Movement - 2 Flashcards
How is there a hierarchy in the movement control of muscles?
The muscles are controlled by the lower alpha motor neurons in the spinal cord → alpha motor neurons are controlled by upper motor neurons in the brainstem → the upper motor neurons are controlled by higher level controllers in the forebrain and motor cortex.
- Is it a problem if some alpha motor neurons are not working?
- Is it a problem if some higher-level controllers (like the upper motor neurons in the brainstem or in the motor cortex) are not working?
- No, it shouldn’t be a really big problem if some alpha motor neurons are not working, since the upper controllers can still send out signals to other alpha motor neurons.
- It is a problem when higher-level controllers are not working, since these are responsible for regulating downstream neurons like the alpha motor neurons. So if the motor cortex is ‘inactive’, the alpha motor neurons also cannot be regulated.
How do we coordinate many muscles without thinking about it?
Rhythmic movements like walking are generated by Central Pattern Generators (CPGs) in the spinal cord. These CPGs can also produce running, which means that CPGs are flexible where the same circuit can produce two types of rhythm.
How do we know that the walking CPG is located in the spinal cord?
When you transection the spinal cord of the cat and you put the cat on a treadmill, the hindlegs will start swinging in automating patterns. And when the muscles in the hindlegs are recorded, a rhythmic pattern is seen of activation of the extensor and flexor muscle.
CPGs are local circuits in the spinal cord. Describe these CPG local circuits.
There are inhibitory interneurons and excitatory neurons in the intermediate gray matter that coordinate alpha motor neurons.
Explain how the walking CPG is generated.
It’s important to remember that in order to initiate a walking CPG, a rhytmhic movement needs to be generated where first the extensor is activated and at the same time the flexor is inactivated. This is then followed by activation of the flexor and inactivation of the extensor, etc. etc.
In order to do this, the following needs to occur:
- An excitatory neuron activates the alpha motor neuron that activates the extensor muscle. At the same time the excitatory neuron activates an inhibitory interneuron that synapses on alpha motor neurons of the flexor muscle, so that the flexor muscle is inactivated.
- Then there’s the flexor and extensor module, between these two modules is a mutual inhibitory circuit, which directly inhibits alpha motor neurons of the flexor and extensor muscles → generating a rhytmic alternating pattern.
What is meant by the fact that CPGs are built for flexibility?
Limbs must work in different rhythms during different gaits.
How could one locomotor CPG achieve these different rhythms of limbs during different gaits?
CPG neurons have a midline for the left and right sides.
- During walk → left excitatory neurons of left muscle is active → activates inhibitory interneuron that inhibits right excitatory neurons of the right muscle.
- During trot → left excitatory neuron is active → activates inhibitory interneuron on the right side → different rhythm
- During bound gait → left excitatory neuron excites another left excitatory neuron → excites an excitatory neuron on the right side.
What upper motor brain region control the locomotion of CPGs?
This is controlled by the mesencephalic locomotor region called the reticular formation (RF).
What happens when you are waiting for a red trafficking light and the light turns green?
This is noticed by the forebrain, which activates the midbrain locomotor region (→ RF). This activates walking CPGs, where alpha motor neurons are activated to innervate the muscles of the legs.
There are 5 main systems that constitute the upper motor neurons in the brain. Name these.
- Reticulospinal
- Vestibulospinal
- Rubrospinal
- Tectospinal
- Corticospinal
Describe for the following systems that constitute the upper motor system what brain regions are involved and what their function is.
- Reticulospinal
- Vestibulospinal
- Rubrospinal
- Tectospinal
- Corticospinal
- Reticulospinal → pons and medullary RF → send information to the spinal cord local motor neuron circuits for locomotion.
- Vestibulospinal → inner ear system important for posture and balance.
- Rubrospinal → not strongly present in humans, so we don’t understand it properly.
- Tectospinal → tectum (inferior colliculus) receives hearing and visual information → reorientates your head based on the sound and visual information by controlling neck muscles.
- Corticospinal → motorcortex that regulates spinal circuits.
Describe the vestibulospinal tract.
The vestibulospinal tract controls posture by inner ear sensing of location. The information is projected down into the axial parts of the spinal cord, which innervate axial muscles for posture control.
Describe the reticulospinal tract.
The reticulospinal tract leaves from the midbrain and controls postural adjustments and initiates locomotion.
- To what region in the spinal cord do axons from the midbrain project to?
- To what region in the spinal cord do axons from the motor cortex project to?
- Axons from the midbrain project to the medial ventral white matter.
- Axons from the motor cortex project to the lateral white matter.
Describe the pathway of cortical control of voluntary movements and skilled movements.
Motor cortex → cell bodies of neurons in the motor cortex are the longest neurons in our bodies → in caudal medulla there’s (pyramidal) decussation → ventral and lateral corticospinal tract → innervation of some lower motor neurons and local circuit neurons in the medial lateral spinal cord.
The cortex contains cortical spinal neurons that project to the spinal cord. How are these cells called and in what layer of the cortex do they reside?
Pyramidal cells in layer 5 provide output from the cortex to the brainstem and spinal cord.
Upper motor neurons have directional tuning. What is this?
The motor cortex neurons selectively fire when movements are made in one particular direction.
Describe the characteristics of the corticobulbar tract.
The corticobulbar tract controls facial movement (e.g. facial expression and voluntary saccades)
From the lateral edges of the motor cortex, neurons of the corticobulbar tract project to the motor nuclei of cortical nerves (pons, medulla) → trigeminal motor nucleus (jaw), facial motor nucleus (face muscles) and hypoglossal nucleus (tongue).
What happens during postural coordination and postural correction when you move your hands?
If you move your hands, your posture needs to be corrected in order to stay balanced. So before the hands even move, the leg muscle (gastrocnemius) is already activated. This is done by a feed-forward circuit. This can be seen in the picture.
What pathway is responsible for postural coordination? Also describe this pathway.
The cortico-reticulo-spinal pathway → couples corticospinal tract (fine distal limb movements) with reticulospinal tract (postural adjustments).
From the premotor cortical neurons, information is projected to the reticulo formation, which activates the reticulospinal tract neurons and axial musculature.
What areas are involved in the visually guided reach-to-grasp (seeing something and then grabbing it)?
- Visual cortex → seeing the object.
- Posterior parietal cortex → observing and integrating signals that you need for executing the action.
- Motor cortices
- Primary motor cortex → start moving hand
Two other structures are important → basal ganglia and cerebellum.
What is a key reason to leave the basal ganglia and cerebellum out of the neural control of movement?
A key reason for this, is that there’s lots of information that comes from lesion experiments. These experiments have showns the following:
- In Parkinson’s Disease, the system in the brain is broken (especially basal ganglia and cerebellum), which results in tremor and jerky movements. This would imply that normally the system is involved in inhibiting tremor.
- If there’s a lesion halfway in the system, it’s very hard to deduct the role of this lesioned area. Since the cerebellum and basal ganglia are in the middle of the system, it’s very hard to deduct their role.
The cerebellum has an organized structure. Describe what cells can be found here.
There are Purkinje cells with long dendrites. Climbing fibers are wrapped around these long dendrites and are able to excite the Purkinje cells. The Purkinje cells are GABAergic and inhibit deep cerebellar nuclei neurons.
Is the cerebellum involved in the feedback controlled parametric adjustment of movement control?
Yes, it’s thought that the cerebellum is involved in the parametric adjustment of the actual result.
