Tourette's Syndrome

What is TS?

A neurodevelopmental disorder characterised by the repetition of tics and movements

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What are tics?

Involuntary, repetitive, stereotyped behaviours that can occur many times in a single day

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What can motor tics range from?

Simple repetitive movements (blinking, grimacing) to complex coordinated action sequences, such as the repetition of another's actions (echopraxia)

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What do verbal tics consist of?

Grunting, repetition of words or utterances, production of inappropriate or obscene utterances, repetition of another words or actions

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Freeman (2000)

Studies 3500 cases, TS is commonly associated with ADHD and OCD

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What are tics difficult to do?

Distinguish from other repetitive behaviours such as compulsions or hyperactive movements in ADHD

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What does TS affect?

1% children, typicall presents during childhood (onset 3-6 years)

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Ratio of TS?

4:1 ratio boys: girls in child samples but 1:1 in adults

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What is the percentage of children with Tic?

70% of children with TS have very mild tics or are free of tics by 18 years of age

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What does this suggesT?

Compensatory adaption

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What does theory of TS disorder implicate?

The inhibition and altered projection in a cortico-striatal-thalmo-cortical pathway

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What does disinhibition of CSTC circuits give rise to what?

impairment of executive or cognitive control of motor behaviour, characterised by a reduced behavioural inhibition (e.g., Channon et al., 2009)

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What is it suggested?

Motor tics are the result of increased signals from the thalamus to the motor cortex

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The brain circuits underlying tics in TS are unknown but thought to involve what?

Cortico/amygdalo-striato-thalmo-cortical (CSTC) loop hyperactivity

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What is the cause of these signals?

The reduction in thalamus regulation by inhibitory neurtransmitting in the basal ganglia (Albin and Mink, 2006)

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CSTC networks in what?

disorganised in adults with TS, relative to matched controls. Abnormalities in functional connectivity were correlated with Tic severity. (Worbe et al, 2012)

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Functional MRI of TS shows what?

primary hyperactivity of excitatory somatosensory, insular and efferent motor output circuits, which elicits premonitory urges and tics (Bohlhalter et al, 2006)

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What is the secondary hypoactivity?

motor suppressing executive control circuits (Swerdlow and Sutherland, 2005)

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What does TMS confirm?

Both disorders involve no cortical inhibition and disinhibition (Ziemann et al, 1997)

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Peterson et al (2004)

5% decrease in caudate volume, reduced inhibition in striatum leading to hyper excitation of cortical motor areas

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What common features do tics and habits share?

Habitual behaviours, similarly to tics, are driven by contextual cues through stimulus-response associations and insensitivty to the desirability of the outcome

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Tourettes Association (2018)

People with tourettes often feel uncomfortable when trying to ‘suppress’ a tic, it can cause them to have an uncomfortable ‘tickling’ sensation or described as a knot in the stomach.

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People with tourettes say they feel better when?

letting the tic out. This could be a stimulus and response, for example, People with tourettes syndrome have an uncomfortable feeling then they tic

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This is positively reinforced by what?

This is positively reinforced by the fact that they no longer have the uncomfortable feeling

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The basal ganglia have been shown to contribute to what?

Habit formation and SR learning

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What is the learning characterised by?

two key features: Lack of awareness of the algorithm learned and a slow rate of acquisition

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Habits are formed where?

in the basal ganglia through the repeated process of evolution and selection of an action

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What does the BG do?

brings together these functions in part by promoting the building up of performance units made up of multiple acts that can be implemented in a particular temporal order – sensorimotor form of chunking (Miller, 1956)

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Behaviour is what?

Chunked and results in the formation of a habit

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TS has been particularly been associated with dysfunctional signalling by what?

neuromodulator dopamine, which is strongly linked to mechanisms of reinforcement learning (Schultz, 1997)

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What happens in the normally developed brain?

the movement and the goal/reward response is coupled with a reduction in the activity of chronergically active inhibitory neurons (GABA) and the release of dopamine in the basal ganglia

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What is suggested about tics?

are created into habits through this system as the mechanism is active without conscious awareness (greybiel, 2008)

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Studies using animal models have directly investigated the relationship between who?

Impaired GABA function within the striatum and the occurence of Motor tics

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Post mortem studies indicate what?

loss can primarily be attributed to a specific reduction in the number of striatal GABAergic interneurons

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McCairn et al (2009)

Focal micro-injections of GABA A antagonist bicuculline to the sensorimotor putamen of behaving primates to induce stereotyped tics similar to those observed in tourettes syndrome

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What did results demonstrate?

focal disinhibition of the sensorimotor striatum leads to formation of repetitive motor tics, confined to single or small groups of muscles predominantly in the orofacial region

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Bronfield et al (2013)

published a review about tics being induced by striatal disinhibition in both monkeys and rats

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When did Tics begin?

several minutes after a microinjection of bicuculline, where injections to the anterior or posterior striatum led to tics in either the forelimb or the hindlimb.

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Therefore this suggests what?

striatal disinhibition in the rat may be used to model motor tics such as observed in TS.

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TS have neuro-genetically developed?

Less fast spiking neurons

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Neurons are only representative in how many striatal neurons?

1%

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They have what?

a broad and large inhibitory effect on the surrounding output neurons, medium – spikey – neurons or MSNs (Tremblay, 2015)

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What do these results strongly support?

pathophysiological hypothesis of an inhibitory dysfunction inside the anterior striatum, which can affect different cortico‐BG circuits

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What does this lead to?

motor tics and behavioral disorders expressed in GTS. – This supports previous findings by Bronfield et al (2013

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What did targeting these neurons do?

Injections with bicuclare, models of direct and indirect can be created

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What are direct pathways promoting?

motor behaviour by actively inhibiting the basal ganglia’s two GABAergic output nuclei- the internal globus pallidus (GPI) and substantia nigrapars reticula (SNR)

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Project to where?

thalamocortical and brain stem motor circuits. The reduction in inhibitory signals leaving the basal ganglia results in disinhibition of these circuits

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What does the indirect pathway include?

the additional synapses onto the external globus palldus and subthalamic nucleus, increases the activity of the basal ganglia’s output nuclei.

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When does this increase in activity result in?

SUppression of the thalamocortical circuitry and ultimately inhibits movement

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It has been suggested that the reduction of inhibition of MSN neurons populate what?

striatum (caused by the absence of inhibitory FS neurons causes increased inhibition of the globus pallidus internal, this is via the direct pathway

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What happens in the indirect pathway?

the globus pallidus external is overexcited and therefore increases its inhibition of the sub-thalamic-nucleus

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What is the STN?

only excitory synaptic terminal in the BG and its inhibition stops the exitry release of glutamate to the globus pallidus internal

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Therefore?

the globus pallidus internal is vastly inhibited and underactive as as such stops regulating the thalamus

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What does the dysregulation allow?

large and incorrect signals to be sent from the thalamus to the cortex (Joo Lee, 2017)

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Using diffusion weighted imaging what has been shown?

Connectivity of areas are altered in TS patients

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CSTC networks in the TS group were

characterised by a loss of ‘hub’ regions that facilitate fast and efficient transfer of information over long distances within brain networks

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What did healthy controls show what?

mixture of nodes which represent local connections and hubs which represent long distance and interhemisphere communications TS show very little hubs

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What does this suggest?

communication issues may also be involved in the development of lies

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What showed a heightened communication between striatum, thalamus and SMA?

- Increased connectivity represented by fractional antrosophy (or the tendency of water to diffuse in one direction over another)

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Sowell (2011)

- Evidence that tourettes patients exhibit differences in cortical thickness

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This is most evident in which areas?

somatosensory and motor cortex regions that represent facial and lingusitical areas.

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What has not yet been applied to TS?

CSTC circuit

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What was predicted?

application of these technique to the study of children with Tourette syndromes would reveal thinning in sensorimotor cortices, the portions of CSTC circuits that control movement and vocalisation in direct proportion to the severity of tic symptoms.

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When was the thinning most prominent?

ventral portions of the sensory and motor homunculi that control the facial, orolingual and laryngeal musculature that is commonly involved in tic symptoms.

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Correlations of Cortical thickness in sensorimotor regions with tic symproms suggest?

these brain regions are important in the pathogenesis of Tourette syndrome

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Cortical thinning are inversly correlated with tic severity suggested what?

the differences are not to do with adaptions to the disorder but instead are core to the development of the disorder

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Ozonoff et al (1996)

Found that TS participants were sig impaired on a negative prime task (measure of inhibition) compared to controls: RT slower in Ts

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Zapparoli et al (2015)

No-go trials, Ts exhibited greater activation.

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What did a control go/no go task suggest?

- A control Go/No-Go task, which involved devaluation of cueing stimuli, was used to measure to measure response rates where high rates on this task would indicate working memory deficits or deficient response inhibition.

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Tourette's Syndrome

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