reflexes.

Stretch reflex is the best example for monosynaptic reflex and it is elicited by stimulation of muscle spindle.

2. Polysynaptic Reflexes

Reflexes having more than one synapse in the reflex arc are called polysynaptic reflexes.

Examples are flexor reflexes (withdrawal reflexes).

V. Classification Depending Upon Whether Somatic or Visceral Reflexes

1. Somatic Reflexes

Somatic reflexes are the reflexes, for which reflex arc is formed by somatic nerve fibers.

Somatic reflexes involve the participation of skeletal muscles.

And there may be flexion or extension at different joints during these reflexes.

2. Visceral or Autonomic Reflexes

Visceral or autonomic reflexes are the reflexes, for which at least a part of reflex arc is formed by autonomic nerve fibers.

Visceral reflexes involve participation of smooth muscle or cardiac muscle.

Visceral reflexes include pupillary reflexes, gastrointestinal reflexes, cardiovascular reflexes, respiratory reflexes, etc.

Some reflexes like swallowing, coughing or vomiting are considered as visceral reflexes.

However, these reflexes involve participation of skeletal muscles also.

VI. Classification Depending Upon Clinical Basis

Depending upon clinical basis reflexes are classified into four types:

  1. Superficial reflexes.
  2. Deep reflexes.
  3. Visceral reflexes.
  4. Pathological reflexes.

4 Superficial Reflexes

Superficial reflexes are the reflexes, which are elicited from surface of body.

Superficial reflexes are elicited from:

  1. Cornea (corneal reflex) and conjunctiva (conjunctival reflex) of eyeball.
  2. Mucous membrane (mucous membrane reflexes).
  3. Skin (cutaneous reflexes).

Superficial Reflexes Elicited from Eye and Mucous Membrane

Reflex Method of Eliciting Reflex Response Afferent Nerve Center Efferent Nerve
Corneal reflex Touching cornea with a wisp of cotton Closure of eyelids (blinking) Ophthalmic branch of V cranial nerve Pons: Trigeminal nucleus VII cranial nerve
Conjunctival reflex Touching conjunctiva with wisp of cotton Closure of eyelids (blinking) Ophthalmic branch of V cranial nerve Pons: Trigeminal nucleus VII cranial nerve
Nasal reflex (sneezing reflex) Stimulating the nasal mucosa with a wisp of cotton Sneezing V cranial nerve Medulla: Nucleus tractus solitarius V, VII, IX and X cranial nerves
Pharyngeal reflex (gag reflex) Touching roof of mouth, back of tongue, uvula, tonsils or back of throat pharynx with a wisp of cotton or any other object Elevation of soft palate and retching (strong involuntary effort to vomit) or gagging (opening of mouth) IX cranial nerve Medulla: Nucleus tractus solitarius X cranial nerve

Superficial Reflexes Elicited from Skin (Cutaneous Reflexes)

Reflex Method of Eliciting Reflex Response Afferent Nerve Center (Spinal Segments) Efferent Nerve
1. Scapular reflex Stroking the skin at interscapular space Contraction of scapular muscles and drawing in of scapula Suprascapular nerve C5 and C6 Suprascapular nerve
2. Upper abdominal reflex Stroking the abdominal wall below the costal margin (supraumbilical level) Ipsilateral contraction of abdominal muscle and movement of umbilicus towards site of stroke T7 and T8 spinal nerves T7 and T8 T7 and T8 spinal nerves
3. Middle abdominal reflex Scratching the abdominal wall near umbilicus (umbilical level) T9 and T10 spinal nerves T9 and T10 T9 and T10 spinal nerves
4. Lower abdominal reflex Stroking the abdominal wall at umbilical and iliac level (infraumbilical level) T11 and T12 spinal nerves T11 and T12 T11 and T12 spinal nerves
5. Cremasteric reflex Scratching the skin at upper and inner aspect of thigh Elevation of testicles L1 and L2 spinal nerves L1 and L2 L1 and L2 spinal nerves
6. Gluteal reflex Stroking the skin over buttock Contraction of gluteus muscles Posterior femoral cutaneous nerve S1 to S3 Inferior gluteal nerve
7. Plantar reflex Stroking the sole Plantar flexion and adduction of toes Sciatic nerve L5 to S1 Sciatic nerve
8. Bulbocavernosus reflex Stroking the dorsum of glans penis Contraction of bulbocavernosus Perineal nerve S3 and S4 Perineal nerve
9. Anal reflex Stroking the perianal region Contraction of anal sphincter Inferior rectal nerve S3 and S4 Inferior rectal nerve

C = Cervical, T = Thoracic, L = Lumbar, S = Sacral.

5 Deep Reflexes or Tendon Reflexes

Deep reflexes or tendon reflexes are elicited from deeper structures beneath the skin such as tendon.

Deep Tendon Reflexes

Reflex Method of Eliciting Reflex Response Afferent Nerve Center Efferent Nerve
1. Jaw jerk Tapping the middle of the chin with slightly opened mouth Closure of mouth V cranial nerve Pons: V cranial nerve nuclei V cranial nerve
2. Biceps jerk Tapping the biceps tendon Flexion of forearm Musculocutaneous nerve 5th and 6th cervical spinal segments Musculocutaneous nerve
3. Triceps jerk Tapping the triceps tendon Tapping the triceps tendon Radial nerve 6th and 7th cervical spinal segments Radial nerve
4. Supinator jerk or brachioradialis jerk or radial periosteal reflex Tapping the tendon over distal end (styloid process) of radius Supination and flexion of forearm Radial nerve 5th and 6th cervical spinal segments Radial nerve
5. Knee jerk or patellar tendon reflex Tapping the patellar tendon Extension of knee due to contraction of quadriceps muscle Femoral nerve 2nd, 3rd and 4th lumbar spinal segments Femoral nerve
6. Ankle jerk or Achilles tendon reflex Tapping the Achilles tendon Plantar flexion of foot Tibial nerve 1st and 2nd sacral spinal segments Tibial nerve

6 Visceral Reflexes

Visceral reflexes are the reflexes arising from pupil and the visceral organs.

Visceral Reflexes Include

  1. Pupillary reflexes in which the size of pupil is altered.
  2. Oculocardiac reflex in which heart rate decreases by pressure applied over eyeball.
  3. Carotid sinus reflex in which the pressure over carotid sinus in neck by tight collar dress decreases heart rate and blood pressure.

7 Pathological Reflexes

Pathological reflexes are the reflexes that are elicited only in pathological conditions.

Three pathological reflexes are well known.

1. Babinski Reflex

Babinski reflex is the abnormal plantar reflex.

It is also called Babinski sign or phenomenon.

In normal plantar reflex, a gentle scratch over the outer edge of the sole of foot causes plantar flexion and adduction of all toes.

But in Babinski reflex, there is dorsiflexion of great toe and fanning of other toes.

Babinski reflex is present in upper motor neuron lesion particularly in lesion of corticospinal (pyramidal) tracts.

It is noticed in some physiological conditions also.

It is present in infants up to 2 years because of incomplete myelination of pyramidal tracts.

In adults this reflex may be elicited in deep sleep and in old age.

2. Clonus

Clonus is a series of rapid and repeated involuntary jerky movements, which occur while eliciting a deep reflex.

It occurs in upper motor neuron lesion.

When a deep reflex is elicited in a normal person, the contractions of a muscle or group of muscles are smooth and continuous.

But in upper motor neuron lesion clonus occurs.

It is because of hypertonicity of muscles and exaggeration of deep reflexes.

Clonus is well seen in calf muscles producing ankle clonus and quadriceps producing patella clonus.

Clonus is also seen in wrists, fingers, jaw and elbow.

3. Pendular Movements

Pendular movements are slow oscillatory movements instead of brisk movements which are developed while eliciting a tendon jerk.

Pendular movements are very common while eliciting the knee jerk in patients affected by cerebellar lesion.

Such movements are similar to movements of clock’s pendulum hence the name pendular movements.

8 Properties of Reflexes

1. One-Way Conduction: Bell-Magendie Law

During any reflex activity, impulses are transmitted in only one direction through the reflex arc as per Bell-Magendie law.

Impulses pass from receptors to the center and then from center to effector organ.

2. Reaction Time

Reaction time is the time interval between application of stimulus and onset of reflex.

It depends upon the length of afferent and efferent nerve fibers, velocity of impulse through these fibers and central delay.

Central delay is the delay at the synapse.

It is also called synaptic delay.

3. Summation

Summation (fusion of effects) in reflex action is of two types:

1. Spatial Summation

When two afferent nerve fibers supplying a muscle are stimulated separately with subliminal stimulus, there is no response.

But the muscle contracts when both the nerve fibers are stimulated together with same strength of stimulus.

It is called spatial summation.

2. Temporal Summation

When one nerve fiber is stimulated repeatedly with subliminal stimuli, these stimuli are summed up to give response in the muscle.

It is called temporal summation.

Thus, both spatial summation and temporal summation play an important role in the facilitation of responses during the reflex activity.

4. Recruitment

Recruitment is defined as the successive activation of additional motor units with progressive increase in force of muscular contraction.

When an excitatory nerve is stimulated for a long time, there is a gradual increase in the response of reflex activities.

It is due to the activation of more and more motor units.

Recruitment is similar to the effect of temporal summation.

5. After Discharge

After discharge is the persistence or continuation of response for some time even after cessation of stimulus.

When a reflex action is elicited continuously for some time and then the stimulation is stopped, the reflex activity (contraction) will be continued for some time even after the stoppage of the stimulus.

It is because of the discharge of impulses from center even after stoppage of stimulus.

Internuncial neurons are responsible for after discharge.

6. Rebound Phenomenon

Reflex activities can be inhibited forcefully for some time.

But, when inhibition is suddenly removed, the reflex activity becomes more forceful than before inhibition.

It is called rebound phenomenon.

Reason for this state of over excitation is not known.

7. Fatigue

When a reflex activity is continuously elicited for a long time, the response is reduced slowly and at one stage, the response does not occur.

This type of failure to give response to the stimulus is called fatigue.

Center or synapse of the reflex arc is the first seat of fatigue.

9 Reciprocal Inhibition and Reciprocal Innervation

Reciprocal Inhibition

Reciprocal inhibition is a process during reflex activity, by which there is relaxation of one group of muscles during contraction of their antagonistic muscles.

When a flexor reflex is elicited, flexor muscles are excited (contracted) and the extensor muscles are inhibited (relaxed) in that side.

This phenomenon is called the reciprocal inhibition.

Reciprocal inhibition occurs because of reciprocal innervation.

Reciprocal Innervation

Reciprocal innervation is a type of innervation of muscles, because of which contraction of a group of muscles is accompanied by relaxation of their antagonistic muscles.

Afferent nerve fibers, which produce flexor reflex in a limb, have connections with motor neurons supplying flexor muscles and the motor neurons supplying the extensor muscles of same side.

Afferent nerve excites the motor neurons, which supply the flexor muscles.

Simultaneously, it inhibits the motor neurons supplying extensor muscles through an interneuron.

Accordingly, flexor muscles contract and extensor muscles relax resulting in flexion of the limb.

Crossed Extensor Reflex

Crossed extensor reflex is a withdrawal reflex in which flexors of withdrawing limb are excited (contracting) and extensors are inhibited (relaxed).

And the opposite movements occur in another limb.

For example, while eliciting a flexor reflex activity in a limb, that limb is flexed.

Simultaneously the opposite limb is extended.

Flexors are excited and extensors are inhibited in this limb, but in the opposite limb, flexors are inhibited and extensors are excited.

This type of crossed extensor reflex is because of reciprocal inhibition.

Significance of Reciprocal Inhibition

Reciprocal inhibition and reciprocal innervation are very important in spinal reflexes, which are involved in locomotion.

It helps in the forward movement of one limb while causing the backward movement of the opposite limb.

10 Applied Physiology: Reflexes in Motor Neuron Lesion

Upper Motor Neuron Lesion

During upper motor neuron lesion, all the superficial reflexes are lost.

Deep reflexes are exaggerated and Babinski’s sign is present.

Lower Motor Neuron Lesion

During lower motor lesion, all the superficial and deep reflexes are lost.