Descending Motor Control

Question 1

Descending systems

Answer 1

Motor Cortex: planning, initiating and directing voluntary movements (modulated by the basal ganglia - involved in initiating movements and suppressing unwanted movements)

Brainstem centres: rhythmic, stereotyped movements and postural control (modulated by the cerebellum - coordination of ongoing movement)

Leads down to the local circuit neurons (sensorimotor integration and central pattern generation) and motor neuron pools (which lead to skeletal muscles)

Question 2

Sensorimotor control loop

Answer 2

1. visual information required to locate target
2. frontal lobe motor areas plan the reach and command the movement
3. spinal cord carries information to hand
4. motor neurons carry message to muscles of the hand and forearm
5. sensory receptors on the fingers send message to sensory cortex saying that the cup has been grasped
6. spinal cord carries sensory information to brain (ventral roots)
7. basal ganglia judge grasp forces, and cerebellum corrects movement errors
   8/ sensory cortex receives the message that the cup has been grasped

Question 3

Structure of spinal cord

Answer 3

• the white matter is on the outside
• ventral horn: where the alpha motor neurons reside
• dorsal horn: where sensory afferent fibres project back to the brain and the brainstem

Question 4

contralateral organisation

Answer 4

upper motor neurons in the cerebral cortex would cross the body inline and innervate lower motor neurons on the left side of the body

Question 5

2 types of descending tracts

Answer 5

1. Corticobulbar tract: axon bundle originating in M1 and terminating in the brainstem: involved in controlling muscles of face
   - -> terminates higher up than the cortical spinal tract
2. corticospinal tract: axon bundle originating in M1 and terminating in the spinal cord
   - controls proximal and distal limb muscles
   - 90% of corticospinal axons decussate in the medulla and form the lateral corticospinal tract
   - remaining 10% terminate ipsilaterally and form the ventral corticospinal tract

Question 6

primary motor cortex distinguishing features

Answer 6

can be distinguished from other motor areas in 2 ways

• architectonics: output layer 5 contains distinctive large-diameter pyramidal neurons
• electrophysiology: low intensity electrical stimulation elicits movements

Question 7

Functional organisation of the primary motor cortex:

Answer 7

• topographic organisation of M1: neighbouring muscles are represented in neighbouring brain tissue
• the homunculus looks at how much cortical territory is devoted to different body parts

Question 8

What do motor maps represent?

Answer 8

One-many mapping: even the smallest currents capable of eliciting a response activate several different muscles

Many-one mapping: stimulation of many individual sites over a relatively wide region of m1 can generate contractions in a single muscle

Question 9

directional tuning in the primary motor cortex

Answer 9

• individual neurons are selectively responsive to upcoming reaches or movements in a particular spatial direction

Question 10

Premotor cortex

Answer 10

support motor functions
help control movement both indirectly via reciprocal connections with M1 and directly via axons projecting through corticobulbar and corticospinal tracts (30% of axons in the corticospinal tract originate in premotor cortex)

Question 11

Brainstem circuits

Answer 11

support posture maintenance and control

anticipatory feedback control