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MD1 Neuroscience > Functional Organisation of Spinal Cord > Flashcards

Functional Organisation of Spinal Cord Flashcards

1
Q

What are the functional roles of the spinal cord?

A 
Reflexes
- Segmental reflexes
- Inter-segmental reflexes
- Pattern generators
Processing and transmission of somatosensory information
- Tactile
- Nociceptive
Relay of descending motor command
Relay of descending autonomic commands
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2
Q

What part of the spinal cord to segmental reflexes involve?

A

Small amounts

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3
Q

What is an example of an activity that a pattern generator controls?

A

Walking

Have to extend and flex opposite compartments of legs, 180 degrees out of phase

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4
Q

Do people lose autonomic control with spinal injury?

A

Yes

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5
Q

What is the spinal cord surrounded by?

A

Meninges

CSF

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6
Q

In which regions is the spinal cord larger?

A

Brachial

Lumbo-sacral

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7
Q

Why do nerve roots travel increasingly long distances to exit into the periphery?

A

Because vertebral column grows more than spinal cord

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8
Q

What happens at the lower end of the vertebral column?

A

Lots of space at bottom where there’s just roots where spinal nerves are exiting
Relatively large volume of CSF at end that’s not covered by bone

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9
Q

Where is CSF sampled from?

A

Between L3 and L4

Don’t damage spinal cord this way

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10
Q

What happens in the grey matter of the spinal cord?

A

Processing

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11
Q

What is the structure of grey and white matter in the spinal cord compared to the brain?

A

Opposite
White matter on outside
Grey matter on inside

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12
Q

What sort of information do the dorsal columns of the spinal cord carry?

A

Ascending information

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13
Q

Describe the cervical region of the spinal cord

A

Lots of neurons in grey matter

Large motor neurons, and many because have a lot of muscles to innervate in that area

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14
Q

Describe the thoracic region of the spinal cord

A

Fewer muscles to innhervale
Fewer sensory fibres
Smaller grey matter area

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15
Q

Describe the lumbar region of the spinal cord

A

Grey matter increases

Lots of muscles and sensory information

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16
Q

What happens to white matter tracts as you go down the spinal cord?

A

Decrease as you go down
Less sensory fibres coming in from lower down
Fewer motor neurons going out

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17
Q

What does the dotty appearance of the sacral region represent in a CT scan?

A

Cauda equina

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18
Q

How is autonomic function distributed through the spinal cord?

A

Differentially
Sympathetic preganglia in thoracic and lumbar regions
Parasympathetic in sacral region

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19
Q

Describe the route of A-beta fibres into the spinal cord

A 
Large fast conducting myelinated fibre
Encapsulated ending
Comes into dorsal root ganglion
Makes some local connections in spinal cord
Mainly takes information up to the brain
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20
Q

Describe the route of A-alpha fibres into the spinal cord

A

Mechanoreceptors in muscle tell about what muscle is doing
Fast, myelinated
Enters dorsal root ganglion
Makes some local connections in spinal cord
Travels up to brain

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21
Q

Describe the route of C fibres into the spinal cord

A

Thin, unmyelinated
Cell bodies in dorsal root ganglion
Connections in spinal cord almost all local or in segment or two
Second neuron carries information up to brain

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22
Q

What other ascending tracts are there in the spinal cord, other than somatosensory information?

A

Nociceptor information

Information to cerebellum

23
Q

What descending tracts are there in the spinal cord?

A

Mostly from motor cortical region

Others motor information from brain stem

24
Q

What are alpha motor neurons?

A

Neurons innervating skeletal muscle

Contract it

25
Q

How are alpha motor neurons organised in the spinal cord?

A

Two groups

  • Medial
  • Lateral
26
Q

Where else does the medial to lateral topography of motor information happen?

A

Across all levels

27
Q

How can you contract a muscle?

A

Only via a motor neuron

28
Q

What can a motor neuron do?

A

Only make a muscle contract

29
Q

How is the only way to make a muscle contract in terms of neuronal pathways?

A

Make a change in activity in the final common pathway

30
Q

Where can changes in motor neuron activity come from?

A

Range of areas in brain

31
Q

What is the topography of motor neurons in the spinal cord?

A

Medial motor neurons innervate proximal muscles
Lateral motor neurons innervate distal muscles
Dorsal-ventral mapping; eg: biceps motor neurons more dorsal than triceps motor neurons

32
Q

Define a motor pool

A

The group of motor neurons innervating the entire muscle

33
Q

Describe a motor unit

A

Each motor neuron innervates several fibres
Size varies
- 3-4 to 100+ fibres

34
Q

Describe functional antagonism in muscles, and how reflexes work in respect to this

A

Almost all muscles, especially those moving skeleton, work in antagonistic pairs functionally
Reflex circuits respect this

35
Q

How does the nervous system know what a muscle is doing?

A

Sensory structures embedded in muscles

  • Muscle spindles
  • Golgi tendon organs
36
Q

Describe muscle spindles

A

Muscle sensory organ
Modified muscle fibres
Form specialised relationship with sensory fibre - encapsulates ending
Still maintain motor innervation

37
Q

What do gamma motor neurons do?

A

Only contract muscle spindles

38
Q

What do group I and II afferent axons do?

A

Sensory fibres
Respond to length/stretch
More stretch > more firing
Certain fibres respond more to change in stretch, other respond to just how long muscle is

39
Q

Describe Golgi tendon organs

A 
Embedded in tendons
Nerve afferent enmeshes itself in collagen of tendon
Tell about force of muscle
Mechanoreceptors responding to force
Relatively high threshold
No motor innervation
Not encapsulating like muscle spindle
40
Q

Why do we have two different muscle sensors?

A

Need to independently say how long muscle is and how much force it’s producing

41
Q

What can be evaluated through a monosynaptic stretch reflex?

A

All parts of motor system
Via testing in region of interest
If something wrong in reflex > something might be wrong in what controls it - brain

42
Q

Describe how a monosynaptic stretch reflex works

A

One central synapse
Muscles work in antagonistic pairs so two circuits
Tap tendon
Large number of muscle spindles project to spinal cord
Main projection to motor neurons innervating same muscle
Forms closed circuit
Sensory nerve has excitatory action on motor neuron
Excites stretched muscle > muscle contracts = negative feedback loop
Have to relax antagonistic muscle at same time
Same sensory fibre excites interneuron
Interneuron = inhibitory
Inhibits motor neurons of antagonist muscle
Antagonist muscle relaxes

43
Q

How many motor neurons does a single muscle spindle synapse with?

A

Probably most if not all of motor pool

44
Q

What is the role of reflex circuits in motor control?

A

Sits at bottom of motor neuron hierarchy

Forms basis of complex movements

45
Q

What does a monosynaptic feedback do physiologically?

A

Maintain muscle position
Load pulls muscle down
Spindle signal excites alpha motor neuron to contract muscle back
Relaxes antagonist muscle via inhibitory interneuron

46
Q

Describe how a Golgi tendon organ works?

A

In series with muscle - in between bone and muscle
All inputs go through interneuron
Force increases
Golgi tendon organ activated
Excites inhibitory interneuron
Interneuron reduces amount of activity in motor neuron from same muscle as Golgi tendon organ
Maintains force by bringing it back down if it goes up
At same time Golgi tendon organ excites antagonist muscle via excitatory interneuron

47
Q

What is a physiological example of a Golgi tendon organ working?

A

If you want to maintain position over long time, muscle might fatigue
Remains same length so muscle spindles don’t detect anything but force decreases
Golgi tendon organ now less active
Reduces activity of inhibitory interneuron
Alpha motor neurons more excited
Increases amount of contraction
Restores force
Relaxes antagonist muscle

48
Q

Descrube the cross-extensor reflex

A 
Initiated by something noxious; eg: nociceptors
Activates flexors of limb
Reduces activity of extensors of limb
In walking situation, if you flex one leg, it's no longer supporting weight of body
Other leg has to extend
- Activate extensors
- Inhibit flexors
Chains of interneurons make that happen
49
Q

Describe the connections of the interneurons in a cross-extensor reflex along the spinal cord

A

A lot cross-extensor reflexes traverse over several segments
A lot of interneurons connect up and down spinal cord
Interneurons that connect to medial muscle motor neurons tend to have long connections
- Proximal muscles do most of stability
Lateral motor neurons have shorter interneuron connections
- Use distal muscles; eg: those in hand, to do different things at same time - don’t want a lot of synchronisation

50
Q

Describe the spinal reflex experiment with the frog with its brain removed

A

Put little patch of vinegar on upper limb - acts as irritant
Hind leg flicks paper off
Purely from spinal cord circuits
Move forelimb to different position and activate same reflex > hind leg moves to where irritant is now and flicks it off
- Spinal cord has some mapping of where limbs in space are
Reflexes complex and adaptable

51
Q

Describe the relation between brain and spinal cord motor function

A

Segmental control of muscle sits at bottom of hierarchy of other muscle controls
Information comes down from cortex and brain stem
Sensory information from muscle spindles and Golgi tendon organs also goes to brain

52
Q

What effect do descending signals from brain have on spinal cord?

A

Inhibitory

53
Q

How does the brain exert motor control?

A

Removes some of inhibition

Maybe adds a little excitation as well

54
Q

What happens to a reflex if there’s a lesion disrupting upper motor control?

A

Lose inhibitory signal
Brisk and exaggerated response in reflex
Level at which exaggerated reflexes start shows lesion is just above it

MD1 Neuroscience (55 decks)

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