Functional Organisation of Spinal Cord Flashcards
What are the functional roles of the spinal cord?
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
What part of the spinal cord to segmental reflexes involve?
Small amounts
What is an example of an activity that a pattern generator controls?
Walking
Have to extend and flex opposite compartments of legs, 180 degrees out of phase
Do people lose autonomic control with spinal injury?
Yes
What is the spinal cord surrounded by?
Meninges
CSF
In which regions is the spinal cord larger?
Brachial
Lumbo-sacral
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?
Processing
What is the structure of grey and white matter in the spinal cord compared to the brain?
Opposite
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?
Differentially
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?
Inhibitory
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
