Spinal reflexes Flashcards
Why are interneurons important?
Eg. to allow limb movement (shut off antagonistic muscles).
What four sources provide input to spinal interneurons?
Primary sensory axons (e.g. Ia and Ib fibres)
Descending axons from the brain
Collaterals (branches) of LMNs
Other interneurons
What two modalities may an interneuron be?
Excitatory or inhibitory
What do inhibitory interneurons do?
Inhibit the antagonistic myotatic reflex
Inhibit antagonistic muscles
Describe a reflex arc allowing muscle contraction
AP from Ia sensory neuron activates corresponding aMN and an inhibitory interneuron. Inhibitory interneuron inhibits antagonistic aMN.
What kind of neuron transmits sensory info from a muscle spindle?
Ia
What two neurons do UMN innervate to cause muscle contraction?
aMN and inhibitory interneuron to inhibit antagonistic aMN
What do excitatory interneurons mediate?
Flexor reflex
Crossed extensor reflex
What is the flexor reflex?
When noxious stimuli is sensed limb flexes away from it.
What is the crossed extensor reflex?
Noxious stimuli causes the opposite limb to extend.
How do the flexor and crossed extensor reflexes combine?
Noxious stimuli- Ipsilateral limb flexes away and contralateral limb extends to support (eg stand on one leg)
Describe the neuronal pattern of flexor and crossed extensor reflexes.
Ad detects noxious stimuli -> Activates excitatory interneurons to ipsilateral flexor and contralateral extensor aMN + inhibitory interneurons to ipsilateral extensor and contralateral flexor aMN
What is an essential feature of an excitatory interneuron?
Oscillation/pacemaker activity
What does a Spinal Central Pattern Generator (CPG) do?
Command Rhythmic, Alternating, Activity That Moves a Limb
Explain how oscillation regulates Spinal Central Pattern Generator (CPG).
EI1 fires a burst of AP which cause burst of AP in flexor aMN. EI1 also sends AP to II1 which inhibits EI2 (extensor) until first oscillation has finished.
When EI1 finishes, EI2 takes over to stimulate extensor and inhibit flexor.
What two things initiate controlled body movement?
Sensory input
Internal mechanisms telling the body it needs to move
What is the hierarchy of motor control?
High (strategy), middle (tactics), low (execution)
What is high (strategy) motor control and what structures does it involve?
What is the aim of the movement and how can it be achieved?
Neocortical association areas
BG
What is middle (tactics) motor control and what structures does it involve?
What sequence of muscle contractions/relaxation is needed.
Motor cortex
Cerebellum
What is low (execution) motor control and what structures does it involve?
Activation of MN and interneurons
Brain stem
Spinal cord
What are the two main descending spinal pathways?
Lateral
Ventromedial
What is the most important lateral descending spinal pathway?
Lateral corticospinal tract
Rubrospinal tract
What is the most important ventromedial descending spinal pathway?
Ventral corticospinal tract
What do the lateral descending spinal pathways do?
Voluntary control of distal muscles and skilled movement.
Controlled by cortex
What do the ventromedial descending spinal pathways do?
Control of posture and locomotion.
Controlled by brain stem
What is another name for the corticospinal tracts?
Pyramidal tracts
Why are the pyramidal tracts called as such?
Because the UMN come together to for a tract in the pyramids of the medulla (base of medulla).
Where do most (80%) UMN decussate?
Pyramids (pyramidal decussation) to form the lateral corticospinal tract.
Where does 20% UMN decussate?
Descend ipsilaterally to form the ventral corticospinal tract and decussate in the spinal cord.
Where do UMN synapse onto LMN?
Ventral horn
What does the rubrospinal tract do?
Exerts control over limb flexor muscle, exciting LMNs of those muscles
Where does the rubrospinal tract arise?
Red nucleus of the midbrain- receives input from the motor cortex and cerebellum
Where does the red nucleus of the midbrain receives input from?
Motor cortex and cerebellum
Where do axons of the rubrospinal tract decussate?
Ventral tegmental decussation in the midbrain
Where do axons of the rubrospinal tract terminate?
Ventral horn
What happens if the lateral corticospinal tract is lesioned?
Initially loss of ‘fractionated’ movements (i.e. shoulders, elbow, wrist and fingers cannot be moved independently) but with time the rubrospinal tract can compensate.
What four tracts make up the ventromedial pathways?
Vestibulospinal tract
Tectospinal tract
Pontine (medial) reticulospinal tract
Medullary (lateral) reticulospinal tract
What does the vestibulospinal tract do?
Hold an upright position by innervating extensor antigravity muscles such as leg, back and neck.
Where do the nerves of the vestibulospinal tract arise and what is their input?
Vestibular nuclei in pons
Input from vestibular labyrinth via CNVIII
What does the tectospinal tract do?
Orientation of head and eyes
Where do the nerves of the tectospinal tract arise and what is their input?
Superior colliculus (aka optic tectum) of midbrain. Retina and visual cortex
What does the Pontine (medial) reticulospinal tract do?
Enhances antigravity muscles
What does the Medullary (lateral) reticulospinal tract do?
Oppose the pontine reticulospinal tract and allow relaxation of antigravity muscles.
Where do both of the reticulospinal tracts originate?
Reticular formation
