Spinal reflexes

Question 1

What does a motor unit consist of?

Answer 1

motor neuron with axon and axon branches and muscle fibres

Question 2

The fewer the number of fibres in a motor unit…

Answer 2

the smaller the twitch and greater the control of movement

Question 3

Define tetanic contraction

Answer 3

Twitches frequent enough to give a smooth contraction

Question 4

Define tetanus

Answer 4

fused twitches

Question 5

Define tetanus fusion frequency

Answer 5

The frequency at which motor axon must fire at to induce tetanus in motor unit

Question 6

What are the 3 main types of synaptic inputs present in the lower motor neuron

Answer 6

1- descending tracts in spinal cord from UMN
2- input from local interneurons
3- input from local sensory nerve fibres via reflexes

Question 7

What does the pyramidal system refer to?

Answer 7

the upper motor neuron cell bodies that project from the frontal lobe to spinal cord via corticospinal tract

Question 8

What does the extrapyramidal system refer to?

Answer 8

Cell bodies in the brainstem that project down to the spinal cord

Question 9

Define reflex

Answer 9

Involuntary motor action triggered by sensory input

Question 10

Where are reflexes ‘stored’

Answer 10

Dorsal and ventral grey matter of spinal cord in pattern on synaptic connections between inputs, output and interneurons

Question 11

Why is the knee-jerk reflex called a homonymous reflex?

Answer 11

Because stretch of quadriceps muscle activates the same (quads) muscle to contract

Question 12

What are the 5 main monosynaptic reflexes?

Answer 12

knee
ankle
biceps
triceps
supinator

Question 13

Why are monosynaptic reflexes so named?

Answer 13

No interneurons are involved in the reflex

Question 14

Why is it hard to suppress tendon jerk reflexes?

Answer 14

There are no interneurons involved, so direct connection makes it hard for brain to suppress.

Question 15

What is meant by reciprocal inhibition in terms of reflexes?

Answer 15

When tendon is tapped, extensor reflex also inhibits antagonist flexor via inhibitory interneuron

Question 16

What receptor mediates all tendon reflexes?

Answer 16

Muscle spindle

Question 17

Define proprioceptor

Answer 17

A receptor that responds to movement of body, not external stimulus

Question 18

What does muscle spindle consist of?

Answer 18

Complex stretch receptor inside connective tissue sheath

Question 19

What is the main sensory nerve fibre from muscle spindle?

Answer 19

1a afferent

Question 20

What activates muscle spindle?

Answer 20

Changes in muscle length causes increase in firing rate

Question 21

Why does tapping tendon cause reflex?

Answer 21

Muscle stretched a small amount, sufficient to briefly increase firing of spindles

Question 22

What is spatial summation?

Answer 22

Two or more synapses active simultaneously make a motor neuron fire

Question 23

What is temporal summation?

Answer 23

Single la nerve fibre fires high frequency burst of action potentials which add together to trigger action potential

Question 24

Describe structure of muscle spindle (3)

Answer 24

Consists of skeletal muscle fibres that have become modified inside capsule
Contractile tissue is at the end
Centre of fibre is without actin or myosin but full of cell bodies, so is very easily stretched

Question 25

Where do la afferents coil around muscle spindle?

Answer 25

Around central region which doesn't have connective tissue

Question 26

What motor neuron supplies muscle spindle?

Answer 26

Gamma motor neuron

Question 27

What is the effect of a gamma motor neuron on muscle spindle?

Answer 27

Causes end of intrafusal fibres to contract and therefore stretch the centre, increasing its response

Question 28

How are gamma motor neurons activated?

Answer 28

driven from descending pathways such as corticospinal tract

Question 29

Effect of pathological increases in gamma activity?

Answer 29

Hyperactive tendon reflexes

Question 30

Function of muscle spindles (other than reflex)

Answer 30

provides continuous feedback to motor neurons and continually adjust the motor neuron output during normal movement

Question 31

Function of monosynaptic reflex

Answer 31

Negative feedback system to maintain constant muscle length despite fatigue

Question 32

How does monosynaptic reflex keep muscle length constant despite fatigue?

Answer 32

When fatigue sets in, arm begins to droop. This stretches the muscle, causing an increase in muscle spindle activity. Increased stretch of muscle means increased frequency of action potentials to spinal cord Increases synaptic input of motor neurons- higher frequency of firing increased contraction

Question 33

Why are monosynaptic reflexes important in leg extensor muscles?

Answer 33

Fatigue in leg muscles would result in falling if not for reflexes Continuous feedback from muscle spindles plus high speed action potential conduction gives tendon jerk reflex shortest possible delay

Question 34

What type of receptor is golgi tendon organ? What is it activated by?

Answer 34

Proprioceptor | Activated by muscle tension

Question 35

Does the golgi organ have an interneuron in its connection?

Answer 35

Yes

Question 36

What type of interneuron is present in golgi tendon organ?

Answer 36

glycinegic inhibitory neuron

Question 37

How does the golgi tendon reflex act when muscle contracts strongly

Answer 37

Muscle contracts strongly enough to increase tension in tendon to extent it may damage muscle, GTO switches off by inhibitory action

Question 38

What nerve fibres mediate reflex withdrawal from painful stimulus?

Answer 38

A-delta (small myelinated nociceptor afferents)

Question 39

What do A-delta act on?

Answer 39

Act on interneurons in spinal cord

Question 40

Why can flexion reflex be suppressed by the brain?

Answer 40

Because flexion reflex relies on several interneurons, pain can be overcome in certain circumstances

Question 41

What is meant by crossed extensor reflex?

Answer 41

Activation of extensors in other leg during a flexion reflex

Question 42

How is crossed extensor reflex produced?

Answer 42

Branches of afferent nerve fibres cross from stimulated side of body to contralateral side of spinal cord Synapse with interneurons which excite or inhibit alpha motor neurons to muscles of contralateral leg

Question 43

What is meant by muscle tone?

Answer 43

Small amounts of contraction in muscles when passively moved

Question 44

What happens to muscle tone when there is lower motor neurone damage?

Answer 44

Tone becomes reduced or absent | Flaccid paralysis

Question 45

What happens to muscle tone when there is upper motor neurone damage?

Answer 45

Exaggerated reflexes Pathological increase in muscle tone Spasticity