Topic: The importance of Glial Cells in Normal Brain Function

Recall

Notes

Neurons

• The brain mediates its function by transmission of electrical signals

• Pre-synaptic terminals

  • Electrical signal leads to calcium influx through voltage-gated calcium channels → triggers release of neurotransmitter into synaptic cleft

• At the post-synaptic terminal a chemical signal is received and converted into an electrical signal

  • Summative input from all dendrites causes action potential initiation at the axon hillock if the threshold potential for sodium channel opening is achieved.

  • Action potentials propagate along the axon as an electrical signal. At the presynaptic terminal the electrical signal is converted into a chemical signal and neurotransmitter is released

• Passive and active propagation of membrane potential

Passive:

• Inject some positive charge

  • Electrotonic spread in both directions

• Equilibrium potential is now out of whack → this effects the leak channels

• The further away you get from the current injection, the less positive the charge

  • The speed of this depends on the number of channels available that allow the positive charge to leak out

• How is voltage across membrane affected after injection?

  1. Internal Resistance

  2. Membrane Resistance

  • Thus, you never reach the end of the axon

<aside> 💡 Passive Signal Propagation - Electrotonic spread of depolarisation

• Changes in membrane potential take time

• Voltage dissipates over distance → membranes are like ‘leaky pipes’

• Current leakage depends on membrane resistance and internal resistance

</aside>

Active:

• Stimulating above threshold for sodium gated channels, where signal is renewed

  • Conduction all along bare axon ; repeated regeneration of depolarisation through voltage-gated channels

<aside> 💡 Active Signal Propagation - Repeated regeneration of depolarisation

• Voltage gated ion channels required for signal to cover distance

</aside>

Back to Passive Propagation:

• These constants describe how distance and time affect membrane potential

Active propagation of membrane potential & conduction velocity

• Conduction velocity = Speed of impulse propagation

  • Depends on diameter & internal resistance

  • Depends on membrane resistance and capacitance

  • is directly proportionate to the length constant (larger $\lambda$ → Current spreads further)

  • Is inversely proportionate to the time constant (smaller time → charge spreads faster)

• How to increase conduction velocity

  • Increase axon diameter (d) → Larger axon → Faster CV

    • Axon diameter increase = internal resistance decrease

  • Increase insulation → add thicker myelin = faster CV

    • Myelin thickness increase = membrane resistance increase = capacitance decrease

<aside> 💡 Axon diameter: 0.1 - 20 um in humans, up to 1mm (squid giant axon)

Myelin (insulation) 0 to >100 myelin sheath are found wrapped around human axons

</aside>

Oligodendrocytes

• Myelin = lipid rich insulation of axons

• Myelin

  • Is produced by oligodendrocytes (CNS) or Schwann cells (PNS)

    • Oligodendrocytes myelinate multiple axons at the same time

    • Schwann cells only myelinate one axon at a time

  • Consists of condensed phospholipid bilayers helically wrapped around axons (lipid content 70-85% dry weight)

  • Has a distinct phospholipid composition on cytoplasm and extracellular space facing side

  • Cytoplasm is extruded, and extracellular layers are crosslinked by myelin lipid proteins during condensation

• Note: the g-ratio describes the thickness of the myelin sheath relative to the axon size with lower g-ratios associated with faster conduction velocity

g - ratio=\frac{r}{R}

• r = axon radius

• R = Myelinated axon radius

• Saltatory impulse propagation speeds up active signal propagation

  • Conduction along unmyelinated axons is slow 0.5-2 m/s (c-fibres)

  • Saltatory conduction along myelinated axons is fast < 150 m/s (motor neurons)

  • = Saltatory conduction → signal jumps from node to node

  

  • In myelinated axons, myelinated regions are interspaced with Nodes of Ranvier where voltage gated sodium channels are clustered and the APs renewed

  • Regions of slow and fast conduction are alternated → saltatory (jumping conduction)

• Myelin alters neuronal conduction properties (velocity and shape of AP) and provides metabolic support

  • Electrical insulation → enables saltatory impulse propagation (decrease capacitance and increase membrane resistance)

  • Potassium buffering → promotes sufficiently rapid recovery from repetitive firing → seizure prevention

  • Trophic support → provides energy requires to sustain repetitive action potential firing

• Nodes of Ranvier

  • Complex protein/membrane structures formed by neurons and oligodendrocytes/Schwann cells together

  • Node of Ranvier = Clustered voltage-gated sodium channels

  • Paranode: Attachment of myelin membrane via Caspr, Contactin & NF155

  • Juxtaparanode: Clustered voltage-gated potassium channels, Na+/K+ ATPase

  • Internode: Compact Myelin, Na+/K+ ATPase

• Long axons need trophic support to function properly

  • Axonal transport is slow and axons need a lot of energy

  • Oligodendrocytes provide trophic support to meet the energy need of active neurons

  1. Oligodendrocytes transfer glycolysis products pyruvate and lactate to axons through MCT transporters which is metabolised in neuronal mitochondria (axon) to generate energy required for impulse propagation (Krebs Cycle and Oxidative Phosphorylation)

  2. Exosomes transfer proteins and RNAs between oligodendrocytes and neurons = communication

  • Trophic Support

• Myelin secures functional connectivity

  • Myelin largely develops postnatally until ~20 years of age, but changes continuously throughout life

  • Myelin is essential for coordinates connectivity of brain regions and between brain and peripheral organs

  • MRI imaging has linked higher IQs to extensive myelination → may preserve circuit activity required to form memories

  • Sensory and social deprivation reduce myelination in the associated brain region (except optic nerve → photons reduce firing)

  • Adaptive myelination is critical for learning because it accurately times signal arrival from distant neuronal sources

• The brains ability to adapt is key to learning and memory

  • Oligodendrocytes are involved in learning

    • To learn new skills you develop new circuit activity, strengthen connections and synchronise timing of electric signal propagation between distant brain regions and/or peripheral organs to improve the accuracy needed to master the new skill

    • Inhibition of oligodendrocytes differentiation in adults interferes with new skill acquisition and impairs learning and memory

  • A pool of committed glial precursor cells is retained throughout life

  • Neuronal activity recruits oligodendrocytes precursor cells → new myelin

  • Prolonged decrease in axonal firing leads to decreased myelination of an axon

  <aside> 💡 Myrf = transcription factor driving oligodendrocyte maturation and myelination

  </aside>

• How activity dependent myelination can contribute to learning and memory

  • Oligodendrocytes influence neuronal signal propagation

• What fires together wires together

  • Oligodendrocytes simultaneously transmit and receive input from multiple axons and regulate synchrony of neighbouring axon

Oligodendrocytes and Schwann Cells

Astrocytes

• Morphologically diverse controllers of CNS microenvironment

  • Radial glia

    • During embryonic development

  • Based on the distribution in gray and white brain matter

    • Protoplasmic astrocytes

    • Fibrous astrocytes

  • In different regions of the CNS

    • Bergmann glia (cerebellum)

    • Muller glia (retina)

    • Tanycytes (hypothalamus)

    • Pituicytes (Neurohypophisis)

    • Velate astrocytes (cerebellum)

  • “Epithelium-like” covering astrocytes

    • Ependymocytes

    • Choroid plexus cells

    • Retinal pigment epithelial cells

    • Surface-associated astrocytes

  • In human cortex

    • Interlaminar astrocytes

    • Varicose projection astrocytes

  • Based on their anatomical localisation close to blood vessels

    • Perivascular astrocytes (in parenchyma)

    • Marginal astrocytes (at the interface with the meninges)

<aside> 💡 One astrocyte occupies one region alone.

• each astrocyte has local control of homeostasis within the defined area its processes reach.

• Extensive interaction and communication through gap junctions with other astrocytes along regional boundaries

ASTROCYTES ARE EXCLUSIVE TO THE CNS

</aside>

• Astrocytes → architects and master regulators of brain homeostasis

  • Astrocytes are the most abundant cell type in the brain, they outnumber neurons 5:1

  • A single astrocyte can make up to 2 million connections with all other cells in their area (100 x a neuron)

  • Star shaped glial cells: protoplasmic, interlaminar, vericose astrocytes in grey matter, fibrous astrocytes in white matter

  • Astrocytes regulate neuro-vascular junction and particularly ion, fluid, pH and energy homeostasis (pan-glial syncytium)

  • Contribute to neuronal synapse function (tripartite synapse), oligodendrocyte control of conduction velocity

  • Function in injury response (reactive astrocytes, glial scar) and development (radial glia - neuron placement and removal)

• Astrocytes contribute to learning, memory and higher cognition

  • They communicate through Ca2+ waves triggered by neurotransmitters, gliotransmitters or insult

  • Astrocytic Ca2+ waves are graded local broadcast signals NOT all or nothing responses like neurons

  • NB: Astrocyte number, size and connections is proportional to brain size and cognitive capabilities (compared to rodents, human astrocytes are ~3-fold larger and make > 10-fold more connections)

  • Mouse experiment

    • human embryonic astrocytes implanted into the brain of a mouse

      • Better memory, navigation and object recognition

      • Improves LTP

  

• Astrocytes can regulate synapse formation, function and decay

  

  • The term tripartite synapse recognises the physical proximity and integration of astrocytes in synapse formation and function

• Astrocytes regulate nutrient supply and osmotic homeostasis at the neuro-vascular junction

  • Local regulation of blood flow (capillaries)

  • Control over nutrient uptake and waste disposal

  • Regulation of ion, pH and water homeostasis

  • Contribute to blood brain barrier integrity by affecting endothelial cell tight junctions

  • Astrocyte Ca2+ levels control the release of:

    • Vasodilators (PLE2) and vasoconstrictors (20-HETE)

    • Which act on contractile pericytes lining the capillaries mediating:

      • Vasodilation → vessel radius increase, resistance decrease, perfusion increase, nutrient and O2 increase

      • Vasoconstriction → vessel radius decreas, resistance increase, perfusion decrease, nutrient and O2 decrease

    

• Astrocytes regulate nutrient supply and osmotic homeostasis

  • Astrocytes and oligodendrocytes can form a pan-glial syncytium (a connected network)

• At Nodes of Ranvier, projecting fibrous astrocytes support oligodendrocytes in potassium buffering and influence myelin integrity

• Astrocytes - execute the injury response

  • Following insults astrocytes undergo substantial Ca2+ induced remodeling to become activated astrocytes and express high levels of glial acidic fibrillary protein (GFAP)

  • Astrocyte activation is graded to match local insult severity

  • Astrocyte activation is induced by numerous factors including cytokines, hypoxia, ROS, excess NTs, toxins and observed in most neurological diseases

  • When the insult is severe activated astrocytes proliferate and shield off the area by forming a glial scar

  • The glial scar reduces impact on neighbouring brain areas, but negatively affects reinnervation and recovery

  • Astrocyte dysfunction is implicated in pathologies ranging from schizophrenia and autism to epilepsy and stroke. Astrocytes are increasingly recognised as targets for modern neurotherapeutics

Microglia

• The resident immune cell in the CNS

• Microglia are small and few (>10% of all CNS cells) but immensely powerful

• Microglia are of hematopoietic origin (blood derived) and infiltrate the brain from the yolk sac during development

• Microglia self-renew as an independent population throughout life and expand rapidly following insult/activation

• Highly motile cells

  • Constant surveillance of the environment

  • Phagocytosis of damaged cells (eat up damage)

  • Synapse formation and pruning

  • Active synapse shielding

• Microglia can actively remove synapses or even tag entire neurons or glial cells for cell death through the complement system (C1q, CR3)

• Note: Microglia populate the brain even before astrocytes or oligodendrocytes develop and actively contribute to early neuron remodeling and myelin development

• Microglia modulate general anesthesia dose, duration, analgesia and hypothermia

• Microglia - health maintenance and disease control

  

• Microglia recruit astrocytes to coordinate a glial response to stress

  • During activation, microglia change from ramified to amoeboid morphology due to its active phagocytosis, but this does not predict whether their cytokine response is pro-inflammatory or anti-inflammatory which is amplified by astrocytes

    <aside> 💡 Microglial response is GRADED, not all or nothing

    </aside>

• Microglia activation reduces BBB integrity

  • Pro-inflammatory microglia → reduce astrocyte support of BBB integrity → leukocyte infiltration (macrophages, T-cells)

  • Anti-inflammatory microglia → promotes astrocyte support of BBB integrity and release protective neurotrophin

    • As more microglial functions are revealed, they are recognised as targets for modern neurotherapies

<aside> 📌 SUMMARY:

</aside>

Date: July 31, 2024

Topic: Emerging Neurotherapeutics

Recall

Notes

Gene Therapy and Genetic Disease

• What is a gene?

  • A gene is part of a chromosomal DNA that encodes a specific protein

    • This general definition is no longer sufficient as non-coding regions (RNAs) have very important functions

  

• Cells and Genome

  • The human body contains about 100 trillion cells → Each cell contains 3 billion base pairs on 3 metres of DNA → Each human cell contains ~ 25.000 protein coding genes → Hundreds of cell types (morphological and functional diversity) → Each cell type expresses a characteristic subset of genes

  • = REGULATION OF GENE EXPRESSION

  • The developmental complexity does not scale with the number of protein coding genes but the sophistication of regulation

    

• Brief History of Gene Therapy

  

• A huge success story - Leber’s congenital Amaurosis

  • Leber’s congenital amaurosis

    • Is caused by a loss of function mutation in the retintal pigment epithelium 65 (RPE65) gene

    • Is an early onset severe retinal dystrophy and responsible for 10%-20% of all childhood blindness

• Luxturna

  • is a AAV2 mediated RPE65 gene replacement therapy to restore vision in children with Leber’s congenital amaurosis

  • Was the first FDA approved in vivo gene therapy

• When should one consider gene therapy? All things to consider:

  

  • Gene therapies are rapidly evolving neurotherapeutics, but inherent risks demand to restrict use for devastating or terminal diseases, after individual risk / benefit evaluation and when no other treatment is available

• Gene therapies in clinical trials

  • Monogenic diseases

    • Caused by a single, defined gene defect

    • Largely environment and lifestyle independent

    • 100% heritable

      • E.g. Huntington’s, Leukodystrophies, SMA

  • Polygenic diseases

    • Multiple genetic alterations combined cause disease

    • Environmental and lifestyle triggers disease

    • Less than 100% heritable disease

    • E.g. Gliomas, MS, Parkinson’s

• Therapeutic Gene Delivery Approaches

  • in-vivo gene therapy

    • Delivers the therapeutic DNA or gene therapy vector directly into the patient

  • ex-vivo - cell-based gene delivery

    • Extract patient’s stem/progenitor cells

      • Add gene therapy vector to stem/progenitor cells in a dish (ex vivo)

    • Modify genome with therapeutic DNA (replacement, regulation, gene editing)

    • Expand and test modified cells in a dish

    • Return modified cells to the patient

Gene Therapy Vectors

• Viral vectors matching gene therapy approach and disease

  • ex vivo → integrating viral vector

    • Host chromosome integration - passed on with cell division

  • in vivo → episomal viral vector

    • Do not integrate into host genome - lost in divisions

    • Episomal is much safer, but used to target terminally differentiated cells that do not divide any longer

  

• Vectors for therapeutic gene delivery

  • Non-viral gene delivery

    • ‘Naked’ DNA/RNA (vector free)

      • Pressure (Gene gun)

      • Ultrasound (sonoporation)

      • Electric (electrotransfer)

    • Packaged DNA/RNA (in a non-viral vector)

      • Lipid nano-particles

      • Cell-penetrating peptides

      • Cationic polymers & Liposomes

  • Viral vectors

    • Integrating viral vectors

      • Lentivirus and retrovirus

    • Episomal viral vectors

      • Adenovirus

      • Herpes-simplex-virus

      • Adeno-associated-virus

<aside> 💡 The properties of the gene therapy vector should match the pathophysiological requirements dictated by the disease.

</aside>

In vivo non-viral gene therapy

• Matching gene therapy approach and disease

  • Example of a targeted, in vivo, vector free gene therapy for profound hearing loss

    • Close-field gene electrotransfer with a cochlea implant:

      • Delivers neurotrophin DNA/RNA to cells at the electrodes to stimulate neurite outgrowth to the electrodes

      • Closes the neural gap between the cochlear implant and auditory neurons to improve cochlear implant performance and thus hearing

      • Safe and efficient use of naked DNA or mRNA (reduced packaging constraints and is regulatory permissive)

• Viral vectors in gene therapy are replication incompetent

  • Viruses evolved towards efficient gene delivery

    • Several different virus vectors have been trialed for therapeutic gene delivery to the CNS

      • Lentivirus, retrovirus, Alphavirus

      • Adenovirus, Vaccinia virus, Herpes-Simplex-virus, Adeno-associated virus

    • Superior safety efficiency profile

  • In all viral vectors essential genes for viral life cycle are removed and replaced with a therapeutic gene expression casette containing promoter, gene of interest, and termination signal

• Replication incompetent viral vectors

  • Wildtype virus (AAV)

  

  • Gene therapy vector (AAV)

    • Instead a promoter and a therapeutic gene

    • There is still endocytosis, translocation to nucleus etc

    • But the promoter will lead to transcription of only the therapeutic gene, so only that expresses, and no viral genes are expressed

  

• Requirements of Viral Gene Therapy Vectors

  1. Host cell tropism - uptake by the cell

     1. Depends on the virus interaction with the host cell membrane

  2. Gene expression - transcription in the target cell

     1. The promoter must match transcription factors in the target

  3. Immune response - against vector or transgene

     1. Evade immune response in a gene therapy

     2. Promote immune response in immunotherapies (i.e. CAR-T cells)

• Modifying Host Cell Tropism

  • Natural discovery

    • Which vectors infect which cell or viruses

  • Capsid shuffling

    • Shuffle around some proteins of different ones to create new properties

  • Rational design

    • Can we rationally design them to specifically target certain cell types or receptors

  • Peptide display

    • Other ways we can avoid an immune response

  • Experiment → promoters can restrict transgene expression

    • Synapsin promoter injected into brain and targets neurons

    • GFAP targets astrocytes

    • Mbp targets oligodendrocytes

    • Thus: different promoters can target very different outcomes

• Gene therapies for CNS Disorders

  • Brain encased in skull

    • Access difficult / volume constraints

  • Blood Brain Barrier

    • Eliminates most vector choices

  • Most neural cells do not divide

    • Limits vector choice

  • Neurons arranged into interacting circuits

Gene Therapy / Genetic Diseases

• The first CNS gene therapies

  • Leukodystrophies are rare monogenic white matter diseases

    • but combined the prevalence of approximately 1:75 000 births is significant

    • Primarily affects oligodendrocytes and astrocytes development or survival

    • Canavan disease first gene therapy attempt for CNS disorder

• Metachromatic Leukodystrophy (MLD) is a devastating autosomal recessive white matter disease

  • caused by mutations in arylsulfatase (ARSA) → toxic accumulation of sulfatides in the CNS and spinal cord

  • Late infantile MLD (onset before 2 years of age) is the most common form with damage to oligodendrocyte myelin resulting in rapid progressive patient decline and usually death before adolescence

  • Autosomal → gene defect on autosome (NOT sex chromosome)

  • Recessive → One healthy copy is sufficient to prevent the disease, Both father and mother need to be carriers for the disease to manifest

  • NB: 25% of offspring will be affected by the disease, 25% will be healthy and not carry the disease, 50% will be healthy but carry the disease

  • Progressively worsening symptoms of MLD:

    • Loss of the ability to detect sensations (touch, sound, heat, pain, vision)

    • Loss of motor skills (walking, moving, speaking, swallowing)

    • Stiff, rigid muscles, poor muscle function, and paralysis

    • Loss of bladder and bowel function

    • Seizures, Ataxia, Spasticity

Ex vivo gene therapy

• First approved ex vivo gene therapy for CNS disease

  • Autologous HSC-GT for late-infantile MLD

• Autologous hematopoietic stem cell - gene therapy (HSC-GT) for late infantile MLD

  1. HSCs are transformed with Lentivirus-ARSA ex vivo, checked and expanded and re-introduced

  2. Autologous (donor = patient) HSC repopulate hematopoietic system in myeloablated patient (no graft vs host disease or rejection)

  3. Monocyte derived macrophages enter diseased CNS, persist expressing the therapeutic transgene → sulfatides → cross correction

• Chimeric Antigen Receptor T- Cells advancing into brain tumours

  • CAR T-cell therapies are autologous ex vivo gene therapies to treat cancer and autoimmune disease

INSERT NOTES ABOUT CRISPR AND THE FOLLOWING SLIDE WHEN THE LECTURE HAS BEEN AMENDED!

In vivo non-viral gene therapy

• Antisense Oligonucleotide to treat Spinal Muscular Atrophy

  • Antisense oligonucleotide (ASO)

    • Are short, synthetic oligonucleotides (DNA or DNA analogs)

    • eliminate, reduce or modify mRNAs (distinct mechanisms)

    • are very stable and slow release but struggle to cross the BBB

  • Survival of motor neurons 2 (SMN2)

    • Is a mutated gene duplication of SMN1 with unknown funciton

    • Shows frequent exon7 skipping → exon 7 absent in 90% of SMA2 mRNA leading to a non-functional protein

    • Copy number varies in the population

      • The higher the SMN2 copy number the better the compensation for missing SMN1

• Spinal Muscular Atrophy treatment

  • Among the most expensive drugs in the world

    • SMN1 Targeted therapy

      • Single shot

    

    • SMN2 Targeted therapy

  

  

  • There are 37 FDA approved gene therapies currently

<aside> 📌 SUMMARY:

</aside>

Date: July 31, 2024

Topic: Motor Neurons and Motor Control

Recall

Notes

• Neural centres responsible for movement control

  • Upper motor neurons = control of the local circuit neurons and alpha-motor neurons

  • Lower motor neurons = neurons which send their axons directly to skeletal muscles

  • Local-circuit neurons are located in the spinal cord in the motor nuclei of the brainstem cranial nerves they regulate activity of the lower motor neurons

  • Cerebellum and basal ganglia → regulate activity of the upper motor neurons without direct access to either the local circuit neurons or lower motor neurons

Lower Motor Neurons

• Lower motor neurons are neurons which send their axons directly to skeletal muscles

  • Usually meant alpha-motor neurons however y-motor neurons controlling muscle spindle sensitivity are also lower motor neurons

  • Axons from motor neurons located in the spinal cord travel to muscles via the ventral roots and peripheral nerves

  • Lower motor neurons in the brainstem are located in the motor nuclei and axons travels via cranial nerves

    • NB: Upper motor neurons could also be located in the brainstem

  • All commands for movement (reflexive or voluntary) are ultimately conveyed to muscles only by lower motor neurons → idea of “final common path” because no other cells have direct access to muscles - the path must involve lower motor neurons

• Motor neuron - muscle relationship

  • Each lower motor neuron innervates muscle fibres within a single muscle

  • Individual motor axons branch within muscles on synapse on many muscles fibres

  • Each muscle fibre is innervated only by one single alpha-motor neuron

  • An action potential generated in the axon brings to the threshold and activate all muscle fibres it innervates

• All motor neurons innervating a single muscle are called motor neuron pool for that muscle and are grouped together into one cluster

  • The motor neuron pools that innervate distal parts of the extremities (fingers and toes) lie farthest from the midline

Motor Unit

<aside> 💡 A motor unit is made up of a motor neuron and the skeletal muscle fibres innervated by that axon.

</aside>

• Fibres are typically distributed over a relatively wide area within the muscle

  • To ensure that the contractile force is spread evenly

  • To ensure that local damage to motor neurons or their axons will not have significant effects on muscle contraction

• Activation of one motor unit corresponds to the smallest amount of force the muscle can produce

• Types of motor units

  • Motor units vary in size - both in regard to cell body size of motor neuron and number of fibres it innervates

  • Small alpha-motor neurons innervate relatively few muscle fibres to form motor units that generate small forces

  • Large alpha-motor neurons innervate larger number of more powerful muscle fibres

  • Motor units differ in the types of muscle fibres that they innervate

  • Small alpha-motor neurons have lowest activation thresholds and thus are first to be recruited

• Henneman’s size principle of motor unit recruitment

  • In 1960s Elwood Henneman from Harvard Medical School observed that gradual increase in muscle tension results from the recruitment of motor units in a fixed order according to their size

  • During a weak contraction only low threshold small size S motor units are activated

    • As synaptic activity driving a motor neuron pool increases, the FR units are recruited

    • To reach the max force finally the largest size FF units are recruited last

  • This systematic relationship is known as the size principle

• Strength of muscle contraction is regulated by means of discharge rate and number of active motor units

  • Motor neurons and action potentials transmitted by axon

  • The more motor neurons, the larger the muscle contraction

Local Circuit Neurons

• Are interneurons, which are responsible for activation of alpha-motor neurons

  • Located close to where corresponding alpha-motor neurons are (in spinal cord or in motor nuclei of brainstem cranial nerves)

  • Receive descending projections from higher centres

  • Mediate sensory-motor reflexes

  • Maintain interconnections for rhythmical and stereotyped behaviour

Even without inputs from the brain the local circuit neurons can control involuntary highly coordinated limb movements like walking (has been demonstrated in animals, some success has been seen using electrical stimulation in humans)

Upper motor neurons (UMNs)

• Cell bodies located in the cerebral cortex or brainstem

• Upper motor neurons in the cortex are essential for initiation of voluntary movements

• Essential for complex spatiotemporal sequences of skilled movements

• Axons synapse with the local circuit neurons and in rare cases (mostly for distal muscles) directly with lower motor neurons

• Upper motor neurons in the brainstem are involved in regulation of muscle tone, control of posture and balance in response to vestibular, auditory, visual and somatic sensory inputs

Cerebellum and basal ganglia

• Cerebellum and basal ganglia are called complex circuits and they

  • Do NOT contain any type of motor neurons

  • Do NOT have direct access to either local circuit neurons or lower motor neurons

  • Regulate activity of upper motor neurons

• Cerebellum

  • Largest subsystem detecting and attenuating the difference between expected and actual movement - ‘motor error’

  • Mediates real-time ongoing error correction (feedback control)

  • Responsible for long term reduction of errors (motor learning)

• Basal ganglia

  • Supress unwanted movements

  • prepare upper motor neuron circuits for initiation of movement

  • malfunction can lead to Parkinson’s and Huntington’s disease

Hierarchical organisation of movement control

Normal Function of motor neurons, reflexes and reflex control

Spinal reflex

• An involuntary response to activation of a sensory receptor that is mediated through spinal pathways

• The concept of a “reflex” is changing

  • Once perceived as hard wired, but now even the simplest reflex is viewed as highly modifiable

• Now the concept of reflex modulation predominates

  • A reflex response depends on the context/task being performed

  • Reflex are incorporated with the voluntary motor command

• Spinal reflex arc

  • Only difference is how many neurons in the spinal cord are involved

    • Mono-synaptic obviously means one synapse, and the synapse is directly on the motor neurons

    • Polysynaptic involves interneurons, could be one or a whole network of them

  • Spinal cord integrating center is the same for both; every synapse in the spinal cord is heavily modifiable

Sensory systems controlling reflexes

• Propioception

  • Meaning one’s own, individual, and perception, in the sense of

    • The relative position of neighbouring parts of your body

    • Position of limbs and other body parts in space

    • Strength of effort being employed in movement

  • Specialised mechanoreceptors

    • Muscle spindles

    • Golgi tendon organs

    • Joint receptors

  To be able to provide this information signals from specialised proprioceptors have to be integrated with signals from other receptor types and sensory systems:

  • Vestibular sensory system

  • Skin mechanoreceptors may provide propriocetive information to signal body part location by sensing pattern of skin stretch

    • Skin stretch tells us where our fingers are, what angle each joint is bent

  • Visual system, very hard without the visual system e.g., imagine using your hand to grab something in the dark (also plays an important role continuously calibrating the proprioceptive system)

    • Can identify errors and then calibrate the system

• Proprioception - Muscle Spindles

  • Extrafusal muscle fibres - true force producing fibres of the muscle

  • Intrafusal muscle fibres - part of the sensory organ - muscle spindles. Keep sensory elements stretched to be able to maintain sensitivity to changes in stretch regardless of the overall muscle length

    • Primary endings - show rapidly adapting responses to changes in muscle length. Provide info about velocity of movement

    • Secondary endings - produce sustained response to muscle length, thus largely provide information about extent of muscle strength

    • Gamma motor neurons activate intrafusal muscle fibres and by changing tension significantly impact on sensitivity of muscle spindles

    • Alpha-motor neurons activate extrafusal (force producing) muscle fibres

  • The highest density of muscle spindles is in extraocular muscles, intrinsic muscles of the hand and muscles of the neck

  • Muscle spindles are not present in the middle ear muscles

  

Co-activation of alpha and gamma motor neurons

• Muscle spindles respond to stretch, but muscle contraction shortens muscle rather than stretches it

  • When you contract muscles, you also change the length of muscle spindles? CLARIFY

Functions of spinal reflexes

• Rapidly respond to perturbations

  • Allow very fast initiation of corrective responses following an unexpected perturbation e.g. Stretch reflex

• Contribute to the motor control and movement adjustments

  • Take care of the details of movement execution to unload higher control centres

Reflexes can be highly organised and modulated accordingly to the task

• A perturbation of one arm causes an excitatory reflex response in the contralateral elbow extensor muscle when the contralateral limb is used to prevent the body from moving forward by grasping the table

• The same stimulus produces an inhibitory response in the muscle when the contralateral hand holds filled cup.

• Muscle stretch reflex

  • Biological function of the stretch reflex is to maintain muscle at a desired length

  • From the control POV stretch reflex is a feedback control mechanism

    • Deviation from a desired length is detected by muscle spindles. The increase or decrease in stretch of muscle spindles alter their discharge rate, which directly translates into excitation of alpha-motor neurons and muscle contraction.

    • The induced muscle contraction will return muscle to the desired length and limb to its initial position restoring muscle spindle activity to a background level

  • During neurological testing, the input is mostly from the afferent muscle spindles.

  • Normally muscles are always under some degree of stretch, this reflex circuit mediated by group II muscle spindle afferents is responsible for the steady level of muscle tension in muscle called muscle tone.

  

Proprioception - Golgi tendon organs (GTO)

• Golgi tendon organs are formed by branches of group 1b afferents distributed among collagen fibres that form tendons. They provide information about muscle tension.

  • Group b is slightly smaller in diameter than 1a (muscle spindles)

• GTOs are arranged in series with a small number (10-20) of extrafusal muscle fibres. Population of afferents provide accurate sample of tension which exists in a whole muscle.

• Golgi tendon organs → A negative feedback system to regulate muscle tension

  • Golgi tendon organ circuit is a negative feedback system to regulate muscle tension

    • Contacts 1b inhibitory interneurons in local circuit

  • GTO control system tends to maintain a steady level of force, counteracting effects that diminish muscle force, for example, fatigue

  • It plays a protective role at large forces

  • 1b inhibitory interneurons receive modulatory synaptic inputs from various sources including upper motor neurons, joint receptors, muscle spindles and cutaneous receptors

<aside> 💡 **Muscle spindle system is a feedback control system that monitors and maintains muscle length and thus keeps limbs in a desired position.

Golgi tendon organ system is a feedback control system that monitors and maintains muscle force.**

</aside>

• Protective reflexes mediated by GTO

  • Reflex gets stronger as the load becomes heavier.

• Protective reflexes: flexion reflex pathways

  • triggered by cutaneous nociceptors

  • Polysynaptic pathway

  • Excitation of ipsilateral flexors and inhibition of extensors

  • Inhibition of contralateral flexors and excitation of extensors, thus providing postural support during withdrawal

  • Descending pathways regulate suppression of the reflex

  • Following damage to descending pathways and after removing inhibition other types of stimuli can trigger the flexion reflex

<aside> 📌 SUMMARY:

</aside>

Date: August 1, 2024

Topic: Motor Control Diseases

Recall

Notes

• Diseases affecting motor system - sites of pathology

  • Motor neuron cells are destroyed

  • Peripheral neuropathies (axons and myelination are affected) → outside brain and spinal cord

  • Neuromuscular junction → physiologically very important, site of many diseases

    • it is a chemical synapse where an AP is transferred from a neuron that activates muscle fibre

    • Motor neuron sustains the life of muscle fibres

    • Acetylcholine acts on many vesicles ?

      • Some stats say that full recovery of required acetylcholine if simulation does not exceed 30 impulses per second?

Examples of commonly known diseases affecting motor system

• Motor neuron diseases (primarily effect cell body of motor neurons)

  • Amyotrophic lateral sclerosis (ALS) affects Upper and Lower motor neurons

  • Primary lateral sclerosis (PLS) affects Upper motor neurons

  • Progressive muscular atrophy (PTA) affects lower motor neurons

• Peripheral demyelinating diseases (damage to the myeline sheath/schwann cells)

  • Guillain-Barre Syndrome (GBS) is an acute idiopathic autoimmune demyelinating diseases of the PNS that is characterised by acute flaccid ascending neuromuscular paralysis. Starts with a microbial infection

  • Charcot-Marie-Tooth disease (CMT) (hereditary disorder)

• Diseases of the neuromuscular junction

  • Myasthenia gravis (MG) (autoimmune disease)

  • Botulism (caused by Clostridium botulinum bacterial toxin)

• Primary muscle disease (myopathies)

  • Myopathies are a heterogenous group of disorders primarily affecting the skeletal muscle structure, metabolism or membrane channel function

  • The muscular dystrophies are a group of genetic diseases characterised by progressive weakness and degeneration of the skeletal muscles that control movement

  • Some forms of MD are seen in infancy or childhood, while others may not appear until middle age or later. Some forms can affect cardiac muscle

    • Duchenne Muscular Dystrophy (mutation in dystrophin gene)

      • Most common form of MD, and primarily affects boys.

      • Caused by absence of dystrophin, a protein involved in maintaining the integrity of muscle

      • Onset is between 3 and 5 years and the disorder progresses rapidly

      • Most boys are unable to walk by age 12 and later need a respirator to breathe

    • Myotonic Muscular Dystrophy (mutation in DMPK and CNBP genes)

      • Myotonic MD is the disorder’s most common adult form, and is typified by prolonged muscle spasms, cataracts, cardiac abnormalities and endocrine disturbances. Individuals with myotonic MD have long, thin faces, drooping eyelids and a swan-like neck

• Diseases affecting motor control - basal ganglia

  • Parkinson’s disease is a progressive incurable neurogenerative disease affecting normal function of the basal ganglia, by removing excitatory dopaminergic inputs from substantia nigra compacta

  • Parkinsonism is a general term that refers to a group of neurological disorders that cause movement problems similar to those seen in Parkinson’s disease, such as tremors, slow movement and stiffness.

    • Early in the disease process, it is often hard to know whether a person has Parkinson’s or a syndrome that mimics it. A wide range of causes may lead to the onset of these symptoms, such as drugs, toxins and metabolic diseases

  • Huntington’s disease is an inherited disorder that results in death of brain cells affecting normal function of the basal ganglia, by removing inhibitory control loops resulting in hyperactivity and uncontrollable unwanted movements. Huntington’s disease occurs when there are more than 35 CAG (cytosine-adenine-guanine) triplet repeats (codon for glutamine amino-acid) on the gene coding for the huntington protein (HTT)

• Examples of commonly known diseases affecting motor system

  • Cerebral palsy is a permanent movement disorder due to abnormal development, not progressive

  • Tetanus is an infectious disease caused by bacterium clostridium tetani characterised by severe muscle spams. The bacteria lives in soil and infection is often associated with rusted objects

    • Spasms may be so severe they result in torn ligaments or even bone fractures → often begins from jaw and facial muscles (lockjaw)

  • Polio is an infectious disease caused by the poliovirus. It may cause severe damage of motor neurons, which may result in temporary or permanent paralyses

Motor neuron dysfunction

• Consequence of diminished descending control of spinal motor neurons

  • Whilst input from the upper motor neuron sis essential for initiation of voluntary movements is excitatory, the majority of inputs controlling spinal reflexes are inhibitory, supressing reflexes when they are not meaningful

  • Thus the reduction in descending input to spinal interneurons result in exaggerated unrestricted flow of excitation reaching motor neurons

  • Also the intrinsic movement excitability may increase to compensate for the reduction of functional activation of the spinal cord

• Signs and symptom of UMN dysfunction

  • Hyperreflexia - exaggerated reflexes

  • Spasticity - muscular hypertonicity with increased tendon reflexes; unlike rigidity it is velocity dependent, i.e., the faster the muscle is stretched the greater resistance and more reflex activity; affects movement in one direction

  • Rigidity - an increased muscle tone leading to a resistance to passive movement throughout the range of motion in both directions. Residual muscle tone or tonus is partial contraction of the muscles during resting state. It is present in a normal muscle

    • it is not a typical sign of UMN damage, but it results from dysregulation of UMN function originating from the basal ganglia

  • Clasp-knife phenomenon - a manifestation of corticospinal spasticity in which there is a sudden release of the resistance to passive flexion/extension typically near the end of the range of joint movement

  • Clonus - muscular spasm involving a series of brisk repeated rhythmic, monophasic (i.e., unidirectional) contractions and relaxations of a group of muscles

  • Myoclonus - very rapid, shock-like contractions of a group of muscles, which are irregular in rhythm and amplitude

  • contracture - a permanent structural shortening of a muscle or joint usually in response to prolonged hypertonic spasticity producing deformity

  • Babinski sign - reversal of cutaneous flexor reflex

    • Following the removal of the descending corticospinal pathways, stroking the sole of the foot may cause an abnormal fanning of the toes and the extension of the big toe

    • Used as a diagnostic tool

    • infants will also show an extensor response - a baby’s smaller toes will fan out

      • This happens because the corticospinal pathways that run from the brain down the spinal cord are not fully myelinated at this age, so the reflex is not inhibited by the cerebral cortex

      • The extensor response disappears and gives way to the flexor response around 12-24 months of age

  

  • Due to loss of voluntary control

    • Loss of dexterity

    • Slowness

    • Clumsiness

<aside> 💡 Symptom = subjective Sign = Objective (doctor and patient can see it)

</aside>

• Signs and symptoms of LMN degeneration

  • Weakened reflexes

  • Flaccidity (decreased muscle tone)

  • Muscle cramps

  • Fasciculation (a brief spontaneous contraction affecting a small number of muscle fibres, involuntary contraction of muscle fibres often seen as flickering of movement under the skin)

  • Muscle wasting

    • Little dip

    

Motor Neuron Diseases

• See definitions earlier

• Epidemiology

  • 8.7 / 100 000 Australians prevalence in 2015

  • About 1900 Australians currently suffer from MND

  • Each day 2 people in Aus are diagnosed with MND

  • Males > females 2:1

  • Sporadic 90-95%

  • 5-10% inherited

  • Onset usually >40 years; 58% < 65 years

  • Total cost is 1.1 million per patient

  • Avg life expectancy is 27 months, 10% surviving longer than 10 years

• Amyotrophic lateral sclerosis (ALS)

  • Named by Jean Martin Charcot in 1874

  • Degeneration of the motor neurons (UMN and LMN) in motor cortex, brainstem and spinal cord

  • Lateral identifies the affected area of the spinal cord

  • Typical LMN signs (weakness, wasting, fasciculations)

  • Typical UMN signs (spasticity, hyperreflexia, Babinski sign)

  • Typically viewed as disease affecting the motor system with no compromise of cognitive abilities

    • Some studies indicate about 25% of patients show some cognitive changes in the frontal lobe region and 3-5% will have fronto-temporal dementia

  • Typically NOT affected:

    • Cerebellular function

    • Sensory function

    • Oculomotor function

    • Autonomic nervous system

    • Bowel and bladder system

    • Sexual function and sexuality

    • Cognitive ability

  • Causes

    • Not known, sporadic in 90-95%

    • Takes 9-15 months for someone to be diagnosed with ALS from time they begin to notice symptoms

    • Possible environmental risks:

      • Exposure to heavy metals, solvents and agricultural chemicals

      • Smoking in postmenopausal women but not men

      • Professional high impact sports

      • military service

    • 5-10% genetic

    • Major gene mutations

      • SOD1 encodes synthesis of CuZn-superoxide dismutase

      • C9ORF72 protein sound in many regions of the brain, most common mutation associated with ALS

      • DCTn1 encodes dynactin. Role is implied in both ALS and FTD

      • TARDBP gene encoding TDP-43 protein. It is transcriptional repressor, associated with several neurodegenerative diseases

  • Treatment - no cure, just therapy to improve quality of life

    • Riluzole, blocks TTX-sensitive sodium channels and decrease glutamate release

      • Delays the onset of ventilator-dependence or tracheostomy in some patients

      • Prolongs overall survival by 203 months

    • Edaravone was originally marketed for use in strike patients. It was approved recently in Aus, its approval states that it is effective within 2 years of onset. Is a drug with antioxidant properties

    • AMX0035, made up of two components

      • Tauroursodeoxycholic acid

      • Sodium phenylbutyrate

      • Thought to increase the threshold for cell death by blocking key cell death pathways

      • Its efficiency is still debated

  • ALS symptomatic treatment

    • Spasticity - Baclofen, Diazepam and stretching-exercise

    • Fasciculations - Lorazepam; decrease caffeine and nicotine intake

    • Respiratory insufficiency - non-invasive positive pressure ventilation

    • Dysphagia - percutaneous endoscopic gastronomy feeding tube

    • Sialorrhoea (hypersalivation) - anticholinergics, scopolamine

    • Pain - NSAIDs

    • Depression - SSRIs, tricyclic antidepressants

• Progressive bulbar palsy

  • Primarily bulbar palsy primarily affects motor neurons in brainstem

  • Symptoms include:

    • Pharyngeal muscle weakness (involved with swallowing), weak jaw and facial muscles, progressive loss of speech, and tongue muscle atrophy

    • Patients are at increased risk of choking and aspiration pneumonia, which is caused by the passage of liquids and food through the vocal folds and into the lower airways and lungs

    • Limb weakness with both lower and upper motor neuron signs is often evident but less prominent

    • Patients have outbursts of laughing or crying (emotionally lability)

    • In about 25% of patients with ALS, early symptoms begin with bulbar involvement

    • Life expectance between 6 months and 3 years from diagnosis

• Pseudobulbar palsy

  • Pseudobulbar palsy shares many symptoms of progressive bulbar palsy but is characterised by selective degeneration of upper motor neurons that transmit signals to the lower motor neurons in the brain stem

  • Symptoms include:

    • Progressive loss of ability to speak chew and swallow

    • Progressive weakness in facial muscles

    • May develop a gravelly voice and increased gag reflex

    • The tongue may become immoble

    • Outbursts of laughing and crying

• Primary lateral sclerosis

  • PLS affects UMNs of arms, leg and face

  • Affects legs first, followed by body trunk, arms and hands, and finally the bulbar muscles

  • PLS is more common in men than women

  • Symptoms progress gradually over the years, leading to progressive stiffness and clumsiness of the affected muscles

  • Disorder is not fatal

  • Sometimes considered a variant of ALS, but big differences are that there is a sparing of lower motor neurons, the slow rate of disease progression and normal lifespan

• Progressive muscular atrophy (PMA) (non hereditary)

  • Progressive (spinal) muscular atrophy is marked by slow but progressive degeneration of only the lower motor neurons

  • Diagnosed by exclusion, mostly effects men

  • Half of patients will live more than 5 years after diagnosis

  • Weakness is typically seen first in the hands and then spreads in to the lower body, where it can be severe

  • Other symptoms may include

    • Muscle wasting, fasciculations, and muscle cramps

    • Loss of dexterity

    • The trunk muscles and respiration may become affected

    • Exposure to cold can worsen symptoms

    • Disease develops into ALS in many instances

    • Bulbar signs

• Spinal Muscular Atrophy

  • Is a hereditary disease affecting the lower motor neurons

  • Autosomal recessive disorder, caused by deficits in SMN1 gene which makes a protein important for the survival of motor neurons

  • The muscle weakness is often more severe in the trunk and upper leg and arm muscles than in muscles of the hands and feet

  • SMA in children can be further classified into several variants, based on ages of onset, severity and progression of symptoms, however, all of them are caused by defects in the SMN1 gene

• Post-polio syndrome (PPS)

  • Polio = Acute contagious viral disease spreading through human faecal matter

  • May cause severe damage of motor neurons, but strictly speaking it is not a motor neuron disease due to its broad effects

  • Some forms of it may cause paralyses, temporarily or permanently

  • Post-polio syndrome is a condition that can strike polio survivors decades after their recovery

  • The survival motor neurons expand the amount of muscle made that each controls

  • PPS and Post-Polio muscular atrophy are thought to occur when the surviving motor neurons are lost in the aging process or through injury or illness

  • it is suggested that PPS is latent weakness among muscles previously affected by polio and not a new MND

  • Symptoms are similar to progressive muscular atrophy and appear most often among muscle groups affected by the intiial disease

  • Doctors estimate that 25-50% of survivors of paralytic polio usually develop PPS

  • normally not life threatening