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Foundations of Medicine > Spinal Reflexes > Flashcards

Spinal Reflexes Flashcards

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1
Q

What is the stretch reflex?

A

The stretch reflex is the contraction of a muscle that occurs in response to its stretch.

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2
Q

Is the stretch reflex controlled by the brain?

A

No

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3
Q

What type of response is the stretch reflex?

A

A monosynaptic response that is transmitted to the spinal cord.

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4
Q

What are reflexes?

A

Reflexes are automatic, subconscious responses to changes within or outside the body.

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5
Q

What is the function of reflexes?

A

They function to maintain the homeostasis (autonomic reflexes), which include breathing, blood pressure regulation and heartbeat.

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6
Q

What is the stretch reflex also known as?

A

The stretch reflex is also referred to as the deep tendon reflex or myotatic reflex.

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7
Q

What is the stretch reflex a response to?

A

A muscle being passively stretched e.g. by a tendon hammer, or a sudden change in the ground surface.

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8
Q

What can activate the stretch reflex?

A

External forces such - load placed on the muscle

Internal forces - the motor neurons being stimulated from within

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9
Q

What is an example of an external force causing the stretch reflex

A

someone holding a plate, and someone serving them some food.

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10
Q

What is an example of an internal force causing the stretch reflex?

A

An example of the latter is shivering that occurs when the person is cold, and is instigated by the internal neurons of the muscle.

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11
Q

How do alpha motor neurons resist stretching?

A

By causing contraction

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12
Q

How do gamma motor neurons work in the stretch reflex?

A

By controlling the sensitivity of the reflex.

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13
Q

What does a muscle stretch stimulate?

A

Muscle spindles

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14
Q

Steps of stretch reflex

A
  1. Add load to muscle
  2. Muscle and muscle spindle stretch as arm drops
  3. Reflex contraction initiated by muscle spindle restores arm position
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15
Q

How many connects do spindle sensory afferent make via dividing?

A

2 in the spinal cord
1 in the brain

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16
Q

How does the patella tendon work?

A

Tapping on the inelastic tendon causes the muscle fibres to stretch activating the sensory nerves in the muscle spindle.

This increases the number of action potentials that are sent through the afferent neurone into the dorsal horn of the spinal cord.

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17
Q

What are muscle spindles?

A

Proprioceptors that consist of intrafusal muscle fibers enclosed in a sheath

They are non-contractile, and serve as receptive surfaces.

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18
Q

Where can muscle spindles be found?

A

Embedded within a muscle are the muscle spindles.

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19
Q

What are the intrafusal fibres found in a muscle spindle?

A

Nuclear bag and nuclear chain are the subtypes)

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20
Q

What are the contractile proteins of normal muscles?

A

Actin and myosin

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21
Q

What are the two types of afferent nerve endings that penetrate muscle spindles?

A

Primary sensory fibres of type Ia and secondary sensory fibres of type II.

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22
Q

Why was the stretch arch developed?

A

To prevent tearing that can occur due to vigorous movement.

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23
Q

Describe the stages of the stretch arch in detail

A

Once the muscle spindle is stretched, the impulse is sent back to the muscle very quickly, and protects it from being pulled forcefully or beyond its normal range of motion.

When a reflex takes place, all of the synergistic muscles (those that cause the same movement) also contract while antagonistic muscles are inhibited.

The decrease in the simultaneous contraction of the opposing muscles reduces the likelihood of injury.

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24
Q

How does the stretch reflex cause the antagonist muscle group to relax?

A

Once a stretch reflex occurs, the impulse is sent from the stretched muscle spindle, to the alpha motor neuron. The alpha motor neuron is split. Hence it is able to cause contraction in the synergistic group, and relaxation in the antagonistic group.

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25
What would happen if the antagonist muscle did not relax during the reflex muscle?
Both groups of muscles would contract, resulting in no corrective movement.
26
How does the stretch reflex establish a rapid response?
Skips the brain, and follows the simple neural loop connecting the muscle to the spinal cord and back.
27
What is a reflex mediated by the golgi tendon known as?
The inverse stretch reflex.
28
What nerves cause the reflex action mediated by the Golgi tendon organs?
1B afferent nerves from the Golgi tendon organs (GTOs) - which monitor muscle tension - Muscle contracts and shortens -this pulls on the tendon and the sensory 1B afferent nerves from the GTOs firing of APs
29
What does muscle contraction and shortening as a result of the golgi tendon cause?
- The activation of the inhibitory neurones to the agonist muscle causing the contraction strength to decrease - Activates excitatory interneurons causing the antagonist muscle to contract. - Information about muscle tension ascends through the dorsal route to the somatosensory cortex.
30
Why does the golgi tendon use a clasp-knife refelx?
- Because greatly increasing tension in tendon leads to a collapse of resistance
31
Describe how the golgi tendon works with an example
Weightlifting - agonist quadriceps contracted - pulls hard on the tendon - GTO afferents increase firing - synapse with inhibitory interneurone - - reduces motoneurone firing - muscle inhibited and relaxes rapidly
32
What are both the golgi tendon and the stretch reflex examples of?
Both are examples of ipsilateral reflexes, meaning the reflex occurs on the same side of the body as the stimulus.
33
What is the role of interneurons?
Connect spinal motor and sensory neurons and can communicate with each other, forming circuits of various complexity.
34
How can interneurons be described?
Multipolar, just like motor neurons.
35
What is a contralateral reflex?
The crossed extensor reflex is a contralateral reflex that allows the body to compensate on one side for a stimulus on the other.
36
Give an example of a contralateral reflex
When one foot steps on a nail, the crossed extensor reflex shifts the body’s weight onto the other foot, protecting and withdrawing the foot on the nail.
37
What is the withdrawal reflex?
The withdrawal reflex and the more-specific pain withdrawal reflex involve withdrawal in response to a stimulus (or pain).
38
How does the withdrawal reflex work?
When pain receptors, called nociceptors, are stimulated, reciprocal innervations stimulate the flexors to withdraw and inhibit the extensors to ensure they are unable to prevent flexion and withdrawal.
39
What type of reflex is the with drawl reflex and what is its function?
The withdrawal reflex (nociceptive or flexor withdrawal reflex) is a spinal reflex intended to protect the body from damaging stimuli.
40
What are the characteristics of the with drawl reflex?
Polysynaptic Causes the stimulation of: Sensory Association Motor neurons
41
What happens when someone touches something hot?
1. Heat stimulates temperature and danger receptors in the skin, triggering a sensory impulse that travels to the central nervous system. 2. The sensory neuron then synapses with interneurons that connect to motor neurons. 3.Some of these send motor impulses to the flexors to allow withdrawal.
42
What is reciporal innervation with regards to the withdrawal reflex
When motor neurons send inhibitory impulses to the extensors so flexion is not inhibited.
43
What are the two unique features of the withdrawal reflex?
1. The body can be trained to override that reflex. 2. An unconscious body (or even drunk or drugged bodies) will not exhibit the reflex
44
What are the higher brain centres?
Cerebrum Brainstem Cerebellum
45
What are spinal reflexes?
Simple building blocks for movement
46
What is a reflex arc?
47
What is the simplest reflex and where is it found?
The stretch reflex - found in all muscles
48
What is an example of the stretch reflex?
Patella tendon aka knee-jerk reflex
49
What type of response is the stretch reflex?
A monosynaptic response that is transmitted to the spinal cord. Not controlled by brain.
50
What are the five components of a reflex arc?
A receptor – muscle spindle An afferent fibre – muscle spindle afferent An integration centre – lamina IX of spinal cord An efferent fibre – α-motoneurones An effector – muscle
51
What are the stages of the patella tendon stretch reflex?
1. Muscle spindle is stretched (caused by the tap stimulus in the knee jerk reflex). 2. When a stretch is detected it causes action potentials to be fired by 1α afferent fibres. AP's in 1α afferents project through the dorsal root of the spinal cord. 3. These then synapse within the spinal cord with α-motoneurones which innervate extrafusal fibres. 4. The antagonistic muscle is inhibited and the agonist muscle contracts i.e. in the knee jerk reflex the quadriceps contract and the hamstrings relax.
52
What are muscle spindles responsible for?
Detecting the length of the muscles fibres.
53
What is responsible for monitoring the sensitivity of the stretch reflex?
Gamma motoneurones – to tightening or relaxing of muscle fibres within the muscle spindle.
54
What does stretching the muscle cause?
Stimulates muscle spindles, causes reflex muscle contraction, muscles shorten to previous lenght.
55
In a stretch reflex, why is force transmitted to muscle fibres rather than tendon fibres?
Muscle fibres are more elastic and therefore are able to stretch
56
What are spindle sensory afferents divided up into?
Three types of connections: Two in the spinal cord One in the brain
57
What is a monosynaptic reflex?
Many spindle sensory afferents contact α-motorneurons in the stretch muscle causing rapid contraction of the agonist muscle. This is a monosynaptic reflex - one synapse no interneurons involved.
58
Where do spindle afferent firing travel to get to the brain?
Up the dorsal columns to thalamus and somatosensory cortex to tell the brain about length of muscles
59
What is reciprocal inhibition?
Spindle sensory afferents from the stretched spindle connect indirectly with the antagonist muscle. When agonist contracts, the antagonist relaxes (stretches) This happens because spindle afferents activate inhibitory motor neuron which decrease activation of α-motoneurones to the antagonist muscle which then relaxes. This is recipcoral inhibition
60
What do golgi tendon organs monitor?
Muscle tension to prevent muscle damage
61
What is the inverse stretch reflex?
An inhibitory effect on the muscle resulting from the muscle tension stimulating Golgi tendon organs (GTO) of the muscle, and hence it is self-induced
62
What causes the inverse stretch reflex?
1β afferent nerves from the Golgi Tendon Organs Muscle contracts and shortens, pulls on the tendon and the sensory 1β afferent nerves from the GTOs increasing firing of action potentials.
63
How does the golgi tendon organ work? (after AP firing)
1. Activation of inhibitory interneurons to the agonist muscle and a decrease in contraction strength 2. Activation of excitatory interneurones to antagonist muscles. 3. Information about muscle tension ascends in the dorsal columns to the somatosensory cortex.
64
What do flexor/withdrawal reflexes use?
Information from pain receptors (nociceptors) in the skin, muscles and joint
65
What are the features of withdrawal reflexes?
Polysynaptic Protective
66
What do withdrawal reflexes cause?
Withdrawal the affected part of the body away from painful stimulus and in towards the body so they **flex** the affected part
67
In the flexor/withdrawal reflex what does increased action potentials in nociceptor nerves cause?
1. Activity in the flexor muscles of affected part via excitatory interneurons 2. Antagonistic receptors are inhibited - via excitatory and inhibitory interneurons 3. Excitatory interneurons cross the spinal cord and excite the contralateral extensors 4. Other interneurons cross the spinal cord, synapse with inhibitory neurons and they inhibit the contralateral flexors - helps maintain upright posture 5, Sensory information ascends to the brain in the contralateral spinothalamic tract.
68
What does the flexor reflex cause?
Ipsilateral flexion in response to pain.
69
Why are nociceptive sensory fibres slower than muscle spindle afferents?
Nociceptive sensory fibres have a smaller diameter than muscle spindle afferents and so conduct more slowly
70
How can the GTO reflex can be over-ridden by voluntary input from the CNS?
1. Neuron from Golgi tendon organ fires 2. Motor neuron is inhibited 3. Muscle relaxes 4. load is released
71
How many synapses does each α motoneurone receive?
>10,000 synapses Many are from the thalamus & cortex - cause excitatory & inhibitory postsynaptic potentials (EPSPs and IPSPs). Some make direct contact, but most act through interneurones Each α motoneurone has to integrate these signals. Their net effect at the cell body is summed (total excitation - total inhibition).
72
What has a lesser effect on dendrites?
Membrane potential changes on distant dendrites have less effect than those from nearer the cell body
73
How can you override the GTO when holding something heavy?
When holding something heavy, but very important, you think “I don’t want to drop this” Descending voluntary excitation of  motoneurones overrides the inhibition from the GTOs and maintains muscle contraction – so preventing the GTO reflex
74
How can you override the GTO when holding something heavy?
When holding something heavy, but very important, you think “I don’t want to drop this” Descending voluntary excitation of α motoneurones overrides the inhibition from the GTOs and maintains muscle contraction – so preventing the GTO reflex
75
Can the stretch reflex be overriden?
Yes
76
When can a stretch reflex be hard to evoke?
Strong descending inhibition in an anxious patient hyperpolarizes α-motoneurones and the stretch reflex can be hard to evoke
77
How can a descending inhibition be overcome?
High activity in upper motor neurones spreads to and depolarises lower level motorneurones
78
What is the clinical relevance of reflexes?
Important in assessing the integrity of the whole spinal cord circuit, includes - afferent nerves, balance of synaptic inputs to the motoneurones, motoneurones, neuro-muscular junction and muscles. Help spinal level localisation of a problem reflexes evoked above, but NOT below a given level may localise a problem eg. segmental trauma to the spine. In the stretch reflex: spindle input is highly localised and affects only α-motoneurones at one or two spinal segments
79
What does facilitation do?
If one finger touches a hot plate - your hand withdraws quickly but if your whole hand contacted a hot surface, you would withdraw your whole arm, extend the contralateral arm. This is Facilitation which enhances the effectiveness of sensory inputs. Pain fibre activity facilitates the muscle spindle activity by maintaining nearby α-motoneurones in a more depolarised state.
80
Where does facilitation occur?
Occurs between the same stimuli, pain fibres (above) and between different stimuli

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