Neurology 8 - Motor Pathways

Question 1

Define functional segregation

Answer 1

• The motor system is organised into a number of different areas that control different aspects of movement

Question 2

Define hierarchial organisation

Answer 2

• High order areas in motor control are involved in more complex tasks (deciding movements and coordinating muscle activity)
• Lower level areas perform lower level tasks

Question 3

Describe the motor system hierarchy

Answer 3

• Primary motor cortex and nonprimary motor cortex have output to the basal ganglia, and via the thalamus
• Also input to the cerebellum, which modifies this information to allow you to carry out fine motor activities
• Input is visual, auditory.ect and routed to the primary motor cortex
• Brainstem innervates muscles of the head and neck, and the spinal cord the rest of the body

Question 4

Describe the location and function of the primary motor cortex

Answer 4

• Location: precentral gyrus, anterior to the central sulcus in the frontal lobe
• Function: control fine, discrete, precise voluntary movement
• Provide descending signals to execute movement
• Somatotopic mapping (lower limbs supplied by anterior cerebral artery, it is lower in the motor area)

Question 5

List the descending motor pathways

Answer 5

• Lateral corticospinal tract (changes side in the medulla, controls movement of limbs)
• Anterior corticospinal tract (changes side in the spinal cord, controls movement of axial muscles in the trunk)

Question 6

Describe the corticobulbar pathway to the tongue

Answer 6

• From the head region of the motor cortex
• Via the genu of the internal capsule
• Synapses in the hypoglossal nucleus, continues with the hypoglossal nerve

Question 7

What is the location and function of the premotor cortex?

Answer 7

• Located in the frontal lobe anterior to MI
• Involved in planning movements and regulating externally cued movements
• e.g. Seeing an apple and reaching out for it requires moving a body part relative to another body part (intra-personal space) and movement of the body in the environment (extra-personal space)

Question 8

What is the location and function of the supplementary motor area?

Answer 8

• Located in the frontal lobe anterior to MI, medially
• Involved in planning complex movements and programming sequencing of movements
• Regulates internally driven movements, so becomes more active when thinking about a movement beforehand

Question 9

Which areas are in the association cortex?

Answer 9

• Brain areas not strictly motor areas as their activity does not correlate with motor output/act
• Posterior parietal cortex: ensures movements are targeted accurately to objects in external space
• Prefrontal cortex: involved in selection of appropriate movements for a particular course of action (based on previous experience)

Question 10

Define lower motor neuron

Answer 10

• Motor neurons in the ventral horn of the spinal cord and brainstem (bulbar)
• A nerve cell that connects the CNS to the muscle

Question 11

Define upper motor neuron

Answer 11

• Corticospinal and corticobulbar (betz cells)
• A neuron that starts in the motor cortex of the brain and terminates within the medulla (another part of the brain) or within the spinal cord

Question 12

Define pyramidal

Answer 12

Lateral corticospinal tract (a tract of motor neurones in the pyramid of the medulla)

Question 13

Define extrapyramidal

Answer 13

• Relating to or denoting motor nerves that descend from the cortex to the spine but are not part of the pyramidal system
• Motor nerves in the basal ganglia/ cerebellum

Question 14

List the consequences of an upper motor neuron lesion

Answer 14

Loss of function (negative signs)

• Paresis (graded weakness)
• Paralysis (complete loss of muscle activity)

Increased abnormal motor function due to loss of inhibitory descending inputs (positive signs)

• Spasticity (increased muscle tone)
• Hyper-reflexia
• Clonus (abnormal oscillatory - repeated - muscle contraction)

Babinskis sign

Question 15

What is apraxia?

Answer 15

• Disorder of skilled movement, where patients have lost information about how to perform skilled movements, though they haven’t lost muscle function
• Lesion of inferior parietal lobe/frontal lobe
• Stroke and dementia most common causes

Question 16

List the effects of a lower motor neuron lesion

Answer 16

• Weakness
• Hypotonia (reduced muscle tone)
• Hyporeflexia (reduced reflexes)
• Muscle atrophy
• Fasciculations: damaged motor units produce spontaneous action potentials, resulting in a visible twitch
• Fibrillations: spontaneous twitching of individual muscle fibres; recorded during needle electromyography examination

Question 17

What is motor neuron disease?

Answer 17

• Progressive neurodegenerative disorder of the motor system (takes out both upper and lower motor neurones)
• Spectrum of disorders
• Amyotrophic Lateral Sclerosis (ALS) is the most common form

Question 18

List the upper motor neuron signs in motor neuron disease

Answer 18

• Increased muscle tone (tongue and limbs)
• Brisk limbs and jaw reflexes
• Babinskis sign
• Loss of dexterity
• Dysarthria (problems speaking)
• Dysphagia (problems eating)

Question 19

List the lower motor neuron signs of motor neuron disease

Answer 19

• Weakness
• Muscle wasting
• Tongue fasciculations and wasting
• Nasal speech
• Dysphagia

Question 20

What is included in the basal ganglia?

Answer 20

• Caudate nucleus (most anteriorly)
• Lentiform nucleus (putamen and external globus pallidus)
• Subthalamic nucleus
• Substantia nigra
• Ventral pallidum, claustrum, nucleus accumbens, nucleus basalis of Meynert
• Striatum (caudate, putamen and globus pallidus)

Question 21

List the functions of the basal ganglia

Answer 21

• Elaborating associated movements (e.g. swinging arms when walking; changing facial expression to match emotions)
• Moderating and coordinating movement (suppressing unwanted movements)
• Performing movements in order

Question 22

Describe the basal ganglia circuitry

Answer 22

• Not entirely clear
• Cortex input to the striatum (caudate, external globus pallidus and putamen)
• Links with the globus pallidus, substantia nigra and subthalamic nucleus
• Thalamus and supplementary motor area are also linked

Question 23

Describe the neuropathology of Parkinsons disease

Answer 23

• Neurodegenertation of the dopaminergic neurons that originate in the substantia nigra and project to the striatum
• Damage to nigrostriatal fibres

Question 24

List the main motor signs of Parkinsons

Answer 24

• Bradykinesia (slowness of movements)
• Hypomimic face (expressionless)
• Akinesia (difficulty initiating movements)
• Rigidity (muscle tone increase)
• Tremor at rest starting in one hand and then spreading
• Slight kyphosis
• Difficulty walking, spontaneous increase to a running pace

Question 25

What is huntingtons disease?

Answer 25

• Genetic neurodegenerative disorder
• Autosomal dominant, chromosome 4
• CAG repeat (35 repeats)
• Degeneration of GABAergic neurons in the striatum, caudate and putamen

Question 26

List the motor signs of Huntingtons disease

Answer 26

• Choreic movements (rapid jerky involuntary movements of the body, hands and face first)
• Speech impairment
• Difficulty swallowing
• Unsteady gait
• Cognitive decline and dementia

Question 27

List the divisions of the cerebellum and their functions

Answer 27

• Vestibulocerebellum (gait, posture and equilibrium + head and eye movement coordination)
• Spinocerebellum (speech, limb movements, adjust muscle tone)
• Cerebrocerebellum (skilled movements, attention, processing of language and emotional control)

Question 28

List the layers of the cerebellum and describe its circuitry

Answer 28

• 3 layer structure
• Molecular layer is the outermost layer
• Purkinje cells in the piriform layer
• Granular layer is the inner layer, containing smaller granular neurons
• Input to purkinje cells via climbing fibres projected from the inferior olive
• Input to the granular cells via mossy fibre
• 1 output fibre to the central nucleus from the purkinje cells

Question 29

What is the result of vestibulocellular syndrome?

Answer 29

• Damage causes syndrome similar to vestibular disease

- Gait ataxia and tendency to fall even when sitting

Question 30

What is the result of spinocerebellar syndrome?

Answer 30

• Damage affects mainly legs, causes abnormal gait and stance (wide based)
• Associated with chronic alcoholism

Question 31

What is the result of cerebrocerebellar or lateral cerebellar syndrome?

Answer 31

Affects mainly arms, skilled coordinated movements and speech

Question 32

List the main signs of cerebellar dysfunction

Answer 32

Defecits are apparent only on movement

• Ataxia (wide based staggering gate)
• Dysmetria (loss of ability to gage distance and force)
• Intention tremor (thinking about an action initiates a tremor)
• Dysdiadochokinesia (inability to form rapidly alternating movements)
• Scanning speech (staccato speech delivery)

Question 33

Describe the pathway taken in the lateral corticospinal tract

Answer 33

• Primary motor cortex passes through the internal capsule to the midbrain, through the cerebral peduncle and then the pons.
• In the medulla, comes through the pyramids and decussates through the base of the medulla
• Descends in the spinal cord to project to the ventral root, where it synapses with the alpha motor neuron.

Question 34

Describe the pathway taken by nerves in the anterior corticospinal pathway

Answer 34

• Do not decussate in the medulla

- Goes through the anterior corticospinal tract to the spinal cord, where it synapses at the spinal cord level