Functional Organisation of Spinal Cord

Question 1

What are the functional roles of the spinal cord?

Answer 1

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

Question 2

What part of the spinal cord to segmental reflexes involve?

Answer 2

Small amounts

Question 3

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

Answer 3

Walking

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

Question 4

Do people lose autonomic control with spinal injury?

Answer 4

Yes

Question 5

What is the spinal cord surrounded by?

Answer 5

Meninges

CSF

Question 6

In which regions is the spinal cord larger?

Answer 6

Brachial

Lumbo-sacral

Question 7

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

Answer 7

Because vertebral column grows more than spinal cord

Question 8

What happens at the lower end of the vertebral column?

Answer 8

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

Question 9

Where is CSF sampled from?

Answer 9

Between L3 and L4

Don’t damage spinal cord this way

Question 10

What happens in the grey matter of the spinal cord?

Answer 10

Processing

Question 11

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

Answer 11

Opposite
White matter on outside
Grey matter on inside

Question 12

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

Answer 12

Ascending information

Question 13

Describe the cervical region of the spinal cord

Answer 13

Lots of neurons in grey matter

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

Question 14

Describe the thoracic region of the spinal cord

Answer 14

Fewer muscles to innhervale
Fewer sensory fibres
Smaller grey matter area

Question 15

Describe the lumbar region of the spinal cord

Answer 15

Grey matter increases

Lots of muscles and sensory information

Question 16

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

Answer 16

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

Question 17

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

Answer 17

Cauda equina

Question 18

How is autonomic function distributed through the spinal cord?

Answer 18

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

Question 19

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

Answer 19

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

Question 20

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

Answer 20

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

Question 21

Describe the route of C fibres into the spinal cord

Answer 21

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

Question 22

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

Answer 22

Nociceptor information

Information to cerebellum

Question 23

What descending tracts are there in the spinal cord?

Answer 23

Mostly from motor cortical region

Others motor information from brain stem

Question 24

What are alpha motor neurons?

Answer 24

Neurons innervating skeletal muscle

Contract it

Question 25

How are alpha motor neurons organised in the spinal cord?

Answer 25

Two groups - Medial - Lateral

Question 26

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

Answer 26

Across all levels

Question 27

How can you contract a muscle?

Answer 27

Only via a motor neuron

Question 28

What can a motor neuron do?

Answer 28

Only make a muscle contract

Question 29

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

Answer 29

Make a change in activity in the final common pathway

Question 30

Where can changes in motor neuron activity come from?

Answer 30

Range of areas in brain

Question 31

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

Answer 31

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

Question 32

Define a motor pool

Answer 32

The group of motor neurons innervating the entire muscle

Question 33

Describe a motor unit

Answer 33

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

Question 34

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

Answer 34

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

Question 35

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

Answer 35

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

Question 36

Describe muscle spindles

Answer 36

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

Question 37

What do gamma motor neurons do?

Answer 37

Only contract muscle spindles

Question 38

What do group I and II afferent axons do?

Answer 38

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

Question 39

Describe Golgi tendon organs

Answer 39

``` 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 ```

Question 40

Why do we have two different muscle sensors?

Answer 40

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

Question 41

What can be evaluated through a monosynaptic stretch reflex?

Answer 41

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

Question 42

Describe how a monosynaptic stretch reflex works

Answer 42

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

Question 43

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

Answer 43

Probably most if not all of motor pool

Question 44

What is the role of reflex circuits in motor control?

Answer 44

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

Question 45

What does a monosynaptic feedback do physiologically?

Answer 45

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

Question 46

Describe how a Golgi tendon organ works?

Answer 46

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

Question 47

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

Answer 47

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

Question 48

Descrube the cross-extensor reflex

Answer 48

``` 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 ```

Question 49

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

Answer 49

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

Question 50

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

Answer 50

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

Question 51

Describe the relation between brain and spinal cord motor function

Answer 51

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

Question 52

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

Answer 52

Inhibitory

Question 53

How does the brain exert motor control?

Answer 53

Removes some of inhibition | Maybe adds a little excitation as well

Question 54

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

Answer 54

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