W5: Brain And Control Of Movements 1

Question 1

What are upper motoneurones?

Answer 1

They have a motor efferent whose entire axon resides within the CNS.

Question 2

What are lower motoneurons?

Answer 2

Responsible for bringing about contraction and change in muscle length. They have a motor efferent whose axon travels in peripheral nerves.

Question 3

How do the 2 connect?

Answer 3

Upper can never contact muscles directly. They always produce their actions by influencing lower motoneurons.

Question 4

How are movements produced by the motor system?

Answer 4

Common to all movements - the body’s motor system releases itself from a tonic descending inhibition from the brain that normally operates to suppress any and all movements. The default position in the body is tonic descending inhibition on the motor system.

Question 5

What does the cerebral cortex do?

Answer 5

Represents the highest centre for processing of sensory information and for motor commands.

Question 6

What are voluntary motor commands issued through?

Answer 6

The motor cortex, which is located in the frontal lobe occupying the pre-central gyrus

Question 7

What is the pre-central gyrus responsible for?

Answer 7

Activating muscles in the body. Gives signals from all parts of the body.

Question 8

What brings about the motor homunculus?

Answer 8

The cells of the pre-central gyrus relationship to each other bring about the motor homunculus.

Question 9

Other names for pre-central gyrus

Answer 9

Motor strip, motor cortex, pre-central gyrus, M1

Question 10

Key motor cortical interconnections

Answer 10

• the motor strip receives signals from the pre motor area and the supplementary motor area
• between the supplementary motor areas and the pre motor areas, there is a lot of exchange of information between them. Both of these are fed information from the posterior parietal cortex
• motor cortex gives the final command via descending fibre tracts
• the variety of descending motor fibres coming from the pre-central gyrus of the brain constituted the pyramidal tract. These are all apomotoneurons.
• pyramidal tract then carries the signal to the lower motorneurones which bring about the response in muscles

Question 11

What are pyramidal tracts?

Answer 11

Axons of apomotoneurones from the motor cortex.

Question 12

Motor homunculus

Answer 12

Neurones of the motor strip are grouped according to skeletal muscles they command. The motor map of cortical neurones can be reconstructed into a disproportional caricature of a distorted muscle man known as the motor homunculus.

Question 13

Which side of the motor cortex controls the left side of the body?

Answer 13

The right cerebral motor cortex of the brain controls skeletal muscles of the left side of the body and vice versa.

Question 14

Neurones of the brain at the level of the pre-central gyrus have axons which leave the cerebral cortex down the descending fibre tracts. These will carry on until the level of the medulla, then what happens?

Answer 14

75-85% of fibres cross over onto the other side. These fibre tracts of the brain will now bring about control of muscles on the opposite side.
20-25% of fibres that do not decussate will stay on the same side. When they get to their destination, they decussate at the site of their target.

So the cerebral cortex has 100% decussate on to the contralateral side.

Question 15

What are descending fibre tracts formed from?

Answer 15

Axons of neurones with cell bodies in motor nuclei of the brain. Descending systems of the brain are upper motoneurons.

Question 16

What are the two classes of descending fibre tracts?

Answer 16

Cortical descending fibre tracts (pyramidal) and non-cortical descending tracts (extra-pyramidal)

Question 17

Describe the cortical descending fibre tracts (pyramidal)

Answer 17

• originate in the motor areas of the cerebral cortex

- primary motor cortex in the pre-central gyrus

Question 18

Describe the non-cortical descending tracts (extra-pyramidal)

Answer 18

• originates in the sub-cortical areas of the brain

- midbrain, brainstem, pons, medulla

Question 19

What are the different tracts?

Answer 19

Lateral corticospinal tract (75-85% of descending fibres that originate from the cerebral cortex)
Ventral corticospinal tract (15-25% decussate to the other side to join the fibres of the lateral corticospinal tract)
Rubrospinal tract (start from brainstem red nuclei)
Lateral vestibulospinal tract
Medullary reticulospinal tract
Pontine reticulospinal tract
Medial longitudinal fasciculus
Tectospinal tract

Question 20

Pathway from upper to lower motoneurons

Answer 20

Upper motoneurons carry the commands either from the cerebral cortex or the brainstem and their fibre tracts constitute the descending fibre tracts. These will terminate on cell bodies of lower motoneurones, located in the ventral horn of the spinal cord or in cranial nerve motor nuclei. The axon of the lower motoneurons approaching muscles are alpha-motoneurones, bringing about contraction via the NMJ. Lower motoneurons are either cranial motoneurons or spinal motoneurons.

Question 21

What does resting muscle tone produce?

Answer 21

Maintenance of posture

Question 22

Where do all motoneurones have their cell bodies?

Answer 22

All motorneurones have their cell bodies in the CNS.

Question 23

Where are the cell bodies of lower motoneurones located?

Answer 23

In the ventral horn of the spinal cord or in cranial nerve motor nuclei