lecture 5- motor control

Question 1

descending systems (upper motor neurons)

Answer 1

motor cortex- planning, initiating and directing voluntary movements

brainstem centres- basic movements and postural control

Question 2

what are the two further structures in the brain that relate to movement?

Answer 2

basal ganglia- gating proper initiation of movement
cerebellum- sensory motor coordination of ongoing movement

Question 3

spinal cord and brainstem circuits

Answer 3

local circuit neurons- lower motor neuron integration (sensory inputs)
=>
motor neuron pools- lower motor neurons => skeletal muscles

Question 4

the motor cortex

Answer 4

• strip of cells just behind the frontal lobes, which runs across both hemispheres
• premotor and primary motor cortex in the frontal lobes
• responsible for planning and more precise control in voluntary movements

Question 5

organisation of the motor cortex

Answer 5

• contralateral control (opposite sides)
• somatotopically organised (Penfield, 1930)
• muscles are disproportionately represented

Question 6

motor mapping

Answer 6

• wilder penfield (1937) and the Montreal procedure
• pioneering surgery for epilepsy
• pre-surgery stimulation allows for mapping of the brain
• map doesnt represent individual muscles/body parts, but co-ordinated groups of muscles
  -lesions to motor cortex regions cause deficits in muscle groups

Question 7

sensory motor talents in the cortex?

Answer 7

• key motor areas have more representation in the motor cortex?
• in animals:
  
  • representation of the paws in raccoons (welker and seidenstein, 1959)
  • representations of whiskers in rats and mice (woolsey and van der loos, 1970)
  • representations of the nose in the star-nose mole (Catania and Kaas, 1995)
    in people, due to practice?
• elbert et al. (1995): larger representations for left-hand digits in string players

Question 8

the basal ganglia

Answer 8

• preparation for movement initiation
• suppression of unwanted movement
• dopaminergic synapses and their receptors are a key part of this region
• major structures:
  
  • striatum (caudate nucleus, putamen)
  • pallidum (Globus pallidus, substantia nigra)
  • thalamus
  • subthalamic nucleus

Question 9

the basal ganglia cont

Answer 9

• the direct pathway in the basal ganglia
• initiation of intended movement
• the regions are linked in a circuit of inhibitory and excitatory impulses
• some regions will be constantly inhibited/activated (tonic)
• others will be inhibited/activated for brief periods of time only (transient)
• correct regulation of this is vital in allowing movement to begin

Question 10

state- tonic= constant activation
transient= brief bursts of activation

Question 11

dopamine: the nigrostriatal pathway

Answer 11

• substantia nigra to striatum
• involved in the basal ganglia loop and the initiation of movement

Question 12

the direct pathway

Answer 12

• nigrostriatal pathway- links substantia nigra and striatum (cuadate/putamen)
• provides additional excitatory impulses
• more likely to inhibit the globus pallidus
• allows the frontal cortex can initiate movement

Question 13

disorders of the basal ganglia: movement initiation

Answer 13

• the direct pathway: involved in the release of the upper motor neurons from inhibition
  -linked to parkinsons disease
  
  • hypokinesis (reduced initiation of movement)

Question 14

dopamine and Parkinson’s disease

Answer 14

• neurodegenerative progressive disorder
• described by Dr james parkinson in 1817
• the most recognised symptoms are those that affect movement:
  
  • tremor
    -slowness of movement
    -rigidity
  • postural instability

Question 15

dopamine and Parkinson’s disease cont.

Answer 15

• the substantia nigra gets its name from the melanin in the cells
• shading observable in healthy individual
• reduced shading in those with parkinson’s
• melanin containing cells are dying off
• causes? unclear- combination of several genetic and environmental factors

Question 16

parkinson’s disease and the direct pathway

Answer 16

• death of the dopaminergic neurons in the nigrostriatal pathway
• reduction of activation of the striatum
• the striatum cannot inhibit the globus pallidus
• the thalamus continues to be inhibited
• movement cannot be easily initiated

Question 17

treatment of parkinson’s disease

Answer 17

• L-Dopa
• artificial precursor to dopamine, which can cross the blood brain barrier
• increases the concentration of dopamine in the synaptic cleft (agonist of dopamine)
• as the cells continue to die, however, this treatment will gradually become less effective

also: side effects due to increased dopamine in other pathways

Question 18

the indirect pathway

Answer 18

• works to stop unwanted movement
• by increasing the ability of the globus pallidus (internal) to inhibit the thalamus
• external segment inhibits the internal segment, preventing it from inhibiting the thalamus
• external segment also inhibits the subthalamic nucleus, which excites the internal segment, further preventing it from inhibiting the thalamus

Question 19

the indirect pathway cont

Answer 19

• cerebral cortex/ striatum inhibit the external segment and excites subthalamic nucleus
• internal segment can inhibit thalamus
• thalamus doesnt signal to cortex- movement is not initiated

Question 20

disorders of the basal ganglia: movement inhibition

Answer 20

• the indirect pathway: involved in increasing the level of inhibition
  
  • linked to huntingtons disease: hyperkinetic (increased unwanted movement)

Question 21

Huntingtons disease

Answer 21

• huntingtons chorea/ Huntintons disease
• described by George Huntington in 1872
• genetic disorder caused by the mutation of the huntington gene on chromosome 4
• symptoms usually begin mid-life (30-50 years),
  
  • changes in mood
  • problems with co-ordination
• decline in both physical and mental capacities
• results in death within 10-20 years

Question 22

Huntingtons disease cont

Answer 22

• profound but selective atrophy of the caudate and putamen
• some associated degeneration of the frontal and temporal cortices
• alteration in mood- usually depression
• change in personality- increase in irritability, suspiciousness, eccentric behaviour
• motor symptoms:
  -rapid, jerky motion with no clear purpose
  
  • can involve a finger or a whole extremity, speech apparatus

Question 23

Huntingtons disease: treatment

Answer 23

• there is currently no cure for Huntingtons disease
• the drug tetrabenazine is used to treat the chorea (the involuntary movements) associated with it
• antagonist of dopamine
  
  • blocks dopamine receptors and depletes other amines
• side effects include Parkinsonism and depression

Question 24

muscles and the lower motor neurons

Answer 24

three types of muscles:
- skeletal/striate muscles: movement of the bones
- smooth muscle: forms organs such as stomach and bladder
- cardiac muscles (a mix of smooth and striated): contracts the heart

Question 25

the lower motor neurons

Answer 25

• muscles are arranged in antagonistic pairs-each making only one movement
• contraction of the skeletal muscles is initiated by the lower motor neurons
• the cell bodies of the LMNs are found in the spinal cord and brainstem
• their axons run down to particular muscles, to form the neuromuscular junction

Question 26

the neuromuscular junction

Answer 26

• motor neuron synapses with skeletal muscle fibres
• ACh acts as an excitatory NT at this synapse
  
  • nicotinic receptors
• each motor neuron innervates several fibres within one muscle
• the motor neuron pool: all the neurons innervating one muscle
• the motor unit: all the fibres in a muscle innervated by one neuron.

Question 27

the lower motor neurons

Answer 27

• the size of the motor unit relates to the type movement that can be produced
• when they innervate many fibres, movements will be coarser eg biceps
• when only a few fibres are innervated by one axon, movements are more precise eg eye movements

Question 28

organisation of the lower motor neurons

Answer 28

• somatotopic organisation of the LMSs:
• in the spinal cord, neurons that act on the muscles of the trunk are located medially
• the neurons for the arms and legs are found more laterally

Question 29

amyotrophic lateral sclerosis (ALS)

Answer 29

• descriptions dating back to 1824
• also known as Motor Neurone Disease or Lou Gehrigs disease
• first link between symptoms and the the neurological disorder by Jean-Martin Charcot (1874)
• causes unknown in 90% of cases
• a degeneration of both upper motor neurons and lower motor neurons

Question 30

amyotrophic lateral sclerosis (ALS) cont

Answer 30

• progressive neurodegenerative disease
• neurons in the brain and spinal cord begin to die
• begins with weakness in the affected regions which gradually worsens over time as muscles waste
  
  • different classifications according to which neurons are affected first
• sensory nerves and cognitive abilities are preserved
• symptoms and rate of progression vary between people

Question 31

augmented/alternative communication

Answer 31

• one of the last muscular groups to be affected are the extraocular muscles (eye movements)
• augmentative/alternative communication (AAC) systems use this to help people with ALS communicate
• various technologies allow eye movements to be used like a computer mouse