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NAB > 7b. Spinal Cord and Muscles > Flashcards

7b. Spinal Cord and Muscles Flashcards

1
Q

Organisation of the Ventral Horn of the Spinal Cord

A

Distal limb motor neurones are lateral

Proximal limb motor neurones are medial

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2
Q

Motor Neurone Pool

A

All 200-500 α motor neurones that innervate a given muscle

Located close together in the ventral horn

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3
Q

Motor Unit

A

Single α motor neurone and all the muscle fibres it branches to innervate

Basic unit of force production

Each muscle contains 200-500 motor units as they are innervated by 200-500 α motor neurones

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4
Q

3 Mechanisms of Controlling Contraction Force

A
  • Type of motor unit stimulated
  • Rate coding
  • Motor unit recruitment
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5
Q

3 Types of Motor Unit

A
  • Slow
  • Fast, fatigue resistant
  • Fast fatigable
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6
Q

Types of Motor Unit

- Slow Motor Unit Anatomical Characteristics

A
  • Small fibres
  • Few fibres per unit
  • Highly vascular
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7
Q

Types of Motor Unit

- Slow Motor Unit Biochemical Characteristics

A
  • Oxidative metabolism

- Abundant myoglobin (red)

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8
Q

Types of Motor Unit

- Slow Motor Unit Physiological Characteristics

A
  • Slow twitch
  • Low tension generation
  • Fatigue resistant
  • Slow, small diameter axons
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9
Q

Types of Motor Unit

- Slow Motor Unit Use

A

Continuous generation of small forces

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10
Q

Types of Motor Unit

- Fast Fatiguable Motor Unit Anatomical Characteristics

A
  • Large fibres
  • Many fibres per unit
  • Few capillaries
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11
Q

Types of Motor Unit

- Fast Fatiguable Motor Unit Biochemistry Characteristics

A
  • Glycolytic metabolism

- Little myoglobin (white)

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12
Q

Types of Motor Unit

- Fast Fatiguable Motor Unit Physiology Characteristics

A
  • Fast twitch
  • High tension generation
  • Fatigable
  • Fast, large diameter axons
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13
Q

Types of Motor Unit

- Fast Fatiguable Motor Unit Use

A

High forces over a short period of time

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14
Q

Rate Coding

A

Rate coding can be used to control generation of low-medium forces

An increase in action potential firing frequency generates more twitches.

However, muscle go into tetanus at quite low frequencies, so motor unit recruitment is used to generate higher forces

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15
Q

Motor Unit Recruitment

A

Motor unit recruitment can be used to control force generation

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16
Q

Motor Unit Recruitment

- Size Principle

A

Low force motor units are recruited first then higher force motor units

Therefore force always increments by the finest available motor unit so is as smooth as possible

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17
Q

Motor Unit Recruitment

- Developmental Plasticity

A

Motor neurones with low firing threshold innervate few muscle fibres and induce them to become slow twitch, low force fatigue resistant fibres.

Motor neurones with high firing threshold innervate many fibres and induce them to become fast twitch, high force and fatiguable

Avoids the need for the brain to control motor neurones independently as inputs to motor neurones innervating a given muscle will automatically recruit motor neurones that generate the lowest force first, as they are low threshold

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18
Q

Motor Neurone Damage

- Result

A

Flaccid Paralysis

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19
Q

Motor Neurone Damage

- Degeneration

A

Motor neurone disease (ALS)

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20
Q

Motor Neurone Damage

- Autoimmune disease

A

Guillain Barre syndrome, where motor neurones demyelinate

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21
Q

Motor Neurone Innervation

- 3 Sources

A
  • Muscle spindle afferents
  • Descending fibres
  • Spinal interneurones
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22
Q

Proprioception

  • Definition
  • Receptors
A

Sense of position or movement of the body

  • Proprioceptors
  • Ruffini endings (exteroceptors)
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23
Q

Proprioceptors

- Description

A

Sensory fibres in muscles and joints

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24
Q

Peripheral Sensory Fibres

- 3 Types

A
  • Proprioceptors (muscles, tendons and joints)
  • Exteroceptors (skin)
  • Teloceptors (special sense organs)
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25
Q

Number of Sensory and Motor Neurones in Muscle Nerves

A

Sensory neurones massively outnumber motor neurones in muscle nerves

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26
Q

Proprioceptors

- 3 Types

A
  • Muscle spindle affernets
  • Golgi tendon organ afferents
  • Join receptors
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27
Q

Proprioceptors

- 3 Functions

A
  • Spinal reflexes
  • Proprioception
  • Provide information for supra spinal motor systems
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28
Q

Muscle Spindle Afferents

- Receptor Type

A

Proprioceptors that detect:

  • Muscle length
  • Change in muscle length

Signal:

  • Muscle length
  • Changes in muscle length
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29
Q

2 Types of Muscle Fibre

A
  • Extrafusal muscle fibre

- Intrafusal muscle fibre

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30
Q

Extrafusal Muscle Fibre

A
  • Large
  • Not present in muscle spindles
  • Contract to generate tension
31
Q

Intrafusal Muscle Fibre

- Characteristics

A
  • Small
  • Present in muscle spindles
  • Contract to alter sensitivity of sensory neurone endings in the spindle
  • Only the ends are striated and contractile
32
Q

Muscle Spindles

- Number

A

20-100 in each muscle

33
Q

Muscle Spindles

- Number of Intrafusal Fibres

A

10-12

34
Q

Intrafusal Muscle Fibre

- 2 Types

A
  • Bag fibre

- Chain fibre

35
Q

Intrafusal Muscle Fibre

- Bag Fibre Structure

A
  • Swollen centre containing many nuclei

- Striated contractile ends

36
Q

Intrafusal Muscle Fibre

- Chain Fibre Structure

A
  • Uniform diameter
  • Many nuclei in a line
  • Uniform striations
37
Q

Intrafusal Muscle Fibre

- 2 Types of Afferent

A
  • Primary Ia spindle afferent

- Secondary IIa spindle afferent

38
Q

Intrafusal Muscle Fibre

- Primary Ia Spindle Afferents

A
  • Large
  • Fast conducting
  • Spiral around centre of bag and chain fibre
  • Terminal fibres are called annulospiral endings
  • Equivalent to Aα motor neurones
  • Rapidly adapting
  • Very sensitive
  • Carry mainly dynamic responses to change in muscle length
39
Q

Intrafusal Muscle Fibre

- Secondary IIa Spindle Afferents

A
  • Small
  • Slow conducting
  • Terminate adjacent to the central region of the intrafusal muscle fibre
  • Equivalent to Aβ motor neurones
  • Carry mainly static response to muscle length
40
Q

Intrafusal Muscle Fibre

- Bag Fibre Stretch Response

A

Both a dynamic and static response to stretch

Dynamic:
Dynamic response to change in muscle length, as non-contractile centre stretch first.
Because primary Ia afferents are in the central region, stretch gives strong rapid increase in action potential frequency

Static:
Static response to muscle length as stretch is relieved as contractile ends elongate, decreasing action potential frequency

41
Q

Intrafusal Muscle Fibre

- Chain Fibre Stretch Response

A

Very small dynamic response to stretch.

Static response to stretch as the whole fibre is contractile

42
Q

Intrafusal Muscle Fibre

- Efferent Innervation Function

A

Controls the sensitivity of the terminals by triggering contraction.

Contraction elongates the ends of the spindle, elongating the central region and sensitising the response of afferent fibres innervating it.

Allows muscle spindles to gave a similar sensitivity to length changes from different starting points, which is a type of adaptation

43
Q

Intrafusal Muscle Fibre

- Efferent Innervation Neurones

A

Gamma motor neurones:

  • Small diameter
  • Slow conducting
44
Q

Gamma Motor Neurone

- 2 Types

A
  • Static gamma motor neurones

- Dynamic gamma motor neurones

45
Q

Intrafusal Muscle Fibre

- Afferent Firing Frequency

A

Could be increased by:

  • Muscle stretch
  • Increased efferent innervation

Therefore muscle length is ambiguous

46
Q

Intrafusal Muscle Fibre

- Efferent Copy

A

Efferent copy is a record of recent gamma motor neurone activity, which allows the brain to determine muscle length

47
Q

Golgi Tendon Organs

- Receptor Type

A

Proprioceptors that detect tendon tension

48
Q

Golgi Tendon Organs

- Activation

A

Active tension in the tendon brought about by muscle contraction.

Passive stretch of the muscle doesn’t activate Golgi tendon organs as muscle elasticity prevents tension on the tendon

49
Q

Joint Receptors

- Receptor Type

A

Proprioceptors that signal joint position and movement, especially at extremes

50
Q

Stretch Reflex

- Function

A

Muscles contract in response to being stretched

51
Q

Stretch Reflex

- Exceptions

A
  • Eye muscles, which contract against a fixed load

- Intrinsic tongue muscles

52
Q

Stretch Reflex

- Sensory Arm

A

Muscle stretch stimulates muscle spindle afferents

53
Q

Stretch Reflex

- Synapse

A

Muscle spindle afferent synapses with motor neurones to several different muscles in the spinal cord

54
Q

Stretch Reflex

- Motor Arm

A

α motor neurones to trigger contraction of the:

  • Stretched muscle
  • Synergist muscles
55
Q

Stretch Reflex

- Spinal Interneurones

A

None = monosynaptic reflex

56
Q

Stretch Reflex

- Use

A

Postural control

57
Q

Disadvantages of Stretch Reflex Negative Feedback

A
  • Gain <1

- Delays

58
Q

Gain

A

Stretch should elicit contraction to precisely counteract, giving a gain of 1.

When measured, gain<1

completely cancel it out

59
Q

Damage to Descending Motor Systems

- Characteristics

A

Causes spastic paralysis, where:

  • Stretch is exaggerated so strong responses are evoked
  • Oscillating muscle contractions following fast muscle stretch = myoclonus
60
Q

Damage to Descending Motor Systems

- 2 Causes

A
  • Cerebral palsy

- Stroke

61
Q

Stretch Reflex

- Role

A

Brain may be able to control gain of the stretch reflex so that it suits the movement required, through adjusting spindle afferent sensitivity using efferent innervation.

  • Slow, precise movements have low gain stretch reflexes to allow negative feedback
  • Rapid movement shave high gain stretch reflexes to allow feedforward predictive mechanisms
62
Q

Renshaw Cells

- Pathway

A

Motor neurones give off intraspinal branches (recurrent collaterals), that innervate Renshaw cells.

Renshaaw cells project to the spinal cord and inhibit the motor neurone that excited it

63
Q

Renshaw Cells

- Function

A

Mediate recurrent inhibition

Regulate pattern of motor neurone discharge to prevent jerkiness and tremor

64
Q

Nociceptor Withdraw Reflex

- Function

A

Nociceptor activation trigger movement of the Boyd part away from the stimulus

65
Q

Nociceptor Withdraw Reflex

- Sensory Arm

A

C-fibres and A-delta fibres

Terminate in superficial forsal horn, in the substantia gelatinosa

66
Q

Nociceptor Withdraw Reflex

- Interneurones

A
  1. Excitatory interneurones in the superficial dorsal horn, in the substantial gelatinosa
  2. Inhibitory internueones in the intermediate zone of the spinal cord that are activated by excitatory interneurones
67
Q

Nociceptor Withdraw Reflex

- Motor Arm

A

Secondary interneurones synapse with motor neurones that generate reflex flexor contraction and extensor inhibition

68
Q

Nociceptor Withdraw Reflex

- Properties

A
  • Spatial summation
  • Temporal summation
  • Local sign, where contracted muscle differ depending on the stimulus location
69
Q

Tendon Organ Reflex

- Sensory Arm

A

Tendon organ afferents

70
Q

Tendon Organ Reflex

- Interneurones

A
  1. Excitatory interneurone
  2. Inhibitory interneurone, excited by the excitatory interneurone

The presence of 2 interneurones allows context dependence:

  • Static postures = tendon organ reflex evokes inhibition of parent muscle
  • Locomotion = tendon organ reflex evokes excitation of parent muscle, supporting contraction against load
71
Q

Tendon Organ Reflex

- Motor Arm

A

Motor neurones

72
Q

Spinal Reflexes

- Importance

A
  • Show developmental changes so give an indication of neural development
  • Changes upon brain damage
73
Q

Spinal Reflexes

- Developmental Changes

A

Infantile grasp reflex:

  • Grasping object placed in hand
  • Disappears at 6-9 months

Reflex stepping:

  • Appears at 4 months
  • Different sequence appears at 1 year

Babinski’s sign:

  • Curling of toes in response to scratching the sole
  • Infants = upward curl
  • Adults = downward curl
74
Q

Spinal Reflexes

- Brain Damage

A

Loss of supra spinal control allows the brainstem and spinal cord to control movement entirely.

  • Exaggerated stretch reflexes (spasticity) and myoclonus
  • Babinki’s sign reverts to infant form
  • Clasp knife reflex, where limbs snap into either extension or flexion

NAB (37 decks)

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