Spinal interneurons and spinal reflexes Flashcards
- gross structure of spinal cord - cell biology of interneurons - biochemistry relexes
What is the central nervous system composed of?
brain and the spinal cord.
The spinal cord is a crucial part of the central nervous system.
what is the peripheral nervous system composed of
cranial nerves and spinal nerves
What are the three main classes of cells found in the spinal cord?
- Relay neurons (interneurons)
- Sensory neurons
- Motor neurons
Where are the cell bodies of sensory neurons located?
The cell bodies of sensory neurons are located outside the spinal cord in the dorsal root ganglion and axons in spinal cord
Where do the axons of motor neurons project?
The axons of motor neurons project outside the spinal cord (cell bodies inside SC)
How is the spinal cord organized in humans?
The spinal cord is organized into four segments:
1. Cervical segments
2. Thoracic segments
3. Lumbar segments
4. Sacral segments
What are the two main compartments of the spinal cord?
White matter (contains axons/tracts)
Gray matter (contains cell bodies)
spinal cord cross section and segments
- dorsal root ganglion: posterior root: 1 st order sensory neurons provide information into SC
- anterior root: axons of motor neurons leave SC here
layers of the SC
- 10 layers (Rexed’s laminae LI-LX)
- Dorsal horn: LI-LVI (sensory)
- Intermediate: LVII-VII (internuerons)
- ventral horn: LIX (motor)
What are motor pools?
clusters of motor neurons that control different muscles and are anatomically clustered.
- clustered along rostrocaudal axis
where are motor neurons innervating axial and distal muscles located ?
along mediolateral axis of ventral horn
What is the function of interneurons in the spinal cord?
Interneurons process sensory information and can perform various functions including inhibition of motor neurons.
What is the role of Renshaw cells?
- inhibitory interneurons that provide negative feedback control to motor neurons to reduce contraction
- receive input from motor neurons collaterals and inhibit same MN pool
- important in descending control
What is the function of la inhibitory interneurons?
- reciprocal inhibition
- main input from la afferent
- prevents synergistic and antagonistic muscles working against each other
- sense stretch from muscle spindle receptors
- reduce contraction of antagonistic muscle
- important in stretch reflex
what is the function of Ib inhibitory neurons
- main input from Ib afferents
- non-reciprocal inhibition
- prevent excess muscle elongation
- sense stretch through golgi tendon organs
- reduce contraction of synergistic muscles
- important for tendon reflex
how are interneurons classified via genetics
- 11 progenitor domains that give rise to cardinal classes have unique transcription factors
- Renshaw - V1
- Ia from V1 and V2b
what are the difference between the two interneuron classes
- V1 - inhibit flexors
- V2b - inhibit extensors
What are the two main groups of ascending tracts in the spinal cord?
- Dorsal column tracts
- Spinothalamic tracts
What type of information do dorsal column tracts convey?
information about fine tactile discrimination and vibration, control fine movements
What type of information do spinothalamic tracts carry?
information about pain, temperature, and crude touch (pressure), for quick movements / reactions
What are the two main groups of descending tracts from the brain to the spinal cord?
- Pyramidal tracts
- Extrapyramidal tracts
what are the pydramidal tracts
- lateral corticospinal tract
- ventral corticospinal tracts
Where do corticospinal tracts originate?
Corticospinal tracts originate in upper motor neurons located in the primary motor cortex and premotor cortices.
- Broadmann 4 and 6
what are the two streams the corticospinal tracts go through
- lateral: cross-over in pyramids
- anterior : cross-over in spinal cord
why are cortiocospinal tracts important
- control voluntary contralateral movements
lateral corticospinal tract
- fine motor control e.g. fingers, limbs
- UMNs inervate single muscles or a small set of muscles
ventral corticospinal tract
- controls axial muscles
e.g. neck, trunk muscles
what are the extrapydramidal tracts
- reticulospinal tract
- rubrospinal tract
- vestibulospinal tract
- teectospinal tract
reticulospinal tract
- posture control
rubrospinal tract
- originates in red nucleus, transmits signals to cerebellum and motor cortex
- control of muscle tone, arm muscles
vestibulospinal tracts
- receive information from vestibulococlear cranial nerve about angular and linear head accelerations
what are the two main parts of the vestibulospinal tract
- medial: stabilises head
- lateral: controls ‘antigravity’ leg muscles
tectospinal tract
- originates in superior colliculus
- co-ordinates orientation of neck, head and eyes
what is superior colliculus for
multisensory and sensimotor integration
- called optic tectum in non-mammals
stretch reflexes
- myotatic reflexes
- don’t need brain for these
e.g. patellar reflex (knee) - refelx still intact even if SC cut
- if dorsal root cut then abolished
circuitry for stretch reflexes - monosynaptic
- la afferent fibers excited by muscle elongation
- excites motor neurons from same synergistic muscles to contract
- arc reflexes
circuitry for stretch reflex - disynaptic
- la afferent fibers excited by muscle elonggation
- excites la inhibitory internuerons
- la inhibitory interneurons inhibit muscle contraction in antagonistic muscles
- reciprocal inhibition
withdrawl reflex - ipsilateral
- polysnaptic reflex
- nociceptive Adelta fibres excite spinal sensory neurons
- spinal sensory neurons indirectly (via dorsal horn) promote muscle contraction via inhibitory and excitatory interneurons for reflex on contralateral side
withdrawl reflex contralateral side
- spinal sensory neurons contact commisural neurons
- commisural neurons indirectly promote flexor muscle relaxation and extensor muscle contraction
renshaw cells for descending control
- via single projection onto renshaw cells
- produce recurrent inhibition of activated muscles through motor neuron collaterals
- also inhibit la inhibitory interneurons, indirectly excites antagonist muscles
tendon reflex
- Ib inhibitory and excitatory interneurons modulation by behavioural state
- Ib stimulation induces non-reciprocal inhibition of muscles at rest
- induces excitation of muscles during walking
What is the significance of decussation in corticospinal tracts?
Decussation allows corticospinal tracts to control voluntary movement on the opposite side of the body.
What does decussation in the corticospinal tracts refer to?
The crossing over to control lateral movement of the opposite side of the body
What is the primary function of the lateral corticospinal tract?
Control fine movements
What type of muscles does the ventral corticospinal tract control?
Medial proximal muscles like the neck and trunk
Why are the extra pyramidal tracts important?
Control automatic movements and muscle tone
They also regulate reflexes and orienting responses to stimuli.
What is the reticulospinal tract important for?
Control of posture and gross movements
What does the rubrospinal tract control?
Muscle tone and arm muscles for large swinging movements
What is the function of the vestibulospinal tract?
Control reflexes and body balance
Where does the vestibulospinal tract originate?
Vestibular nuclei of the pons
What information does the vestibulospinal tract convey?
Angular velocity and linear acceleration of the head
What are the two main parts of the vestibulospinal tract?
- Middle part stabilizes head position by controlling neck muscles
- Lateral tract controls extensor muscles for legs
What is the tectospinal tract important for?
Sensory motor integration and orienting reflexes
What reflex is exemplified by the patellar reflex?
Stretch or myotactic reflex
What defines a reflex as a monosynaptic reflex?
It only requires one synapse
What happens when the patellar tendon is struck?
Induces a change in muscle length that excites motor neurons
What is the purpose of the withdrawal reflex?
To withdraw a body part from harmful stimuli
What are the two components of the withdrawal reflex?
- Ipsilateral component
- Contralateral component
What do ipsilateral sensory neurons do in the withdrawal reflex?
Inhibit extensor muscles and excite flexor muscles
What do contralateral sensory neurons do in the withdrawal reflex?
Excite extensor muscles and inhibit flexor muscles
What are Renshaw cells important for?
Negative feedback control
How do Renshaw cells function in relation to motor neurons?
They provide recurrent inhibition and can be controlled by the brain
What is the tendon reflex?
A reflex that can change according to behavioral state
How does the activity of interneurons change during locomotion?
They switch from inhibiting motor neurons to increasing excitability
What can the same sensory stimulus provoke according to behavioral state?
Two opposite motor reactions
