Reflex Physiology
Definition of Reflex
Reflex: involuntary and nearly instantaneous movement in
response to a stimulus
Reflex arc: the neural arc utilized in a reflex action; an impulse
travels centrally over afferent fibers to a nerve center, and the
response outward to an effector organ or part over efferent
fibers
The neural path of a reflex.
Classifications of Reflexes
1. By early development
–
Innate or Acquired
2. By type of motor response
–
Somatic or Visceral
3. By complexity of neural circuit
–
Monosynaptic or Polysynaptic
4. By site of information processing
–
Spinal or Cranial
Reflex classifications
• Innate reflexes
– Result from connections that form between neurons
during development
• Acquired reflexes
– Learned, and typically more complex
Reflex classifications
• Cranial reflexes
– Reflexes processed in the brain
• Spinal reflexes
– Interconnections and processing events occur in the
spinal cord
Reflex classifications
• Somatic reflexes
– Control skeletal muscle
• Visceral reflexes (autonomic reflexes)
– Control activities of other systems
Reflex classifications
• Monosynaptic reflex
– Sensory neuron synapses directly on a motor neuron
• Polysynaptic reflex
– At least one interneuron between sensory afferent
and motor efferent
– Longer delay between stimulus and response
Spinal Reflexes
• Range in increasing order of complexity:
– monosynaptic reflexes
– polysynaptic reflexes
– intersegmental reflex arcs:
• many segments interact
• produce highly variable motor response
Methods of Classifying Reflexes
Neural Organization and Simple Reflexes
Reflex activity
• 5 components of
a reflex arc
– Receptor
– Sensory neuron
– Integration center
(CNS)
– Motor neuron
– Effector
Monosynaptic Reflexes
• Stretch reflex automatically monitors skeletal
muscle length and tone
– Patellar (knee jerk) reflex
• Sensory receptors are muscle spindles
• Postural reflex maintains upright position
Monosynaptic Reflexes
• Have least delay
between sensory
input and motor
output:
– e.g., stretch
reflex (such as
patellar reflex)
• Completed in
20–40 msec
Components of the Stretch Reflex
Muscle Spindles
• The receptors in stretch
reflexes
• Bundles of small,
specialized intrafusal
muscle fibers:
– innervated by sensory and
motor neurons
• Surrounded by extrafusal
muscle fibers:
– which maintain tone and
contract muscle
Intrafusal Fibers
Postural Reflexes
• Postural reflexes:
– stretch reflexes
– maintain normal upright posture
• Stretched muscle responds by contracting:
– automatically maintain balance
Polysynaptic Reflexes
• More complicated than monosynaptic reflexes
• Interneurons control more than 1 muscle
group
• Produce either EPSPs or IPSPs
Polysynaptic reflexes
•
•
•
•
Involve pools of interneurons
Are intersegmental in distribution
Involve reciprocal inhibition
Have reverberating circuits to prolong the motor
response
• Several reflexes may cooperate to produce a
coordinated response
Polysynaptic reflexes
• Produce more complicated responses
– Tendon reflex
– Withdrawal reflexes
– Flexor reflex
– Crossed extensor reflex
The Tendon Reflex
• Prevents skeletal muscles from:
– developing too much tension
– tearing or breaking tendons
• Sensory receptors unlike muscle spindles or
proprioceptors
Withdrawal Reflexes
• Move body part away
from stimulus (pain or
pressure):
– e.g., flexor reflex:
• pulls hand away from hot
stove
• Strength and extent of
response:
– depends on intensity
and location of stimulus
Control of spinal reflexes
• Brain can facilitate or inhibit motor patterns
based in spinal cord
• Motor control involves a series of interacting
levels
– Monosynaptic reflexes are the lowest level
– Brain centers that modulate or build on motor
patterns are the highest
Reinforcement and inhibition
• Reinforcement = facilitation that enhances
spinal reflexes
• Spinal reflexes can also be inhibited
– Babinski reflex replaced by planter reflex
Reciprocal Inhibition
• For flexor reflex to work:
– the stretch reflex of antagonistic (extensor)
muscle must be inhibited (reciprocal inhibition) by
interneurons in spinal cord
Crossed Extensor Reflexes
• Occur simultaneously,
coordinated with flexor
reflex
• e.g., flexor reflex causes
leg to pull up:
– crossed extensor reflex
straightens other leg
– to receive body weight
– maintained by
reverberating circuits
Integration and Control of Spinal Reflexes
• Though reflex behaviors are automatic:
– processing centers in brain can facilitate or inhibit
reflex motor patterns based in spinal cord
• Higher centers of brain incorporate lower,
reflexive motor patterns
• Automatic reflexes:
– can be activated by brain as needed
– use few nerve impulses to control complex motor
functions
– walking, running, jumping
Superficial reflexes
• Stroking of the skin elicits muscle contraction
– Involves functional upper motor pathways as well as cord level
reflex arcs
• Plantar reflex (L4-S2)…Babinski is normal in infants
– Usually indicative of CNS damage in adults
• Abdominal reflex (T8-T12)
– Absent with corticospinal lesion
Spinal Cord Trauma: Transection
• Cross sectioning of the spinal cord at any level
results in total motor and sensory loss in
regions inferior to the cut
• Paraplegia – transection between T1 and L1
• Quadriplegia – transection in the cervical
region