Reflexes

CNS mediates reflexes

Reflex arc

  • In higher animals, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord

  • This characteristic allows reflex actions to occur relatively quickly by activating spinal motor neurons without the delay of routing signals through the brain, although the brain will receive sensory input while the reflex action occurs

General concepts about reflexes

  • Rapid, predictable response to stimulus 

  • Learned or inborn

    • Suckling reflex – newborns (inborn)

    • Driving a car or reacting in sports (learned)

  • Prevents us from thinking about small details 

    • Posture 

Reflex classification 

  • Somatic 

    • These activate skeletal muscle

  • Autonomic 

    • These activate visceral effectors (cardiac muscle)

Somatic reflexes

  • Mediated by the spinal cord- no direct involvement of the brain

  • May occur even if brain is damage

  • Can be used as a diagnostic tool

  • Spastic vs flaccid paralysis

Components of reflex arc

  • Receptors 🡪 Afferent neuron 🡪 Integration center 🡪 Efferent neuron 🡪 Effector (muscle)

Monosynaptic Vs. Polysynaptic 

  • Reflexes can be monosynaptic or polysynaptic depending on how many synapses occur in the “integration center.”  The greater the number of synapses, the more complex and slow the reflex.

    • Monosynaptic

      • bicep reflex, patellar reflex, tricep reflex

    • Polysynaptic

      • Withdraw reflex (step on something sharp and you immediately withdraw your foot

      • unlike the monosynaptic, you are able to consciously inhibit this pathway

Structures involved in somatic reflexes

  • Muscle spindles

    • Sense the length of muscle

    • Sensitive to stretch

    • Send self-excitatory signals

  • Golgi Tendon Organ (GTO)

    • Located in the tendon

    • Senses tension on muscle and associated tendon

    • Sensory fiber sends information to CNS via sensory fiber

    • Self-inhibitory

Stretch Reflex

  • Caused by stretch in muscle- involves muscle spindles

  • Steps

    • Muscle stretched in quad

    • Spindle senses stretch, sends afferent signal to spinal cord

    • Afferent neuron synapses with motor neuron of quad and inhibitory neuron of hamstring

    • Result- quad contracts and the hamstring relaxes

  • Overall effect- stretched muscle will be activated and antagonist will be inhibited

  • Clinical Implication- Shows spinal cord is intact at that point and excitability of the spinal cord

Crossed-Extensor reflex

  • Accompanies flexor reflex

  • Important in maintaining balance

  • Ipsilateral withdrawal and contralateral extension

    • Afferent sensory fibers from right arm to spinal cord interneurons

    • Efferent motor response to right arm, exciting stretched muscle and inhibiting antagonist -> arm flexes

    • Efferent motor response to left arm, inhibiting flexors and exciting extensors -> arm extends

Reflexes as diagnostic tool

  • Hypo-reflexia

    • Occurs when there is an injury to a lower motor neuron

    • Expected reflexes

      • Less than expected or non-existent

    • Why?

      • Sensory signal cannot get to spinal cord

      • Damage to motor neuron not allowing signal to reach effector

  • Hyper-reflexia

    • Occurs when there is an injury to an upper motor neuron

    • Expected reflexes

      • Greater than expected

    • Why?

      • Reflex information cannot be modified by CNS