Reflexes Flashcards
Define a reflex?
An involuntary stereotyped response to a stimulus.
What are tendon jerks?
Reflex muscle contractions produced by muscle stretch. When we tap we displace and stretch the muscle.
Describe the monosynaptic circuit for the knee jerk reflex?
- Stimulus - tap stretches the muscle.
- Muscle spindle receptors are activated.
- Conduction along afferent sensory fibres (1-alpha fibres). These are the fastest conducting fibres.
- Transmission at synapse between 1-alpha fibres and motor neurons in the ventral horn.
- Conduction along efferent motor fibre to flexors and extensors.
- Neuromuscular transmission (ACh).
- Excitation-contraction coupling.
- Twitch contraction of skeletal.
How can reflexes be reinforced? How does this work?
Grit teeth or Jendrassiks manoeuvre.
This increases the excitability of motor neurons in the motor neuron pool by inhibiting the descending pathways.
Causes distraction so patient doesnt inhibit the response consciously.
Causes the patient to shift position and slightly contract. This means the motor neurons are closer to threshold and it is easier to reach threshold as the resting membrane potential is increased, more motor neurons can now fire and ellicit a response.
What is the stretch reflex important for?
Main determinant of muscle tone and important in postural muscles.
Describe the stretch reflex?
When a muscle is held stationary a force is generated to support the limb. If there is an additional force (e.g. a weight dropped in the hand) the muscle stretches and muscle spindles are activated.
This increases alpha motor neuron activity and causes the reflex to occur and more contraction causes the arm to be stabilised.
How is muscle tone tested?
By assessing passive movement.
How does increased muscle tone affect reflexes?
Increased tone causes hyperflexia due to increased sensitivity.
How does decreased muscle tone affect reflexes?
Decreased tone causes hyporeflexia or absent reflexes due to decreased sensitivity.
How would you illicit the plantar reflex?
Use a blunt object and run it up the sole of the foot and across the ball of the foot, lateral to medial.
What is Babinskis sign? Where would you expect to see it?
When the toes fan up (instead of curling down) when illiciting the plantar reflex.
Seen in children aged 0-12months and those with damage to brain and descending pathways.
What is ‘dying back’ neuropathy?
The most common polyneuropathy.
Causes axonal degeneration.
Distal terminals of the longest nerves are affected first.
So sensation in feet is first, then hands. Symptoms progress proximally.
What causes dying back neuropathies?
Toxic or metabolic conditions e.g. diabetes.
How is 2-point discrimination sensed on the skin?
By free nerve endings.
Why when the points are close is it difficult to determine whether it was 1 prick or 2?
As receptive fields of free nerve endings overlap making it difficult to distinguish.
