Neurology 8 - Motor Pathways Flashcards
Define functional segregation
- The motor system is organised into a number of different areas that control different aspects of movement
Define hierarchial organisation
- 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
Describe the motor system hierarchy
- 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
Describe the location and function of the primary motor cortex
- 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)
List the descending motor pathways
- 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)
Describe the corticobulbar pathway to the tongue
- 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
What is the location and function of the premotor cortex?
- 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)
What is the location and function of the supplementary motor area?
- 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
Which areas are in the association cortex?
- 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)
Define lower motor neuron
- Motor neurons in the ventral horn of the spinal cord and brainstem (bulbar)
- A nerve cell that connects the CNS to the muscle
Define upper motor neuron
- 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
Define pyramidal
Lateral corticospinal tract (a tract of motor neurones in the pyramid of the medulla)
Define extrapyramidal
- 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
List the consequences of an upper motor neuron lesion
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
What is apraxia?
- 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
List the effects of a lower motor neuron lesion
- 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
What is motor neuron disease?
- 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
List the upper motor neuron signs in motor neuron disease
- Increased muscle tone (tongue and limbs)
- Brisk limbs and jaw reflexes
- Babinskis sign
- Loss of dexterity
- Dysarthria (problems speaking)
- Dysphagia (problems eating)
List the lower motor neuron signs of motor neuron disease
- Weakness
- Muscle wasting
- Tongue fasciculations and wasting
- Nasal speech
- Dysphagia
What is included in the basal ganglia?
- 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)
List the functions of the basal ganglia
- 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
Describe the basal ganglia circuitry
- 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
Describe the neuropathology of Parkinsons disease
- Neurodegenertation of the dopaminergic neurons that originate in the substantia nigra and project to the striatum
- Damage to nigrostriatal fibres
List the main motor signs of Parkinsons
- 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
What is huntingtons disease?
- Genetic neurodegenerative disorder
- Autosomal dominant, chromosome 4
- CAG repeat (35 repeats)
- Degeneration of GABAergic neurons in the striatum, caudate and putamen
List the motor signs of Huntingtons disease
- Choreic movements (rapid jerky involuntary movements of the body, hands and face first)
- Speech impairment
- Difficulty swallowing
- Unsteady gait
- Cognitive decline and dementia
List the divisions of the cerebellum and their functions
- 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)
List the layers of the cerebellum and describe its circuitry
- 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
What is the result of vestibulocellular syndrome?
- Damage causes syndrome similar to vestibular disease
- Gait ataxia and tendency to fall even when sitting
What is the result of spinocerebellar syndrome?
- Damage affects mainly legs, causes abnormal gait and stance (wide based)
- Associated with chronic alcoholism
What is the result of cerebrocerebellar or lateral cerebellar syndrome?
Affects mainly arms, skilled coordinated movements and speech
List the main signs of cerebellar dysfunction
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)
Describe the pathway taken in the lateral corticospinal tract
- 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.
Describe the pathway taken by nerves in the anterior corticospinal pathway
- Do not decussate in the medulla
- Goes through the anterior corticospinal tract to the spinal cord, where it synapses at the spinal cord level
