Synaptic inputs to MNs Flashcards
(18 cards)
Where do synaptic inputs to motorneurons mainly occur/
On the dendrites and soma
What are the two main types of input to MNs?
Ionotropic - fast
Neuromodulatory - slow
Where are APs generated in the MN and what is this dependent on?
- At the axon hillock
- Dependent on intrinsic electrical properties and balance between excitatory and inhibitory inputs
What are the main sources of input to the MNs?
CPG - excitatory and inhibitory inputs, control locomotion
Descending inputs - Mainly excitatory with some evidence for inhibitory tracts
Sensory inputs - exteroreceptive and proprioceptive inputs serve to provoke reflex outputs
Motor Neurons - Contact each other (some electrically coupled) as well as interneurons within the spinal cord to create collateral feedback circuits
What is Brown’s hypothesis hypothesis?
- Antagonistic ‘half-centres’ containing MNs for flexors and extensors of each joint are connected by reciprocal inhibition
- ‘tonic background excitation’ which is in intrinsic to circuit activates these half-centres
What is axial locomotion?
Used for swimming, involves contraction of left and right myotomes for swimming
How to tadpoles swim?
Alternate contraction sequence propelled from head to tail propelling thrust
What are the characteristics for descending interneurons?
- Glutaminergic
- Electrically coupled with mutually excitatory positive and negative feedback
- Fire once with a singular borad AP
- Relatively positive
Why is it significannt that dINs activate both NMDA and non-NMDA receptors?
NMDA: slow rise and fall (~200ms)
non-NMDA: fast rise and fall (~50ms)
Combine to produce fast rise and slow fall which is longer than the swim cycle resulting in summation and tonic excitation
What evidence exists that dIN partners are connected by gap junctions?
- Exciting one dIN partner it excites the other producing a positive feedback loop which sustains swimming
- If you hyperpolarise one dIN then the other also becomes hyperpolarised, this effect is removed with a gap junction blocker
What are the characteristics of commisural inhibitory interneurons?
- Fire once per cycle during swims
- Provide reciprocal inhibition responsable for left to right alterations
- Make glycinergic synapses (strychnine sensitive) with contralateral neurons
What are the similarities between mamalian and tadpole CPGs?
1) Flexor/extensor half centres coupled by strychnine sensitive reciprocal inhibition
2) Excitation involves activation of both AMPA and NMDA type glutamate receptors
What is the structure of the mammalian CPG believed to be?
Hierarchical organisation
- Rhythm generating core (F/E) of oscillation
- Pattern generating layer sculpts pattern in more detail
- Motor neurons receive inputs
What are the two categories of descending inputs?
Pyramidal tracts - anterior and lateral corticospinal tracts
Extrapyramidal tracts - have a particular morphology and can enter the spinal cord in stereotyped positions
What are the 4 extrapyramidal tracts?
Rubrospinal - emanates from red nucleus which activates flexors and forelimbs
Olivospinal - eminates from inferior olive, related to cerebellum control acting as an error detector. Olive compares afferent to efferent and corrects mismatch
Reticulospinal - From reticular formation, integrates motor activity, involved in locomotor control and inhibition of movement
Vestibulospinal - From balance organs, activates extensor motor neurons
What are the two main afferent inputs to alpha motor neurons?
Exteroreceptive - cutaneous (touch) inputs which project via the dorsal horn interneurons (polysynaptic)
Proprioceptive - Golgi tendon organs (1b afferents) and muscle spindles (1a afferents) (monosynaptic)
What is a secondary way in which 1a afferents enact inhibitory reflex control?
- Project to inhibitory alpha motor neuron which inhibits the antagonistic muscle
What are Renshaw neurons?
- Activated by motor neuron collateral processes , inhibit the firing of motor neurons in order to terminate a burst of discharge
- Inhibit the firing of the neuron which has activated them and also inhibit the inhibitory neuron of the other side providing disinhibition
