Functional organisation of spinal cord Flashcards
List the functional roles of the spinal cord?
Segmental reflexes
Inter-segmental reflexes
Pattern generators
Processing and transmission of somatosensory information
Relay of descending motor command
Relay of descending autonomic commands
What is the spinal cord surrounded by?
Meninges and CSF
At what level is a lumbar puncture performed? Why?
L3/4
The spinal cord terminates at L1/2
Describe the organisation of grey and white matter in the spinal cord?
White matter external
Grey matter internal
How is the grey matter arranged in the spinal cord?
Dorsal horns
Ventral horns
How does the arrangement of the spinal cord change along its length?
Proportion of grey and white matter changes
More white matter towards the top, as more fibres have joined closer to the brain
Only a thin layer of white matter in the sacral region
Where do nociceptors enter the spinal cord?
Where do they make their connections?
Enter at dorsal root
Make connections in dorsal horn
What do nociceptor fibres do after they have made their connection in the dorsal horn?
Project across spinal cord at the level that they enter
Travel up to brain on contralateral side in the anterolateral system
Where do mechanoreceptor fibres enter the spinal cord?
Where do they make their connections?
Enter at the dorsal root
Make some local connections, the project up to the brain ipsilaterally
Where do muscle spindle fibres enter the spinal cord?
Where do they make their connections?
Dorsal root
Make connections locally, then project up to brain ipsilaterally
Is most of the white matter in the spinal cord ascending or descending tracts?
Ascending (sensory) tracts
What is located in the ventral horn of the spinal cord?
Motor neurons
Describe the topography of motor nerves within the spinal cord?
Medial: innervate more proximal muscles
Lateral: innervate more distal muscles
There is also topographical organisation superiorly and inferiorly
What are a-motor neurons?
The main motor neurons that cause muscles to contract
Why are motor neurons referred to as the ‘final common pathway’?
They are at the bottom of a massive structure/pathway
Describe the pathway of nociceptor, muscle spindle and mechanoreceptor pathways within the spinal cord?
Define a motor unit?
Define a motor neuron pool?
Motor unit: a collection of muscle fibres innervated by a single motor neuron
Morot neuron pool: all of the motor neurons that innervate a particular muscle
What is the role of the muscle spindle?
Detect stretch within a muscle
This tells us about muscle length
Are muscle spindles intrafusal fibres or extrafusal fibres?
Intrafusal fibres
Where are muscle spindles found within muscles?
Intrafusal fibres
Within connective tissue sheath/capsule
Which types of muscles have muscle spindles?
All muscles except the extra-ocular muscles
Are muscle spindles contractile?
Can contract, but their main function is to detect muscle stretch
How do muscle spindles receive information about stretch?
Group I and II afferent axons wrap around swollen parts of the fibres
Transduce stretch into nerve impulses
What are the two sensory structures within muscles?
Muscle spindles
Golgi tendon organs
What is the function of golgi tendon organs?
Detect muscle force
Describe the arrangement of muscle spindle and golgi tendon organs in relation to muscle fibres (ie. in series or parallel)?
Golgi tendon organs are in series with muscles
Muscle spindles are in parallel with muscles
How can muscle spindle function be tested clinically?
Monosynaptic stretch reflexes
Describe the neurology of the monosynaptic stretch reflex?
Pull tendon > muscle spindles detect stretch > AP travels to dorsal root via spindle afferent > spindle afferent makes an excitatory connection with motor neurons that innervate the same muscle > muscle contraction
At the same time, the spindle afferent connects with inhibitory motor neuron > contraction inhibited in anatgonist muscle
Describe the response to placing an increased load on a muscle?
Describe how golgi tendon organs respond to an increase in force?
GTO detects increased force > transmits AP along Golgi afferent > makes connection within inhibitory interneuron > muscle contraction inhibited
At the same time, the Golgi afferent links to an excitatory interneuron, which cause the antagonist muscle to contract
Is the response of the Golgi tendon organ a monosynaptic reflex?
No
Interneurons are involved
Is the response of muscle spindles a monosynaptic reflex?
Yes
There are no interneurons involved
What type of reflex is the cross-extensor reflex?
Bilateral
In which type of animal is the bilateral cross-extensor reflex important?
Quadrupeds
Which type of receptors are involved in the bilateral cross-extensor reflex?
Nociceptors
(This is a withdrawal reflex)
Describe how the cross-extensor reflex occurs?
Are interneurons all the same length?
No, there is variation in the length of connections
Where are longer and shorter interneurons found within the spinal cord?
Why is the arrangement like this?
Shorter connections tend to be found laterally, as distal muscles are generally used for dextrous, complex movement
Longer connections tend to be found medially, as proximal muscles that maintain posture require lots of connections between muscles, so inter-segmental coordinations and connections are required
Is the distinction between upper and lower motor neurons a functional or anatomical one?
Functional
What are upper and lower motor neurons?
Lower motor neurons innervate muscles (final common pathway)
Upper motor neurons include any neurons that affect the excitability of lower motor neurons and therefore affect the outcome of the final common pathway
Where might upper motor neurons be found?
Brain stem
Cortex
Basal ganglia
Are most interneurons excitatory or inhibitory?
Inhibitory
What happens when there is a lesion in the descending motor pathway (lower motor neurons)?
Lower motor neurons below the lesion lose their inhibitory input > become more excitable and maybe spontaneously active
What happens to reflexes in stroke?
Exxagerated reflexes contraletrally
