Reflexes

Key terms

Reflex action: involuntary response to a sensory stimulus

Reflex arc

• Receptor

  • Detects stimulus

  • Creates action potential in sensory neurone

• Sensory neurone

  • Carries impulse to spinal cord

• Relay neurone

  • Connects sensory neurone to motor neurone

  • Within spinal cord or brain

• Motor neurone

  • Carries impulse to effector

  • Carry out appropriate response

Reflex arc involved in withdrawal of hand

• Stimulus

• Sensory neurone

• Relay neurone

• Motor neurone

• Effector

• Response

Spinal cord

• Column of nervous tissues

  • Running up back

• Surrounded by spine

• Intervals along spinal cord

  • Pairs of neurones emerge

Knee-jerk reflex

• Spinal reflex

  • Neural circuit only goes up to spinal cord
    

• Leg tapped just below the kneecap

• Stretches patellar tendon

  • Acts as a stimulus

• Stimulus initiates reflex arc

  • Causes extensor muscle to contract

• Relay neurone inhibits motor neurone of flexor muscle

  • Causes it to relax

• Contraction coordinated with relaxation of antagonistic flexor hamstring muscle

  • Causes leg to kick
    

• Leg extends once and comes to rest

  • Absence of reflex indicates nervous problems

  • Multiple oscillation may show cerebellar disease

• Helps maintain posture and balance

Blinking reflex

• Corneal reflex

• Involuntary blinking of eyelids

• Cornea is stimulated

• Used to keep cornea safe from damage

  • Due to dust or insects

• Blinking due to over-bright light

  • Optical reflex

• Blinking reflex

  • Cranial reflex

  • Sounds greater than 40-60 dB
    

• Cornea irritated by foreign body

• Stimulus triggers impulse along sensory neurone

• Impulse passes through a relay neurone

  • In lower brain stem

• Impulses sent along branches of motor neurone

  • Initiate motor response

  • Close to eyelids

• Reflex initiates consensual response

  • Both eyes closed in response to stimulus

• Blinking reflex is rapid

  • One tenth of a second

• Doctors test for blinking reflex

  • Unconscious patients

• Reflex present, lower brain stem is functioning

  • Cannot diagnose person as brain-dead

Measuring reaction time

• Suitable scale placed onto ruler

• Converts distance dropped by ruler into reaction time

• Can measure effect of caffeine concentration on reaction time

Survival importance

• Reflexes essential for survival

  • Avoid body being harmed, reduce severity of damage

• Being involuntary responses

  • Decision making regions of brain not involved

  • Brain able to deal with complex responses

  • Prevents brain from being overloaded with situations

    • Where response always same

• Not having to be learnt

  • Present at birth

  • Provide immediate protection

• Extremely fast

  • Reflex arc very short

  • Involves one or two synapses

    • Slowest part of nervous transmission

• Everyday actions

  • Keep us upright

  • Control digestion