How is direction of movement encoded by our neurons?
Lots of neurons fire for a movement and they have preferred directions for which they will fire stronger.
So the overall movement direction is population vector based on the integrated activity of lots of neurons.
Explain the concept of predictive posture?
You spinal tracts are involved in feedback and feed forward systems to correct and predict postural instability:
- Vestibulospinal acts as feedback, detecting instability and correcting for it
- Reticulospinal carries info from the reticular formation to stablize for a movement before you make it (Feed Forward)
What are the symptoms of UMN damage?
- Spinal Shock (Spinal hypotonia due to loss of cortical input)
- Babinski sign (Plantar extension instead of flexion)
- Spasticity (increased muscle tone, hyperreflexia & clonus due to loss of UMN suppression of LMNs)
- Loss of fine finger movements
List the basal ganglia?
Input = Caudate nucleus and putamen
Output = Globus Pallidus & Substantia Nigra
(Anatomically the globus pallidus and putamen together form the lentiform nucleus)
Whats the function of the basal ganglia?
They’re involved in intitiating movements.
This occurs as part of a positive feedback loop.
Basically they focus widespread cortical activity from the somatosensory, premotor and motor parts of the cerebrum to the supplementary motor area.
Its beleived the GO signal for a movement is when the Basal Ganglia boost the SMA beyond a certain thrreshold
Explain the process by which the Basal ganglia boos SMA activity:
This is known as the direct pathway
- Medium spiny neurons in putamen excited by signals from cerebrum motor cortices
- Putamen inhibits GPinterna & the Substantia nigra
- GPinterna & SN then STOP inhibiting the Thalamus’ VLo Neurons
- Thalamus’ VLo neurons are free to excite the SMA
This is the process for body movements, the process for eye movements is basically the same except the caudate nucleus rather than the putamen is used
If the SMA is boosted to enact an action by the basal ganglia Direct Pathway. What does the indirect pathway do?
IT suppresses competeting or innapropriate actions
- Striated nuclei (Caudate/putamen) inhibit the GPexterna
- GPexterna ceases inhibiting the STN, SN & GPinterna
- STN excites GPinterna & SN (STN also recieves excitatory fibres direct from the motor cortex)
- GPinterna & SN inhibit the VLo
- Thus the VLo cant boost the SMA
- innapropriate actions are inhibited
Name two disorders of the basal ganglia?
What are the symptoms of Parkinson’s?
- Difficulty initiating voluntary movement
- Increased muscle tone
- Tremors of the hand & Jaw
What causes Parkinson’s?
Neuronal Degeneration in the substantia nigra, particular its dopaminergic inputs to the striatum.
Dopamine is involved in enhancing the basal ganglia’s direct & indirect pathways without it it can be impossible to initiate the right movements and stop the wrong ones.
What are the symptoms of huntington’s?
- Personality disorders
The characteristic chorea is spontaneous, uncontrolled, rapid flicks and major movements with no purpose
What causes huntington’s?
Its a hereditary genetic disorder
It occurs by profound loss of the caudate, putamen and globus pallidus
You lose the inhibitory effect of the basal ganglia and lots of violent, innapropriate movements are initiated
What is the function of the cerebellum in movement?
Fine Tuning complex movements
There’s a cortico-pontine-cerebellar projection through which the cerebellum passes instructions on direction, timing and force of movements. Making use of motor learning stored in the cerebellum.
So what would happen with a cerebellar lesion
Uncoordinated inaccurate movements (ataxia)
This can also be caused by alcohol depressing the cerebellum