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MD2002 > Reflexes > Flashcards

Reflexes Flashcards

1
Q

What is a reflex ?

A

A specific, involuntary, unpremeditated, “built-in” response to a particular stimulus.

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2
Q

Give an example of an acquired reflex ?

A

Many operations during car driving

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3
Q

What is the relation between homeostasis and reflexes ?

A

Reflexes are a component of homeostatic control systems

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4
Q

Are we conscious of our reflexes ?

A

We are conscious of some of them

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5
Q

Describe the steps of the homeostatic mechanism initiated as a result of decreased room temperature and identify the reflex as part of this mechanism.

A

Decreased room temperature –> Increased heat loss from body –> Decreased body temperature –>
Reflexes: 1) (curling up –> Decreased heat loss from body) + 2) (constriction of skin blood vessels –> Decreased heat loss from body) + 3) (shivering –> Increased heat production) –> Return of body temperature towards original value.

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6
Q

What is the reflex arc ? What are the components of a reflex arc ?

A

The pathway mediating a reflex

Stimulus: Detectable change in internal/ external environment
Receptor: Detects the stimulus
Integrating center: Receives the signal and compares it to set point
Effector: Produces a response
Response: Sometimes gives negative feedback (back to stimulus)

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7
Q

Is the spinal/motor reflex conscious ?

A

No, it does not involve conscious thought

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8
Q

What is the sensory, and what is the efferent component of the spinal/motor reflex ?

A

Sensory component = sensory receptor

Effector component = motor outflow from the spinal cord to the appropriate muscles

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9
Q

What is a monosynaptic reflex ? Give an example of monosynaptic reflex.

A

A simple reflex that involves transmission of information from a sensory neuron to the appropriate motor neuron across a single synapse in the spinal cord.

E.g. Knee-jerk reflex

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10
Q

How are Axons routed to the CNS ?

A

Insides nerves

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11
Q

How many axons are there per nerve ?

A

Rypically there are many axons from many neurons inside one nerve

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12
Q

Is is possible for single nerves to carry both sensory and motor axons ? How is this useful ?

A

Yes

Because information can travel in two directions in a nerve.

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13
Q

What do the principal differences between different axons lie in ?

A
  1. Diameter

2. Myelination

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14
Q

Why is it that, if you record the activity in a nerve following a stimulation, you get multiple peaks ?

A

Because there are different fibre types within a nerve, all of which have different properties.

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15
Q

What are the types of nerve fiber types, when classified by conduction velocity ?

A 
Aα
Aβ
Aγ
Aδ
B
C
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16
Q

What is the function of the different nerve fiber types (different types based on conduction velocity) ?

A 
Aα: Proprioception, somatic motor
Aβ: Touch, pressure
Aγ: Motor to muscle spindles
Aδ: Pain Temperature
B: Preganglionic automic
C: Postganglionic sympathetic, pain and heat, cold and pressure
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17
Q

State the conduction velocity (m/s), and diameter of the different nerve fiber types (different types based on conduction velocity).

A 
Aα: 70-120 m/s and 12-20  µm
Aβ: 30-70 m/s and 5-12 µm
Aγ: 15-30 m/s and 3-6 µm
Aδ: 12-30 m/s and 2-5 µm
B: 3-15 m/s and 1-3 µm
C: 0.5-2 m/s and 0.1-2 µm
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18
Q

What are the different nerve fibre types based on sensory function ? What is the origin of each type ? What is the equivalent of each type on the classification system based on conduction velocity ?

A 
IA: Muscle spindles (Aα) 
IB: Golgi tendon organs (Aα) 
II: Muscle spindles; touch and pressure receptors (Aβ and Aγ):
III: Pain and temperature receptors (Aδ)
IV: Pain and other receptors (C)
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19
Q

What do alpha motor neurons innervate ?

A

Skeletal muscle

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20
Q

How many muscle fibers can one neuron innervate ?
How many motor neurons can innervate one muscle fiber?
How many motor neurons can innervate a muscle ?

A

Each neuron will innervate multiple muscle fibres, but each muscle fibre will only be innervated by one motor neuron
Many neurons can innervate a whole muscle (neighbouring fibres may be innervated by different neurons)

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21
Q

What are the components of a motor unit ?

A

A motor neuron and the muscle fibres it innervates.

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22
Q

What is the NT of motor units ?

A

ACh

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23
Q

How many muscle fibers per motor neuron do the smallest motor units innervate ?
How many muscle fibers per motor neuron do the largest motor units innervate ?

A

Smallest innervate 5-10 muscle fibres per motor neuron

Largest innervate ~1000 muscle fibres per motor neuron

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24
Q

What does the actual spectrum of motor unit sizes within a given muscle depend on ?

A
  1. The precision of control required (small motor units gives highly precise i.e. fingers, eyes)
  2. The force of contraction required or a low precision (large motor units ie postural muscles or calf muscles)
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25
Q

What is the intensity of a particular muscle’s contraction graded by ?

A
  1. Alterations in the frequency with which any single motor neuron fires action potentials
  2. Recruitment of α-motor neurons: Increasing the number of motor units recruited in a given muscle increases tension
26
Q

Are smaller or larger diameter neurons easier to activate/excited ?

A

Smaller diameter neurons

27
Q

Where are small motor neurons, and large motor neurons found respectively ?
Link the location they are found at, with their distinctive properties.

A

Small motor neurons innervate slow oxidative muscle fibres (small motor units) so can give low power but sustained effort
Large motor neurons innervate fast glycolytic muscle fibres (large motor units) give high power but fatigue quickly

28
Q

What are the main characteristics of muscle reflexes ?

A
  • Fast and automatic
  • Do not require conscious intervention for movement control.
  • Produce stereotyped response to a given stimulus
29
Q

What is a polysynaptic reflex ?

Give an example of one.

A

A reflex action that involves an electrical impulse being transferred from a sensory neuron to a motor neuron via at least one connecting neuron (interneuron) in the spinal cord.
For example, stimulation of pain receptors in the skin initiates a withdrawal reflex

30
Q

What is the difference between intra and intersegmental reflexes ?

A
  1. Intrasegmental
    the reflex arc is restricted to one or two spinal cord segments
  2. Intersegmental
    the reflex arc involves a number of spinal cord segments
31
Q

What are the Muscle Reflex time components ?

A
  1. Activation of sensory receptors (start the clock)
  2. Conduction to CNS along nerve fibres from
    receptors
  3. Transmission across neuron/neuron synapses in
    CNS (1-2ms)
  4. Conduction from CNS along nerve fibres to muscle
  5. Activation of muscle contractile machinery (5ms)
32
Q

What is the duration of the knee-jerk reflex ?

A

20-25 mS

33
Q

What are Muscle stretch reflexes ? How are they mediated ?

A

Enable muscle to oppose external forces that tend to elongate them.

Mediated by sense organs within muscles known as:
– Muscle spindles (mounted in parallel to the extrafusal muscle fibres)

34
Q

What are muscle spindles ? What is its components ? What is the function of mitotic spindles ?

A

Peripheral nerve endings of afferent fibres wrapped around modified muscle fibres (ie intrafusal fibres).. Function as stretch receptors.

Nuclear chain fibre + Nuclear bag fibre

Monitor length (nuclear chain fibre + nuclear bag fibre) and rate of change in length (nuclear bag fibre).

35
Q

What are the spindle fibre types ? Give the main features of each, including innervation.

A

CHAIN FIBRES:

  • Innervated by both IA and II sensory nerve fibres. IA fibres give rate of change and Type II fibres give total length.
  • Most common fibre type in the spindle.
  • So called because the fibres nuclei are arranged in a chain in the central portion.

BAG FIBRES:

  • Only innervated by Type 1A fibres so only give rate of change in length information.
  • Only one or two per spindle
  • So called because of the swelling in the equatorial region which houses the nuclei.
36
Q

What is the rate of firing of muscle spindles determined by ?

A
  1. Magnitude of stretch

2. Speed of stretch.

37
Q

Do mitotic spindles exist by themselves ?

A

No, several together wrapped by connective tissue.

38
Q

What is the difference between intrafusal and extrafusal muscle fibers ?

A

INTRAFUSAL MUSCLE FIBERS:
skeletal muscle fibers that serve as specialized sensory organs (proprioceptors) that detect the amount and rate of change in length of a muscle. Constitute the muscle spindle and are innervated by two axons, one sensory and one motor.

EXTRAFUSAL MUSCLE FIBERS:
contract, generating skeletal movement and are innervated by alpha motor neurons.

39
Q

Explain what happens to muscle spindles in muscle stretch.

A

-Extrafusal and intrafusal muscle fibres both lengthen
-This movement activates receptor endings, allowing ion exchange to take place which brings about receptor potential
-Action potentials are fired (contraction)
The more change we get in receptors, the more/faster stretch, the greater the rate of firing of AP receptors

40
Q

What is the correlation between extent of stretch and rate of firing of AP.

A

The more change we get in receptors, the more/faster stretch, the greater the rate of firing of AP receptors.

41
Q

How do we avoid “having a loosening of spindles and eventually, reducing of sensitivity and no firing of Action Potentials” which could result from constant shortening of muscle in contraction ?

A

THROUGH ALPHA GAMMA COACTIVATION

The muscle starts at a certain length, encoded by the firing of a Ia afferent. When the muscle is stretched, the muscle spindle stretches and the Ia afferent fires more strongly (more APs). When the muscle is released from the stretch and contracts (alpha motor neuron connected to extrafusal muscle fiber bring about shortening/contraction of extrafusal muscle fiber: this movement activates receptor endings, allowing ion exchange to take place which brings about receptor potential, following by firing of APs), the muscle spindle becomes slack, causing the Ia afferent to fall silent (slower/inexistant rate of firing). The muscle spindle is rendered insensitive to further stretches of muscle.

To restore sensitivity, gamma motor neurons fire and cause the spindle to contract, thereby becoming taut and able to signal the muscle length again.
So when the CNS instructs a muscle to contract, it not only sends the appropriate signals to the alpha motor neurons, it also instructs gamma motor neurons to contract the intrafusal fibers appropriately

42
Q

Describe the effect of effect of muscle length on spindle sensory output, especially for the IA and II type muscle fibers ?

A

IA:

  • Muscle stretches, increase in rate of firing as stretching takes place
  • When steady at fixed longer length, new rate of firing that is quicker from shorter length but not as quick as dynamic change

II:

  • Slow rate of firing at shorter length
  • Rate increases up to a rapid rate at longer length (and stays like that)
43
Q

Are there more intrafusal or extrafusal muscle fibers ?

A

Fewer intrafusal fibers than extrafusal fibers

44
Q

What is the function of the knee-jerk reflex ? Is it a monosynaptic or polysynaptic reflex ?

A

Maintains balance and posture

Monosynaptic

45
Q

How do skeletal muscle react to being stretched ?

A

They contract when artificially extended by tapping the tendon.

46
Q

Which structures are the reflexes mediated by ?

A

By muscle spindles

47
Q

What is reciprocal innervation ? How does it work ?

A

Influence of spindle sensory nerve fibres from one (homonymous) muscle on motor neurones projecting to antagonist muscles relaxes antagonist muscles and facilitates shortening of homonymous muscle. Requires the involvement of a local inhibitory neuron in the reflex onto the antagonist muscles.

48
Q

How is length monitoring achieved ?

A

By activation of both principal (group IA) sensory fibres and secondary (group II) sensory fibres on both bag and chain intrafusal fibre types.

49
Q

How is rate of change in length and duration of change achieved ?

A

By activation of only principal (group IA) sensory fibres on both bag and chain intrafusal fibre types.

50
Q

How are bag-type fibers innervated ?

A

Only innervated by Type 1A fibres (not type II)

51
Q

What are Golgi tendon organs ? What is their function ?

A

Afferent nerve fibres (mechanoreceptor) wrapped around collagen bundles.

FUNCTION: Monitor the tension in the muscle – are in series with the muscle fibres. Compliment the information from the muscle spindles in the CNS.

52
Q

What type of muscle fibers are Golgi tendon organs ?

A

Type IB sensory fibre afferent

53
Q

How do Golgi tendon organs basically work to monitor tension ? Does this work with both stretch and contraction ?

A

-As tendon is put under increasing tension, the more the collagen fibres lock onto, and stretch the type IB sensory fibres. This increases the receptor/generator potential and speeds signal frequency (resulting in an AP). Hence during normal use GTO’s are thought to regulate the amount of tension that develops in a muscle and, in extreme conditions, to protect the muscle and tendon from damage.
Yes, Golgi tendon organs monitor tension which may result from both stretch or contraction.

54
Q

Is tendon stretch typically greater with active contraction or passive stretch ?

A

Tendon stretch is typically greater with active contraction than passive stretch.

55
Q

Describe the role of Golgi tendon organs in a reflex arc.

A
  • Provide reflex inhibition to the motor neurons in the tendon they are located (ie their homonymous MN’s), via a disynaptic reflex
  • The more alpha MN’s are activated by the CNS, the more the muscles contract and the more tension develops. As tension develops the GTO’s increase their rate of firing and so increase the inhibition to the alpha MN’s. This consequently reduces the tension in the muscle.
56
Q

Based on the information given in this chapter, what is skeletal muscle contraction a combination of ?

A
  1. Alpha MN mediated contraction (under the control of the CNS).
  2. Gamma MN mediated spindle fibre contraction enabling accurate feedback.
  3. GTO mediated inhibitory feedback to the alpha MNs.
57
Q

What are some reflexes involving special senses ? Define each.

A
  • Rotatory nystagmus: Eye movements driven by moving visual images.
  • Post Rotatory nystagmus: Eye movements driven by the movement of fluid in the semi-circular canals of the inner ear.
  • Interpretation of movement within images: on the retina – used to make postural adjustments to preserve balance.
58
Q

Describe the main features of the baroreceptor reflex, as a result of both increased arterial pressure and decreased arterial pressure.

A

Increased Arterial Pressure (sensed by arterial baroreceptors) –> Increased Firing Rate –> Reflex via medullary cardiovascular center: Increased PSNS outflow to heart and decreased SNS outflow to heart, arterioles, and veins –> Decreased Arterial Pressure (towards normal)

Haemorrhage –> Decreased Arterial Pressure (sensed by arterial baroreceptors) –> Decreased Firing Rate –> Reflex via medullary cardiovascular center: Decreased PSNS discharge to heart and increased SNS discharge to heart, arterioles, and veins –> Increased Arterial Pressure (towards normal)

59
Q

What is meant by vestibular system ?

A

Sensory system that provides the leading contribution to the sense of balance and spatial orientation for the purpose of coordinating movement with balance.

60
Q

What are some structures other types of reflex (other than baroreceptor reflexes and special sense reflexes) are concerned with ?

A

– Glands
– Heart
– Gut
– Capillary system

MD2002 (77 decks)

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