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Brain and behaviour > Spinal reflexes > Flashcards

Spinal reflexes Flashcards

1
Q

What does a motor unit consist of?

A

motor neuron with axon and axon branches and muscle fibres

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2
Q

The fewer the number of fibres in a motor unit…

A

the smaller the twitch and greater the control of movement

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3
Q

Define tetanic contraction

A

Twitches frequent enough to give a smooth contraction

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4
Q

Define tetanus

A

fused twitches

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5
Q

Define tetanus fusion frequency

A

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

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6
Q

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

A

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

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7
Q

What does the pyramidal system refer to?

A

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

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8
Q

What does the extrapyramidal system refer to?

A

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

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9
Q

Define reflex

A

Involuntary motor action triggered by sensory input

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10
Q

Where are reflexes ‘stored’

A

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

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11
Q

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

A

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

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12
Q

What are the 5 main monosynaptic reflexes?

A 
knee
ankle
biceps
triceps
supinator
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13
Q

Why are monosynaptic reflexes so named?

A

No interneurons are involved in the reflex

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14
Q

Why is it hard to suppress tendon jerk reflexes?

A

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

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15
Q

What is meant by reciprocal inhibition in terms of reflexes?

A

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

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16
Q

What receptor mediates all tendon reflexes?

A

Muscle spindle

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17
Q

Define proprioceptor

A

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

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18
Q

What does muscle spindle consist of?

A

Complex stretch receptor inside connective tissue sheath

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19
Q

What is the main sensory nerve fibre from muscle spindle?

A

1a afferent

20
Q

What activates muscle spindle?

A

Changes in muscle length causes increase in firing rate

21
Q

Why does tapping tendon cause reflex?

A

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

22
Q

What is spatial summation?

A

Two or more synapses active simultaneously make a motor neuron fire

23
Q

What is temporal summation?

A

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

24
Q

Describe structure of muscle spindle (3)

A

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

25
Q

Where do la afferents coil around muscle spindle?

A

Around central region which doesn’t have connective tissue

26
Q

What motor neuron supplies muscle spindle?

A

Gamma motor neuron

27
Q

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

A

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

28
Q

How are gamma motor neurons activated?

A

driven from descending pathways such as corticospinal tract

29
Q

Effect of pathological increases in gamma activity?

A

Hyperactive tendon reflexes

30
Q

Function of muscle spindles (other than reflex)

A

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

31
Q

Function of monosynaptic reflex

A

Negative feedback system to maintain constant muscle length despite fatigue

32
Q

How does monosynaptic reflex keep muscle length constant despite fatigue?

A

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

33
Q

Why are monosynaptic reflexes important in leg extensor muscles?

A

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

34
Q

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

A

Proprioceptor

Activated by muscle tension

35
Q

Does the golgi organ have an interneuron in its connection?

A

Yes

36
Q

What type of interneuron is present in golgi tendon organ?

A

glycinegic inhibitory neuron

37
Q

How does the golgi tendon reflex act when muscle contracts strongly

A

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

38
Q

What nerve fibres mediate reflex withdrawal from painful stimulus?

A

A-delta (small myelinated nociceptor afferents)

39
Q

What do A-delta act on?

A

Act on interneurons in spinal cord

40
Q

Why can flexion reflex be suppressed by the brain?

A

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

41
Q

What is meant by crossed extensor reflex?

A

Activation of extensors in other leg during a flexion reflex

42
Q

How is crossed extensor reflex produced?

A

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

43
Q

What is meant by muscle tone?

A

Small amounts of contraction in muscles when passively moved

44
Q

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

A

Tone becomes reduced or absent

Flaccid paralysis

45
Q

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

A

Exaggerated reflexes
Pathological increase in muscle tone
Spasticity

Brain and behaviour (22 decks)

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