Motor control 1

Question 1

What are the components of the motor system?

Answer 1

Cerebral cortex and voluntary movement - the motor cortex and sensory input, right beside each other.
Descending motor pathways - lateral is voluntary, ventromedial arises from brainstem, unconscious control.
Spinal cord - motor neurones, sensory input, local reflexes.

Question 2

What is a simple reflex movement?

Answer 2

Output from spinal cord causes muscle contraction and movement, this is detected by sensory receptors.
Sensory consequences of movement feeds back into the spinal cord.

Question 3

What is the basal ganglia loop?

Answer 3

Receives input from the premotor and supplementary motor cortex, and feeds back via the thalamus.
It has hierarchical organisation and somatotopic representation.
It is dysfunctional in Parkinson’s and Huntingdon’s.

Question 4

What is the cerebellum loop?

Answer 4

The cerebral cortex inputs into the cerebellum, which then inputs to the brain stem and then to the spinal cord to cause muscle contraction.
The cerebral cortex can also stimulate the spinal cord directly.

Question 5

What are the types of movement?

Answer 5

Reflex
Rhythmic motor patterns.
Voluntary

Question 6

What are reflexes?

Answer 6

Protective e.g. limb withdrawal
Motor patterns are generated in the spinal cord.
Closed loop

Question 7

What are rhythmic motor patterns?

Answer 7

e.g. chewing, walking, breathing.
A combination of reflex and voluntary

Question 8

What are voluntary movements?

Answer 8

Purposeful, goal-directed.
Command originates from higher centres.
Open loop.

Question 9

What are lower motor neurones?

Answer 9

Alpha motor neurone.
a-MN alone directly control muscle contraction.

Question 10

What inputs is spinal motor neuron activity governed by?

Answer 10

Sensory input - local feedback control via dorsal roots.
Spinal interneurones - circuitry generating motor programmes.
These two are reflexes.
Upper motor neurones - initiation and control.

Question 11

Can spinal cords act in isolation?

Answer 11

Spinal cord circuits can generate movements in isolation.
Even when the descending influences are severed.
Central pattern generators - circuits within the spinal cord are responsible.

Question 12

What is descending input from upper motor neurones?

Answer 12

Sophisticated, adaptable, patterns of movement - voluntary or unconscious.
Involves input descending from the brain - the cortex or brainstem.
Signals from the brain are superimposed on top of the intrinsic circuitry of the spinal cord - modulating it - not a reflex.

Question 13

What is the musculature?

Answer 13

Posture, innervated by medial motor neurones:
Proximal - elbow, knee.
Axial - trunk muscles.
Fine motor, innervated by lateral motor neurones:
Distal - hands, feet, digits.

Question 14

What are the lateral and proximal pathways of the spinal cord?

Answer 14

Lateral - direct and indirect (via red nucleus in brainstem) pathway to the spinal cord.
Proximal - indirect pathway via the brainstem to the spinal cord: reticular nuclei and superior colliculus (eyes) and vestibular nuclei (balance).

Question 15

What are the descending tracts of the spinal cord?

Answer 15

Lateral pathways:
Corticospinal tract
Rubrospinal tract

Ventromedial pathways:
Vestibulospinal tract
Tectospinal tract
Pontine reticulospinal tract
Medullary reticulospinal tract

Question 16

What does lateral mean?

Answer 16

Away from the midline of the spinal cord (side).
The corticospinal tract and rubrospinal tract are lateral.
See picture.

Question 17

What does ventromedial mean?

Answer 17

The bottom of the middle of the spinal cord, towards the front of the body.
See picture

Question 18

What are the spinal cord horns?

Answer 18

Dorsal horn is the sensory input.
Ventral horn is motor output, where the alpha motor neurones are found.
The lateral horn is only found in T1-L2, and is responsible for sympathetic motor control.

Question 19

What is the corticospinal tract?

Answer 19

Or Pyramidal tract.
A direct line contralateral projection from cortex to lateral spinal motor neurones.
Monosynaptic contact with a-MNs.
Innervate a-MNs controlling distal muscles, particularly flexors.
Most axons arise from neurones with cell bodies in the motor cortex.

Question 20

What is the rubrospinal tract?

Answer 20

Contralateral projections from red nucleus running down the lateral column of the spinal cord.
Similar role to corticospinal tract but smaller component of the lateral pathway.

Question 21

What are ventromedial motor pathways?

Answer 21

Extra pyramidal tracts.
Originate from brain stem nuclei.
Both contra and ipsilateral descending projections.
Control of motor output to proximal and axial muscles.
Control of body position and posture.

Question 22

What are the pathways of the ventromedial pathways?

Answer 22

Reticular nuclei: Pontine reticulo-spinal and medullary reticulo-spinal.
Superior colliculus and vestibular nuclei: Vestibulo-spinal and tecto-spinal.
Balance, body position and visual input.
Modulate spinal reflexes to modulate posture.

Question 23

What is the pontine reticulospinal tract?

Answer 23

Enhances anti-gravity reflexes of spinal cord.
Facilitates leg extensors to maintain standing posture.

Question 24

What is the medullary reticulospinal tract?

Answer 24

Opposing effect of pontine reticulospinal tract.
Frees antigravity muscles from reflex control - allows voluntary override.

Question 25

What is the vestibulospinal tract?

Answer 25

Relays gravitational sensory information from vestibular labyrinth (inner ear) and stretch receptors in axial muscles. Maintains head and neck position and legs.

Question 26

What is the tectospinal tract?

Answer 26

Relays visual sensory information from retina and visual cortex. Orientates head and eyes to visual and auditory stimuli.

Question 27

What does control of voluntary movement involve?

Answer 27

Most of the neocortex. Movement involves sensory input, planning, deciding the appropriate action, holding the plan in memory and execution.

Question 28

What is the structure of the neocortex?

Answer 28

Area 4 - M1, primary motor cortex Area 6 - SMA (supplementary motor area) and PMA (pre-motor area) Prefrontal cortex - thinking about making movement. Sensory cortex, right next to motor cortex

Question 29

What is the primary motor cortex?

Answer 29

Area 4/ M1. Most direct link, using alpha motor neurones. Lowest stimulus threshold required to prodice movement - strong synaptic link. Control of distal musculature - fine motor control.

Question 30

What is the supplementary motor area?

Answer 30

In area 6, fronto/medial. PMA is in area 6 lateral. SMA does not have as strong a synaptic link, so has higher stimulus threshold. Produces movement by stimulating the primary motor cortex. Role in planning and initiation. Bi-manual coordination - left and right halves.

Question 31

What is the premotor cortex?

Answer 31

Control of proximal musculature - posture and balance. Control of movement sequencing. Preparation for movement, initiation.

Question 32

What are upper motor neurones?

Answer 32

Primary motor cortical output neurones. Axons in the corticospinal tract. 50% of corticospinal tract axons. Pyramidal, cell body in cortical layer V. Somatotopically organised. Activate small groups of muscles. Neurone individually encode the force or direction of movement.

Question 33

What is somatotopic representation?

Answer 33

Larger areas (like hands, lips, face) = more neurons = finer control or sensitivity Smaller areas (like the trunk or legs) = less sensitivity or motor precision

Question 34

What is damage to upper motor neurones?

Answer 34

Initial muscle weakness. Eventual spasticity - increased resistance to passive movement. Increases muscle tone, reflex responses. Affects side contralateral to damage. Recovery is possible as the primary motor cortex circuitry has adaptive alterations.