Functional Organisation of Spinal Cord Flashcards Preview

MD1 Neuroscience > Functional Organisation of Spinal Cord > Flashcards

Flashcards in Functional Organisation of Spinal Cord Deck (54):

What are the functional roles of the spinal cord?

- 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


What part of the spinal cord to segmental reflexes involve?

Small amounts


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

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


Do people lose autonomic control with spinal injury?



What is the spinal cord surrounded by?



In which regions is the spinal cord larger?



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

Because vertebral column grows more than spinal cord


What happens at the lower end of the vertebral column?

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


Where is CSF sampled from?

Between L3 and L4
Don't damage spinal cord this way


What happens in the grey matter of the spinal cord?



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

White matter on outside
Grey matter on inside


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

Ascending information


Describe the cervical region of the spinal cord

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


Describe the thoracic region of the spinal cord

Fewer muscles to innhervale
Fewer sensory fibres
Smaller grey matter area


Describe the lumbar region of the spinal cord

Grey matter increases
Lots of muscles and sensory information


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

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


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

Cauda equina


How is autonomic function distributed through the spinal cord?

Sympathetic preganglia in thoracic and lumbar regions
Parasympathetic in sacral region


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

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


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

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


Describe the route of C fibres into the spinal cord

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


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

Nociceptor information
Information to cerebellum


What descending tracts are there in the spinal cord?

Mostly from motor cortical region
Others motor information from brain stem


What are alpha motor neurons?

Neurons innervating skeletal muscle
Contract it


How are alpha motor neurons organised in the spinal cord?

Two groups
- Medial
- Lateral


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

Across all levels


How can you contract a muscle?

Only via a motor neuron


What can a motor neuron do?

Only make a muscle contract


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

Make a change in activity in the final common pathway


Where can changes in motor neuron activity come from?

Range of areas in brain


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

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


Define a motor pool

The group of motor neurons innervating the entire muscle


Describe a motor unit

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


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

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


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

Sensory structures embedded in muscles
- Muscle spindles
- Golgi tendon organs


Describe muscle spindles

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


What do gamma motor neurons do?

Only contract muscle spindles


What do group I and II afferent axons do?

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


Describe Golgi tendon organs

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


Why do we have two different muscle sensors?

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


What can be evaluated through a monosynaptic stretch reflex?

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


Describe how a monosynaptic stretch reflex works

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


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

Probably most if not all of motor pool


What is the role of reflex circuits in motor control?

Sits at bottom of motor neuron hierarchy
Forms basis of complex movements


What does a monosynaptic feedback do physiologically?

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


Describe how a Golgi tendon organ works?

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


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

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


Descrube the cross-extensor reflex

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


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

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


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

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


Describe the relation between brain and spinal cord motor function

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


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



How does the brain exert motor control?

Removes some of inhibition
Maybe adds a little excitation as well


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

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

Decks in MD1 Neuroscience Class (55):