Mednotes

L14 Nervous System Motor Systems 1 _ 2

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tagsnervoussystem, lecture
aliasesL14 Nervous System Motor Systems 1 _ 2

26 min read

Overview

A four-lecture block on the motor system, worked from muscle back to cortex: the lower motor neuron (LMN) and the spinal machinery around it, the upper motor neuron (UMN) comprising brainstem motor systems and motor cortex, then the basal ganglia and the cerebellum. One organising diagram recurs throughout: motor cortex projects through the internal capsule as the corticobulbar tract (to cranial nerve nuclei) and the corticospinal or pyramidal tract (to the spinal motor column); these, with brainstem motor systems, drive spinal interneuron networks and alpha-motor neurons, which activate muscle. Cerebellum and basal ganglia sit off to the side, connected to motor cortex through the thalamus, and do not project to muscle themselves. Sensory feedback returns from muscle to spinal networks, brainstem, cerebellum and motor cortex. The clinical payoff is the LMN versus UMN sign table and the movement disorders produced by basal ganglia and cerebellar damage. Recommended background is HUBS 191 lectures 23 to 26 and the ELM 2 musculoskeletal physiology lectures; the texts listed are Kandel and Schwartz, Guyton and Hall, and Ganong.

Lower motor neuron and the motor unit

  • The lower motor neuron is the alpha-motor neuron: cell body in the spinal cord, axon out through a peripheral nerve to synaptic terminals at neuromuscular junctions.
  • The motor unit is the motor neuron together with the muscle fibres it drives. The diagram shows one motor neuron axon branching so that its terminals form two motor units, each on a distinct set of muscle fibres within the same muscle.
  • A lesion at LMN level blocks activation of the muscle even when upper motor neuron, brainstem, cerebellar and basal ganglia input are all intact; the pathway diagram crosses out the alpha-motor neuron box and bars the connection to muscle. Sensory feedback is unaffected.

Muscle spindle and the tendon (stretch) reflex

  • The muscle spindle reports muscle length (static response) and rate of change of muscle length (dynamic response). Muscle length gives joint angle, which gives position of the limb in space, that is, proprioception.
  • Structure: an intrafusal fibre inside a capsule with a Ia afferent ending wrapped around it. The sensory ending carries stretch-sensitive ion channels on cytoskeletal strands spanning the membrane.
  • Reflex circuit: striking the tendon stretches the muscle, the Ia afferent enters the cord (also ascending in the dorsal column pathway) and makes an excitatory synapse onto the alpha-motor neuron, whose axon returns to the muscle. Descending motor commands are crossed out on the diagram, that is, they are not required for the basic monosynaptic reflex. Gamma-motor neurons set spindle sensitivity.
  • Normal purpose: to defend muscle length against rapid, brief stretch.
  • Clinically: reduced with a peripheral nerve lesion (sensory axon or LMN), increased with CNS (UMN) lesions.

Muscle tone

  • Tone is the resistance of muscle to stretch, tested with the person relaxed so that voluntary contraction is avoided.
  • Resting tone has three sources: muscle structure and tissue properties; tonic activity in the stretch reflex arc (tonic, not phasic activation of the reflex); incomplete relaxation.
  • Other causes of hypotonia besides LMN lesions, given beside the sign table: cerebellar disease and muscle disease.

Muscle atrophy

  • Mechanism: reduced activation of muscle leads to reduced contractile proteins and so to reduced muscle bulk. The diagram shows proteolysis increased and protein synthesis decreased, shrinking the free subunit pool.
  • Two types are named on the slide: denervation atrophy, defined as loss of LMN input, and disuse atrophy, which is named but not defined.
  • Illustrated by long-standing poliomyelitis, with visible wasting of one leg compared with the other.

LMN lesions

  • Example disorders: trauma; disease, specifically motor neuron disease (amyotrophic lateral sclerosis), polio and Guillain-Barre syndrome.
  • Signs, built up across the lecture: power reduced, tendon reflexes reduced, muscle tone reduced, muscle atrophy yes (denervation), plantar response normal (flexor, or none).

Two transcript flags on the LMN lesion slides

  • Slide 8 carries a small rotated news clipping overlapping the slide, only partly legible. It appears to report that one to three people affected by the Havelock North campylobacter outbreak could develop Guillain-Barre syndrome, quoting Professor Michael Baker of Otago public health that the risk of progression from campylobacter infection to GBS is about one in 1700 in New Zealand, and describing GBS as the immune system attacking nerves that control movement and feeling, developing one to two weeks after infection. Several phrases could not be confirmed because of the rotation and small size.
  • Slide 42 prints “Motor neuron disease*” with an asterisk and no visible footnote anywhere on that slide.

Spinal cord as a mini-brain

Spinal circuits between the upper and lower motor neuron perform complex functions in their own right:

  1. Withdrawal reflex. A nociceptive skin stimulus drives, through inhibitory and excitatory spinal interneurons, ipsilateral flexors active with extensors relaxed and contralateral extensors active with flexors relaxed (the crossed-extensor reflex). A UMN lesion removes the regulation of this reflex, giving flexor spasms.
  2. Rhythmic motor patterns such as locomotion. A UMN lesion in humans loses spontaneous rhythm generation.
  3. Processing of descending UMN commands for voluntary movement.

Corticospinal and corticobulbar tracts

  • Corticobulbar fibres leave for cranial nerve and brainstem motor nuclei; corticospinal tract fibres continue down the spinal cord. Both pass through the internal capsule, where an inset shows arm representation zones (M1, M2, M3, M4 and LPMCd arm).
  • The lateral corticospinal tract supplies distal and proximal muscle motor neuron pools.
  • 85% of fibres cross (decussate) at the medullary pyramids, which is why it is called the pyramidal tract.
  • 15% remain uncrossed as the medial or ventral corticospinal tract, mainly to proximal muscles.
  • Because proximal muscles (including forehead) receive both crossed and uncrossed input, more function is preserved in them after unilateral damage.

Corticomotoneuronal component:

  • Most lateral CST axons synapse on spinal interneurons, which in turn synapse on alpha-motor neurons.
  • A small number synapse directly on alpha-motor neurons, a monosynaptic input; these are the corticomotoneuronal axons.
  • They are particularly prominent in primates and contribute to the high level of fine distal control of the digits. The diagram shows them acting on extensor pools (ECR, ECU, EDC, ED4,5, ED2,3) and flexor pools (FCU, PL, FCR, FDS), with inhibitory interneurons to antagonist pools.

Brainstem motor systems

StructureTractRole
Red nucleusRubrospinalInter-joint coordination; locomotion (restless legs syndrome)
Superior colliculus (tectum)TectospinalReceives visual pathway input; directs rapid orientation towards moving objects
Reticular formation, pontine partReticulospinalEnhances antigravity reflexes (lower limb extensors, upper limb flexors)
Reticular formation, medullary partReticulospinalSuppresses reflexes to allow voluntary movement
Vestibular nucleiVestibulospinalIn from vestibular apparatus, neck proprioceptors and cerebellum; out to various postural reflexes and sense of head position

The tectospinal, reticulospinal and vestibulospinal tracts converge as the medial motor pathway onto the medial motor columns of the ventral horn, which serve proximal muscles.

Primary motor cortex

Four defining criteria:

  1. Cytoarchitectonics. Agranular cortex (area 4) as against granular cortex (area 17), and the presence of gigantopyramidal (Betz) cells. Area 4 lies in the precentral gyrus, immediately anterior to the central sulcus (Rolandic fissure).
  2. High density of corticospinal cell bodies.
  3. Low threshold for electrically evoked movement.
  4. Somatotopic body representation, running approximately leg to arm to hand to face. Muscle representations are intermingled and multiple, cortical area is proportional to amount and type of usage, training expands a representation, and reassignment after stroke is raised as a question. The homunculus runs medial to lateral: hip, trunk, shoulder, elbow, wrist, fingers and hand, neck, brow, eyelid and eyeball, face, lips, jaw, tongue, and swallowing, mastication, salivation, vocalisation.

Functions, framed as voluntary motor commands:

  • Regulates muscle force: higher cortical neuron firing frequency produces greater spinal alpha-motor neuron activity.
  • Controls movement direction: each neuron is most active for a certain preferred direction, and the net movement is determined by summed activity across all neurons.

Secondary motor cortex

  • Premotor cortex (PMC) is lateral area 6; supplementary motor area (SMA) is medial area 6.
  • Both contribute to the corticospinal tract.
  • Compared with primary motor cortex they have a higher threshold for electrical stimulation and less organised somatotopy.
  • Higher order functions: SMA for initiation of movement, suppression of unwanted movement and bimanual coordination; PMC for visual-motor coordination, hand-mouth coordination and mirror neurons.

UMN lesions

  • Example disorders: stroke (CVA), trauma, multiple sclerosis. The lesion diagram marks possible sites at cortex, internal capsule, brainstem and spinal cord.
  • Clinical demonstrations shown: reduced power and distal dexterity on a fine hand and finger task; an enhanced (brisk) knee-jerk reflex; the Babinski sign on plantar testing.
  • The complex of signs is called spastic hypertonia, annotated with resistance proportional to velocity, and explicitly contrasted with Parkinsonian rigidity.

LMN versus UMN signs

SignLMN lesionUMN lesion
Power↓↓
Tendon reflexes↓↑
Muscle tone↓↑
Muscle atrophyYes (denervation)Mild (disuse)
Plantar responseNormal (flexor, or none)Extensor (Babinski)

Annotations printed beside the table: for the LMN column, other causes of hypotonia are cerebellar disease and muscle disease; for the UMN column, spastic catch, clasp-knife phenomenon and clonus.

Three uncaptioned photographs (slides 36 to 38)

Placed between the completed UMN sign table and the slide on UMN variations, these carry no caption or label at all: an elderly person reclining in an armchair, appearing frail and unresponsive; two hands resting on a green surface with the nearer fingers held in a flexed, claw-like posture; a close-up of an older adult’s face with a strained, open-mouthed expression. The clinical point each is meant to make is not stated on the slides and cannot be determined from the images.

Variations in the UMN picture, and localisation

The UMN clinical picture depends on two things.

Time since injury:

  • Immediately, flaccid (hypotonic) paralysis.
  • Spasticity develops over hours to weeks.

Injury location:

  • Cortical or subcortical: as in the table, plus a tendency to flexed posture in the upper limb and extended posture in the lower limb.
  • Brainstem: as in the table, plus various extreme postures depending on the level.
  • Spinal cord (paraplegia, quadriplegia): muscle groups innervated from below the lesion show UMN signs plus increased withdrawal reflex (flexor spasms) and increased autonomic reflexes; muscles innervated at the lesion level, if it lies in the cervical or lumbar enlargements, show LMN signs.

Localising the lesion by what accompanies the UMN signs:

  • Cortical lesion: UMN signs plus language, perceptual or frontal abnormalities.
  • Brainstem lesion: UMN signs plus cranial nerve deficits.
  • Spinal cord lesion: UMN signs plus sensory pathway involvement and possible local nerve root (LMN) signs.

Basal ganglia: components and circuitry

Components, on a coronal section:

  • Striatum = caudate + putamen, containing medium spiny neurons.
  • Globus pallidus, external (GPe) and internal (GPi).
  • Substantia nigra, pars compacta and pars reticulata.
  • Subthalamic nucleus. Thalamus and internal capsule are labelled for reference.

Circuit, with red arrows inhibitory and green excitatory:

  • Cerebral cortex excites caudate and putamen, which express D1 and D2 dopamine receptors.
  • Direct pathway: striatum (D1) inhibits internal globus pallidus, which disinhibits thalamus, which excites cortex.
  • Indirect pathway: striatum (D2) inhibits external globus pallidus, which inhibits subthalamic nucleus, which excites internal globus pallidus, which inhibits thalamus.
  • Substantia nigra pars compacta supplies dopaminergic input to the striatum.

The same two routes are restated as the extrapyramidal GO and NO-GO pathways:

  • GO pathway: when activated it increases excitation of motor cortex neurons. Dopaminergic input to the neostriatum (DA+) inhibits globus pallidus internus, raising activity at intralaminar thalamus (VA, VL), cortex and LMNs.
  • NO-GO pathway: when activated it decreases excitation of motor cortex neurons. Routed via GPe and the subthalamic nucleus to GPi, with reduced dopamine (DA-) at the neostriatum, producing reduced activity at thalamus and cortex.

Basal ganglia: functions

  • Motor: helps motor cortex select the appropriate motor cortex neurons for voluntary movement, covering initiation, scaling (amplitude and speed) and sequencing. Abnormal functioning gives Parkinson’s disease and Huntington’s disease.
  • Cognitive and psychological: helps prefrontal cortex organise thoughts, helps the limbic system organise emotions, and supports reward-mediated learning and attention. Abnormal functioning appears in drug addiction, ADHD, psychosis, Parkinson’s disease and Huntington’s disease.
  • Dopamine in the basal ganglia serves learning and attention by reinforcing preceding actions that produced reward, which links it to drug addiction and ADHD.
  • The functional loop drawn is dopamine into basal ganglia, basal ganglia to and from thalamus, thalamus to and from cortex, cortex back to basal ganglia.

Parkinson’s disease

  • The slide lists neurological disorders of the elderly as 1. stroke, 2. Alzheimer’s, 3. PD, with Parkinson’s disease third on that list. No ranking criterion is stated on the slide. Onset is approximately 45 to 70.
  • Signs:
    • Resting tremor, pill-rolling, 4-6 Hz, described as unique to PD.
    • Rigidity, lead-pipe, or cogwheel when combined with tremor; not velocity dependent.
    • Akinesia and bradykinesia, that is lack of movement and slow movement, with mask-like face, quiet speech and micrographia.
    • Posture and gait abnormalities: forward leaning posture, postural instability, retropulsion, shuffling gait, falls.
    • Dementia, depression, autonomic dysfunction.
  • Mechanism of the movement disorder: generating movement requires progressive selection of a subset of motor cortex neurons, the basal ganglia contribute to that selection, and striatal dopamine is needed to allow some pathways through the basal ganglia to be selected above others. Without dopamine all pathways are equally active, giving co-activation of agonists and antagonists (rigidity) and difficulty getting the main agonists active enough to trigger movement (akinesia, bradykinesia). Tremor is not always present and its mechanism is described as mysterious.
  • Pathology: death of dopamine neurons, mechanism uncertain, with oxidative stress raised as a possibility. Histology and gross brainstem sections show depigmentation of the substantia nigra pars compacta.
  • Causes: idiopathic in over 80% of cases, with environmental factors and polygenic predisposition; single-gene genetic forms; specific toxin; trauma.
  • Treatment:
    • Pharmacological: replace dopamine with L-DOPA, taken up by surviving presynaptic terminals and converted to dopamine for packaging and release; or direct dopamine receptor agonists acting on postsynaptic D1/D2 receptors, bypassing the need for synthesis.
    • Surgical: deep brain stimulation of basal ganglia nuclei, shown on MRI as electrode tracks with a chest-wall battery pack.
    • Experimental: dopamine cell replacement by stem-cell transplant, with [18F]-DOPA PET comparing asymmetric striatal uptake and a graft site in PD against symmetric uptake in a normal subject.

Gait change can be the presenting sign noticed by a bystander. The slides carry a news item in which Billy Connolly, aged 71, was told by a Tasmanian doctor who saw him walking through a hotel lobby that his gait suggested early Parkinson's disease; blood tests followed and he was diagnosed.

Patient resources listed: the video “Paula”, a parkinsons.org.uk article on wearing-off and dyskinesia, and the Cure Parkinson’s NZ film “One in 37” at onein37.org.nz.

Huntington’s disease

  • Pathology: progressive death of medium spiny neurons in caudate and putamen. Coronal sections show marked enlargement of the lateral ventricles and loss of caudate bulk.
  • Genetics: inherited, autosomal dominant, caused by too many repeated CAG triplets in the Huntingtin gene. Rare in the population as a whole but common in affected families, a founder effect.
  • Consequence: unbalanced activity of surviving neurons.
    • Motor cortex deficit: chorea, that is excessive, spontaneous, irregular, random, brief, abrupt, non-repetitive movements, with eventual loss of voluntary movement.
    • Other cortex: dementia, agitation, mania, irritability, paranoia.
  • Treatment: none currently; possible future approaches are stem cells and gene therapy.

Cerebellum: anatomy and subregions

Gross anatomy on sagittal section: anterior lobe, posterior lobe and the nodulus, with the flocculus lying more laterally, sitting behind midbrain, pons and medulla.

SubregionNormal functionSigns when damaged
SpinocerebellumPosture, balance, locomotionTrunk ataxia, broad-based gait
CerebrocerebellumPlanning and feedback control of limbsHypotonia, intention tremor, limb ataxia
VestibulocerebellumPosture, eye movementFalls, vertigo, disordered eye movement

The cross-section also labels the vermis, the cerebellar hemisphere, and the deep nuclei, named as fastigial, interposed and dentate. Connections are drawn from the deep nuclei to the spinal cord, cerebral cortex and vestibular nuclei; the slide lists the three nuclei and the three targets but does not show which nucleus connects to which target.

Cerebellum: cortical microcircuitry

  • Mossy fibre input carries sensory information (vision, proprioception) to granule cells.
  • Granule cell parallel fibres make excitatory synapses onto Purkinje cells.
  • Climbing fibre input, from cerebral cortex via the inferior olive, also synapses onto Purkinje cells.
  • Purkinje cells inhibit the deep cerebellar nucleus.
  • The deep cerebellar nucleus is the cerebellar output, to thalamus, brainstem and spinal cord.
  • An inset shows error-signal and LTD circuitry involving the climbing fibre, inferior olivary nucleus, red nucleus, and cerebellar and vestibular nuclei.

Cerebellum: function

The structure suggests a calculation machine:

  • Simple connections: two input channels, one output.
  • Parallel architecture.
  • Huge combinatorial power, 5x10^10 neurons, equal to all the rest of the brain.

What it is calculating is left open: timing, coordinate system transformations, and probably prediction on the basis of motor command output plus sensory feedback plus prior learning. A coordinate-transformation model is shown running from vestibular sensory frame and neck-muscle motor frame through covariant embedding (motor intention) and a covariant-contravariant transformation to sensorimotor transformation and motor execution. This slide is immediately repeated with no additional or different content (transcript flag on slide 65).

Role in motor control: process sensory information for use by motor systems, and use it to

  1. help plan movement of eyes, trunk and limbs, since nervous system activity is always behind reality and prediction is therefore vital;
  2. make online corrections;
  3. support motor learning;
  4. possibly contribute to cognitive functions.

Cerebellum: clinical signs

  • Disorders: stroke, infection, alcoholic cerebellar degeneration, tumours, genetic causes.
  • Signs:
    • Ataxia: incoordination between body segments, jerky movements, with dysdiadochokinesia (reduced rapid alternating movements).
    • Intention tremor: tremor during movement.
    • Dysmetria: inaccuracy in distance.
    • Nystagmus: involuntary rhythmic eye movements.
    • Titubation: tremor of the trunk.
  • Treatment: in acquired forms, with many possible causes, it may be possible to treat the cause, for example a vitamin deficiency; hereditary forms get symptomatic treatment and physiotherapy.

Self-test

  1. Define the lower motor neuron and describe what a motor unit consists of.
  2. List the example disorders given for LMN lesions.
  3. State what the muscle spindle reports and explain why that information matters for proprioception.
  4. Describe the circuit of the tendon (stretch) reflex, and state how it changes with a peripheral nerve lesion and with a CNS lesion.
  5. Define muscle tone and list the three contributors to resting tone.
  6. Besides LMN lesions, what other causes of hypotonia are given?
  7. Describe the general mechanism of muscle atrophy, and state how the slides define denervation atrophy.
  8. List three complex functions of spinal circuits and state what a UMN lesion does to each.
  9. Describe the course of the corticospinal and corticobulbar tracts, including the proportions that cross and the clinical consequence of that arrangement.
  10. Define corticomotoneuronal axons and explain their functional significance.
  11. List the brainstem motor structures, name each one’s tract and summarise its role.
  12. Give the four criteria by which primary motor cortex is defined.
  13. Explain how primary motor cortex regulates muscle force and how it controls movement direction.
  14. Distinguish premotor cortex from supplementary motor area by location and by function.
  15. List the example disorders given for UMN lesions and the anatomical levels at which such a lesion can sit.
  16. Explain how the UMN clinical picture varies with time since injury and with lesion location.
  17. A patient has UMN signs. What additional findings would point to a cortical, a brainstem and a spinal cord lesion respectively?
  18. Distinguish LMN from UMN lesions across the five signs in the table.
  19. Describe spastic hypertonia and distinguish it from Parkinsonian rigidity.
  20. Describe the direct and indirect basal ganglia pathways, and say what each does to excitation of motor cortex.
  21. List the motor and the cognitive functions attributed to the basal ganglia.
  22. List the signs of Parkinson’s disease, with the specific features given for tremor and rigidity.
  23. Explain why loss of striatal dopamine produces rigidity and akinesia.
  24. Summarise the pathology, causes and treatment options for Parkinson’s disease.
  25. Summarise the genetics, pathology, signs and treatment of Huntington’s disease.
  26. Name the three cerebellar subregions, with the normal function and the clinical picture of each.
  27. Describe cerebellar cortical microcircuitry from its two inputs through to its output.
  28. List the signs of cerebellar damage, with the causes and treatment approaches given.
  29. Describe what the cerebellum’s structure suggests about its function, and list the ways it uses sensory information in motor control.
  30. A patient has weakness of one hand with brisk reflexes, increased tone and an extensor plantar response. A second has weakness with reduced reflexes, low tone and obvious wasting. Identify the lesion level in each and explain, from the circuits covered, why the reflex, tone and atrophy findings differ.

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