Week 9: Spinal Reflexes Flashcards
What are UMNs in the brain needed for?
- initiation
- integration
- co-ordination
What are LMNs in the spinal cord needed for?
- simple reflexes
- pattern generation
What are the 3 types of spinal reflexes?
- Myotatic stretch reflex
- agonist response
- antagonist response
- load response - Inverse myotatic reflex
- Crossed extensor/flexor withdrawal reflex
What is a reflex?
a fast, predictable automatic response to a change in the environment or a stimulus
What are the 5 functional components of a reflex arc?
- Sensory receptor –> fires action potential upon stimulus
- Sensory afferent neuron: carries action potential to spinal cord
- Integration centre: relay sensory –> motor
- Motor neuron
- Effector organ
What are the two different types of reflexes?
Reflexes can be:
- Monosynaptic: 1 synapse, rapid
- Polysynaptic: 2 or more synapses, slower but more complex, involves interneurons
The myotatic stretch reflex occurs in response to what?
muscle contraction in response to stretching (change in length) within the muscle
Which receptors are involved in the myotatic stretch reflex?
Muscle spindle - these detect change in length
What are the functions of the myotatic stretch reflex?
- antigravity
- posture (helps us remain upright)
- movement
Explain the 6 sequence of events in the agonist response of the myotatic stretch reflex using the example of the patellar tap reflex?
- Tap quadriceps tendon & stretch quadriceps muscle
- Activation of the muscle spindle & increased firing of 1a afferent
- Afferent terminals synapse directly with LMNs in ventral horn of SC and excite the alpha motor neuron
- Increased alpha motor neuron efferent axon activity
- Contraction of the agonist homonymous quadricep muscle
- Extension of lower leg
What do muscle spindles do in the myotatic stretch reflex?
detect a change in stretch and send afferent fibres to the spinal cord
What are the afferent fibres from muscle spindles in the myotatic stretch reflex called?
1a afferents
Is the myotatic stretch reflex monosynaptic or polysynaptic?
monosynaptic as it only requires two neurones
Does the myotatic reflex occur ipsilaterally or contralaterally?
ipsilaterally - i.e tap on left leg causes extension of left leg
What is the antagonist response of the myotatic stretch reflex?
reciprocal inhibition of antagonist muscle groups during the myotatic reflex
What does reciprocal inhibition mean?
reciprocal inhibition means the antagonist muscle does the opposite - in the patellar tendon reflex, the quadricep contracts so the hamstring muscle will relax/be inhibited
Explain the antagonist response in the patellar tendon reflex (the myotatic stretch reflex)
- Tapping the patellar tendon activates the 1a afferent
- Leads to direct excitation of motor neurons innervating agonist (extensor) muscle groups
- Indirect inhibition via the 1a inhibitory interneuron of motor neurons innervating antagonist (flexor) muscle groups
- we activate the inhibitory interneuron so we have decreased firing on the antagonist/flexor muscle
- hamstrings flex
Is the antagonist response of the myotatic stretch reflex direct or indirect?
indirect (polysynaptic) as it involves three neurons
Explain the myotatic stretch reflex when you add a load e.g to arm muscle groups
Addition of a load leads to:
- Muscle spindles detect change in length and send the 1a afferents to connect with the LMNs in the ventral horn. These LMNs increase firing to contract the bicep muscle
- Decreased firing in LMNs to triceps to allow it to relax
Is there excitation or inhibition of:
- The agonist/flexor
- The antagonist/extensor
- There is direct excitation of agonist/flexor muscle groups
- There is indirect inhibition via the 1a inhibitor interneuron of antagonist/extensor muscle
When is the inverse myotatic (golgi tendon) stretch reflex activated?
In response to muscle contraction to avoid tendon damage
controls the TENSION of an active muscle
Which receptor is used in the inverse myotatic stretch reflex?
golgi tendon organ
What are the function of the inverse myotatic/golgi tendon stretch reflex?
- tension feedback
- overload protection
- kicks in when tension in muscle is too high and it needs to relax to prevent destruction of fibres
- is exact opposite to myotatic reflex
Explain the inverse myotatic (golgi tendon) reflex
- Increased tension is sensed by golgi tendon and excites 1b afferents
- This leads to indirect inhibition via inhibitory interneurones of motor neurones innervating the homonymous muscle causing relaxation
- Simultaneously, there is indirect excitation of motor neurones innervating antagonist muscle groups causing contraction
So there is indirect inhibition to the agonist muscle and indirect excitation of the antagonist muscle
What does homonymous mean?
same muscle that was stretched
What is the crossed extensor/flexor withdrawal reflex
- noxious (harmful/unpleasant) cutaneous stimulation causes flexion withdrawal from the offending stimulus
- simultaneous extension of contralateral limb (crossed extensor) may occur for weight bearing
What are the functions of the crossed extensor/flexor withdrawal reflex?
- damage limitation of avoidance
- maintained balance on limb withdrawal
Explain the sequence of events on activation of the cutaneous nociceptor?
- Increased activity in A-delta & C afferents
- Polysynaptic activation of ipsilateral flexors
- Polysynaptic inhibition of ipsilateral extensors
- Polysynaptic inhibition of contralateral flexors
- Polysynaptic excitation of contralateral extensors
This enables automatic maintenance of balance during the reflex
Explain what happens when you step on glass
- If you step on glass, you need to remove your leg from the painful stimulus
- Pain receptors initiate firing. These give rise to different afferent fibres called alpha delta or C afferents
- Interneurons activate ipsilateral flexors so we can withdraw from stimulus
- Another interneuron will cross the midline of the SC and communicate with the other side
- This activates quadriceps (extensors) and inhibits hamstring (flexor)
- So we decrease the activity of the contralateral hamstring
Which sensory afferents are involved in:
- Myotatic (stretch) reflex
- Inverse myotatic
- Flexor withdrawel
- 1a afferents
- 1b afferents
- A-delta, C afferents
What is the sensory stimulus for:
- Myotatic (stretch) reflex
- Inverse myotatic
- Flexor withdrawel
- Change in muscle length
- Change in muscle tension
- Painful or damaging stimuli
Which receptors are involved in:
- Myotatic (stretch) reflex
- Inverse myotatic
- Flexor withdrawel
- Muscle spindles
- Golgi tendons
- Nocioceptors
