Week 4 - D - Pharmacology - UMNs, interneurones and spinal reflexes

Question 1

What in the spinal cord relays information between the afferent and efferent motor neurons?

Answer 1

The interneurons relay information between the afferent motor neurons from muscle to spinal cord and efferent motor neurons from brain to spinal cord

Question 2

Apart from primary sensory axons, what other areas do interneurones receive information from? (2 others)

Answer 2

1. Primary sensory axons 2. Descending axons from the brain 3. Collaterals from the LMNs that link the extrafusal muscle fibres to the intrafusal muscle fibres 4. Other interneurons

Question 3

What motor neurons supply the extrafusal and intrafusal muscle fibres? (what sensation do the intrafusal muscle fibres relay)

Answer 3

Alpha/beta motor neurons inneverate the extrafusal muscle fibres Gamma motor neurons innervate the intrafusal muscle fibres and cary out proprioception

Question 4

What is the myotatic reflex also known as and describe the reflex?

Answer 4

Myotatic reflex aka the stretch reflex When a muscle lengthens, the muscle spindle is stretched and its nerve activity increases. This increases alpha motor neuron activity, causing the muscle fibers to contract and thus resist the stretching. A secondary set of neurons also causes the opposing muscle to relax.

Question 5

Describe the inverse myotatic reflex?

Answer 5

When a muscle is being very stretched, to prevent overstretching the reverse myotatic reflex will cause the muscle to relax to prevent rupture of the tendon. The afferent neurones synapse on inhibitory interneurons which in turn synapse on the alpha motoneurons of the homonymous muscle forming the basis of the inverse myotatic reflex Due to the inhibitory interneurones synapsing on the same contracting muscle, this will hypepolarizze the muscle stopping contraction.

Question 6

What inhibitory neurotrasmitter do the interneurones release in the reverse myotatic reflex? What effect does this neurotransmitter have to bring about muscle relaxation?

Answer 6

The inhibitory interneuron releases inhibitory neurotransmitter glycine which causes chloride ions to enter the cells As chloride is negative this hyperpolarizes the cell leading to no action potential and muscle relaxation

Question 7

Myotatic reflex causes the homonymous extensor muscle (quadriceps) to contract, but for the leg to extend the antagonist flexor muscle (hamstring) must simultaneously relax WHat mediates the reciprocal inhibition between extensor and flexor muscles?

Answer 7

The interneurones mediate the flexor and extensor muscle contraction in the legs - interneurones can be excitatory or inhibitory remember

Question 8

What is the type of sensory fibre that innervates the muscle spindle for these reflexes to occur?

Answer 8

This would be the type 1a (I alpha) - carries sensation of proprioception

Question 9

The myotatic response only consists of 2 neurones - the afferent and efferent neurones How many does the reciprocal inhibition response consist of?

Answer 9

Consists of 3 neurones Afferent, interneuron, efferent

Question 10

Inhibitory interneurons mediate: the inverse myotatic response reciprocal inhibition between extensor and flexor muscles Excitatory interneurons mediate the flexor reflex the crossed extensor reflex What is the flexor reflex? These reflex happen in response to a noxious stimulus, what is a noxious stimulus?

Answer 10

The flexor reflex is where excititatory interneurones cause contraction of the flexor muscles in response to a noxious stimulus and relaxation of the extensor muscles of the same region (ie arm, leg, thigh) due to excitatory and inhibitory interneurones A noxious stimulus is “an actually or potentially tissue damaging event.” It is a prerequisite for nociception, which itself is a prerequisite for nociceptive pain.

Question 11

What is the reflex by which noxious stiimulus will cause the contralateral limb to be in extension?

Answer 11

This is the crossed extensor reflex

Question 12

How does the crossed extensor reflex work?

Answer 12

Contraction of extensor muscles via excitatory interneurones and Relaxation of flexor muscles via excitatory and inhibitory interneurones

Question 13

What does the crossed extensor reflex enchance?

Answer 13

It enhances postural support during withdrawal of a foot from a painful stimulus - you do not fall over

Question 14

If someone is stabbed with a pin in the left foot, what happens with the flexor reflex and crossed extensor reflex?

Answer 14

In response to the noxious stimulus Flexor reflex - contraction of flexor muscles in the left leg via excitatory interneurones and relaxation of the extensory muslces in left leg via excitatory and inhibitory interneurones Crossed extensor reflex - contraction of extensor muscles in the right leg via excitatory interneurones and relaxation of flexor muscles in the right leg via excitatory and inhibitory interneurones

Question 15

What do inhibtiory interneurones mediate? What do excitatory interneurones mediate?

Answer 15

INhibitory interneuroes mediate: Inverse myotatic reflex Reciprocal inhibiton of flexors and extensors Excitatory interneurones mediate: Flexor reflex Crossed extensor reflex (both in response to noxious stimulus)

Question 16

The Motor Control Hierarchy Involves at Least Three Levels Motor control can (very simplistically) be viewed as a hierarchy from top (forebrain) to bottom (spinal cord) Strategy (aim of movement), tactics (pre planned movmement), execution (the movmement itself) have different structures in the brain/spinal cord controlling What level of cogntiive function is required for strategy? Tactis? Execution?

Answer 16

High level of cognitive function is required for strategy as this is the ability to understand the aim fo the movement Middle level of cognitive function is required for tactics as this is learned movements due to pre plannning Low level of cognitive function is required for the exectution of the movement

Question 17

Which structures are responsible for strategy? Tactics? Execution? Remember heirarchy of motor controls highest to lowest - strategy, tactics, execution

Answer 17

Strategy - high level of cognitive function - neocortical association areas (different regions of the brain) and basal ganglia Tactics - middle level of cognitive function cerebellum plays a role in coordination and motor learning/ fine tuning movememnt Execution - low level of cognitive function - brainstem and spinal cord are the structures which iniitate this control

Question 18

In relation to major motor pathways, what is the most important motor pathway?

Answer 18

This is the corticospinal tract (pyramidal tract)

Question 19

Where in the brain does the corticospinal pathway begin? (ie where is the motor cortex of the brain)

Answer 19

Motor cortex of the brain is posterior frontal lobe Primary motor area is in the pre central gyrus (posterior frontal lobe)

Question 20

When do fibres decussate in the corticospinal tract? What side of the body would the left hemisphere of the brain control? (does the difference in level of decussation matter)

Answer 20

85% of fibres decussate at the level of the medullary pyramids to form the lateral corticospinal tract 15% of fibres decussate at the the vertebral level to form the anterior corticospinal tract. Irrespective of the level of decussation, left hemisphere controls right musculature and vice versa

Question 21

What is the difference in muscle groups innervated by the lateral and anterior corticospinal tracts?

Answer 21

Lateral corticospinal tract - innervates muscles of the limbs Anterior corticospinal tracts - innervates the axial muscles