reflexes

reflexes

1. definition

   • an involuntary stereotypical response to a stimulus

2. reflex circuits

   • begin with receptors
   • effectors
   • do not reach consciousness

3. simplest circuits in nervous system

4. tendon jerk reflexes

   • reflex muscle contractions produced by muscle stretch
   • can be evoked by many muscles where tendon is accessible
   • circuitry
     • Tapping tendon stretches muscle to which it is attached.
       • his elicits a monosynaptic reflex that produces contraction of the stretched muscle
   • Ia reciprocal inhibition accompanies the tendon jerk reflex
     • Inhibition of antagonist muscle evoked simultaneously with tendon jerk reflex
     • mediated by an interneurone - Ia inhibitory interneurone
     • the pathway is disynaptic i.e. 2 synapses
     • the inhibition operates in each direction between antagonistic muscles at a joint – hence reciprocal inhibition e.g. stretch of quadriceps muscle causes inhibition of hamstring muscle
   • excitation of the muscle spindles is responsible for th jerk reflex
     • tendon jerk reflex is responsible clinically but is in response to an artificial stimuli
     • Normal function of the monosynaptic reflex circuit is the stretch reflex
     • prolonged stretch produces prolonged contraction
     • the stretch reflex acts to maintain a constant muscle length

5. Vestibulo-ocular and pursuit eye movements

   • Both mechanisms produce movement of the eyes to stabilise the visual image
   • Vestibulo-ocular – movements of head relative to object
     • much faster than pursuit – clearer image when shaking head than when waving finger
   • Visual pursuit – movements of object relative to eyes

6. control reflex (closed loop)

   • Control some physiological variable e.g. stretch reflex controls muscle length
   • Stimulus triggers actions which feedback on stimulus e.g. stretch of muscle leads to a reflex response (muscle contraction) which returns the muscle to its original length

7. the protective reflex - open loop

   • Reflex does not affect stimulus e.g. flexion withdrawal reflex
   • Accompanied by crossed extensor reflex (both are polysynaptic)

8. Flexor reflex and crossed extensor reflex

   • mediated by chains of interneurones - polysynaptic (many synapses)
   • excitatory and inhibitory interneurones produce appropriate actions on different motoneurone pools
   • pathway shows divergence - motoneurones with actions at hip and ankle joints also involved

9. the carotid syncs bro receptor reflex

   • • Autonomic reflex regulating blood pressure
   • Controls heart rate and force of contraction (cardiac output)
   • Controls diameter of blood vessels (peripheral resistance)
   • BP = CO x TPR

10. reflex circuits are used by higher centres

    • spinal reflex circuits are used by descending systems to control movement