Unit 5 - Reflexes Flashcards
Name the 3 most important reflexes
Stretch - reciprocal inhibition
Inverse stretch
Withdrawal - crossed extensor reflex
What is the motor unit composed of
The alpha motor neuron and the muscle fibres it innervates
What does each alpha motorneuron supply
Several muscle fibres (extrafusal)
Low innervation ratio for fine movement
What is the innervation ratio for fine movement
Low ratio
What is EACH muscle fibre innervated by
A single alpha motor neuron
What does the force of contraction depend on
Number of motor units recruited
Frequency of AP
what are the 5 elements of spinal reflexes
- Sensory receptor
- Afferent pathway to spinal cord
- Synaptic connection (direct or via interneurons)
- Efferent pathway (alpha motorneuron)
- Effector (muscle)
-ve feedback circuit often exists
What part of the muscle spindle controls the length of muscle (sensory receptors)
Functions
group II afferents - innervate nuclear bag and chain fibres
Physiological basis for static muscle tone
Proprioception
What part of the muscle spindle influences the rate of change of the muscle spindle
How are the afferents activated
What reflex are they responsible for
Ia afferents innervate nuclear bag fibres
Activated by rapid change in length of muscles
Physiological basis of stretch reflex
What are golgi tendon organs sensitive to
muscle TENSION, not length
Ib afferents
Located in tendon
Group II afferents
Innervate nuclear bag and chain fibres
Ia afferents
Nuclear bag fibres
Ib afferents
Golgi tendon organ
Function of alpha motor neurons
Supply extrafusal muscle fibres
function of γ motor neurons
Maintain muscle tone
Overview of structure of muscle spindle
Pathway of stretch reflex
2 neurons - Ia afferent & α motor neuron
Stretch → muscle spindle stretch → Ia activation → alpha motor neuron activation → contraction
e.g. knee jerk reflex
What is the reciprocal inhibition of the knee jerk reflex
Activation of Ia sensory afferent
Synapses with INHIBITORY interneuron in spinal cord
Inhibition of antagonist alpha motor neuron firing
Relaxation of extrafusal muscle fibres of antagonist muscle
MCQ
What is the role of gamma motor neurons
What fibres are involved
- Innervates contractile portions of muscle spindle ends
- Firing of alpha motor neuron causes contraction, but if there is no γ motor neuron then the afferent activity will stop
- Maintains muscle tone during contraction, allowing spindles to work at all muscle lengths
Hence allow you to contract a muscle that is already partially contracted (for a greater weight e.g.)
Nuclear bag fibres can send APs up
What does the inverse stretch reflex protect against
Over stretching
Golgi tendon organ involved
MOA of inverse stretch reflex
- GTO activated by increased tension in the tendon due to contraction of a muscle
- Firing of 1b afferent
- Activation of inhibitory interneuron
- Inhibition of α motor neuron to agonist muscle => relaxation
- Excitatory interneuron excites α motor neuron to antagonist muscle and causes contraction
MCQ
MOA of withdrawal/flexation reflex
- The Aδ nociceptor afferent are free nerve endings that respond to mechanical injury (well myelinated)
- Aδ activation leads to activation of excitatory and inhibitory interneurons
- Activation of α motor neuron
- Contraction of muscle and withdrawal from stimulus
- Reciprocal inhibition
Describe C fibres
Unmyelinated => slow speed of conduction
Aching pain
Where is the crossed extensor reflex seen
Describe its MOA
- In lower limbs
- Contraction of ipsilateral flexor and contralateral extensor
- Relaxation of ipsilateral extensor and contralateral flexor
Allows withdrawal of limb
Where is there damage in lower motor neuron syndrome
Symptoms
Damage to lower motor neurons
Results in paralysis (loss of movement) and paresis (weakness)
Also loss of reflexes and reduced muscle tone, due to loss of sensory motor arcs
Overtime, there is muscle wasting and spontaneous twitching due to denervation
What is Motor Neuron Disease also known as
Where is there damage to
Is there a cause
What is life expectancy
Describe the symptoms
Amyotrophic Lateral Sclerosis (ALS)
Degeneration of alpha motor neurons - overtime also neurons in ventral spinal cord and brainstem
Cause may be genetically linked (10%) or due to inflammation, viruses, toxins
Usually patients die within 5 years of diagnosis
